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Table of ContentsAbout This Book

About the Author vii

CHAPTER I Basic Instruments 1Why Study Instruments?; Aircraft Instrument Requirements;Pitot-Static System Instruments; Gyro Instruments; Compass Systems;Electronic Instruments; Computers in Aircraft

CHAPTER II Powerplant Instruments and Logic Gates 55

Liquid Quantity Measuring Systems; Fuel Flow Indicators;Temperature Measuring Systems; Position Indicating Systems; Tachometers;Oil Pressure Indicators; Torquemeters; Engine Pressure Ratio Indicators;Manifold Pressure Gauges; Primary Power Setting Instruments; Vibration Indicators;Logic Circuits and Digital Systems; Takeoff Warning Systems; Angle of Attack Indicators;Stall Warning Systems; Annunciators; FARs for Warning Systems and Annunciators

CHAPTER III Communication and Navigation Systems 103Radio Fundamentals; Regulations and Standards for Radios;Intercom and Interphone Systems; Communications Radios;Emergency Locator Transmitters (ELTs);Cockpit Voice Recorders and Flight Data Recorders; Navigational Systems;Long Range Navigation Systems; Instrument Landing System (ILS);Microwave Landing System (MLS); Radar Altimeter;Ground Proximity Warning System (GPWS); Weather Radar; Stormscope®;TCAS — Airborne Collision Avoidance System

CHAPTER IV Aircraft Antennas and Autopilots 149Installation and Inspection of Avionics; Antenna Installations;Autopilots and Flight Directors

Appendix A Glossary 193

Appendix B Abbreviations 197

Index 201

iii

Aircraft Technical Books, LLC (970) 726-5111 http://www.ACTechBooks.com

iv


About This Book

This textbook is intended to be used in the instruc-tion of students in an aviation maintenance tech-nician training program. The descriptions, drawingsand graphics in this book are for instructional pur-poses only and should not be used as a technicalreference source for specific maintenance tasks onaircraft or aircraft systems or for other operationalpurposes. Excerpts from Federal Aviation Regula-tions and other sources have been paraphrased andsimplified in order to save space and time.

The author wishes to express his appreciation forpermission to use material from the technical pub-lications of the following aviation companies.

Beechcraft Aircraft Corp. (Wichita, Kansas)Canadair Group, Bombardier Inc. (Montreal,

Quebec, Canada)Cessna Aircraft Co. (Wichita, Kansas)Comant Industries Inc. (Sante Fe Springs,

California)

Dayton-Granger Inc. (Fort Lauderdale, Florida)Dorne and Margolin Inc. (Bohemia, New York)Flight Dynamics Inc. (Portland, Oregon)Piper Aircraft Corp. (Vero Beach, Florida)Sensor Systems (Chatsworth, California)Terra Avionics (Albuquerque, New Mexico)United Technologies — Pratt & Whitney Canada

Inc. (Longueuil, Quebec, Canada)

The Canadair drawings which appear throughoutthis book are the proprietary property of BombardierInc, Canadair Group. As such, it is forbidden to copythese drawings without the express written permis-sion of Bombardier Inc., Canadair Group. Readersare cautioned that the drawings are trainingmaterial only and as such are not subject to revision.They are not to be used in lieu of approved technicalmanual illustrations for the purposes of carrying outany maintenance procedure or any other activity onany Canadair aircraft or any other aircraft.


Vi


About The Author

Max F. Henderson has been teaching AviationMaintenance Technology subjects at Embry-RiddleAeronautical University since 1982. Previous ex-perience includes working as an Electronics Tech-nician in the U.S. Air Force, as a Commercial pilotand A&P mechanic and as a Control Tower Operator.Mr. Henderson holds four FAA certificates:

Commercial Pilot Certificate — Ratings forsingle and multi-engine land, instruments,single-engine seaplanes and gliders.Mechanic Certificate — Airframe andPowerplant ratings

3. Ground Instructor Certificate — Advancedrating

4. Control Tower Operator CertificateDuring his years at Embry-Riddle Aeronautical

University, Mr. Henderson has earned the follow-ing degrees: A.S. Degree in Aircraft Maintenance,B.S. Degree in Professional Aeronautics and aMaster's Degree in Aeronautical Science. Mr.Henderson acquired his interest in aviation fromhis father Floyd B. Henderson whose flying andmaintenance experience on aircraft began in 1932.Mr. Henderson's interest in aviation history isevidenced by a collection of books andphotographs of early aviation dating back to theWorld War I era.

vii


viii


CHAPTER I

Basic Instruments

Chapter one begins with a study of the generalrequirements for aircraft instruments and their in-stallation. The categories and types of instrumentsare covered before beginning a study of specificinstruments.

The instruments examined in chapter one includepitot-static system instruments, gyro instruments,compass systems and electronic instruments. Ref-erence will be made to Federal Aviation Regulationswhich apply to these areas, particularly FAR Part1 Definitions and Abbreviations, FAR Part 23, FARPart 43 and FAR Part 91.

A. Why Study Instruments?

It is important for aircraft technicians to studyaircraft instruments so that they will be able toinspect, install and troubleshoot them properly.There are also occasions when the technician willbe running the aircraft's engines or other systemsand will have to use the instruments himself. Manyinstruments are a part of a larger system and itis necessary to understand the interrelationshipsbetween the various parts of the system.

All certificated aircraft have instruments, al-though in the case of hot air balloons and gliders,

only a few basic instruments may be required. Thenumber and variety of instruments has increasedover the years so that a small single engine airplaneof today has more instruments and more sophis-ticated instruments than airliners had in the 1940sor earlier. Figure 1-1 shows the instrument panelof a Piper Cub from the early 1940s with its sixbasic instruments. This airplane was a small twoplace airplane which did not have an electrical sys-tem, so all the instruments used mechanical meansof operation. The instrument panel shown in figure1-2 is that of a modern single engine airplaneequipped for "blind flying" or IFR flight operations.This airplane has many more instruments and sys-tems that increase the safety of flight and makeit a more efficient means of transportation. Themost important instruments are placed directly infront of the pilot and the radios are grouped togetherin the middle for easy access to the controls.

The most common and important types of aircraftinstruments and avionics systems will be describedin this book along with some FAA requirementsfor testing and installation. The categories foraircraft instruments and the basic FAA require-ments will be covered first.

OILTEMPERATURE

TACHOMETER

AIRSPEED

COMPASS ALTMETER

OILINDICATOR

PRESSURE

Figure 1-1. The instrument panel of a 1940s Piper Cub. (Courtesy Piper Aircraft Corp.)

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B. Aircraft InstrumentRequirements

1. Instrument CategoriesThe instruments found on different types of aircrafthave considerable variety, but they can be categorizedaccording to either how they work or what kindsof information they present to the flight crew.

a. Categories According to Application

Powerplant Instruments — These give infor-mation related to the aircraft's powerplant orpowerplants.

Flight and Navigation Instruments — These giveinformation such as altitude, speed etc. or in-formation required for navigating the aircraft.

3. Systems Instruments — These concern air-craft systems such as electrical, hydraulic,pressurization, bleed air systems etc.

b. Categories According to Means of Operation

Pressure Instruments — These measure thepressure of air, fuel, oil, etc.

Mechanical Instruments — These use a me-chanical system to obtain and/or transmitinformation.

3. Gyro Instruments — These use the principlesof a gyroscope and are primarily used for IFRflight.

4. Electrical and Electronic Instruments — Thisgroup has seen the most change in recentyears due to advances in digital technologyand other related fields.

2. Instrument Placementand Installation

While there is not a standard placement for allthe instruments that might be found on eithera small or large aircraft, some of the most im-portant instruments will have a standard layoutdirectly in front of the pilot. This makes it easierfor the pilot to scan the important instrumentsand it makes it easier to transition to a differenttype of airplane.

Aircraft instruments are manufactured in a num-ber of standard sizes. This mainly applies to theround instruments since some other types comein a wide variety of sizes. The standard sizes forround instruments are:

1" — Often a vacuum gauge on single-engineairplanes

2" Flangeless — Many turbine engine powerpl-ant instruments are this type.

3. 2- 1/4" — A common size for many differentinstruments

4. 3- 1/8" — Considered to be a standard full-sizeinstrument

Figure 1-2. Typical arrangement of instruments and radios in a single engine airplane.

2


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5. 4" — The older style gyro instruments wereoften this size, but it is seldom used on amodern aircraft.

Examples of the 1", 2- 1/4" and 3- 1/8" sizes canbe seen on the instrument panel in figure 1-2.

There are a number of methods used to installinstruments into the aircraft instrument panel. Thethree most common methods are:

Screws — Non-magnetic fine thread machinescrews are used, brass screws with a blackoxide finish are very common.

Circumferential Clamps — These clamps arefastened to the back of the instrument paneland operate somewhat like a hose clamp (fig-ure 1-3).

3. Brackets — small "L" or "U" shaped bracketsare installed on studs to hold the instrumentin place (figure 1-3).

Installing aircraft instruments is often made dif-ficult by the fact that there isn't much room behindthe instrument panel and access is limited. Someaircraft use sliding or hinged panels to improveaccess to this area.

3. Instrument MarkingsAircraft instruments often utilize colored mark-ings so that safe operating values can be indicatedto the pilot. For example, red usually means amaximum or minimum operating limitation forthe airplane or engine. These markings are nor-mally on the face of the instrument, inside thecover glass. It is permissible to apply coloredmarkings with paint to the cover glass, but if

this is done the marks must not interfere withreading the instrument and a white line mustbe applied to the cover glass and case to actas a slippage mark.

When applying or inspecting the markings oninstruments, a suitable reference source must beused, the acceptable sources are:

Approved Aircraft Flight Manual or Pilot's Op-erating Handbook — This is an FAA-approveddocument which is a part of the required air-craft equipment.

Maintenance Manual or Service Manual.

Type Certificate Data Sheet or Specifications.

STC, Manufacturers Service Bulletins, and ADNotes — These would indicate a change fromthe original aircraft requirements.

There are standard meanings for the differentcolors and markings applied to instruments likepowerplant and system instruments, they are:

Red Radial Line — This indicates a maximumor minimum operating limitation. Example: onan oil pressure gauge.

Red Arc — This indicates a prohibited rangeof operation. A common example is the redarc on a tachometer because of vibration prob-lems at certain RPMS.

Yellow Arc — This indicates a caution range.

Green Arc — The normal operating range.

5. Blue Arc or Line — This has a meaning spec-ified by the manufacturer. An example is ablue arc on a manifold pressure gauge forengine operation with a lean mixture.

CIRCUMFERENTIAL CLAMP(A)

BRACKET(B)

Figure 1-3. Two of the common methods used to install instruments in an aircraft instrument panel.

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The colored markings on an airspeed indicator

have different and very specific meanings, this will

be covered later. The colored markings found onan airspeed indicator are shown in figure 1-4, noticethat both arcs and radial lines are used.

4. General Precautions forInstalling Instruments

A number of things must be kept in mind wheninstalling instruments, radios and related systems.Instruments are very delicate and must be handledwith care. If an instrument must be sent to a repairstation or other place for repair, it should be packedin an approved container with any ports or openingsplugged and with a desiccant pack to prevent mois-ture damage if it is sensitive to moisture. Whenshock mounts are required for a particular instal-lation, you should ensure that the shock mountsare in good condition and that the weight of theequipment does not exceed the weight carryingcapability of the shock mounts. Figure 1-5 showsa shock mount unit for aircraft equipment witha bonding jumper installed. Figure 1-6 illustratesthe use of a shock mounted sub-panel for the flightinstruments on a small airplane.

Many types of instruments can be damaged bystatic electricity and magnetic fields. Bondingjumpers, shielded wires and static wicks must beproperly installed and maintained to prevent thesekinds of problems.

Bonding jumpers have a number of functions wheninstalled on an aircraft. Four of these functions are:

A ground return path for aircraft circuits thatuse single wire type systems.

Reducing radio frequency interference in sen-sitive aircraft systems.

Decreasing the possibility of damage due tolightning strikes on control surfaces and otherareas.

Allowing static charges to move around easilyand equalize. This prevents arcing which couldcreate a fire hazard.

A bonding jumper is a small metal braid or metalstrap which electrically connects together two partson the airframe. Some of the recommendations forinstalling and maintaining bonding jumpers are:

Keep them as short as possible but allow forany movement that is necessary as on controlsurfaces.

Clean the contact areas to minimize resistance.This includes removing coatings such as an-odizing.

Do not solder bonding jumpers. It makes thembrittle.

Do not paint bonding jumpers. It makes thembrittle.

Use multiple jumpers on shock mounted elec-trical equipment. One might break.

Ensure that the jumper is compatible withthe structural material and hardware to pre-vent corrosion. Aluminum jumpers are rec-ommended for aluminum aircraft parts andcopper jumpers are recommended for stainlesssteel, cadmium plated steel, brass and bronze.

Some instrument and equipment installations re-quire the use of shielded wire. Don't assume thatboth ends of the wire shield should be connectedto ground. It is sometimes specified that only oneend of the wire shielding be attached to ground.

Figure 1-4. Operating limitations markings on an Figure 1-5. Bonding jumpers are installed on shockairspeed indicator. mounts to ensure proper grounding.

4


SHOCK MOUNTS

GROUND STRAP(INSTALLED ON SHOCK MOUNTDIRECTLY UNDER CONTROL WHEEL)

When installing an instrument that requireslighting, use care to ensure that the light is in-stalled correctly and that it will not short outor cause a problem with another instrument.There are often many small wires behind the in-strument panel for the light fixtures and theymust be routed and tied carefully. There are fourcommon types of instrument lighting systemsfound on aircraft:

1. Eyebrow Lights — These are small semi-cir-cular fixtures that fit over the top of a roundinstrument and look like eyebrows.

Post Lights — These are small round lightsthat install into holes in the instrument panel(figure 1-7).

Internal Lights — These are inside the instru-ment case.

4. Flood Lights — These lights can be aimed atthe instrument panel and are shielded so thatthey don't shine in the pilot's eyes. They areoften fitted in addition to one of the othertypes as a back-up lighting system.

Figure 1-8 shows the fluorescent flood lightingsystem for a corporate jet airplane.

Figure 1-6. The instrument panel for a twin engine airplane showing the shock mounted sub--panel and two kinds ofshock mounts. (Courtesy Cessna Aircraft Co.)


PILOT INSTRUMENT PANELDUAL STRIP

PILOT SIDEFACIA STRIP

PILOT SIDECONSOLE STRIPS

CO-PILOT INSTRUMENT PANELDUAL STRIP

CO-PILOT SIDEFACIA STRIP

CO-PILOT SIDECONSOLE STRIPS

IN

Figure 1-7. A post light type of lighting fixture (item 1) foraircraft instruments. (Courtesy CessnaAircraft Co.)

Precipitation static, also known as P-static, isa build up of static electricity on the aircraft inflight. It can have an adverse effect on the operationof many instruments and radios. P-static is causedby friction between the aircraft structure and par-ticles in the air such as rain, snow, ice and dustparticles. It can also be caused by the hot exhaustof a turbine engine as it exits the large metal tailpipeor exhaust pipe. It cannot be prevented but theproblems can be reduced by installing good staticdischargers on the aircraft. These are normally in-stalled on the trailing edges of main control surfacesand also occasionally on the tips of the wing andhorizontal stabilizer.

5. FAA Regulations for Instruments

The FAA has many regulations that concern theinstallation of instruments in certificated aircraft.The examples that will be given apply to FAR Part23 airplanes although the requirements for othercategories of aircraft are often very similar. Therequirements of FAR Part 91 would apply to anyaircraft being operated under that section of theFARs. The FARs will not be quoted exactly, but

CENTER INSTRUMENT PANELDUAL STRIP

Figure 1-8. Fluorescent lighting arrangement for a corporate jet. (Courtesy Canadair Group, Bombardier Inc.)

6


(CARBURETORAIR CONTROL

CARBURETOR AIRTEMPERATURE BULB

CARBURETOR

THROTTLE VALVE

INTAKE PIPE MANIFOLDPRESSURE

SUPERCHARGERIMPELLER

INTAKE AIR DUCT

CARBURETORHEAT VALVE

THROTTLE

Figure 1-9. An un-supercharged reciprocating aircraftengine.

will be paraphrased in order to simplify the wordingand save space.

Since some of the FAR rules for instrumentsdepend on what type of engine the aircraft uses,it is necessary to discuss the different types ofpowerplants found on certificated aircraft. Areciprocating engine is a piston engine which caneither be supercharged or non-supercharged. Themost common type of reciprocating engine onmodern aircraft is the horizontally opposed typealthough many radial piston engines are still inservice. Figure 1-9 shows an unsupercharged

horizontally opposed reciprocating engine. Figure1-10 shows a radial engine with an internal su-percharger. A turbine engine could be one of fourtypes. The turbojet and turbofan engines are similarin that they are both rated in pounds of thrust.The difference is that with the turbofan engine someairflow bypasses the core of the engine and is actedupon only by the fan section as seen in figure1-11. The turbojet engine illustrated in figure 1-12does not use the bypass principle since all of theintake air passes through the length of the engine.In the FARs, the use of the term turbojet includesturbofan engines. The other two types of turbineengines are the turbo-propeller and turboshaft en-gines. These are both usually rated in horsepowerbecause they deliver power to an output shaft (unliketurbojet and turbofan engines). The difference be-tween the two is that the turboprop engine turnsa propeller while the turboshaft engine powers therotor drive gearbox of a helicopter. The reductiongearbox and output shaft of a turboprop enginecan be seen on the left in figure 1-13.

FAR Part 23

This FAR covers the Airworthiness Standards forNormal, Utility, Acrobatic and Commuter categoryairplanes. Certain instrument requirements are apart of these Airworthiness Standards.

Figure 1-10. A radial piston engine with internal supercharger.

7


HOTEXHAUST

COMBUSTORS EXHAUSTCOMPRESSOR

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FAR 23.841The additional instruments required for pressurized

airplanes are specified in this section. Figure 1-14illustrates these instruments. Pressurized airplanesare required to have instruments to indicate:

Cabin differential pressure

Cabin altitude

3. Rate of change of cabin altitude (cabin rateof climb)

FAR 23.1301

This FAR states that equipment must be labeledas to its identification, function and operating limita-tions. The colored markings on an airspeed indicatorare examples of these required operating limitations.

FAR 23.1303Required Flight and Navigation Instruments

Airspeed indicator

Altimeter

Magnetic direction indicator (compass)

Free air temperature for airplanes with turbineengines

5. Speed warning for turbine engine aircraft orothers when Vmo/Mmo is greater than .8 Vd.

Notice that only the first three would be requiredfor all FAR Part 23 airplanes.

FAR 23.1305Required Powerplant Instruments1. Fuel quantity for each tank.

Figure 1-11. A turbofan engine showing the bypass airflow which bypasses the core of the engine.

TURBINES

Figure 1-12. A turbojet engine with the major sections identified. This type of engine has no bypass airflow.

8


EXHAUST OUTLET I/ AIR INLET COMPRESSOR

PROPELLER DRIVE SHAFT

REDUCTION GEARBOX

FREE (POWER) TURBINE -COMPRESSOR TURBINE

Figure 1-13. A typical turboprop engine with the output shaft on the left. The engine could become a turboshaft engineif it was modified to drive the rotors of a helicopter.

Oil pressure for each engine.

Oil pressure for each turbosupercharger (onlywith separate oil system).

Oil temperature for each engine.

Oil temperature for each turbosupercharger(only with separate oil system).

Tachometer for each engine.

Cylinder head temperature for:

Air-cooled engines with cowl flaps.

Reciprocating engine commuter categoryairplanes.

Fuel pressure if the engine is pump fed.

Manifold pressure for:

Altitude engines.

Reciprocating engine commuter categoryairplanes.

10. Oil quantity for each oil tank (if separate fromengine).

Figure 1-14. The three instruments required for a pres-surized aircraft.

Gas temperature for turbine engines.

Fuel flowmeter for turbine engines.

Torquemeter for turbo-propeller engines.

EGT and carburetor inlet temperature for tur-bosupercharger installations if operating lim-itations can be exceeded.

Figure 1-15 shows a typical tachometer and man-ifold pressure gauge installation for a twin engineairplane with two pointers in each gauge for theleft and right engines.

FAR 23.1321Instrument Installation

Instruments must be plainly visible with min-imum deviation of the pilot's position.

For multi-engine airplanes, identical instru-ments must be arranged to prevent confusion.

Instrument panel vibration must not damagethe instruments.

For airplanes more than 6,000 lbs. maximumweight, the following instruments must be in-stalled so that they are centered about thevertical plane of the pilot's vision in this order:

Primary attitude instrument in the center.

Airspeed indicator adjacent and to the leftof the attitude instrument.

Altitude instrument adjacent and to theright of the attitude instrument.

Direction of flight instrument adjacent andbelow the attitude instrument.

9


The instrument referred to as the primary at-

titude instrument above would be an artificialhorizon on smaller aircraft or an ADI on aircraftwith more modern types of instruments. The direc-tion of flight instrument in item 4 is not a magneticcompass. It would be a directional gyro on simpleairplanes or an HSI on more sophisticated aircraft.In addition to the four standard instruments men-tioned above as part of this "T" configuration,two other instruments are commonly installed instandard positions to make up an arrangementof six instruments. This is illustrated in figure1-16. The actual appearance of the instrumentsin the standard configuration is shown in figure1-17. The gyro instruments in figure 1-17 arethe older style instruments.

FAR 23.1322Warning, Caution and Advisory Lights

There are standard colors specified for certainindicator lights used in airplanes. A fire warninglight for example would be red. The colors specifiedin this FAR are as follows:

RED — Warning — This is used when immediateattention is required.

AMBER — Caution — This is not as serious asa warning.

GREEN — Safe operation — normal operatingrange.

Any other color of indicator light can be used in-cluding white if it differs sufficiently from other colorsand its meaning is specified by the manufacturer.

FAR 23.1381Instrument Lights

Any instrument lights that are installed mustmake the instruments and controls easily readableand must be shielded so that they don't shine inthe pilot's eyes. A cabin dome light is not acceptableas an instrument light.

FAR 23.1541Markings and Placards

The airplane must have all placards required bythe FARs and any additional placards that are re-quired for safe operation if unusual design, handlingor operational characteristics are present. In ad-dition, each marking and placard:

1. Must be displayed in a conspicuous place.

2. Must not be easily erased, disfigured orobscured.

FAR 23.1543Instrument Markings, General Requirements

When markings are on the cover glass, theremust be a means to maintain the alignmentof the glass with the dial (a slippage mark).

Each arc and line must be wide enough andlocated to be clearly visible to the pilot.

FAR 23.1545Airspeed Indicator

The required markings are:

1. For the never exceed speed Vne, a red radialline.

Figure 1-15. The manifold pressure gauge is used inconjunction with a tachometer to set poweron a supercharged engine.

10


AIRSPEEDA/S= INDICATOR

ALT = ALTIMETER

TURNT&B = AND

BANK

ARTIFICIALA/H= HORIZON

VERTICALVSI=SPEED

INDICATOR

DG=DIRECTIONAL GYRO

For the caution range, a yellow arc extendingfrom the red radial line in item 1 to the upperlimit of the green arc in item 3.

For the normal operating range, a green arcwith the lower limit at Vs1 (maximum weight,landing gear and flaps retracted) and the upper

Figure 1-16. Most modern aircraft have the importantflight instruments installed in a standardconfiguration directly in front of the pilot.

limit at Vno, maximum structural cruisingspeed.

For the flap operating range, a white arc withthe lower limit at Vso and the upper limitat Vfe.

For the one-engine-inoperative best rate ofclimb speed, Vy, a blue sector extending fromthe Vy speed at sea level to the Vy speedat 5,000 ft. (or higher altitude as appropriate).

6. For the minimum control speed (one-engine-inoperative) Vmc, a red radial line.

Note: Items 1 through 3 do not apply to an aircraftfor which a maximum operating speed,Vmo/Mmo has been established. Theseaircraft would need a maximum allowableairspeed indicator.

FAR 23.1547Magnetic Direction Indicator

A placard must be installed on or near themagnetic direction indicator to show deviationerror corrections.

The placard must show the calibration in levelflight with the engines running.

Figure 1-17. The types of instruments that might be found in the standard "T" configuration in a small airplane.

11


The placard must state if calibration was madewith radios on or off.

The placard must list the corrections for mag-netic headings in increments of no more than30°.

5. If the operation of electrical equipment cancause a deviation of more than 10 0 , the placardmust identify that equipment.

FAR 23.1549Powerplant Instruments

The required markings are:

Each maximum and minimum safe operatinglimit must be marked with a red radial line.

Each normal operating range must be markedwith a green arc.

Each takeoff and precautionary range mustbe marked with a yellow arc.

Each engine or propeller range that is re-stricted because of vibration stresses must bemarked with red arcs or red lines.

FAR 23.1551Oil Quantity Indicator

It must be marked with sufficient increments toreadily and accurately indicate the quantity of oil.

FAR 23.1553Fuel Quantity Indicator

If the unusable fuel supply for any tank exceedsone gallon or 5% of tank capacity, whichever isgreater, a red arc must be marked to extend fromcalibrated zero to the lowest reading obtainable inlevel flight.

FAR 23.1563Airspeed Placards

There must be an airspeed placard in clear viewof the pilot and as close as practical to the airspeedindicator. This placard must list:

The design maneuvering speed Va.

The maximum landing gear operating speed Vlo.

FAR 43 Appendix AAppliance Major Repairs

Calibration and repair of instruments

Calibration of radio equipment

FAR 43 Appendix DScope and Detail of Items to Inspect For 100 -Hour and Annual Inspections

(4) Instruments - inspect for poor condition,mounting, marking and where practical forimproper operation.

FAR 65.81General Privileges and Limitations

(a) A certificated mechanic may perform or su-pervise the maintenance, preventive mainte-nance or alteration of an aircraft or applianceor a part thereof, for which he is rated butexcluding major repairs to and major alter-ations of propellers and any repair to or al-teration of instruments.

FAR 91.9Powered Civil Aircraft with Standard CategoryU.S. Airworthiness Certificates; Instrument andEquipment Requirements

Visual Flight Rules Day - For VFR, the followingare required:

Airspeed indicator

Altimeter

Magnetic direction indicator

Tachometer for each engine

Oil pressure gauge for each engine

Temperature gauge for each liquid-cooled engine

Oil temperature gauge for each air-cooled engine

Manifold pressure gauge for each altitude engine

Fuel quantity gauge for each tank

Landing gear position indicator if landing gearis retractable

Visual Flight Rules Night - For night VFR in ad-dition to the requirements for VFR day, the followingare required:

Position lights

Anti-collision lights (red or white)

Adequate source of electrical energy for allelectrical equipment

If operating for hire, one electric landing light

5. Spare fuses

Instrument Flight Rules - For IFR, the followingare required

The equipment for VFR day and night as ap-propriate; PLUS

Two-way radio communications equipment

3. Radio navigation equipment

12


BOURDONTUBE/

ANCHORPOINT

POINTERSTAFF

Gyroscopic rate-of-turn indicator (except air-craft with three attitude instruments)

Slip-skid indicator

Sensitive altimeter

Clock which displays hours, minutes and sec-onds

Generator or alternator

Gyroscopic bank and pitch indicator (artificialhorizon)

Gyroscopic direction indicator (DG or equiv-alent)

For flight at and above 24,000 ft. MSL, a DMEsystem

12. For Category II operations, additional equip-ment is required as listed in Appendix A.

A modern digital aircraft clock is shown in figure1-18. The clock required for IFR does not haveto be a digital clock, but it must display hours,minutes and seconds. The older type of aircraftclock was a conventional kind of round clock withthree hands.

C. Pitot-StaticSystem Instruments

1. Methods of Pressure MeasurementPressure instruments are those which obtain read-ings by measuring the pressure at one or moreplaces in terms of a liquid or gas pressure.

a. Pressure Measuring SensorsThere are three common types of sensors that canbe used to mechanically measure a fluid pressure.

Bourdon TubeA bourdon tube is a curved, hollow tube madeof a springy metal (often a specially treated brassmaterial) (figure 1-19).

One end of the tube is anchored and the otheris free to move and fastened through linkages tothe indicator pointer or similar device. As pressureis applied to the inside of the bourdon tube, itwill try to straighten out as a result of the imbalanceof forces on the walls of the curved tube. Thismotion is transmitted to the indicator needle orpointer through gears and linkages. The bourdontube is used to measure relatively high pressure,typically 20 PSI or more.

DiaphragmA diaphragm is a thin, lens shaped hollow metalcontainer. It is used to measure relatively low pres-sures, as in an altimeter or airspeed indicator. Typi-cally, it is made of a very thin, springy metal(beryllium copper is often used). One side isanchored and the other side is connected throughgears and linkages to the pointer. See figure 1-20.

3) BellowsA bellows is somewhat similar to a diaphragm,

but it is longer and has accordion folds to typicallyallow a greater range of motion. It measures relatively

Figure 1-18. A digital aircraft clock able to display hours,minutes and seconds. (Courtesy CanadairGroup, Bombardier Inc.

Figure 1-19. The bourdon tube pressure measuringdevice for instruments.

13


PRESSURECAPSULE

PRESSURE\ENTRANCE

/ PRESSUREENTRANCE

BELLOWS

-n -/

low pressures and a common use is to measure

differential pressure. It that case the bellows wouldbe divided into two separate chambers with a dif-ferent pressure source connected to each one asseen in figure 1-21.

b. Units of Measure for Pressure

Many different units of measure are used for pres-sure. In aviation, a number of units are in commonuse. Some examples are listed below.

Pounds per Square Inch — PSI

Inches of Mercury — in. Hg

Millibars — Mbar

Kilopascals — kPa

c. Types of Pressure MeasurementPressure is measured compared to some referencevalue. Standard abbreviations are used to designatewhat this reference level is for a particular pressuremeasurement. The three common designations are:

Absolute Pressure — This is a pressure com-pared to a perfect vacuum. No pressure canbe below a vacuum, so there is never a negativeabsolute pressure. Examples of this designa-tion are PSIA and in. Hg absolute.

Gauge Pressure — This is a pressure com-pared to ambient pressure, usually outsideatmospheric pressure, as in PSIG, etc.

3. Differential Pressure — This is used to des-ignate two different pressures in the aircraftthat are compared to each other, as in PSID.

Examples of these using aircraft instruments arethe manifold pressure gauge which measures ab-solute pressure, the oil pressure gauge measuresgauge pressure and the cabin differential pressuregauge is calibrated in PSID.

We will now examine the basic aircraft flight in-

struments that operate as pressure instruments.These are often referred to as the pitot-static in-struments because they utilize pitot pressure andstatic pressure.

Standard Atmosphere TableAltimeters and other instruments obtain readingsby measurements of the pressure at various al-titudes. The average or standard pressures andtemperatures at different altitudes are containedin a table of the Standard Atmosphere. The valuesgiven in this table are accepted by virtually allcountries of the world for scientific and aviationpurposes (figure 1-22).

AltimetersThe altimeter is a basic required instrument forall certificated aircraft. It measures the atmosphericpressure and displays it as altitude in feet. Thisaltitude is called mean sea level (MSL) since it isreferenced to the average level of the major oceans.The measurement of altitude is based on the stand-ard atmosphere table and the changes in pressurewith altitude changes that it gives.

a. Non-Sensitive and Sensitive Altimeters

1) Non-Sensitive Altimeter

A non-sensitive altimeter has only one pointer andit makes one complete revolution for each 10,000ft. in altitude. If the pointer was on the number 8,

Figure 1-20. The diaphragm or aneroid pressure Figure 1-21. An example of a bellows being used tomeasuring device. measure a differential pressure.

14


that would represent 8,000 ft.; but it could also in-dicate 18,000 ft. For this reason, this type of altimeteris only used for small, simple aircraft that don't operateat high altitudes. A non-sensitive altimeter is il-lustrated in figure 1-23.

2) Sensitive Altimeter

A sensitive altimeter is more sensitive and easierto read than the non-sensitive type. A sensitivealtimeter may have three separate pointers or asingle pointer and a drum readout.

On an altimeter with three pointers, the longestpointer makes one revolution for each 1,000 ft.,the second pointer makes one revolution for each10,000 ft., and the shortest pointer makes onerevolution for each 100,000 ft. The three pointer

ALTITUDEFT.

DENSITYRATIO

a

PRESSURERATIO

b

TEMPER-ATURE

°F

SPEED OFSOUND

KNOTS

0 1.0000 1.0000 59.00 661.7

1000 0.9711 0.9644 55.43 659.5

2000 0.9428 0.9298 51.87 657.2

3000 0.9151 .08962 48.30 654.9

4000 0.8881 0.8637 44.74 652.6

5000 0.8617 0.8320 41.17 650.3

6000 0.8359 0.8014 37.60 647.97000 0.8106 0.7716 34.04 645.6

8000 .07860 0.7428 30.47 643.3

9000 0.7620 0.7148 26.90 640.9

10000 0.7385 0.6877 23.34 638.615000 0.6292 0.5643 5.51 626.7

20000 0.5328 0.4595 -12.32 614.6

25000 0.4481 0.3711 -30.15 602.2

30000 0.3741 0.2970 -47.98 589.535000 0.3099 0.2353 -65.82 576.6

*36089 0.2971 0.2234 -69.70 573.8

40000 0.2462 0.1851 -69.70 573.8

45000 0.1936 0.1455 -69.70 573.850000 0.1522 0.1145 -69.70 573.8

55000 0.1197 0.0900 -69.70 573.8

60000 0.0941 0.0708 -69.70 573.8

65000 0.0740 0.0557 -69.70 573.870000 0.0582 0.0438 -69.70 573.8

75000 0.0458 0.0344 -69.70 573.8

80000 0.0360 0.0271 -69.70 573.8

85000 0.0280 0.0213 -64.80 577.490000 0.0217 0.0168 -56.57 583.4

95000 0.0169 0.0134 -48.34 589.3

100000 0.0132 0.0107 -40.11 595.2

* GEOPOTENTIAL OF THE TROPOPAUSE

Figure 1-22. A reference table of the Standard Atmos-phere.

sensitive altimeter is shown in figure 1-24. A majorproblem with this type of altimeter is that the smal-lest pointer can be covered up by one of the othertwo pointers which makes it easy to misread thealtimeter. The newer kind of three pointer altimeter

Figure 1-23. A simple non-sensitive aircraft altimeter.(Courtesy Piper Aircraft Corp.)

Figure 1-24. The older style three-pointer sensitive al-timeter.

15


seen in figure 1-25 has been modified to make it

easier to read. The pointer that makes one revolutionin 100,000 ft. has been extended to the outsideof the face with a small triangle on the end soit cannot be covered up. In addition, a small windowshows a striped pattern below about 15.000 ft. (athigher altitudes the striped symbol disappears). Thesensitive altimeter with one pointer as seen in figure1-26 uses the drum readout and a single pointerwhich makes one revolution for each 1,000 ft. Thedrum readout may have three or five digits displayed,but in any case the combination of the two permitsaccurate reading of the altimeter.

b. Altimeter Mechanism

The internal mechanism of the altimeter consistsof three diaphragms in series. This creates an al-timeter which is sensitive to very small changesin pressure and which has a large enough rangeof measurement to read altitudes of 35,000 to50,000 ft. or more.

Inside the case of the altimeter is a bimetallicdevice which gives automatic compensation for tem-perature changes to ensure accurate readings.There is a knob on the front of the altimeter whichoperates a mechanism that compensates for non-standard atmospheric pressure. The standard at-mosphere table gives the standard pressure at sealevel as 29.92 in. Hg, but the actual atmospherepressure in any given location seldom matches the

Figure 1-25. The newer style three-pointer sensitive al-timeter uses a different pointer and a stripedsymbol that is visible below about 15,000 ft.as aids to make reading the altimeter easier.

values in the table. Weather systems with higher

or lower pressure are constantly moving across thesurface of the earth.

It is due to these variations in pressure thatthe knob on the front of the altimeter must beadjusted to the current barometric pressure set-ting for the area in which it is operating. Thereis a small window on the altimeter which displaysthe current barometric pressure setting. It is com-monly called the Kollsman window. If for examplea pilot received an altimeter setting of 30.01 froman air traffic controller, that number would beset into the Kollsman window of the altimeter.When the pilot sets the altimeter to the correctsetting before takeoff, the indication on the al-timeter will show the height of that airport abovesea level. Outside the United Sates, a differentunit of measurement is often used for barometricpressure settings. This unit of measurement ismillibars (MB) and the altimeters found in manymodern aircraft have two Kollsman windows: onefor inches of mercury and one for millibars. Figure1-26 shows the two windows and the barometricpressure setting knob.

Altimeter OperationThe diaphragms of an altimeter are sealed at thefactory. The case of the altimeter is connected tothe static ports on the airplane. The static pressureoutside the airplane is conducted to the instrumentcase by tubing and hose. If the airplane climbs,the reduced pressure in the case causes the di-aphragms to expand and move the pointers to in-dicate a higher altitude.

The altimeter uses three diaphragms in seriesto increase the range of measurement. The mech-anism uses very delicate gearing which is designedin such a way that a very small movement of thediaphragm causes a large motion of the pointeror pointers. Under certain circumstances it hasa tendency to stick and some aircraft have a vibratoron the instrument panel or in the altimeter to pre-vent this sticking.

Encoding AltimeterAn altimeter related device which is found on manyaircraft is the encoding altimeter system. The pur-pose of this system is to send information con-cerning the aircraft's altitude to a radar systemon the ground so that it can be displayed on aradar scope. The data is transmitted to the groundusing a special aircraft radio called a transponder.An encoding altimeter system (also known as ModeC), supplies the electrical signal to the transponderthat contains this altitude data. An aircraft encoding

16


ALTIMETER07 INDICATION

DIAPHRAGM

Set Knob

STATICPRESSURE-

Indicator

altimeter and transponder are required for threedifferent conditions:

Flight operations within a TCA (terminal con-trol area) or Class B airspace.

Flight operations within an ARSA (airport radarservice area) or Class C airspace.

Flight operations above 10,000 ft. MSL andover 2500 ft. AGL.

The connections for the encoding feature of analtimeter and an altimeter vibrator can be seenin figure 1-27.

Airspeed IndicatorsAirspeed indicators are required on all certificatedaircraft except free balloons. This instrument givesthe pilot an indication of his speed through theair, it does not measure groundspeed. The airspeedindicator is a pitot-static system instrument thatis connected to both the pitot pressure source andthe static pressure source. It measures the dif-ference between these two pressures as indicatedairspeed.

On aircraft operated in the United States, theunit of measurement for airspeed indicators is nau-tical miles per hour (knots), statute miles per hour(MPH) or both on the same instrument.

a. Types of Airspeed

There are three types of airspeed associated withthe airspeed indicator. This is because it is subjectto a number of errors or inaccuracies. The threetypes of airspeed are:

Indicated Airspeed — This is the reading onthe instrument

Calibrated Airspeed — This is indicated air-speed which has been corrected for positionand instrument error. The pitot tube ismounted at a fixed angle to the longitudinalaxis of the aircraft and at slow speeds andhigh angles of attack there will be an error.The pilot determines the correction for cal-ibrated airspeed by consulting a table in thePilot's Operating Handbook, an example ofwhich is shown in figure 1-28.

3. True Airspeed — This is calibrated airspeedwhich has been corrected for altitude and tem-perature effects. At high altitudes the indicatedairspeed will be much less than the actualspeed through the air which is true airspeed.For example, at 41,000 ft. the indicated air-speed is only about half the true airspeed.At sea level on a standard day with the aircraftin level cruise flight, indicated and true air-speed are usually the same.

Figure 1-26. A sensitive altimeter with drum readout showing the static connection to the outside of the diaphragm.

17


BARO SET COUNTER DISPLAYSBARO SET KNOB

VIBRATOR FAIL FLAG

CB-B 26228 V DCBUS 2

C/P ENC ALTVIBRATOR

I700

ALTITUDECOUNTERDISPLAY1I- - -

III

DIFFERENTIAL'

— _ __M I

"S" CAPSULE

ALTITUDEPOINTER

S2 (OR S 3) PRESSURE

fBARO SET COUNTER

DISPLAYSENCODED ALTITUDETO N° 2 TRANSPONDER

OPTICALALTITUDEENCODER

I=.7

MB IN HG

22 91 31

VIBRATOR FAIL FLAG

BARO SET KNOB

FRONT FACE

Figure 1-27. An altimeter installation that includes a vibrator and altitude encoder. (Courtesy Canadair Group, Bombar-dier Inc.)

18


Indicator Case

Indicator

AIRSPEED

INDICATION

DIAPHRAGM

) PITOTPRESSURE

\ \\\\\\\

)) STATIC

PRESSURE

Airspeed Indicator OperationThe airspeed indicator uses a diaphragm thatmeasures the differential pressure between pitotpressure and static pressure. Pitot pressure is con-nected to the inside of the diaphragm and staticpressure to the outside as shown in figure 1-29.A pitot tube has an opening facing forward intothe relative wind so that it measures ram pressureeffects. The static ports simply measure outsideatmospheric pressure. The faster the airplane goes,the greater will be the difference in pressure.

Airspeed Indicator MarkingsThe operating limitations airspeeds that would befound on a general aviation twin-engine airplaneare listed below.

FLAPS UP

KIASKCAS

80 100 120 140 160 180 20081 101 120 139 159 177 196

210206

FLAPS 10°

KIAS 70 80 90 100 120 140 160 175KCAS 72 81 91 100 119 138 157 171

FLAPS 30°

KIAS 60 70 80 90 100 110 125 - - -KCAS 64 72 80 89 98 108 123 - - -

Note: For Illustration only; not to be used for flight planning

Figure 1-28. The table of calibrated airspeed correctionsfor a twin-engine airplane. (CourtesyCessna Aircraft Co.)

Vso — Stall speed with gear and flaps down.This is shown by the bottom of the white arc.

Vsl — Stall speed with gear and flaps up. Thebottom of the green arc.

Vmc — Minimum control speed with criticalengine failed. A red radial line in the lowerrange of indicated airspeeds.

Vfe — Maximum speed with flaps down. Topof white arc.

Vno — Maximum structural cruise. Top ofgreen arc or bottom of yellow arc.

Vne — Never exceed speed. a red radial lineat the highest permitted airspeed, also thetop of the yellow arc.

7. Vyse — Best rate of climb speed with one en-gine inoperative. A blue line or sector.

The aircraft should only be operated in the yellowarc range with caution and in smooth air. Figure1-30 shows the colored markings for the airspeedindicator on a single-engine airplane. A twin-engineairplane would have the additional markings forVmc and Vyse. The principle of critical engine andVmc is illustrated in figure 1-31. The critical engineis the engine whose failure would produce the mostadverse effect on the handling characteristics ofthe airplane. Since the flight controls become moreeffective at higher speeds, the concept of Vmc in-volves high engine power and slow speeds suchas occurs during takeoff. During initial climb aftertakeoff, the thrust from the propellers is offsetto the right side as shown in figure 1-31 if both

Figure 1-29. The two pressure connections to the diaphragm in an airspeed indicator.

19


propellers rotate clockwise. In this case, the critical

engine is the left engine since the thrust line forthe right engine is farther from the longitudinalaxis of the airplane. Some twin engine airplanesare designed so that the propellers rotate in op-posite directions. In that case, the two enginesare equally critical. The Vmc speed would be thesame no matter which engine failed. The reasonthat the Vmc marking is red is that maintaininga speed above Vmc can be very important to preventloss of control. If an engine fails after takeoff andthe airplane is below Vmc speed, the airplane willstart to turn and roll and the motion cannot bestopped with full opposite control inputs. This hascaused many accidents and many deaths. Vmcis a critical operating limitation that the pilot mustkeep in mind. The speed for Vyse is obviouslyhigher than Vmc, it is the speed the pilot woulduse after an engine failure to get the best rateof climb possible.

A table of airspeed indicator operating limitationsmarkings for a small twin engine airplane is shownin figure 1-32.

All of the important operating limitationsairspeeds will not be shown on the airspeed in-dicator. Some of them will be displayed on a placardnext to the airspeed indicator like that seen infigure 1-33 and others will only be found in thePilot's Operating Handbook or Airplane FlightManual.

\\\\\ / ,240 40

220 AIR 'ED 60 lik4st*

200 . 80, if.

180 1004,,160 1 120

AtfAli 1 dm.

wtisanittOl"

Figure 1-30. The airspeed indicator for a single engineairplane showing the colored operatinglimitations markings.

Figure 1-31. An illustration of why the left engine is thecritical engine when both propellers turnclockwise. At slow speeds, the thrust linesfrom the propellers are offset to the rightwhich places the thrust line of the rightengine farther from the center line of theaircraft. Vmc is the slowest speed at whichcontrol can be maintained when the criticalengine fails.

MARKING KIAS VALUEOR RANGE

SIGNIFICANCE

Red Radial 65 Minimum Control Speed

White Arc 58-125

Full Flap Operating Range.Lower limit is maximumweight Vs, in landingconfiguration. Upper limitis maximum speedpermissible with flapsextended.

Green Arc 66-175

Normal Operating Range.Lower limit is maximumweight Vs at most forwardC.G. with flaps retracted.Upper limit is maximumstructural cruising speed.

Blue Radial 97Single Engine Best Rate-of-Climb Speed at MaximumWeight.

Yellow Arc 175-210Operations must beconducted with cautionand only in smooth air.

Red Radial 210Maximum speed for alloperations.

Note: For Illustration purposes only; not to be used for flight planning.

Figure 1-32. A table of airspeed indicator markings for atwin engine airplane. This table is found inthe Pilot's Operating Handbook or Ap-proved Airplane Flight Manual. (CourtesyCessna Aircraft Co.)

20


MAX SPEED KIASMANEUVERING 148EXTEND GEAR 175RETRACT GEAR 150GEAR DOWN 210

Note: For Illustration purposes only; not to be used for flight planning.

Maximum Allowable Airspeed IndicatorOn high speed aircraft that operate at high altitudes,it is not practical to use the colored markings onthe airspeed indicator because the values changesignificantly as the aircraft climbs to higher altitudes.

These aircraft would use a different type ofairspeed indicator called the maximum allowableairspeed indicator. It is found on turbopropairplanes, bizjets and air carrier jets.

The maximum allowable airspeed indicator hastwo pointers; one gives the normal indicatedairspeed and the second one, which has a stripedor checked pattern, is connected to a separatediaphragm and it shows the Vmo speed at the ex-isting altitude. The two pointers can be seen infigure 1-34. On a jet airplane, the maximum al-lowable airspeed indicator will also usually includea Mach number display as shown in figure 1-34.

MachmeterA machmeter is required for jet airplanes becausethey will have a maximum safe operating speed ex-pressed in Mach numbers. This is in addition tothe maximum safe indicated airspeed. This is neces-sary because if the safe Mach number is exceeded,the airplane can become difficult or impossible to

Figure 1-33. The airspeed limitations placard that wouldbe installed near the airspeed indicator fora twin engine airplane. (Courtesy CessnaAircraft Co.)

control. Mach number indicates the ratio of theaircraft's true airspeed compared to the speed ofsound. A speed of Mach 0.8 means the aircraftis traveling at 80% of the speed of sound. The speedof sound is about 760 MPH at sea level, but it isaffected by temperature. As the temperaturedecreases, the speed of sound is also decreased.Since temperature decreases with an increase inaltitude, the speed of sound is reduced at higheraltitudes. At 35,000 ft. the speed of sound wouldbe about 660 MPH. If an airplane climbed from sealevel to 35,000 ft. at a constant true airspeed, theMach number would steadily increase. Most civilianjet airplanes have an Mmo (maximum operatingMach number) of about 0.8 to 0.87. As an airplaneapproaches its critical Mach number, a shock wavebegins to form on the upper surface of the wingas seen in figure 1-35. This will occur at flight speedsbelow Mach 1 because the air accelerates as it movesover the top of the wing. This shock wave disruptsairflow and causes the center of pressure to shiftaft. As aircraft speed gets closer to Mmo, the shockwave gets stronger until a point is reached wherecontrol of the aircraft may be lost. A loss of controlat high speed and high altitude can and has resultedin the loss of the aircraft—it is an important operatinglimitation for jet airplanes. The machmeter instru-ment has a diaphragm like a normal airspeed in-dicator that is connected to pitot and static pressure.It also has an additional diaphragm which modifiesthe movement of the pointer to compensate for theeffects of altitude and temperature (figure 1-36).

5. Vertical Speed IndicatorThe last of the three basic pitot-static system in-struments is identified by several names: a verticalspeed indicator (VSI), a rate of climb indicator (ROC)

VMO POINTER

MACH NUMBER DISPLAY(OBSCURED BY "OFF" FLAG WHENINSTRUMENT UN POWERED ORELECTRICALLY MALFUNCTIONING)

AIRSPEED POINTER —MANUALLY SETTABLE BUGS

Figure 1-34. The combined maximum allowable airspeed indicator and Machmeter for a corporate jet airplane. (Cour-tesy Canadair Group, Bombardier Inc.)

21


1.00 NORMALSHOCK

SUPERSONICREGION

0.90 0.95 1.00,, 1.02

0.85

0.950.90

0.85

FLIGHT MACH NUMBER 0.85

STATIC1' PRESSUREALTITUDE DIAPHRAGM

(SEALED)

PITOT 11 PRESSURE

INDICATOR CASE

and a vertical velocity indicator (VVI). This instru-

ment measures the rate of change of static pressure.

Since static pressure decreases with an increasein altitude, it can measure the rate of climb ordescent of the aircraft. The unit of measurementis feet per minute (FPM) as seen on the instrumentface in figure 1-37.

a. Basic Operation of VSIStatic pressure is connected directly to the insideof the diaphragm, but it is connected to the casepressure (outside of the diaphragm) by a small orificeor restrictor opening as indicated in figure 1-38.If the airplane climbs, the pressure will decreasemore rapidly inside the diaphragm than outside

Figure 1-35. A shock wave will form on the upper surfaceof the wing when the airplane is traveling atless than the speed of sound.

of it. The diaphragm will be compressed and thatwill move the pointer to show the rate of change

of altitude in feet per minute. The opposite occursduring descents. When the aircraft levels off, thepressures equalize and the pointer returns to zero.The instrument case usually includes an overpres-sure valve to prevent damage at rates of climb ordescent in excess of the maximum reading for thatinstrument.

Because of the very small size of the restrictoropening, the pointer tends to react slowly to rapidchanges in altitude. This problem is reduced inthe instrument called an Instantaneous VerticalSpeed Indicator (IVSI).

Instantaneous Vertical Speed IndicatorThe IVSI has two little cylinders with pistons andsprings which can be seen in figure 1-39. Theyare called accelerometer operated dashpots. Whenthe aircraft pitches up or down suddenly, the pistonswill move and force air into or out of the diaphragmto get the pointer moving quickly. In a steady rateclimb or descent, they will have no effect on thepointer.

VariometerThe variometer is a very sensitive version of theVSI which is used by gliders. The mechanism usesa movable vane in a small air tight metal container.

Figure 1-36. The Machmeter has an extra diaphragm to compensate for altitude effects.

22


STATICPRESSURE

RESTRICTION

The vane divides the container into two chambers.One chamber is connected to the static port andthe other is connected to an air bottle. The airstorage bottle is used because it will hold moreair than a small instrument case and this improvessensitivity. With the variometer, even small changesin rate of climb or descent can be detected.

6. AccelerometerThe accelerometer is an instrument that measuresthe "G" forces or acceleration forces on an aircraftin flight. The common application is in an aerobaticairplane where the pilot needs to know how much"G" force the airplane is being subjected to in orderto prevent overstress of the structure.

The mechanism of the accelerometer consists ofa weight which is connected by a cord and pulleysto the shaft that operates the pointer. The internalarrangement of an accelerometer is shown in figure1-40. The weight is supported by a guide shaftwhich only allows it to move up and down relativeto the guide shaft. A positive G acceleration willcause the weight to move downward and rotatethe pointer to show a higher positive G loading.There is a balance spring on the pointer shaft pulleyto balance the forces. The instrument is installedin the airplane so that it measures accelerationalong the vertical axis of the airplane. The normalat rest indication on the ground or in level flightis +1 Gs. The instrument face of an accelerometeris shown in figure 1-41. The instrument has threepointers connected to the operating mechanism.One pointer gives a readout of the current accelera-tion force along the vertical axis. The other twopointers have a ratchet device so that they will

remain at the highest reading recorded for positiveand negative forces. A knob is included on theinstrument to reset the two recording pointers.

7. Pitot-Static Systemsa. Pitot and Static Ports

The system of ports and tubing on the aircraft whichsupplies pitot and static pressure for the instru-ments is called the pitot-static system. The pitottube is an open tube which faces forward into therelative wind in flight. It measures the ram pressureof the airstream. On small airplanes, the pitot tubeis usually installed below the wing. On other aircraft,it is installed on the nose section of the aircraft.

The static ports are openings at right angles tothe relative wind so that they will measure staticpressure and not be affected by the speed of theaircraft. The static ports are most often locatedin pairs along the sides of the fuselage. On someaircraft, the static ports are along the sides or topand bottom surfaces of the pitot tube so that bothpressures are measured with the same probe. Apitot tube which includes static ports and electricheaters to prevent icing is shown in figure 1-42.

With the static ports in pairs on opposite sidesof the fuselage, any errors caused by sideslip willbe eliminated. Aircraft that must operate in adverseweather conditions will require an electrical heatingsystem for the pitot tubes and static ports to preventicing. Air carrier jets and similar types of aircraftusually employ multiple pitot tubes and static portsfor safety. A typical arrangement of this kind withthree pitot tubes and three sets of static ports isillustrated in figure 1-43. Notice that the instru-ments on the left and right sides of the cockpit

Figure 1-37. The vertical speed indicator has two scales:one for climbs and one for descents.

Figure 1-38. The VSI has a restrictor in the connection tothe case which causes a difference betweendiaphragm and case pressure duringclimbs and descents.

23


ACCELEROMETER—ACTUATEDPUMPS OR DASHPOTS

INLET FROMSTATIC PORT

CALIBRATED LEAK

are operated by totally separate pressure sources.

By routinely cross checking the instrument indica-tions from the left and right sides, a faulty indicationcan be identified.

b. Blockage of Ports

When pitot-static lines or ports become blocked byice or other factors, it can cause the instrumentsto give improper readings. If the static pressure isblocked, the altimeter will remain at the current in-dication and the VSI will continue to read zero evenwhen the aircraft climbs or descends. This problemwould usually be noticed by the pilot. The airspeedindicator uses both pitot and static pressure anda blockage in flight could be more difficult to detect.

Let's use the example of an airplane that is flyingat 10,000 ft. when the static ports become blocked.The pressure in the static system will be sealedin and won't change when the aircraft climbs ordescends. If the airplane climbs at a constant speed,the airspeed indicator will show a decreasing

airspeed. If the airplane descends, the airspeed in-

dicator will indicate a higher than actual airspeed.Just the opposite would happen if the pitot tubeiced over or was blocked in some other way inflight. A climb would cause the airspeed indicatorto read higher than it should and a descent wouldcause it to read lower than actual airspeed.

The lines and connections in a pitot-static systemshould be maintained in good condition. Eventhough they don't have to handle high pressures,the instruments are very sensitive to small changesin pressure so that even very small leaks can causeerrors in the instruments.

The tubing and hoses that are used are not verystrong and should be inspected carefully for damage.The fittings and connections should be installedwith care and torqued to specified values.

c. Altimeter System Tests and Inspections

There are some FAR requirements for testing ofaltimeter systems. These will be covered next along

Figure 1-39. The acceleration actuated dashpots in the WS! reduce the lag in pointer movement.

24


PAWL RATCHETS

CONTROL CORD

MAIN PULLEY

MAIN POINTER CENTERING SPRING

RETURN SPRINGS N\4

DRIVER ARM

AUXILIARY POINTER

AUXILIARY POINTER(POSITIVE G INDICATION)

MAINPOINTER

.-----TOP PULLEY

SHAFTS

WEIGHT

.4r----------BOTTOM PULLEY

POINTER RESET SHAFT

AUXILIARY POINTER(NEGATIVE G INDICATION)

Figure 1-40. The internal mechanism of a three-pointer accelerometer.

Figure 1-41. Accelerometer instrument face with threepointers.

with the procedures for other tests which mightnot be required at specific intervals but would beconducted whenever a problem was suspected. Aportable pitot-static system test set that could beused for these tests is seen in figure 1-44.

FAR 91.411Altimeter System and Altitude Reporting Equip-ment Tests and Inspectionsa. No person may operate an airplane, or helicopter,

in controlled airspace under IFR unless:

Within the preceding 24 calendar months,each static pressure system, each altimeterinstrument, and each automatic pressurealtitude reporting system has been testedand inspected and found to comply withAppendix E of Part 43 of this chapter;

Except for the use of system drain andalternate static pressure valves, followingany opening and closing of a static pressuresystem, that system has been tested and

25


HEATER STATICHOLES

HEATER

STATICTUBE

PITOT TUBE

DRAIN HOLE

STATICHOLES

n III

PITOT TUBEPLATE

iht

AIRSPEED MACH ALTITUDE RATE OFCLIMB

RATE OF ALTITUDE MACH AIRSPEEDCLIMB

ALTERNATEAIR

SELECTORVALVES

WARN MACH

CAPTAIN RIGHTFIRST OFFICER SIDE

ICAUXILIARY PTOART-1

CABIN DIFF. PRESSURE

AUTOPILOT

I/

FLIGHTDIRECTOR

FLIGHTRECORDER',

t=fl__

AUXILIARYPITOTTUBE

WMACH

1ST OFFICERPITOT TUBE

LEFTSIDE

STATICPORTS

inspected and found to comply with para-graph (a) Appendices E and F of Part 43of this chapter; and

3. Following installation or maintenance onthe automatic pressure altitude reportingsystem of the ATC transponder where any

error could have been introduced, the entiresystem has been tested, inspected andfound to comply with paragraph (c) Ap-pendix E of part 43 of this chapter.

b. The tests required by paragraph (a.) of thissection must be conducted by:

Figure 1-42. A pitot tube which also contains static ports and electric heating elements.

CAPTAINSPITOTTUBE

Figure 1-43. The pitot-static system for a large jet airplane showing the instruments and equipment operated by thethree pitot tubes and three sets of static ports.

26


The manufacturer of the airplane orhelicopter;

A certificated repair station with appropri-ate rating;

3. A certificated mechanic with an airframerating (but only for the static pressure sys-tem tests and inspections).

FAR 43 Appendix EAltimeter System Test and Inspection

Each person performing the altimeter system testsand inspections required by FAR 91.411 shall com-ply with the following:

a. Static pressure system;

Ensure freedom from entrapped moistureand restrictions.

Determine that leakage is within the tol-erances established in FAR 23.1325 or25.1325 whichever is applicable.

Determine that the static port heater, ifinstalled, is operative.

Ensure that no alterations or deformationof the airframe surface have been madethat would affect the relationship betweenair pressure in the static pressure systemand true ambient static pressure for anyflight condition.

b. Altimeter — omitted here

c. Automatic pressure altitude reporting system— omitted here

FAR 23.1325Static Pressure System

b. If a static pressure system is necessary forthe functioning of instruments, systems or de-vices, it must comply with the provisions ofparagraphs (1) through (3) of this section.

1. The design and installation of the staticpressure system must be such that

Positive drainage of moisture is provided

Chafing of tubing and excessive distor-tion or restriction in bends is avoided;and

iii. The materials used are durable, suitablefor the purpose and protected againstcorrosion.

2. A proof test must be conducted to dem-onstrate the integrity of the static pressuresystem in the following manner

I. Unpressurized Airplanes — Evacuatethe static pressure system to a pressure

differential of approximately 1 inch ofmercury or to a reading of 1,000 ft.above the aircraft elevation at the timeof the test. Without additional pumpingfor a period of one minute, the loss ofindicated altitude must not exceed 100ft. on the altimeter.

ii. Pressurized Airplanes — Evacuate thestatic pressure system until a pressuredifferential equivalent to the maximumcabin pressure differential for which theairplane is type certificated is achieved.Without additional pumping for a periodof 1 minute, the loss of indicated altitudemust not exceed 2 percent of theequivalent altitude or 100 ft., whicheveris greater.

3. Each static pressure port must be de-signed and located so that errors will notresult when the aircraft encounters icingconditions. An anti-icing means or alter-nate static ports may be used to showcompliance.

d. Pitot System Tests

There is no specific test for pitot systems as thereis for static systems other than the normal inspec-tions of the entire aircraft. If a problem is reportedor suspected with a pitot system, there is a generalleak test procedure in AC 43.13-1A, as well as somegeneral guidelines for pitot-static system main-tenance. The procedure for leak testing the pitotsystem is: Apply pressure to the pitot tube to causethe airspeed indicator to show 150 knots. Seal offfor 1 minute and the maximum loss of indicatedairspeed should not exceed 10 knots.

Figure 1-44. A portable pitot-static system tester whichcan be used for leak tests and other main-tenance functions.

27


AC 43.13-1APrecautions in Testing Pitot-Static System

Perform all maintenance and inspections be-fore leak testing.

Use a system diagram.

Check the test unit for leaks before beginningthe test.

Run full range tests only if you are thoroughlyfamiliar with both the aircraft and the testequipment.

Pressure in the pitot system must always beequal to or greater than the pressure in thestatic system.

The rate of change of pressure during testingshould not exceed the limits for any installedinstrument.

7. After testing make sure that the system isreturned to flying condition, such as removingtape from ports and drain holes.

There is an FAR that concerns the altimeter set-ting which is set by the pilot in the Kollsman window.

FAR 91.121Altimeter Settings

Each person operating an aircraft shall maintainthe altitude of the aircraft by reference to an al-timeter that is set:

1. Below 18,000 ft. MSL, to

The current reported altimeter setting ofa station along the route of flight and within100 nautical miles of the aircraft

If there is no station within 100 nm, thenearest appropriate station altimeter setting

iii. If the aircraft has no radio, the elevationof the departure airport or the setting avail-able before takeoff shall be used.

2. At and above 18,000 ft. MSL, the altimetershall be set to 29.92.

e. Air Data Computer Systems

Aircraft that operate at high speeds and high al-titudes can have significant errors in the pitot-staticsystem instruments with the simple probes usedon smaller aircraft. These aircraft will use an AirData Computer (ADC) to operate the airspeed in-dicator, altimeter, VSI and any other systems thatrequire this data. The air data computer is placedin the system between the sensor ports and theinstruments to automatically apply corrections inorder to increase accuracy.

The air data computer has three inputs; pitot

pressure from the pitot tube, static pressure from

the static ports and total air temperature (TAT)from a special TAT probe. The TAT measurementis needed to correct the instrument indications forfriction heating of the air at high speeds. The TATprobe also permits the calculation of SAT (staticair temperature) which is used to apply correctionsfor non-standard temperatures for any flight al-titude. The outputs of the air data computer supplya number of cockpit instruments. The three basicpitot-static instruments are operated by the ADCand often several others are added. The commoninputs and outputs associated with an air datacomputer are shown in figure 1-45.

High speed jet airplanes require a machmeter;this could be a separate instrument or includedwith the airspeed indicator.

Aircraft with an air data computer usually havea digital display on the instrument panel whichgives a calculated true airspeed and total airtemperature or static air temperature. TAT includesthe heating effect of the friction at high speedwhereas SAT is just ambient outside air tempera-ture. The temperature indications are especially im-portant for turbine engines which are affected bythe temperature of the intake air.

The air data computer system automatically com-pensates for both temperature effects and the com-pressibility of the air at higher Mach numbers. Thishelps to ensure accurate instrument readingsthroughout a wide range of altitudes and airspeeds.Air data computers are typically found on turbopropairplanes and jet airplanes.

TATPROBE

► MACHMETER

AIRSPEEDINDICATOR

-IP- ALTIMETER

VSI

I TAS & TAT/SATINDICATOR

I -11.-PITOT TUBE

AIR DATACOMPUTER

STATICPORTS

Figure 1-45. An air data computer provides more ac-curate readings on the pitot-static instru-ments for high performance aircraft. Totalair temperature is measured by a specialprobe and provided as an input to the ADC.

28


D. Gyro InstrumentsThe instruments know as gyro instruments are re-quired for IFR flight and can also be an aid toaccurate flying in VFR conditions. These instru-ments utilize the principles of a spinning gyroscopeto give the pilot information about the aircraft'spitch and roll attitude, heading and rate of turn.A gyroscope is a device which consists of a weightedwheel or rotor which spins at high speed and isheld in an arrangement of hinged mounting ringscalled gimbals (figure 1-46).

The gyro has three axes and one is always thespin axis. Depending on the type of gimbals used,it will be able to move relative to the mountingbase around one or both of the remaining axes.If it has 1 degree of freedom, it can move aroundone axis and if it has two degrees of freedom itcan move around both. A gyro with two degreesof freedom is also called a free gyro.

1. Principles of GyroscopesThere are two main properties of a spinning gyrowhich are of importance to aircraft use, they are:

Figure 1-46. A simple gyroscope with both inner andouter gimbals.

Rigidity in Space — This means that the gyrorotor will try to maintain its position in spaceeven when its mounting base is tilted androtated. This is illustrated in figure 1-47.

Precession — This effect will cause a gyro,when it is acted upon by an outside force,to tilt or rotate as if the disturbing force wasapplied to it 90 degrees ahead in the directionof rotation of the rotor (figure 1-48).

Gyros are also subject to other effects such asoscillation. Oscillation is a problem caused by themass of the gimbals. It can be reduced by makingthe gimbals lighter. A number of things can be doneto make the gyro more stable and more efficient.One way is to concentrate the mass of the rotoron its rim and reduce the mass of the web andshaft which connects it to the bearings. Anotherway to make it more efficient is to increase thespeed of rotation. There are two common methodsused to spin the rotor of an aircraft gyro instrument.Pneumatics makes use of a stream of air directedat the rim of the rotor to make it spin at about8,000 RPM. Electric motors can also be used to spinthe rotor and will usually produce a speed of about24,000 RPM. A gyro can become unstable which iscalled tumbling. Some aircraft gyro instruments havea caging knob or mechanism which is designed toreturn the gyro to a stable condition so that it willgive correct instrument readings. A caging knob canbe seen at the bottom left of figure 1-49. The latesttypes of gyro instruments are non-tumbling andas long as the instrument is in good condition itwill not tumble, even in unusual attitudes. The threecommon types of aircraft gyro instruments are thedirectional gyro (DG), the artificial horizon and theturn and bank instrument.

2. Directional GyroThe directional gyro is the primary heading referencefor IFR flight. The magnetic compass is not a good

Figure 1-47. The rigidity in space characteristic causesthe gyro rotor to try to maintain its orienta-tion in space.

29


FORCEAPPLIEDHERE

FORCEFELTHERE

OUTERGIMBALINNER

GIMBAL

GYROROTOR

CARD

111011111101111IM II11II1111

CAGI NG KNOB

r3

LUBBER LINE

I 0 I 1331

0 = 360° OR NORTH33 = 330' OR NORTH-NORTHWEST3 = 030' OR NORTH-NORTHEAST

heading reference for IFR because it tends to os-cillate and is not as stable as a DG. This instrumentuses a free gyro with a horizontal spin axis. TheDG will drift due to precession errors and mustbe reset every 15 to 20 minutes using the magneticcompass as a reference. Older style DGs had arectangular window on the face through which thenumbers representing the magnetic heading couldbe read. This older style DG presentation is shownin figure 1-50. Newer style DGs show a full compasscard with the indicated magnetic heading underthe index mark at the top of the instrument asseen in figure 1-51.

The newer style DG with a full compass cardis often called a heading indicator. Another name

Figure 1-48. Precession causes a gyro rotor to tilt as if adisturbing force was applied 90° ahead inthe direction of rotation from the actualpoint of application.

Figure 1-49. The rotor in a directional gyro has a horizon-tal spin axis and two degrees of freedom.

for a DG type instrument is a gyro compass. TheDG uses a free gyro because the spin axis mustremain horizontal to give accurate readings. Whenthe airplane banks, for example, the rotor will main-tain its horizontal spin axis. On many modernaircraft a more sophisticated instrument replacesthe DG, this instrument is the horizontal situationindicator or HSI.

The HSI shown in figure 1-52 is an example ofa modern integrated instrument. It has a gyro sta-bilized compass card like a DG that indicates theaircraft's magnetic heading. Unlike the DG however,this compass card is slaved to a remote compassso it does not have to be reset every 15 to 20minutes. The HSI is called an integrated instrumentbecause it combines several different types of dis-plays which would normally be found in separate

Figure 1-50. The old style DG displayed a small windowlike a magnetic compass. This type ofpresentation can be difficult to read.

Figure 1-51. The modern DG or heading indicator has adisplay which shows a complete compasscard. This type is easier to use.

30


OME INDICATOR

FIXED MARKER (45°)

AIRCRAFT SYMBOL(Fixed to glass)

LUBBER LINE

12 8 40i\,\\ 7;477.7-//,

\ 3 6 /°•

n MAGNETIC/TRUE ANNUNCIATOR

HOG SELECT CURSOR

COURSE SELECT CURSOR

• "TO" ARROW("FROM" ARROW opposite

under Mask)

0 3151.1MILES No .I

GLIDESLOPE DEVIATIONPOINTER

VOR/LOC DEVIATION BAR

COMPASS CARD

OUTER,GIMBAL

OUTER GIMBALBEARING

STOP PIN

1STOPS

instruments. The wide split bar in the middle isconnected to navigational radios and tells the pilotwhether to turn right or left to follow the radionavigational signals. The indicators on the rightside and at the top are connected to other naviga-tional radios to provide additional information tothe pilot.

3. Artificial HorizonThe artificial horizon is the pilot's most importantinstrument for IFR flying. As the name implies,it replaces the natural earth horizon that a pilotuses in VFR flying to maintain the correct pitchand roll attitude of the aircraft. When an aircraftis flying in the clouds, the pilot must rely on the

Figure 1-52. The horizontal situation indicator is an integrated instrument that displays many additional kinds ofinformation besides gyro stabilized heading data.

GYRO ROTOR

INNER GIMBAL (Gyro housing)HORIZON INDICATOR coupled to this frame

Figure 1-53. The artificial horizon uses a gyro rotor with a vertical spin axis and two degrees of freedom.

31


GYROROTOR

INNERGIMBAL

BALLERECTOR

\‘n.\1111 11 11 111111111111 11 11 1 11

'AIR EXITSLOT OPENPENDULUM

VALVE

OVA

ROTOR

11111111111111111111111

GYROHOUSING

AIR EXITSLOT CLOSED

r

artificial horizon to determine the aircraft attitude

and prevent a loss of control. The artificial horizon

is also known as a gyro horizon, an attitude in-dicator, an attitude gyro and a bank and pitchinstrument. The arrangement of the spin axis andgimbals for an artificial horizon is illustrated infigure 1-53. The artificial horizon is a free gyrowith a vertical spin axis; this allows it to measurethe angular displacement of the aircraft in bothpitch and roll. The internal mechanism of the ar-tificial horizon must have some means to maintainthe spin axis in a vertical orientation. There aretwo common devices used to accomplish this func-tion, the pendulous vane and ball erector systems.

The ball erector system uses a number of steelballs similar to ball bearings that are free to rollaround on a plate mounted above or below thegyro rotor. One type of erector system that usessteel balls is shown in figure 1-54. When the gyrotilts away from the vertical, the balls roll to thelow side and this produces a force which pushesthe gyro back to the vertical position. This devicewill maintain the required vertical spin axis of thegyro rotor. The ball erector is usually found onelectric motor driven artificial horizons. When anair-driven gyro is used, the pendulous vane erectormechanism is utilized.

The pendulous vanes in this type of erectormechanism are small gravity operated air valves.When the rotor tilts away from the vertical, the

Figure 1-54. The ball erector mechanism uses steel ballsthat move to the low side of the plate tosupply a force that corrects the artificialhorizon back to a vertical spin axis.

vanes move to open the air valves in such a waythat streams of air are directed to push the rotor

back to the vertical position. The swing of the pen-dulous vanes to open and close the air valves canbe seen in figure 1-55.

The artificial horizon has a presentation whichshows an airplane symbol with the horizon behindit. It includes index marks to show the angle ofbank. Some indications of an artificial horizon fordifferent flight attitudes are illustrated in figure1-56. The newer types of artificial horizon havea more user friendly presentation on the instrumentface. Different colors are used above and belowthe horizon line to make it easier to read. Converginglines are placed below the horizon line to createperspective and additional markings for pitch at-titude are included. These features can be seenon the newer type artificial horizon in figure 1-57.

The small knob on the front of the artificial horizonis used to move the airplane symbol up and down

Figure 1-55. The pendulous vanes used in the erectormechanism of an air-driven gyro horizonare opened and closed by gravity.

32


to adjust for different aircraft flight attitudes andfor tall and short pilots. There also may be a controlfor quick erect that can be used to stabilize thegyro if it tumbles. There is a newer type of integratedinstrument which replaces the artificial horizon onmany modern aircraft. This instrument is calledthe attitude director indicator (ADI). It includes com-mand bars that are operated by a flight director.The flight director is a computer which receivessignals from navigational radios and other sourcesand calculates the correct pitch and roll attitudesto keep the aircraft on course or return it to thedesired flight path. The output signals from theflight director computer move the command barson the ADI and the pilot follows these commands.

An example of the use of these command bars onan ADI is found in figure 1-58.

The latest kinds of ADIs use different shapesfor the airplane symbol and the command barsas can be seen in figure 1-59. The airplane symbolis a triangle that resembles a delta wing airplaneand the command bars are two converging trian-gular shapes above the airplane symbol. They oftenuse different colors as well to make it easier todistinguish between the airplane symbol and com-mand bar symbol. Some ADIs also include indicatorsfor other aircraft systems in addition to the flightdirector indicator. The indicator pointer on the rightside of the ADI is operated by the aircraft auto-throttle system. The pointer on the left side is

AIRCRAFT IS FLYING STRAIGHT AND LEVEL. AIRCRAFT IS BANKED 20 DEGREES TO THE RIGHT.(A)

(B)

AIRCRAFT IS PITCHED NOSE DOWN.(C)

Figure 1-56. Indications for an artificial horizon instrument.The airplane is straight and level.The airplane is in a 20° bank to the right with the nose on the horizon.

(C) The wings are level, but the nose is below the horizon.

33


SYMBOLICAIRCRAFTALIGNMENT KNOB

SYMBOLICAIRCRAFT

COMMANDBAR

AIRCRAFT IS FLYING STRAIGHT AND LEVEL ANDALL COMMANDS ARE SATISFIED.

AIRCRAFT IS FLYING STRAIGHT AND LEVEL ANDTHE FLIGHT DIRECTOR IS COMMANDING A CLIMB.

THE AIRCRAFT HAS SATISFIED THE CLIMBCOMMAND.

(C)

operated by a glideslope radio receiver and gives

the pilot information needed for an instrument ILSapproach. An ADI is an integrated instrument thatgives pitch and roll attitude data like an artificialhorizon and additional displays of information fromradio navigation sources and other aircraft systemslike the flight director. On sophisticated aircraft whichhave backup gyro instruments, a turn and bank isnot installed, so the inclinometer is installed at thebottom of the ADI as can be seen in figure 1-59.

4. Turn and BankThe last of the three basic gyro instruments is theoldest and simplest. It is called the turn and bankand it is really two instruments in one. The gyropart of the instrument measures the rate of turnfor the aircraft. The inclinometer or slip-skid in-dicator is a simple mechanical instrument that con-sists of a ball in a liquid filled glass tube. Thistube is curved and the ball reacts to gravity andcentrifugal force. It is used by a pilot to coordinatethe use of aileron and rudder control. If the pilotkeeps the ball centered, the aircraft is being flownin a coordinated manner. This instrument is espe-cially helpful when the aircraft is turning. Whenthe ball is not centered, it means the aircraft is

Figure 1-57. The newer type of artificial horizon uses apresentation that is easier to interpret.

flying a little sideways. The gyro rotor of the turnand bank is designed to measure the rate of turnof the aircraft. It is the only one of the three basicgyro instruments which is a rate gyro.

The other two basic gyro instruments measureangular displacement about the aircraft's axes. Theturn and bank has a gyro with a horizontal spin

Figure 1-58. The command bars on an attitude directorindicator show the pilot the pitch and rollattitude that is needed to satisfy the com-mands of the flight director.

34


ATTITUDE SPHERE

PITCH REFERENCE

ROLL REFERENCE "POINTER"I ROLL ATTITUDE SCALE

FLIGHT DIRECTORMODE ANNUNCIATORS

GO AROUNDANNUNCIATOR

RADIO ALTIMETERDECISION HEIGHTANNUNCIATOR

FLIGHT DIRECTORCUE COMMAND BAR

GLIDESLOPEPOINTER

SPEED COMMAND SCALE

-SPEED COMMAND "DONUT"111

GLIDESLOPESCALE

RADIO ALTIMETER DECISIONHEIGHT READOUT RADIO ALTITUDE READOUT

AIRCRAFT SYMBOL

RADIO ALTIMETERTEST BUTTON

ATTITUDE TEST BUTTON

RATE OF TURN SCALE

RATE OF TURN POINTER

INCLINOMETER

BRIGHTNESS CONTROLFOR DIGITAL READOUTSIN AD1 & HSI

RADIO ALTIMETER DECISIONHEIGHT SET CONTROL

EXPANDED LOCALIZER SCALE

EXPANDED LOCALIZER POINTER

axis and one degree of freedom. The feature thatmakes it a rate gyro are the springs that are con-nected to the gimbals. These springs oppose theprecession force which is caused by the aircraftturning. These features of the turn and bank canbe seen in figure 1-60.

When the aircraft turns, the gimbal holding thegyro rotor tilts over against the tension of the springand moves the pointer to indicate the directionand rate of turn. The turn and bank gives readingsbased on the concept of a standard rate turn. Astandard rate turn is a turning rate of 3° per second.This is also called a 2 minute turn because it would

take 2 minutes to turn 360° at this rate. A standardrate turn is not suitable for a high speed aircraftbecause it would require a steep angle of bank.

These higher speed aircraft would use a 1/2 stand-ard rate turn which is 1- 1/2° per second or a 4minute turn. Turn and banks are manufacturedin both types; 2 minute turn and 4 minute turn,both of which are shown in figure 1-61. The turnand bank indicator is also called a turn and slipindicator and a needle and ball. The face of theinstrument shows a needle to indicate turn directionand rate and a ball which is the slip-skid indicatoror inclinometer.

Figure 1-59. The newer type of ADI uses different shapes for the airplane symbol and the command bars symbol. It alsomay include additional displays for other systems. (Courtesy of Canadair Group, Bombardier Inc.)

35


POINTER

CALIBRATEDCENTERING

SPRING

DASHPOT

The turn and bank is shown in figure 1-61. The

pointer is the rate of turn indicator and the glass

tube is the inclinometer.

The index marks on either side of the centerposition of the pointer on the lower instrumentare called dog houses. When the pointer is linedup with a dog house, it indicates a 2-minute turnon the bottom instrument. A 2-minute turn on theupper instrument would be indicated by a one nee-dle width deflection of the turn needle. The turnand bank is considered to be a backup instrumnetfor the artificial horizon. If the artificial horizonfails, it is possible to fly the aircraft using the turnand bank in its place.

Another gyro instrument called the turn coor-dinator is a modified version of the turn and bank.The only significant difference in the internal mech-anism is the fact that the tilt axis for the gimbalis changed to a 30° angle from the horizontal asshown in figure 1-62. This causes the gyro rotor

to react to rotation around the longitudinal axis

as well as the vertical axis. The turn and bank

only measures rotation rate about the vertical axisso that it cannot be used accurately to level thewings. The turn coordinator is a better back-upinstrument for the artificial horizon for this reason.Since the turn coordinator is not the same as aturn and bank and doesn't give the same kindof information, it has a different appearance sothat pilots won't confuse the two instruments.

The turn coordinator as illustrated in figure 1-63uses a rear view of a small airplane as the indicator.When the wing tip of the airplane symbol is linedup with an index mark, it indicates a standard rateturn for the 2 minute type. The turn coordinatoralso includes an inclinometer, like the turn and bank.

5. Gyro Instrument Power Sources

Aircraft gyro instruments can be powered by elec-tricity or air. The electric gyros can use 14 or

Figure 1-60. The turn-and-bank instrument has a gyro rotor with a horizontal spin axis and one degree of freedom. Italso has a centering spring on the gimbal.

36


TWO-MINUTE TURN INDICATOR DIAL

FOUR-MINUTE TURN INDICATOR DIAL

28 volts DC or several different values of AC. Thegyros that are air driven can use either an airpump or bleed air from turbine engines. Air-drivengyros can either use suction pressure or positivepressure. Those that use suction pressure areusually called vacuum driven gyros. Some olderaircraft used vacuum venturis to power the air-driven gyro instruments. The venturi for gyros ismounted on the fuselage of the aircraft and theairflow caused by the forward motion of the aircraftcreates a low pressure or suction in the throat

Figure 1-61. Both 2-minute and 4-minute turn and bankinstruments are available.

of the venturi. A major problem with using a venturifor IFR flight is that the venturi tends to becomeblocked with ice under some flight conditions.Another disadvantage of the venturi tube is thatthe aircraft must maintain a certain minimumairspeed to generate enough vacuum for the gyros.The gyros will not be spun-up and stable duringtakeoff for example. Examples of 2" and 4" venturis

Figure 1-62. By mounting the gimbal at an angle to thehorizontal, the turn coordinator sensesrotation about both the roll and yaw axes ofthe aircraft.

Figure 1-63. The presentation on the face of the turncoordinator is different from that of a turnand bank so that the two instruments willnot be confused with each other.

37


FOUR INCHVENTURI

TWO INCHVENTURI

TURN AND SLIPINDICATOR

CENTRALAIRFILTER

SUCTIONGAGE

DIRECTIONALGYRO

GYROHORIZON

are found in figure 1-64. The 2" and 4" are notphysical dimensions. They refer to the amount ofsuction in inches of mercury that each is designedto provide.

The most common type of air pump used onmodern airplanes for the gyro instruments is calleda dry air pump. It does not use any oil for sealingor lubrication. It is a vane type pump and the vanesare made of a carbon based material which graduallywears away in service from rubbing against thecylinder walls. Figure 1-65 shows a dry air pumpconnected to operate as a vacuum pump. Noticethat the gyro instruments and gauge are installedin parallel. Figure 1-66 shows the same kind ofdry air pump that has been connected to operateas a positive pressure pump. In the vacuum pump

system the output of the pump is dumped overboardand the cockpit air is filtered before it flows intothe instruments. A filter is required on the regulatorof the vacuum system because air is drawn in atthat point to regulate the vacuum pressure. Anadvantage of the positive pressure system is thatit is better for aircraft that operate at higher altitudesof 15,000 to 18,000 ft. The positive pressure systemrequires a filter on the inlet side of the pump anda filter on the outlet side ahead of the instruments.A filter is not required on the regulator in the positivepressure system. There is also a wet pump forair driven gyros which uses engine oil for coolingand lubrication. It can only be used as a vacuumpump and requires an air/oil separator to returnoil to the airplane's engine. Figure 1-67 shows the

Figure 1-64. A 2 " and 4 " venturi are available to power air-driven gyro instruments. The ratings apply to the numberof inches of Mercury vacuum that are provided, not to physical size.

38


FILTER

PUMP

INLETFILTER REG. INLINE

FILTER

GAGE

PUMP

REQ'D WHEN AIRCRAFTIS PRESSURIZED

OUT

IN SOUT

IN

air/oil separator in a wet pump system as wellas a suction reducer that is used to drop the pres-sure for the turn and slip indicator. An air driventurn and bank or turn and slip requires about2 inches of mercury while the other two basic typesof gyros require 4-5 inches of mercury.

For any kind of air driven gyros, it is very importantto change the filters regularly to ensure that onlyvery clean air reaches the gyro instruments. The in-struments are very delicate and can wear out rapidlyif dust and dirt are allowed to enter with the air

supply. A typical air filter for aircraft gyro instrumentsystems is shown in figure 1-68. The small filterinstalled on vacuum regulator valves can be seenin figure 1-69. The tubing and hose in an air drivengyro system must be checked to make certain thatno restrictions are present which would create higherthan normal resistance to the flow of air. The onlylubricant approved for vacuum system fittings is usu-ally a silicone spray. Thread lubricants and Teflon®tape should not be used as they might get drawninto the system and cause damage.

REG.

Figure 1-65. A gyro instrument vacuum system that uses a dry air pump.

Figure 1-66. A gyro instrument system that uses a dry air pump to supply positive rather than negative pressure.

39


FILTERSUCTION REDUCER

TURN & SLIPINDICATOR CENTRAL

J/--- AIR FILTER

AIRIN

V

GYRO HORIZON DIRECTIONAL GYRO

./— SUCTION REGULATOR SUCTION GAUGE11 AIR< IN

(1

VACUUMPUMP

OIL SEPARATOR/ AIR OVERBOARD4 OR TO DE-ICER

DISTRIBUTOR VALVE

ENGINE OILMETERED INTOPUMP FORCOOLING & SEALING

OIL RETURN.0 TO ENGINE

CRANKCASE

Figure 1-67. A wet pump vacuum system to operate three gyro instruments. A suction reducer is needed in the line tothe turn and slip since it requires less vacuum pressure.

Figure 1-68. A typical replaceable filter used with air-driven gyro instruments.

6. Inspection and Maintenanceof Gyro Systems

Some recommended practices for gyro system main-tenance are:

Check the time it takes for the gyro instru-ments to come up to full speed and stabilize.This should normally be about 2-4 minutes.

Listen for unusual noise when the gyros arespinning. Noise is easier to detect after theengines are stopped.

When power to the gyros is removed, measurethe run-down time. If there is a shorteningof the normal run-down time, it indicates thebearings are getting worn or some other prob-lem exists.

Check tubing and hose condition. They shouldnot be worn or restricted. Check for kinksand dents.

Fittings should be in good condition and withwide radius bends. Do not over tighten.

Use only approved lubricants for fittings. Siliconespray is the most common recommendation.

40


EXPANSION UNITINSTRUMENTLAMP

-JEWEL

JEWEL POST

JEWEL SPRING

COMPENSATINGMAGNET

FLOAT

CONTACT ANDSOCKET ASSEMBLY

LUBBER LINE

CARD —

LENS — OUTER CASEii

FILLER HOLE

PIVOT

COMPENSATING MECHANISM

SENSINGMAGNET

COMPENSATINGSCREWS

Route tubing carefully to avoid rubbing andabrasion.If it becomes necessary to blow the lines toremove dirt or moisture, ensure that instru-ments are completely disconnected. Apply airpressure to instrument end of the lines.

Figure 1-69. A vacuum regulator for gyro instrumentsincludes an air filter.

9. Replace filters at recommended. intervals—more often in dusty conditions or if smokersride in the aircraft.

When installing additional air-driven gyro instru-ments or if a problem is suspected, the load onthe pump should be evaluated. Each gyro instrumentrequires a certain volume of air which is statedin cubic feet per minute (CFM). Add up the re-quirement in CFM for all the instruments and ensurethat it does not exceed the rated CFM for the pump.You must also evaluate the pressure drop require-ments for the instruments and lines. Artificialhorizons and directional gyros usually require 4.0-5.0 in. Hg. The turn and bank requires 2.0-2.5in. Hg. The loss or pressure drop in all the linesand tubing should not exceed 2 in. Hg. If it does,you may have to use larger diameter tubing.

E. Compass Systems1. Magnetic Compass

The aircraft magnetic direction indicator or compassis a completely independent instrument. It does notrequire any electrical or tubing connections. It con-tains a compass card with magnets that line up with

Figure 1-70. The parts of a liquid-filled aircraft magnetic compass.

41


the magnetic flux lines of the earth. Figure 1-70 showsthe following basic parts of a magnetic compass.

A compass card or float which is mountedon jeweled bearings. It has numbers and di-rection markings so that the magnetic headingof the aircraft can be read from the instrument.

The case is filled with a light oil (usually refinedkerosene) which dampens float motion andlubricates the bearings.

A diaphragm or bellows accommodates ther-mal expansion and contraction.

The compensator is two small moveable mag-nets used to adjust the compass for deviationerror.

5. The lubber line is a marker against whichreadings are taken.

The face of a typical liquid filled magnetic compassis shown in figure 1-71. The indicated magneticheading is 035°.

2. Compass ErrorsThe magnetic compass is subject to a number oferrors which affect its operation. These includevariation, deviation, acceleration error, northerlyturning error and oscillation error. Variation erroris simply the fact that a magnetic compass willgive indications based on the magnetic north poleand not the north geographic pole. The normal gridlines on an aeronautical chart are in true directionsbased on the geographic poles and the equator.As can be seen in figure 1-72, the north magneticpole is hundreds of miles from the north geographic

Figure 1-71. The face of a liquid-type magnetic compass.

pole. In most locations, there will be a differencebetween true and magnetic directions. This is varia-tion. Figure 1-73 shows how the amount of variationis drawn on aeronautical charts for pilots to use.There are some locations where true and magneticdirections are the same. This would be along theline called the agonic line. Everywhere else the pilotwould consult the variation markings on the mapand add or subtract the appropriate number ofdegrees to convert from true to magnetic headings.

Acceleration error and north turning error areboth a result of compass dip. The earth is roundso that at high latitudes in the northern hemisphere,the compass card will tilt downward toward thenorth magnetic pole. This compass dip causes bothof these errors. If an airplane is flying east andit accelerates, the compass will momentarily in-dicate a turn to the north. If it decelerates, it willindicate a turn to the south. North turning erroroccurs when the aircraft is flying north or south.If a turn is made from a north heading, the compasswill indicate a turn in the opposite direction momen-tarily and then it will lag behind the actual headingduring the turn. Turns from south will cause thecompass to lead the actual heading or indicate ahigher than actual turning rate. Oscillation erroris caused by the very delicate bearings in the com-pass. In rough air, the compass will oscillate backand forth 40°, 50° or more. The compass may neversettle down as long as the turbulence persists. Thisforces the pilot to have to estimate the actual com-pass reading.

Deviation error is the most important one formaintenance technicians because they usually per-form the checks and adjustments for deviation error.This error is also called magnetic influence errorsince it is caused by magnetic influences withinthe aircraft. All aircraft have some steel parts thatmay have some permanent magnetism. Most aircraftalso have electrical circuits that can produce electro-magnetic fields. Both of these can affect the mag-netic compass and cause errors. The compensatormagnets in the compass are used to adjust thiserror to a minimum. This process is called swingingthe compass. It should be performed wheneverequipment is installed that could cause a changeor when a problem with the compass accuracy issuspected.

Swinging the compass—the basic procedure is:

1. Locate a compass rose on a ramp area whichis accurate and can be used as a reference.A compass rose is a circle with magnetic di-rections indicated as shown in figure 1-74.

42


Configure the aircraft for the checks by turningon electrical equipment and radios, runningthe engines and establishing a level attitude.

Set the compensators to zero (there are twolittle screws labeled N-S and E-W)

Point the aircraft north on the compass roseand adjust the N-S screw to zero error or asclose to zero error as possible.

Point the aircraft east and adjust the E-Wscrew to zero error or as close to zero erroras possible.

6. Point the aircraft south and remove half theerror.

Point the aircraft west and remove half the error.The process so far has averaged the error for

all headings. Now you are ready to record the error.

Point the aircraft on all headings every 30°,and record the compass heading for each.

9. Prepare a placard which lists the deviationerror at least each 30°. Place it on or nearthe compass and make a logbook entry.

The compass correction card is used to recordthe deviation error for the aircraft's compass. Anexample is shown in figure 1-75.

A newer type of compass is called the verticalcard compass. It operates like the other types of

yNorth Geographical

Figure 1-72. Variation error for an aircraft compass is the difference between true headings and magnetic headings. Itis caused by the fact that the north geographic pole and the north magnetic pole are not in the samelocation.

43


0 +5 +10+15 +20 +24-24

+15

..+10

WESTERLYVARIATION

+5-15'

AGONICLINE

-5

Figure 1-73. Lines of variation are drawn on aeronautical charts so that pilots can apply the proper corrections duringflight planning.

Figure 1-74. A compass rose contains the markings andnumbers needed for magnetic direction ref-erences.

magnetic compass but the presentation on the faceof the instrument is a full compass card whichis easier to read. It sometimes eliminates the useof oil and employs eddy current damping. See figure

1-76 for an illustration of the appearance of a verticalcard compass.

The full compass card presentation of the verticalcard compass makes it easier to read. If the pilotwants to turn to a heading of 180°, the presentationmakes it easier to determine if it is quicker to turnleft or right to reach that heading. The compensatorscrews can be seen at the bottom of the verticalcard compass.

3. Flux Gate CompassThe flux gate compass is a special type of remotemounted compass which is more stable than a stand-ard magnetic compass and usually eliminates theproblems of acceleration and north turning errors.

The sensor used with a flux gate compass systemis called a flux valve or flux gate. It is a wheelshaped device made of a ferrous material with threespokes and the rim cut into three equal parts. Theflux valve sensor can be seen in figure 1-77. Theexcitation coil is in the center and the pick up coilsor output coils are installed with one on each spokeof the flux valve core. The excitation coil is suppliedwith AC current with a frequency of 400 Hz. Itis designed so that when the current flow in theexcitation coil is at peak value, the core materialis saturated. When the current falls below peakvalue, the earth's magnetic flux lines cut acrossthe pick up coils and produce an output signal

44


COMPASSCARD

O

FIXEDAIRCRAFTSYMBOL

CORRECTIONSCREWS

LUBBERLINE

PICK UPCOILS

EXCITATIONCOIL

SECTION A-A

in each one. The excitation coil in effect alternatelyblocks out the earth's magnetic field and then allowsit to move across the output coils. This producesan AC output signal from each of the three outputcoils. Since the angle of the earth's flux lines to

FOR N0 3 6

E9 12 15

S18

RADIO

ER

NO RADIO

FOR 21 24W27 30 33

RADIO

ER

NO RADIO

Figure 1-75. The compass correction card is usuallymade up by the mechanic when he swingsthe compass to determine the deviationerror.

Figure 1-76. The vertical card compass displays a com-plete compass card and is easier to readthan the older type. (Courtesy CanadairGroup, Bombardier Inc.)

the flux valve changes for each different heading,the relative values of the three output voltages willbe different for each different heading. This isillustrated in figure 1-78. An electronic componentmeasures the three output signals and derives themagnetic heading of the aircraft.

In order to give accurate readings, the flux gatesensor must normally be maintained in a level, hor-izontal position with respect to the Earth's surface.This leveling can be accomplished in one of two ways.In the first type of flux gate sensor found on aircraft,the sensor is suspended by a pendulous mechanismso that it can remain level when the aircraft attitudeis changed. This type of flux gate has a housingfilled with a light oil to dampen the motions of themoving parts. In the second type of flux gate compasssystem, the flux gate sensor is stabilized by a gyrosystem to keep it level. The output signals from aflux gate sensor are sent to an electronic unit whichamplifies the signals and calculates magnetic heading.The output of this electronic unit is sent to cockpitindicators that require magnetic heading informationand sometimes to navigational systems that requireheading information. The two common cockpit in-struments that receive signals from the flux gate sys-tem are the HSI and the RMI (radio magnetic indicator).The HSI and RMI can be seen in figure 1-79 whichshows the flux gate compass system for a Challenger

Figure 1-77. The flux valve sensor has an excitation coilin the center and three pick up coils on thespokes or arms.

STE

STE

45


airplane. The flux valve sensors themselves are nor-mally installed near the wing tips to keep themaway from magnetic influences in the aircraft. Thelocation of the vertical card magnetic compass isalso shown in figure 1-79.

The HSI and the RMI both have a compass cardwhich indicates the magnetic heading of the aircraft.The heading information comes from a flux gatecompass system. The compass cards on both in-struments are driven by a remote mounted direc-tional gyro. The DG receives signals from the fluxgate compass that automatically reset it to the cor-rect magnetic heading. The remote DG is slavedto the flux gate compass and the compass cardson the instruments are slaved to the remote DG.The connections are shown in figure 1 - 80 whichis a diagram of a flux gate compass system. Inthis system, the pilot never has to reset the in-struments with his magnetic compass in the cockpitunless the flux gate compass system fails.

The face of an RMI is shown in figure 1-81. Thecompass card in this instrument indicates the mag-netic heading of the aircraft as previously described.The RMI can be identified by the two pointers thathave a common pivot point in the center of theinstrument. These pointers are connected to radionavigation systems so that they point toward thelocation of the ground transmitter. The selectorswitches allow each pointer to be connected to anADF or VOR radio receiver.

4. FARs for Compass Systems

There are a number of FARs that relate directlyto compass systems.

FAR 23.1327Magnetic Direction Indicator

Must be installed to prevent influence by air-plane vibrations or magnetic fields.

Maximum deviation in level flight is 10 degreeson any heading.

3. Magnetic non-stabilized may deviate more than10 degrees due to electric heated windshieldetc. if either a stabilized magnetic directionindicator or DG is installed. Deviation over10 degrees requires a placard.

FAR 23.1547Magnetic Direction IndicatorDeviation Placard

Placard must be installed on or near the MDI(compass).

Placard must list calibration for level flightwith engines running.

Placard must state if calibration is for radioson or off.

Calibration increments must be 30 degreesmaximum.

THE AIRCRAFT IS HEADED NORTH THE AIRCRAFT IS HEADED WEST

(A) (B)

Figure 1-78. The changing angle of the earth's flux lines to the flux valve produces a different output signal for eachdifferent heading of the aircraft.

46


0 COMPASS CONTROLS 0

0 .0•Sl AVEO

O O

STANDBY COMPASS

RADIO MAGNETICINDICATOR (RMI)

-(0.\HORIZONTAL SITUATION

INDICATOR (HSI)

COMPASS CONTROLPANEL

FLUX VALVE WS 274

DUAL REMOTE COMPENSATOR

DIRECTIONALGYRO NO. 2

DIRECTIONALGYRO NO. 1

Figure 1-79. The location of the various components of a flux valve compass system are illustrated in this drawing. Thestandby magnetic compass is also shown. (Courtesy Canadair Group, Bombardier Inc.)

47


CO-PILOTSRMI

e e

CO-PILOT COMPASSCONTROL PANEL

FLUX VALVE 2

NO.2 VHFNAV RX

STANDBYCOMPASS

AUTOPILOTCOMPUTER

DUAL REMOTECOMPENSATOR

PILOTSHSI

FLUX VALVE 1

PILOT COMPASSCONTROL PANEL DIRECTIONAL GYRO 1

DIRECTIONAL GYRO 2

PILOTSRMI

® COMPASS CONTROLS

DG

SLAVED

Figure 1-80. The compass cards in the aircraft HSI and RMI instruments are operated by both the flux valve compasssystem and the remote mounted directional gyros. (Courtesy Canadair Group, Bombardier Inc.)

48


SINGLE BAR POINTER(YELLOW)

COMPASS CARD

LUBBERLINE

OFF-WARNINGFLAG

MODE SELECTSWITCHES

DOUBLE BAR POINTER(GREEN)

5. More than 10 degrees deviation for electricheated windshield etc. must be placarded.

F. Electronic Instruments

1. Basic PrinciplesThe term "electronic instruments" is used to referto the latest trend in aircraft instruments. Thisinvolves the use of CRTs (cathode ray tubes orTV screens) to display aircraft instrument infor-mation. Another common term for this system isthe "glass cockpit". The use of CRTs permits agreater use of integrated instruments which displaynumerous types of information on one screen. Italso permits greater flexibility because the methodof displaying the information and the amount ofinformation on each CRT can be changed in flight.It is also claimed that reliability is increased becausecomplex electro-mechanical instruments are re-placed by CRTs that have no moving parts. TheseCRTs are operated by a special type of computercontrol called a symbol generator. The latest gen-

eration of air carrier jets and bizjets was designedto use the glass cockpit displays. This group in-cludes Boeing 757, 767 and 747-400; McDonnellDouglas MD-11 and Gulfstream G-IVs among oth-

ers.

Other aircraft have been retrofitted with glasscockpit displays in their latest versions or as anoption from the factory.

Some aircraft have only one or two CRTs, whileothers with a full glass cockpit system will usesix or more CRTs. The electronic instruments thatmake up a full glass cockpit come in three types:

Electronic attitude director indicators (EADIs)

Electronic horizontal situation indicators(EHSIs)

3. Engine indication and crew alerting system(EICAS)

The appearance of the EADI and EHSI are verysimilar to the electro-mechanical versions that havebeen covered previously. The major difference isthat the display is more versatile and the pilotscan select what types of information they wish

Figure 1-81. The radio magnetic indicator (RM!) has a compass card which indicates the magnetic heading of theaircraft. (Courtesy Canadair Group, Bombardier Inc.)

49


to see and much more information can be presentedwith the electronic version of the instrument. Atypical EADI and EHSI are shown in figure 1-82.The EHSI can be set to a map mode which changesthe appearance of the display to that of a map.An EHSI that is showing the map mode is foundin figure 1-83. The map mode shows an airplanesymbol along with navigational sites, airports andother features on the ground. The map modepresentation shows the aircraft moving across themap in correct relationship to locations on theground. It is a very user friendly display whichshows a large amount of information to the pilotin a way that makes it easier to read than moreconventional displays.

2. EADISome of the information that can be presented onthe EADI other than the basic pitch and roll dataincludes; radio glideslope data, radio localizer data,

Figure 1-82. Electronic flight instruments: EADI on thetop and an EHSI below.

radar altimeter data, autopilot status and aircraftindicated airspeed.

EHSIInformation displayed on the EHSI includes: mag-netic heading, radio steering commands forVOR /INS, radio glideslope data, DME radio dataand weather radar data. The EADI and EHSI areinstalled directly in front of the pilots to matchthe standard "T" configuration.

EICASThe EICAS system usually consists of two largeCRTs installed in the middle of the instrumentpanel. The two CRTs may be arranged verticallyor horizontally depending on the particular aircraftinvolved. The EICAS display screens in figure 1-84are positioned one above the other. The EICAS sys-tem has two main types of information that aredisplayed, as the name implies. The engine indica-tion function displays numerous powerplant instru-ments in standard columns depending on how manyengines the aircraft has. The crew alerting systemfunction consists of many sensors locatedthroughout the aircraft that monitor all the majorsystems such as engines, electrical, hydraulic, bleedair, pressurization, etc. These sensors are monitoredby computer and any faults or abnormal readingsare displayed to the flight crew. On many aircraftthese sophisticated monitoring systems replace ahuman crew member, the flight engineer. This allowsthe aircraft manufacturer to design a large airplanelike a 747-400 that only requires two flight crewmembers instead of three.

An EICAS system that uses two CRTs stackedvertically is probably the most common and willbe described here. This system is shown in figure1-84. The upper screen has a standard presen-tation which displays the primary engineparameters. These are the most important engineinstruments that are used to set power andmonitor the engines. Also on the upper screenis a list of alert and status messages concerningthe aircraft systems.

During routine cruise flight conditions, the lowerscreen is very often blank. If a problem suddenlydeveloped with the hydraulic pressure, for example,the EICAS computer would automatically put a mes-sage on the upper screen and show the hydraulicsystem instruments on the lower screen. The basictheory of this system is that normal readings onthe instruments do not have to be displayed forthe crew. When an abnormal reading occurs, thenit will be displayed to the crew. This reduces theworkload for the two-man cockpit.

50


The primary engine parameters on the upperscreen are in two identical columns because theairplane has two engines. The instruments dis-played are engine pressure ratio (EPR), N1tachometer and exhaust gas temperature (EGT).During engine starts, the EICAS system will auto-matically display the secondary engine parametersas shown here. The EICAS screens can also displayadditional information such as check lists. If anengine flames out during flight, a checklist is auto-matically displayed which shows the acceptable al-titudes and airspeeds for an attempted restart aswell as the checklist to accomplish this task. TheEICAS system is complex and expensive so it hasbeen installed only on the more sophisticatedaircraft. The EADI and EHSI can be found on allclasses of aircraft including small single engineairplanes.

5. Heads Up Displays

The glass cockpit instruments were made possibleby the rapid advances made in microprocessors

and digital computer technology. An even neweritem of advanced cockpit displays is the Heads UpDisplay. The use of a HUD system was developedby the military for combat aircraft. If informationabout important aircraft systems is displayed inthe windshield area, the pilot does not have toshift his attention down to the instrument panelto get this information. The HUD allows the pilotto keep looking out the windshield of the aircraft(head up) and to see the information that is neededprojected onto a special screen in the windshieldarea. A heads up display system for commercialjet airplanes has been developed and is installedin some aircraft at this time. This system is calledthe Heads Up Guidance System (HGS) and the dis-play is shown in figure 1-85.

The HGS screen itself is a special type of glassplate which the pilot can look through even whileinformation is being projected onto the screen. Theimages on the screen are focused at infinity sothe pilot does not have to refocus his eyes to lookat either the world outside the windshield or the

— AIRCRAFTSYMBOL

—SELECTEDNAV SOURCE

DISTANCE AND COURSETO DESIGNATOR

TUNEDVOR/DME

"NORTH-UP" TRU

IDENTIFIER 4

DISPLACEMENTLINE

DESIGNATORRNG100

6.0/190°

NE

+ 0 LA?

MAY

AIRPORTANNUNCIATOR

"TO" WAYPOINT

0

WXRANGE

ETA

MAG/TRUANNUNCIATOR

FMS 2

TO MANZY'llETA 1736DIS 100

WAYPOINT -DISTANCE TO

—"TO" WAYPOINT

Figure 1-83. The appearance of an EHSI when operating in MAP MODE.

51


ALERT MESSAGEFIELD >PRIMARY ENGINE

PARAMETERS

ENGINE OILPARAMETERS

SECONDARY ENGINEPARAMETERS

ENGINEVIRRATION

data on the HGS. The HGS screen is operated bya computer controlled system which has many sen-sors to display different information. On a civilaircraft the information displayed on the HGS isthe same kind of information displayed on an EADI.The first airline to begin using an FAA-approvedHGS system was Alaska Airlines. They retrofittedtheir Boeing 727s with HGS at a cost of about$200,000 for each aircraft. The use of the HGSenables the airline to operate in bad weather con-ditions that might ground aircraft of other airlines.The FAA has approved this operation because ofthe elimination of the need for the pilot to switchhis attention from the instrument panel to the viewout the windshield. The future will no doubt seean increasing use of HGS and other advanced dis-play systems by many other airlines and aircraftoperators. Versions of the heads up display designedfor use in twin-engine turboprop aircraft are alreadybeing developed by several companies.

G. Computers in Aircraft

The rapid advances in computer technology inrecent years have been applied to many differentaircraft systems such as cockpit displays,autopilots, navigational computers, engine controls

Figure 1-84. EICAS display screens. This system is usedon the Boeing 757 and 767.

etc. The aviation maintenance technician that works

on modern aircraft should have a basic under-standing of computers in general and their applica-tion to aircraft systems. The modern digitalcomputer is made possible by the rapid advancesin integrated circuits that have taken place overthe last twenty years. A modern microprocessoris in effect a computer on a small chip of silicon.This small and powerful chip makes possible themanufacture of small but powerful computers.

1. Basic Parts of a Computer

A computer is made up of three basic parts asrelated to their functions. The basic hardware con-sists of input devices, output devices and the CPU(central processing unit). Refer to figure 1-86.

Input devices are things like keyboards, mice,scanners etc. Output devices are CRTs, printers,plotters etc. The central processing unit containsthe brains of the computer. The CPU can be dividedinto three different units by their function. Thecentral control unit directs data from one placeto another and maintains overall control of the

Figure 1-85. The display screen for a heads up guidancesystem (HGS) is a transparent plate thatdisplays the same kinds of information asan ADI, but it allows the pilot to look out thewindshield through the HGS display. (Cour-tesy Flight Dynamics, Inc.)

INPUTS

CPU

CENTRAL CONTROL UNIT

MEMORY

3. ALU

• OUTPUTS

Figure 1-86. The basic parts of a computer. The CPU contains the control unit, the memory and the arithmetic logic unitwhich performs calculations.

52


LRU

operations. The memory stores information on spe-cial computer chips. The ALU is the arithmetic logicunit; it performs the mathematical calculations thatare required. The term peripherals is often usedin discussing computers. Peripherals are the variousinput and output devices, examples of which weregiven above.

The memory of a computer comes in two typesthat are known as RAM and ROM. The RAM orvolatile memory temporarily holds data that is beingacted upon by the computer. It is called volatilebecause it is lost each time the computer is switchedoff. The operator can change and manipulate theRAM memory with keyboard entries and other ac-tions. The ROM or non-volatile memory is sometimescalled hard-wired. The data in the ROM area willnot be lost when the computer is switched off andcannot be altered by a simple keyboard entry. Anexample of ROM is the built-in startup test thatmost computers have. When the computer is firstswitched on, it tests itself for errors and checksto see what peripheral devices are connected toit. An example of RAM could be a term paper thatyou are typing into a computer using a word proces-sor program. If you forget to save the documenton a disk, it will be lost when the computer isswitched off.

2. Some Applications ofComputers in Aircraft

A modern jet airplane may have many different com-puters that perform a variety of functions. The useof digital systems on aircraft is becoming more andmore common because it offers several advantages:

Increased reliability

Faster response

3. Reduced power consumption

Smaller and lighter weight equipment

Lower operating cost

Computers have become so common that theyare now used in many different aircraft systemssuch as autopilot, engine controls, navigation, flightplanning, etc.

3. BITE SystemsOne of the features of the effort to reduce operatingcost is the use of BITE (built-in test equipment).The latest types of aircraft electronic equipmentand computers have special types of test equipmentas a part of the major units. BITE systems oftenprovide three different kinds of tests that can beused to identify and correct faults.

1. Fault Detection — continuous during equip-ment operation

Fault Isolation — faulty equipment can be iso-lated or bypassed

Operational Verification After Defect Repair

The last example is a type of BITE program thatmaintenance personnel would use most often. Afterchanging a piece of equipment which is thoughtto be the cause of the problem, a verification testcan be conducted to ensure that the system isnow operating normally. Running this particulartest usually involves just pushing the appropriatebutton.

To simplify the troubleshooting and repair ofmodern electronic equipment, it is installed in theaircraft in the form of LRUs. An LRU is a linereplaceable unit which means a standard size con-tainer which slides easily in and out of a specialmounting rack. A typical arrangement for LRUsand equipment installations is shown in figure 1 -87.The LRUs use standard types of electrical connectorsand mounting attachments; this makes it easy tolocate and change one in a short period of time.The BITE systems in an aircraft are designed toidentify faulty LRUs so that they can be changedquickly and easily.

5. Digital Data TransmissionThe increasing use of computers and sophisticatedelectronic devices on modern aircraft requires thatthese devices be able to communicate with each otherrapidly and efficiently. This rapid exchange of datais accomplished with digital data transmission using

Figure 1-87. Avionics equipment in modern aircraft isinstalled in special racks that accommodatestandard sizes of line replaceable units orLRUs. This makes changing the LRUs aquick and simple procedure.

53


HSIADI

DIGITAL BUS SYSTEMn_/

FLIGHT AUTO FLIGHT THRUST VHF RADIOMANAGEMENT GUIDANCE MANAGEMENT NAVIGATION ALTIMETER

SYSTEM SYSTEM SYSTEM SYSTEM SYSTEM

0 Enno q o o 0000

° q q 1:111 q q AFDS MODE CONTROL PANEL

110101:glinnnnnnnWIrAtt.: 7,111.1

DIGITAL DATA BUS

INERTIALREFERENCE

SYSTEM

CENTRALAIR DATA

COMPUTER

SYMBOLGENERATOR

digital data busses. Since the various pieces of equip-ment that use this data bus are manufactured bymany different companies, a uniform standard forthe method of data transmission is needed. The stand-ard which is used by this type of equipment on modernair carrier jets and bizjets is ARINC 429.

The initials ARINC stand for Aeronautical RadioIncorporated. This organization has been in exist-ence since the 1930s to provide certain servicesto the airline industry.

The members of ARINC include the major airlines,aircraft manufacturers and equipment manufac-turers. They establish many study groups that in-vestigate emerging technology and suggeststandards that can be applied to new types of equip-ment. When these standards are approved, theywill be followed by all the members of ARINC. In

the case of ARINC 429, this means that computersand similar equipment that utilize digital signals

will be compatible with each other. Since the airlinesoften lead the way in the development of new typesof equipment, the manufacturers of equipment forsmaller aircraft often use ARINC standards also—even if they are not members of ARINC. UnlikeFAA and FCC regulations, ARINC standards arenot laws; but anyone who wants to sell airplanesor equipment to the major airlines will comply withthese accepted industry standards. ARINC 429 hasbeen used as an example here because it appliesto digital information transmission systems usedon aircraft. Many of the newer types of equipmentdescribed earlier such as EADI, EHSI, EICAS, BITE,etc. will use digital data exchange systems thatare designed in accordance with ARINC 429.

Figure 1-88. A digital data bus permits rapid transmission of data between the various electronic systems on the aircraft.The bus itself is a shielded twisted pair conductor which helps to prevent interference.

54


CHAPTER II

Powerplant Instruments and Logic Gates

The information presented in chapter 2 will be inthree major topic areas: powerplant instruments,logic gates and binary numbers, and position in-dicating and annunciator systems.

A. Liquid QuantityMeasuring Systems

Depending on the type of aircraft involved, theremay be just one or there may be many differentliquids carried on the aircraft for which a quantitymeasurement is required. Most of the examplesgiven here will be fuel quantity systems becausethey are the most common. It should be notedthat for each type of system described, it couldbe used to measure fuel, oil, water, hydraulic fluidor some other liquid quantity.

Most small, single engine airplanes only haveone liquid quantity indicating system in the cockpitand that is for fuel. The simplest types of fuel quan-tity systems are those that use mechanical systemsand require no electrical power to give readings.These will be described first.

Sight Glass GaugesThe simplest kind of liquid quantity system is thesight glass gauge. In this system a small glass orplastic tube is connected into the tank so that thelevel of the liquid in the tube matches that in thetank. Markings on the tube itself or a plate behindit indicate the quantity. A sight glass gauge is shownin figure 2-1.

This type of quantity system has no moving parts,but the tank must be located in or near the cockpitarea for it to be practical. It has been used onolder aircraft for fuel and hydraulic fluid quantitysystems.

Float-type Mechanical GaugesA number of different kinds of mechanical floatquantity systems have been used.

A very simple version utilized a float mountedon a metal rod which projected through a holein the gas cap so that the rod would be visiblefrom the cockpit. The fuel tank was located directlyin front of the cockpit in the fuselage so that itcould be easily seen. This type is illustrated byfigure 2-2.

The float was often made of cork and it hadto be coated with a special shellac or varnish sothat it would not sink. Two disadvantages of thisquantity system are that the rod tends to bounceup and down and there are no index markingsat all. A variation of this system was used in manybiplanes where the fuel tank was in the centersection and the rod stuck down below the tankin a clear tube with an indicator fastened to it.This inverted float system with the indicator belowthe tank is shown in figure 2-3.

The gauge called the magnetic direct reading isa float-type gauge which uses a gear system to

Figure 2-1. A sight glass liquid quantity gauge.

Figure 2-2. A mechanical float-type fuel quantity gaugefor a fuselage tank.

55


TANKRESISTOR

• -i- DC POWER

Figure 2-3. A mechanical float fuel quantity gauge for thecenter section of a biplane.

rotate a pointer in a round gauge and uses a mag-netic principle to isolate the glass face cover andpointer from the fuel. This type of gauge was oftenused on high wing airplanes where the fuel tankswere in the butt end of the wing. The gauge wasinstalled so that the float was inside the tank andthe round face of the gauge was visible inside thecockpit. It is shown in figure 2-4.

The float rotated a shaft through a simple gearingsystem. On the end of the shaft was a U-shapedmagnet which rotated along with the shaft. Separat-ing the magnet and shaft from the pointer and

Figure 2-4. Mechanical float-type gauge with a per-manent magnet to isolate the fuel from thecover glass and pointer.

the rest of the gauge in the cockpit was a piece

of aluminum with sealing gaskets. The magnetic

flux traveled through the aluminum and rotateda piece of ferrous metal that in turn rotated thepointer.

Resistance GaugesThe type of fuel quantity gauge most common onmodern small airplanes is similar to the kind usedin cars. It has a float in the tank that moves avariable resistor. The variable resistor alters thecurrent flow in a DC circuit to operate a metermovement that is somewhat similar to those usedin voltmeters and ammeters.

The gauge used with the float operated variableresistor is most often the ratiometer type seen infigure 2-5. This gauge uses two opposing magneticfields so that the pointer reacts to the ratio of currentflow in the two sections. In this way, it is lessaffected by fluctuations in system voltage causedby voltage regulator settings or a weak battery.

Underwing Fuel Quantity IndicatorsMany large aircraft have two totally different typesof fuel quantity measuring systems. One of theseoperates the cockpit gauges and the other is anunderwing fuel quantity system. The underwing sys-tem can only be used on the ground and is mostoften employed by maintenance and service per-sonnel rather than by the flight crew. There arethree kinds of underwing fuel quantity systems,but they all share certain features in common. Theyall utilize a fuel quantity stick of some type whichcan be extended below the bottom surface of thewing. They measure the fuel quantity in terms ofvolume and not mass. These underwing quantitysystems typically require no electrical power to ob-tain readings. This last characteristic would be anadvantage if it was necessary to take fuel readingswhile working on the fuel system.

The oldest and simplest type of underwing fuelquantity system is called a drip stick. It uses a

Figure 2-5. Ratiometer fuel quantity system using a float-operated variable resistor.

56


LATCHING CAM

hollow tube which extends from the bottom of thewing up inside the fuel tank as illustrated in figure2-6. The tube is normally stowed by being pushedup inside the tank until the bottom of the tubeis flush with the bottom surface of the wing whereit is latched in the closed position. In order toobtain a reading, the tube is unlatched and pulleddown until the upper end reaches the top of thefuel level. When fuel begins to drip out the bottomof the tube, a reading is taken using the markingson the outside of the tube. This type of fuel quantitysystem is not used on modern aircraft becauseof the fire danger when fuel is allowed to drip onthe ground or hangar floor.

Another type of underwing fuel quantity stickis the one which uses a clear Lucite® plastic rod.The main features of this device are shown in figure2-7. The rod is made of clear plastic because itobtains readings by transmitting light along therod. The principle involved is the refraction of light.Fuel and air have different light refraction char-acteristics and if a specially shaped quartz tip isinstalled on the top of the rod, it will produce aparticular light pattern when it is positioned atthe top of the fuel level. In order to take a reading,the tube is pulled down from the bottom of thewing until the light pattern on the bottom of therod is focused to a point of light. The reading isthen taken using the markings along the lengthof the rod.

The most common type of underwing fuel quantitystick on modern aircraft is the one which usesa float inside the tank that has a magnet fastenedto it. The upper end of the stick has a magnetwhich will attract the float magnet when they are

Figure 2-6. Older style drip stick underwing fuel quan-tity gauge.

in alignment. This type of underwing fuel quantitystick is shown in figure 2-8.

A fuel reading is taken by unlatching the stickand pulling it down until the float and the top

Figure 2-7. An underwing fuel quantity measuring stickthat utilizes a clear Lucite rod.

Figure 2-8. Underwing fuel quantity stick which employsa float and permanent magnets.

57


115V400HZ

FUEL TANKCAPACITOR

REFERENCECAPACITOR

of the stick are held in position by magnetic at-traction. The reading is then taken from the mark-ings along the tube. The magnetic force is not strongenough to lift the float out of the fuel; so whenthe stick is pushed up, the magnetic attractionis broken and the stick can be pushed up andstowed.

These types of underwing fuel quantity systemsare not usually as accurate as the cockpit fuelquantity system, but they can be used for main-tenance and troubleshooting purposes.

5. Capacitance Quantity IndicatorsThe most common type of liquid quantity measuringsystem used on modern turbine engine aircraft isthe capacitance type. It has the advantage overother quantity systems in that it can give accuratereadings in very large or unusually shaped tanks.Another advantage is the fact that liquid quantityis measured in terms of mass or weight ratherthan in volume. Measuring fuel quantity in massis especially useful with large turbine engine aircraftbecause the power produced by the engines is morea factor of the mass of fuel consumed rather thanthe volume. In very large fuel tanks, the volumeof the fuel will vary considerably due to thermalexpansion and contraction, but the mass wouldremain the same.

The capacitance liquid quantity system gets itsname from the fact that the measuring probes lo-cated in the tank are capacitors. A simplified rep-resentation of this type of fuel quantity system isshown in figure 2-9. In the real system, the probeis usually constructed in the form of two concentricmetal tubes which are the two plates of the capacitor.When this probe is located in a fuel tank, the twoplates of the capacitor will be separated by fuelon the lower end and air on the upper end. Sincefuel and air have different dielectric constant values,the amount of capacitance will change as the fuellevel rises or falls. The dielectric constant for thefuel is also affected by density. Therefore any in-crease in density caused by thermal contractionwill result in an increase in capacitance. The probeswill automatically measure the mass or weight ofthe fuel. A small, symmetrical tank like an engineoil tank may only require one capacitance probeto give accurate readings. A large, tapered wingfuel tank might have 15 or more probes connectedin parallel to ensure accurate readings. Thecapacitance fuel quantity system of figure 2-10 hasa total of 17 capacitance probes. Electronic circuitsmeasure the amount of capacitance in the probes,apply any needed corrections, and send electrical

signals to the cockpit gauges to indicate the fuelquantity in pounds. Capacitance fuel quantity sys-tems usually include a totalizer. The totalizer givesa reading of the total fuel on board the aircraft.Some fuel systems will also give the fuel used sincetakeoff.

B. Fuel Flow IndicatorsThere are a number of different gauges which mightbe used for aircraft fuel systems depending on thetype and complexity of the particular kind of fuelsystem used. All powered aircraft will have a fuelquantity system. On small airplanes with gravityflow fuel systems, this would be the only type offuel system instrument required. Aircraft with pumpfed engines will need a fuel pressure gauge in ad-dition to fuel quantity. Aircraft with fuel-injectedor large radial piston engines and aircraft with tur-bine engines will usually have a fuel flow instrument.Some aircraft with turbine engines that operatein cold temperatures will also have a fuel tempera-ture indicator to guard against the danger of icecrystals in the fuel. The various types of fuel flowindicator systems will be described in this section.

1. Fuel-injected Engine FlowmetersThe type of flowmeter commonly installed on aircraftwith fuel-injected reciprocating engines is not atrue flowmeter at all. The sensor used with thisinstrument system actually measures pressure not

Figure 2-9. Simplified circuit to illustrate the principleof operation for a capacitance liquid quan-tity system.

58


L.H. REARAVIONIC BAY

•0%*% OUTPUT FROM

0‘sSIGNAL CONDITIONER0TO INDICATOR

ws3

SIGNAL FROM PROBETO SIGNAL CONDITIONER

1111111111111111111111111111111111111111111111111111111M111111111

ws200

ws244 ws

277

CAPACITANCETYPE QTY.

TRANSMITTER

Figure 2-10. The major components and their location for a capacitance fuel quantity system on a corporate jet.(Courtesy Canadair Group, Bombardier Inc.)

59


LEFT ENGINE

MSS VAPOR RETURN AND EXCESS FUEL

LIED CROSSFEED FUEL

VENT

- MECHANICAL LINKAGE

ELECTRICAL CONNECTION

RIGHT ENGINE

- IFUELDISTRIBUTION

MANIFOLDFUEL FLOWINDICATOR

_

CODE

THROTTLESFUEL-INJECTIONNOZZLE (TYP)

FUEL/AIRCONTROL UNIT

FUELPRESSURE SWITCH

I i H 4_ J L

J L

THROTTLESWITCHES

FUEL-INJECTIONNOZZLE (TYP)

FUEL/AIRCONTROL UNIT

FUELPRESSURE SWITCH

MIXTURECONTROLS

•

ENGINE DRIVEN FUELPUMP

i PRESSURE SWITCHRELAY

PRESSURE SWITCHRELAY

ENGINE DRIVEN FUELPUMP

flow rate. Since the injector lines and nozzles havea certain restriction to flow, a given pressure sup-plied to the injection system will produce a givenflow rate for normal operating conditions. This typeof instrument uses a Bourdon tube in the gaugewhich is connected by tubing and hose to the fueldivider block on top of the engine. The line leadingfrom the fuel distribution manifold on the enginecan be seen in figure 2-11. The face of the instrumentusually has three different units of measurement:PSI, gallons per hour (GPH) or pounds per hour andpercent of cruise power. The instrument face shownin figure 2-12 has these three units. This instrumentwill give accurate readings for all three of thesevalues as long as everything is operating normally.The pressure at the fuel distribution manifold willbe proportional to the flow rate if the total restric-tions to flow in the system are normal. If thereis any fault in the system which causes the restric-tion to the flow of fuel to increase or decrease,the instrument can give erroneous readings. Forexample, if an injector nozzle was blocked this wouldcause a greater restriction to flow and an indicationof increased fuel consumption when the actual fuel

flow rate would be decreased. A leak in an injectorline would decrease the restriction to flow anddecrease the indicated flow rate on the gauge butthe fuel consumption would actually increase.

2. Vane-type FlowmetersThe vane-type flowmeter uses a sensor like thatin figure 2-13 that is installed in the line that feedsfuel to the engine. The vane is mounted on a shaftso that it will rotate through an arc as the fuelpushes against it. The circular chamber that con-tains the vane has enough clearance between thecylinder walls and the vane that the flow of fuelis not retarded to any significant degree. The vanetype sensor will measure the volumetric flow rateof the fuel.

The vane is rotated against a restraining springso that the amount of rotation of the vane cor-responds to the volumetric flow rate. The cockpitgauge is normally marked to show the flow ratein gallons per hour. The position of the vane inthe sensor is transmitted electrically to the cockpitgauge where it rotates the pointer to the correctreading. The type of electrical system that transmits

Figure 2-11. The fuel flow indicator system for a fuel-injected reciprocating engine that measures pressure at the fueldistributor manifold. (Courtesy Cessna Aircraft Co.)

60


BYPASS VALVE

FUELINLET

FUELOUTLET

METERINGVANE

1

1

1

FUEL bFLOW

INDICATOR

TRANSMITTER

RESTRAINING SPRING

this positional information concerning the vane isa type of synchro system. An example of a vane-typeflowmeter with a synchro system is shown in figure2-14. Since these synchro systems are used withmany other types of aircraft instruments, they willbe described next.

Figure 2-12. Fuel flow indicator with three differentmeasurement units: pounds per hour, PSI

and percent of cruise power.

3. Synchro SystemsThere are three types of synchro systems and theyshare the same basic features and are used forsimilar purposes. A synchro system consists ofa transmitter unit and a receiver unit. The twoare connected to each other by electrical wiring.The transmitter unit contains an input shaft andit can be connected to anything which will rotate

Figure 2-13. A vane-type flow sensor that measuresvolumetric flow rate.

Figure 2-14. Vane-type flowmeter system for a large airplane which includes a synchro system.

61


INDICATORTRANSMITTER

D

this shaft through an arc. The receiver unit hasa shaft which is connected to the pointer in theinstrument. If the shaft in the transmitter unit isrotated 20° to the right, the shaft in the receiverwill also rotate 20° to the right. The operation ofa synchro system causes the receiver unit to movein synchronization with the transmitter unit. A largejet transport may have many different synchro sys-tems for a variety of different instruments. Thetransmitter unit can be connected to anything whichproduces a rotation of the shaft through an arc.Figure 2-15 shows how the rotor in the receiverunit will position itself automatically based on themagnetic field created by the three outer magnets.The three different kinds of synchro systems willnow be described. They do differ in details of con-struction, but the basic operation of all three isas described above.

a. DC Selsyn® Synchro

The transmitter unit in the Selsyn synchro is avariable resistor with three sections as seen in figure2-16. The shaft is connected to the wiper arm. Thethree sections of the variable resistor are connectedby wires to the three coils in the receiver unit. Therotor of the receiver unit is a permanent magnetthat is connected to the instrument pointer. Theposition of the wiper arm in the transmitter de-termines the voltages that are produced by the three

sections of the resistor. The permanent magnet inthe receiver unit will line up with the overall magneticfield produced by the three coils surrounding it.Any rotation of the shaft and wiper arm of the trans-mitter will cause different voltages to be appliedto the receiver unit. A new orientation of the magneticfield in the receiver unit will pull the rotor intothe correct alignment.

Figure 2-15. An illustration of the basic operation of a synchro receiver unit. The rotor will align itself with the resultantfield of the three outer magnetic fields.

Figure 2-16. The DC-powered Selsyn synchro system.

62


1/3

7:6\ \

1/3 —s C71/3

26 V B

400 HZPOWER SUPPLY

TRANSMITTINGMAGNESYN

PERMANENT MAGNET SOFT IRON COREv

PD

)13

INDICATINGMAGNESYN

TOROIDALWINDING

DOWN

Magnesyn® Synchro

The Magnesyn synchro system uses AC power,most often 26 volts AC and 400 Hz. The use ofAC power eliminates the need for a variable resistorand improves reliability because there are nobrushes to wear or get out of adjustment. The con-struction features of the transmitter and receiverunit are similar, which can be seen in figure 2-17.The rotors are permanent magnets and the threesection windings are connected together. The mag-netic field produced in the receiver will pull therotor into a position that corresponds to the rotorposition in the transmitter unit.

Autosyn® Synchro

The only difference between the Autosyn and Mag-nesyn synchros is that the Autosyn uses electromag-nets instead of permanent magnets for the rotors.Figure 2-18 shows an Autosyn synchro system. Italso uses AC power that is most often 26 volts ACat 400 Hz. Many pressure and flow type instrumentson modern jet airplanes use a synchro system totransmit the information to the cockpit gauge.

4. Mass Flowmeters

The latest types of turbine engine aircraft use a flow-meter that gives a reading of the mass flow rate

Figure 2-17. The AC powered Magnesyn synchro system.

Figure 2-18. The AC-powered Autosyn synchro system.

in pounds per hour rather than a volumetric readingin gallons per hour. The mass flow rate is a moreuseful indication for this type of aircraft. Refer tofigure 2-19 for a drawing of the mass flowmeter. Themass flowmeter consists of a motor-driven impeller,a turbine and a synchro system to transmit the datato a cockpit gauge. In order to give accurate readings,the impeller must be driven at a constant speed.This is accomplished with an AC synchronous motoror a similar device. As the fuel flows through theimpeller, it is given a spin or rotation by the spinningimpeller. When the fuel leaves the impeller, it strikesthe turbine which is rotated against a restrainingspring by the spin energy of the fuel. Because adenser fuel would impart more spin energy to theturbine, the degree of rotation of the turbine is ameasure of mass flow rate. The turbine is connectedto the transmitter rotor of a synchro system whichwill cause the pointer on the cockpit gauge to rotateto the proper position to indicate the correct massflow rate. The sensor for this and other types of flow-meters is installed in the fuel system downstreamof the fuel control device so that the flow rate rep-resents the fuel consumption rate for that engine.

5. Computerized Fuel System

The computerized fuel system is a volumetric flowmeasuring system found on some fuel-injectedreciprocating engines. The sensor is a small unit whichis installed on top of the engine in the fuel line thatfeeds the fuel splitter or manifold. The sensor ortransducer contains a small rotor that has the samedensity as the fuel to ensure accurate readings.

As the fuel flows past the rim of the rotor, itspins the rotor at a rate which is proportional tothe volumetric flow rate of the fuel. The rotor hasnotches on its rim which interrupt a light beamfrom a light emitting diode (LED). This light beamfalls on a phototransistor which produces an outputsignal with a frequency that matches the flickerrate of the light beam. The electrical output of thephototransistor is connected to the computer inthe cockpit instrument which processes the dataand displays information for the pilot. The inputsignal to the computer has a frequency which isan indication of flow rate, but the computer cancalculate and display fuel flow, fuel used and fuelremaining in several different units of measurement.By including a computer to process the signal fromthe transducer, the computerized fuel system cangive the pilot a number of different kinds of usefulinformation. The cockpit indicator is usually alighted display like that shown in figure 2-20. Thisis not a CRT but a digital lighted display. Thereare three common types of lighted digital displays

63


DECOUPLINGDISK IMPELLER FUEL

FLOW<=i1

TURBINE

!IMPELLER!MOTOR

available that use light emitting diodes (LEDs), liquidcrystal displays (LCDs), or gas discharge tubes.

6. FARs for Fuel Systems

In chapter 1, the requirements for powerplantinstruments were covered and it would be usefulto look over that section again to review the in-formation appropriate to the instruments coveredin this section. Some additional FARs concerningfuel systems are given here.

FAR 23.993Fuel Systems

Each fuel line must be installed and supportedto prevent vibration and to withstand fuel pres-sure and flight loads.

Where relative motion could exist, fuel linesmust have provisions for flexibility.

Each flexible hose must be approved or shownto be suitable for the particular application.

No flexible hose that might be adverselyaffected by exposure to high temperaturesmay be used where excessive temperatureswill exist during operation or after engineshutdown.

FAR 23.1337Powerplant Instruments

1. Each line carrying flammable fluids underpressure must:

Have restricting orifices or other safety de-vices at the source of pressure to preventthe escape of excessive fluid if a line fails;and

Be installed and located so that the escapeof fluids would not create a hazard.

2. Each powerplant instrument that utilizes flam-mable fluids must be installed and locatedso that the escape of fluid would not createa hazard.

3. Fuel Quantity Indicator — There must be ameans to indicate to the flight crew membersthe quantity of fuel in each tank during flight.An indicator, calibrated in either gallons orpounds, and clearly marked to indicate whichscale is used, may be used. In addition:

Each fuel quantity indicator must be cal-ibrated to read zero during level flight whenthe quantity of fuel remaining in the tankis equal to the unusable fuel supply.

Each exposed sight gauge used as a fuelquantity indicator must be protectedagainst damage.

CALIBRATEDRESTRAINING FLUID FLUID

SPRINGS PASSAGE PASSAGETRANSMITTER B C

115 V.A.C.

C

INDICATOR

rA

I.1

A

C D

MOTORCIRCUIT

Figure 2-19. A fuel flowmeter system that measures the mass flow rate of fuel for a turbine engine.

64


FUN'FLOW

I_ I_I 1_I A I I

OPH

F tilELTRON **WM

WESVS WY

*ESE,

WOW

Sgen

TICSITNT

L'(;;)

BRASS

n-n

IRON

BEFORE HEATING

AFTER HEATING

Each sight gauge that can collect waterand freeze must have a means to allowdrainage on the ground.

Tanks with interconnected outlets and air-spaces may be considered as one tank andneed not have separate indicators.

4. Fuel Flowrneter System — Each metering com-ponent must have a means to bypass fuelif a malfunction of that component severelyrestricts fuel flow.

FAR 23.1553Fuel Quantity Indicator

If the unusable fuel supply for any tank exceedsone gallon, or five percent of tank capacity,whichever is greater, a red arc must be markedon its indicator extending from the calibrated zeroreading to the lowest reading obtainable in levelflight.

FAR 23.1557Miscellaneous Markings and Placards

1. Fuel and oil filler openings.

Each fuel filler opening must be markedon or near the filler cover with the word"fuel" and the acceptable fuel grades.

For pressure fueling systems, the maximumpermissible fueling and defueling pressuresmust be indicated.

c. Oil filler openings must be marked at ornear the filler cover with the word "oil".

C. TemperatureMeasuring Systems

There are a number of common methods of measur-ing temperatures on an aircraft. All of them willhave a limit as to how high a temperature theycan be used to measure. The common types oftemperature measuring systems will be described,but not all of the possible applications can be in-cluded. Aircraft temperature indicators may givereadings in degrees Fahrenheit or in degrees Celsius.Most of the temperatures given here will be indegrees Fahrenheit for ease of comparison.

1. BimetallicThe bimetallic temperature system is limited tomeasuring temperatures up to a maximum of 140°F.The outside air temperature gauge (OAT) or free airtemperature gauge is an example of the bimetallicsystem. The device that reacts to changes in tempera-ture is a bimetallic sensor that consists of two thinstrips of metal joined together. The strips are madeof different metals that have different coefficients ofexpansion. The metals iron and brass are often used.

As the temperature changes, one of the metal stripswill expand or contract more than the other causingthe device to bend and move the indicator pointer.The basic principle of a bimetallic temperature sensoris shown in figure 2-21. The two metal strips are

Figure 2-20. The lighted digital display instrument for acomputerized fuel flow system.

Figure 2-21. An illustration of the basic principle of opera-tion for a bimetallic temperature sensor.

65


TEMPERATUREPROBE

BOURDONTUBE

Afi:10 ADM.

ANCHORPOINT

WHEATSTONE BRIDGE-TYPE RESISTANCE THERMOMETER

often formed into a spiral shape so that a temperaturechange will cause a rotating motion of the sensorstrips. A typical bimetallic outside air temperaturegauge is shown in figure 2-22. This type is ofteninstalled through the windshield.

2. Mechanical BulbThe mechanical bulb utilizes the principle of the in-crease in pressure of a confined gas with temperatureincreases to measure temperatures. As shown in figure2-23, the mechanical bulb system consists of a bour-don tube gauge to measure pressure, a thin-walledbulb which is at the point of measurement and athin tube (capillary tube) to connect them together.

The system is filled with a chemical such as methylchloride which will be part liquid and part gas. Thesystem is sealed with the proper amount of thechemical so that the change in pressure withtemperature changes will give an accurate readingon the bourdon tube gauge. The mechanical bulbsystem is found as an oil temperature indicatorsystem on many smaller airplanes that don't havean electrical system. A mechanical bulb is also foundon some jet engines where it transmits compressorinlet temperature (CIT) data to the fuel control unit.The maximum temperature for this type of systemis about 300°F.

Figure 2-22. An outside air temperature gauge for a smallairplane. This gauge uses the bimetallicprinciple.

3. Wheatstone BridgeThis method of measuring temperature is poweredby electricity and limited to about 300°F. TheWheatstone bridge system is illustrated in figure 2-24.The bridge circuit consists of three fixed resistors

Figure 2-23. A mechanical bulb temperature measuringsystem which measures the vapor pressureof a special chemical.

Figure 2-24. The Wheatstone bridge system is used tomeasure temperatures with a variable resis-tance probe.

66


THE VOLTAGE GENERATED IN A THERMOCOUPLESYSTEM IS PROPORTIONAL TO THE TEMPERATURE

DIFFERENCE BETWEEN THE TWO ENDS.

and one variable resistor. The variable resistor is thetemperature probe which contains a coil of fine nickelwire. As the coil of wire is heated, its resistance increasesand alters the current flow in the bridge which movesthe needle in the gauge. Electromagnetic attraction andrepulsion will move the pointer whenever the currentflow through the meter changes. A disadvantage ofthe Wheatstone bridge is that any added resistancedue to bad connections or any fluctuations in the systemvoltage can cause inaccurate readings. For this reason,it has been largely replaced by the ratiometer.

RatiometerThe ratiometer uses the same kind of electricalpower and the same kind of probe that the Wheat-stone bridge uses. The difference is in the metermovement that moves the indicator pointer. Theratiometer has two opposing magnetic fields thatcombine to produce a resultant field that movesthe pointer. The resultant field is a ratio of thetwo opposing fields so that a lower voltage appliedto the system will not cause inaccurate readings.The ratiometer can measure temperatures up to300°F. and is used for oil temperature indicatorsand other similar requirements. The schematic fora ratiometer is shown in figure 2-25.

ThermocouplesWhen it becomes necessary to measure temper-atures of about 500°F or more the thermocoupleis most often used. The principle of the thermo-couple is shown in figure 2-26. When a junctionof two dissimilar metals is heated, it will producea difference of potential or voltage. The amountof voltage produced is proportional to the temper-ature. The terms "hot junction" and "cold junction"are used with thermocouples. The hot junction iswhere the temperature measurement is being taken,while the cold junction is at the opposite end ofthe wires in the instrument. The voltage outputof the thermocouple is a result of the temperaturedifference between the hot junction and the coldjunction. It sometimes is necessary to compensate

Figure 2-25. The circuit for a ratiometer temperature sys-tem that can operate on 14 or 28 volts DC.

for any temperature variances at the cold junctionin order to obtain accurate readings. Only a fewcombinations of metals are used for thermocouplehot junctions. The metals must not only withstandthe high temperature being measured, but theymust produce a usable amount of voltage. The chartin figure 2-27 shows the three commonly used pairsof metals and their voltage output at various tem-peratures. The actual voltages produced are verylow, particularly at lower temperatures. This is whythe thermocouple system is not usually used fortemperatures below about 400°F. The three typesof thermocouples are the iron-constantan, the cop-per -constantan and the chromel-alumel. Notice thatthe chromel-alumel can measure much higher tem-peratures than the other two types. Thermocoupleleads are available in standard lengths with specificvalues of resistance. The length of a thermocouplelead should not be altered in the field.

An application of the thermocouple on pistonengines is the cylinder head temperature (CHT)gauge. This gauge is used to monitor the coolingof an air-cooled engine. If only one probe is used,it will be installed in the hottest running cylinder.This would usually be a rear cylinder on a hor-izontally opposed engine. One type of CHT probeis a gasket that goes under the spark plug, anothertype fits in a special fitting in the cylinder head.These two types are shown in figures 2-28 and2-29 respectively. It is better to install CHT probesin all the cylinders. In addition to monitoring allthe cylinders, it can be used to troubleshoot sometypes of engine problems. Another application forthe thermocouple is as an exhaust gas temper-ature (EGT) gauge. All turbine engines have an

Figure 2-26. An example of a simple thermocouple sys-tem showing the hot junction and coldjunction.

67


0 300 500 1000

CHROMEL-ALUMEL IOMEL-ALUMEL

42 --I.

IRON-CONSTANTAN

22.50

15-J

COPPER-CONSTANTAN

30

50

EGT gauge or gas temperature gauge becauseturbine engines can be severely damaged by hightemperatures in the turbine sections. A numberof terms and abbreviations are used for the gastemperature gauge on turbine engines. Turbineinlet temperature (TIT) refers to a system wherethe probes are installed ahead of the turbinestages. Inter turbine temperature (ITT) means theprobes are located between the different sectionsof the turbine. Turbine outlet temperature (TOT)and EGT refer to measurements that have theprobes installed downstream of all the turbinestages. These locations and abbreviations areshown in figure 2-30. Turbine engines use mul-tiple thermocouple probes that are connected inparallel to give an average temperature as seenin figures 2-31 and 2-32.

Some aircraft use the term measured gastemperature (MGT) for the required gas temperatureindicator. An MGT gauge for a turbine enginehelicopter is shown in figure 2-33.

On reciprocating engine aircraft, the EGT gaugehas a different purpose. It is used to manually leanthe fuel-air mixture for better economy. The ther-mocouple probe is installed in an exhaust pipe andconnected to a simple gauge in the cockpit. Theactual procedure for leaning the engine will varyfrom one aircraft to another, the example given here

is for purposes of illustration. When ready to setthe fuel-air mixture, the pilot watches the EGT gaugeas the mixture control knob is pulled back. Thetemperature will rise as the mixture is leaned be-cause more efficient combustion is taking place.When the EGT reaches a peak, the proper mixturefor maximum economy has been reached. If onlyone probe is installed for the EGT system, the in-dication is really the average EGT for all the cylindersand they can vary. It is best to have an EGT probefor each cylinder as this gives more informationand permits the use of EGT for troubleshooting en-gine faults. When the EGT gauge is used for leaning,there is usually no redline at all on the gauge. Thiscan be seen on the gauge in figure 2-34 whichis an EGT system for a reciprocating engine airplane.However, a turbo-supercharged reciprocating enginewill have a redline because the turbo-superchargercan be damaged by high temperatures.

In aircraft schematics, the thermocouple wiresare given standard color codes for ease of iden-tification. The standard colors for the wires are:

IRON — BlackCONSTANTAN — YellowCOPPER — RedCHROMEL — White

5. ALUMEL — Green

410DEGREES CELSIUS

Figure 2-27. Voltage output versus temperature for different combinations of thermocouple metals.

68


YELLOW(CONSTANTAN)

BLACK(IRON)

YELLOW(CONSTANTAN)

RED(COPPER)

D. Position Indicating SystemsThere are many different components and systemson aircraft that might have a position indicating sys-tem. There are a relatively small number of differentmethods that are used to obtain these indications.The basic operation of common types of position in-dicating systems will be explained and some specificapplications will be discussed.

Some of the different methods used to operateposition indicating systems are:

Mechanical — Rods, levers, cables, etc.

Microswitches — Sometimes called limitswitches (figure 2-35).

Variable Resistance — Wheatstone bridge andratiometer.

Proximity sensors — Mainly on larger, moremodern aircraft.

5. Synchro Systems — Selsyn, Magnesyn andAutosyn.

Figure 2-29. A cylinder head temperature system thatuses a bayonet-type probe which fits into aspecial recess in the cylinder head.

JJJu JL

Figure 2-28. A cylinder head temperature (CHT) systemfor a small reciprocating engine. The ther-mocouple probe is in the form of a sparkplug gasket.

ITT

TOT OR EGT

Figure 2-30. The measurement points in the turbine sec-tion for turbine inlet temperature, inter-tur-bine temperature, turbine outlettemperature and exhaust gas temperature.

69


ALUMEL —

/*\

CHROMEL + —

AMPLIFIER

ITT

COCKPIT INDICATOR

EGTTHERMOCOUPLE

PROBE

CHROMEL-ALUMELWIRING HARNESS

Figure 2-31. Turbine engine gas temperature systems such as the ITT gauge use multiple probes in parallel and anamplifier to supply signals to the cockpit indicator.

Figure 2-32. Turbine engines use multiple EGT probes of the chrome) / alumel type to take an average EGT for theexhaust section of the tailpipe.

70


300 - 750 °C CONTINUOUS OPERATION

750 - 780 °C TAKEOFF (5 MINUTES)

780 °C MAXIMUM TAKEOFF

900 °C MAXIMUM FOR STARTING(12 SECONDS)

All the systems and components in an aircraftthat might have a position indicating system canbe divided into two categories: those that only havetwo operating positions and those that have manyor an infinite number of different operating posi-tions. Some examples follow:

Landing Gear — 2 Positions

Cabin and Cargo Doors — 2 Positions

Thrust Reversers — 2 Positions

Ground Spoilers — 2 Positions

Trailing Edge Flaps — Many Positions

Flight Control Surfaces — Infinite Positions

7. Trim Tabs — Infinite Positions

On smaller aircraft, the systems that only havetwo operating positions most often use micros-witches. Landing gear position indicators are usual-ly lights that are operated by microswitches onthe landing gear. This type is shown in figure 2-36.A problem with microswitches is that they are proneto damage from rocks, sand, water, etc. that arethrown up onto the landing gear in service. Theyalso have problems with arcing and burning of thesmall contact points. For these reasons, most largeraircraft employ proximity sensors instead of micros-witches. Figure 2-37 illustrates the operation ofone type of proximity sensor. The proximity sensoris a sealed unit that operates in conjunction witha metal target. The proximity sensor produces anelectromagnetic field that is distorted when the tar-get piece of metal moves close to it. This changeis detected by an electronic circuit to give an in-dication of gear-up, gear-down etc. The target nevertouches the proximity sensor and there are no smallcontacts to cause problems. About the only dis-advantages of the proximity sensor systems arethat they cost more and since they use electroniccircuits, adjustment is more complicated than fora simple microswitch.

On simple aircraft, the systems that have manyor an infinite number of operating positions willuse mechanical or variable resistance position in-dicating systems. An example of a mechanical trimtab position indicator is shown in figure 2-38. Thewheel that the pilot rotates to move the trim tabcontains a spiral groove which moves a small wirepointer to show the position of the trim tab. Theratiometer type variable resistance system is usedas a flap position indicator on small airplanes. Itworks like the ratiometer system already describedexcept that the variable resistor is moved by a partof the flap mechanism.

On the more sophisticated aircraft, the systemsthat have many or an infinite number of operatingpositions use synchros. Surface position indicatorsare usually found on bizjets and air carrier jetsto show the flight crew the position of the flightcontrol surfaces. These use a synchro system withthe transmitter unit attached to the mechanicallinkage of the flight control surface and the receiverunit in the cockpit gauge or indicator. This typeof system is shown in figure 2-39.

E. TachometersMost types of aircraft have at least one tachometerto indicate the rotational speed of the engine. Aircraftwith reciprocating engines have tachometers that in-dicate the crankshaft RPM. This is true for engineswith reduction gearing also; the tachometer gives en-gine crankshaft RPM, not propeller RPM. Helicoptershave a tachometer for the main rotor or rotors toenable the pilot to maintain a safe rotor RPM. Turbineengines use tachometers that give readings in percentof RPM rather than actual revolutions per minute.

Figure 2-33. Some gas temperature gauges are labeledMGT for measured gas temperature. Themaximum operating temperatures are oftentime limited for specific operations.

71


REAR VIEW

TAN

ACTUATINGPLUNGER

STATIONARYCONTACT

MOVABLECONTACT

CALIBRATION SCREW (AFT) 3. PROBE

5. EXHAUST STACKTHERMOCOUPLE WIRE 4. CLAMP

6. CALIBRATION SCREW (FORWARD)

Figure 2-34. The components in an EGT system for a twin-engine airplane with reciprocating engines. This instrumentis sometimes referred to as a mixture indicator. (Courtesy Cessna Aircraft Co.)

Split spool turbine engines contain more than onemain shaft in the engine. They usually have twospools or shafts, but there are some turbine engineswith three. The split spool engines will have atachometer for each main shaft. In the case of turbojetand turbofan engines, these are referred to as theN 1 and N2 tachometers. The N 1 tach is the low pressurecompressor tach and N2 is the high pressure com-pressor tach. The low pressure compressor and highpressure compressor sections can be seen in thedrawing of a twin-spool turbojet engine in figure 2-40.Turboprop and turboshaft engines may use differentdesignations for the tachometers such as gas producer

Figure 2-35. A microswitch is designed so that the con-tact points open and close with a very smallmotion of the plunger.

tach and power section tach. The gas producer tachfor a turbine engine helicopter shown in figure 2-41shows some time limited permissible readings above100%. All tachometers have a red radial line to indicatethe maximum permissible RPM. On turbine enginesthis redline is not necessarily at 100%; it could beabove or below the 100% reading. The turbine enginemanufacturer will establish what engine section RPM

is equal to 100%. On one particular model of CF34turbofan engine, a reading of 100% N2 is equal to17,820 RPM. Some reciprocating engine tachometerswill have a red arc which denotes a range of enginespeeds that is prohibited due to vibration problemsat those rotational speeds. A triple tach for a twin-engine helicopter is shown in figure 2-42. It is threetachometers in one to give readings for the powersections of both engines and the main rotor.

1. Mechanical TachometersMost small general aviation aircraft use simplemechanical tachometers that utilize a flexible drivecable similar to the speedometer drive cable in acar. This flexible drive cable is connected to a drivegear in the engine accessory section and the otherend is connected to the tachometer in the cockpit.Older style tachometers used rotating flyweights tomove the pointer in the tachometer instrument asillustrated in figure 2-43. Later mechanicaltachometers use a rotating permanent magnet anda drag cup to move the pointer. A tachometer dragcup is shown in figure 2-44. The small permanent

72


A

MA

IN

BUS

LIMITSWITCHES I

IN

r - -0- -1-1

LU

UP LEFT

DOWN UP NOSE DOWN UP RIGHT DOWN

'- — 1 r 1 - 1 r I 1I II I II I II II 1 II I I

O ulD . NI I N• QUID oNI IN QUID ONII D J LU I DJ L U_. I DJ---

IIIL - - - -J

GEARDOWN

1iL _ _ _ _,

GEARUP

r -FLIGHT 1I

II--o Ii GROUND

SQUATSWITCH

25 • PUMPA MOTOR

HYDRAULICPRESSURE

SWITCHr - 1i II E II I II

Eli), DOWNAND

LOCKEDLIGHTS

LIGHTDIMMING

r DOWNII

1UP I

I

rII

RELAY

.. I TONAV

I I r OPEN 1 I C I LIGHTS

L J I I I 1 I - :L.G.

SELECTORI Ii-0 IL CLOSEDSWITCH

/ UNSAFE THROTTLE

LIGHT SWITCH

WARNINGHORN_ \

AIRCRAFT IS IN THE AIR, LANDING GEAR IS DOWN AND LOCKED, GEAR SELECTOR SWITCH IS INTHE GEAR UP POSITION

Figure 2-36. The landing gear indicating and warning system for a small airplane. A number of microswitches areutilized.

73


TARGET NOT IN PROXIMITY

MAGNETIC FIELD

ELECTRONICSWITCH

METAL TARGET

DETECTOR

OSCILLATOR

SENSITIVESURFACE

PROXIMITYSWITCH

TARGET

PROXIMITY SWITCHSENSITIVESURFACE

LGCU

TARGET IN PROXIMITY

n 28V DC.

0/P

OSCILLATOR 0

.Lr

METAL TARGET(WITH INDUCED EDDY CURRENTS)

DETECTOR

PROXIMITY SWITCH

TYPICAL PROXIMITY SWITCH INSTALLATION(NOSE GEAR WEIGHT ON WHEELS)

LGCU

Figure 2-37. The operation of a proximity sensor installed on the landing gear system of a corporate jet airplane.(Courtesy Canadair Group, Bombardier Inc.)

74


RUDDER TRIMINDICATOR

RUDDER TRIMKNOB

magnet is fastened to the end of the drive mechanismso that it produces a rotating magnetic field. Sur-rounding the magnet is a drag cup made of aluminum.As the magnet rotates, it sets up eddy currents inthe aluminum drag cup and the magnetic fields ofthe eddy currents interact with the rotating field ofthe permanent magnet. The interaction of the twofields causes a torque force or drag force to be appliedto the drag cup which rotates it against spring tensionto move the pointer. The main advantage of the dragcup tachometer is that there is no direct mechanicalconnection between the drive cable and the pointermechanism. This makes it smoother in operation andless likely to break if some minor binding occurs.

2. Tachometer GeneratorsThis type of tachometer system uses an electricalgenerator that is mechanically driven by gears atthe engine and which transmits electrical energy tothe cockpit instrument to give an indication of RPM.

There are both AC and DC tach generator systems,but the DC type is not found on modern aircraft.The DC type had a small DC generator on the engineand a cockpit gauge which indicated the voltage outputof the generator as RPM. The major disadvantage ofthis type of tach generator is that any fault whichcaused a lower than normal voltage would resultin an error in the indicated RPM. The AC tach generatoreliminates that problem by using an AC generatoron the engine and an AC synchronous motor in theindicator. An AC tach generator system is shownin figure 2-45. The primary determining factor inthe motor RPM is the frequency of the AC that powers

Figure 2-38. A mechanical position indicator for the rud-der trim on a twin-engine airplane. (Cour-tesy Cessna Aircraft Co.)

it. A lower voltage caused by loose connections forexample would not have much affect on the indicatedRPM. Both types of tach generator systems use per-manent magnets so that they are totally independentof the electrical systems of the aircraft.

3. Electronic TachometersThere are several different types of electronictachometers used on aircraft. The kind used onsome reciprocating engines is operated by a specialset of points in the engine magneto. This set ofcontact points opens and closes like the normalpoints, but only supplies signals for the tach system.

The points in the magneto are connected by wiringto the cockpit instrument. Since the frequency ofopening of the points is proportional to the engineRPM, an electronic circuit measures the frequencyat which the points open and close and movesthe pointer to indicate the proper RPM of the engine.

Two slightly different kinds of electronictachometers are found on turbine engines. The firsttype is often used as a fan speed sensor to measurethe RPM of the fan section of a turbofan engine.Figure 2-46 shows this type. It uses a sensor whichcontains a coil of wire that generates a magneticfield. The sensor is mounted in the shroud aroundthe fan. As each fan blade goes by, it cuts thefield of the coil and this is sensed and measuredby an electronic circuit. The frequency at whichthe fan blades cut across the field of the sensoris directly proportional to the fan RPM.

Another type of electronic tach used on turbineengines has a gear driven shaft on the engine whichturns a rotor with a permanent magnet embeddedin its rim. The sensor contains a coil which is locatedclose to the rotating magnet. Each time the fieldof the rotating permanent magnet cuts across thecoil, it induces a voltage. The frequency of thissignal is measured by an electronic circuit andused to position the pointer for the correct RPM

indication. This type of tachometer is used in figure2-47 for the N2 indication for a large turbofan engine.

F. Oil Pressure IndicatorsThe oil pressure gauges on small aircraft are usuallythe direct reading type. The oil pressure line is con-nected into an oil passage in the engine and transmitsthat pressure through tubing and hose to the cockpitinstrument which contains a bourdon tube to movethe pointer. Larger aircraft such as corporate tur-boprops, bizjets and air carrier jets will use instru-ments that do not rely on having fluids under pressurein the cockpit area. These aircraft may use a bourdontube or similar pressure sensor, but it will be installed

75


iSURFACE POSITION INDICATOR

AILERON

RUDDER

L RELEVATOR

Figure 2-39. A surface position indicator system for the flight control surfaces. Synchro transmitters and receivers areused to transmit the information from the control surfaces to the cockpit indicator.

76


LOW PRESSURE COMPRESSORAND TURBINE

-HIGH PRESSURECOMPRESSORAND TURBINE

1..00000C3O -,':,O c=,c=)c,..=--,

000 00[ C=. C=> 0 C,,a'f,'D

0 C=,Ca 0 0

70% 102% CONTINUOUS OPERATION

102% 104% TAKEOFF POWER(5 MINUTES)

104% MAXIMUM TAKEOFF

105% ONE ENGINE INOPERATIVE(30 MINUTE LIMIT)

106.5% MAXIMUM ONE ENGINEINOPERATIVE(2-1/2 MINUTE LIMIT)

Figure 2-40. Diagram of a twin-spool turbojet engine showing the low pressure compressor and high pressurecompressor.

Figure 2-41. The label Ng is commonly applied to the gasproducer tachometer for turboprop and tur-boshaft engines. Some maximum values aretime limited and some only apply when oneengine has failed on a twin engine aircraft.

Figure 2-42. Twin-engine helicopters often use a tripletachometer that provides indications forboth engines and the main rotor. The engineRPM is Np for twin spool engines since theindication is for the power turbine section.

on the engine or wherever the pressure source islocated. The information will be transmitted to thecockpit gauges by electrical signals from a synchrotransmitter or similar device that is located at thesensor end of the system. Figure 2-48 illustrates anoil pressure system with an Autosyn synchro. Thesame basic principle of operation would apply to fuelpressure gauges, hydraulic pressure gauges, andsimilar instruments.

77


SECTORGEAR

4s1

FLYWEIGHTS

POINTER

PERMANENT- MAGNET

DRAG CUP

DRIVE CABLE

FLEXIBLEDRIVE

SHAFT

Figure 2-43. Older type of mechanical tachometer that used the centrifugal force of spinning flyweights.

Figure 2-44. The permanent magnet and drag cup of amodern mechanical tachometer.

G. TorquemetersTorquemeters are used to give an indication of thetorque being produced by an engine or the torquebeing delivered to the main rotor drive of a helicopter.Turboprop airplanes and aircraft with turboshaftengines will have a torquemeter installed becauseit is the best way to measure the power beingproduced by these types of turbine engines. Figure

2-49 is a triple torquemeter for a twin engine helicop-ter. It supplies readings of the torque for each engineand the main rotor drive.

Helicopters often have a torquemeter that is lo-cated at the rotor drive gearbox to indicate the torquethat is driving the main rotor. Large radial reciprocat-ing engines like those found in DC-6s and Convair240s also had torquemeters to accurately measurethe power developed by the engines.

There are several techniques used to measuretorque for an aircraft torquemeter instrument. Onetechnique is to put sensors on a driveshaft, likethe main rotor drive shaft of a helicopter. The sensorscan be seen at the bottom of the main rotor driveshaft in figure 2-50 which illustrates this principle.The sensors measure the amount of twist in theshaft which is caused by the torque force. Theelectrical signals from the sensors are processedand used to position the cockpit indicator. Anothertechnique which is used relies on a measurementof torque pressure. The sensor for this type of systemis a small oil filled cylinder with a piston in it.The sensor would be located in the reduction gear-box. The reduction gearbox for a turboprop engineis shown in figure 2-51. The sensor is installedin the reduction gearbox so that the torque reactionforce is applied to the piston and creates a build-upof pressure that is proportional to the torque force.Figure 2-52 shows another example of this typeof torque sensor. The torque pressure is measuredand causes the instrument pointer to show the

78


TACHOMETERINDICATOR TYPICAL ROTOR

DRIVE GEAR RATION2 .343 TO 1 CW.N I .489 TO 1 CW.

VERNIERPOINTER

SPRING

POINTERYOKE

LOCATION TACHOMETERGENERATOR

N2 ACCESSORY DRIVE PADNi ACCESSORY DRIVE CASE

GENERATORFIELD

SYNCHRONOUSMOTOR FIELD

FLUX COUPLING

corresponding torque reading. The cockpit indicatorfor a torquemeter system may use a number ofdifferent units of measure. Those units of measuremost often seen are horsepower, PSI, foot-poundsand percent.

H. Engine PressureRatio Indicators

This type of instrument is used on some kindsof turbojet and turbofan airplane engines. Thoseengines built by Pratt & Whitney and Rolls Royceuse an engine pressure ratio gauge (EPR) as aprimary engine instrument. Jet engines built byGE and Garrett usually do not have an EPR gauge,but use the N i tachometer in its place. As its nameimplies, the engine pressure ratio gauge indicatesthe ratio of two different pressures measured onthe engine. The two pressures are most often calledPt2 and Pt7. The total inlet pressure at the frontof the engine is Pt2. The total outlet pressure atthe aft end of the engine is Pt 7 . Figure 2-53 il-lustrates the location of the probes and the dif-ferential pressure transducer. These pressures arecalled total pressures because the probes measureboth static and dynamic pressure. The probes

operate like pitot tubes since they measure rampressure or total pressure in the airstream. Thetwo probes are connected by tubing to a transducermounted on the engine. The transducer is a dif-ferential pressure device that produces an electricaloutput related to the ratio of Pt7/Pt2. The transducermay use a synchro transmitter that is connectedto the synchro receiver in the cockpit instrument.The Pt2 probe in the front of the engine is proneto icing so it includes a heating system to preventblockage by ice. The transducer and cockpit gaugefor a typical EPR system is shown in figure 2-54.Engine pressure ratio gauges often include an indexmark which can be set manually by the pilot. Whenthe correct power setting for takeoff has been deter-mined, a knob is used to set the bug or indexmark to the correct value on the face of the in-strument. During takeoff, the power is set by liningup the EPR gauge pointer with the bug. This makesit easier to set the correct level of engine powerfor takeoff.

I. Manifold Pressure GaugesManifold pressure gauges are only found on certainreciprocating engines where they are required to

THE THREE-PHASE GENERATOR IS DRIVEN BY THE ENGINE TO PRODUCE AC WHOSEFREQUENCY RELATES TO ENGINE RPM. THE INDICATOR HOLDS A SYNCHRONOUS MOTOR

WHICH DRIVES A MAGNETIC DRAG TACHOMETER MAGNET.

Figure 2-45. AN AC tach generator system.

79


HOTEXHAUST

INLETDUCT

C).

FAN BLADES

ELECTROMAGNETICSENSOR

FORWARD-FAN TURBOFAN ENGINE

Figure 2-46. Electronic tachometer which measures fan speed for the N1 tachometer. The sensors produce an outputsignal each time a fan blade cuts through the magnetic field. The frequency of this output signal ismeasured to provide indications of N1 RPM.

80


PICK-UPCOIL

PERMANENTMAGNET

GEARDRIVENROTOR

OIL PRESSURE INDICATOR(INSTRUMENT PANEL)

D,

CSTATOR

ROTOR

INDICATORPOINTER

A.C. POWER

TO ENGINE OIL PUMP ..

OIL PRESSURE TRANSMITTER (ENGINE)

STATOR PRESSURE CONNECTOR

ROTOR

I

DIAPHRAGM

•

VENT ---o-i-

OPEN TO ATMOSPHERE

Figure 2-47. Tachometer system for the high pressure compressor of a turbofan engine. The gear-driven rotor from theaccessory section has a permanent magnet which induces signals in a pick-up coil.

Figure 2-48. An oil pressure indicating system for a large airplane.

81


INNER SHAFT

MAIN ROTORDRIVE SHAFT

DRIVEGEAR

SENSORS

PLANET CRANKSHAFTGEARS (SUN) GEAR

RINGGEAR

TORQUEPRESSURE

GAUGE

Figure 2-49. A triple torquemeter for a twin-enginehelicopter. The torque for both engines andthe main rotor is given on the same instru-ment with readings in percent.

Figure 2-50. Example of a main rotor mast torque systemfor a helicopter. The two sensors measurethe amount of twist in the main rotor driveshaft as an indication of torque.

accurately set engine power. Aircraft with super-charged engines and aircraft with constant-speedpropellers will have manifold pressure gauges. Themanifold pressure gauge measures the absolutepressure in inches of mercury at a specific pointin the induction system of the engine. Figure 2-55shows the location of the manifold pressure (MAP)measurement for a radial supercharged engine. Fig-ure 2-56 shows the location of the MAP connectionin the induction system of a turbocharged horizon-tally opposed reciprocating engine. The pressureis measured downstream of the carburetor or fuelcontrol unit and downstream of the superchargerif so equipped. The pressure measuring port inthe induction system is connected by tubing andhose to a bellows or diaphragm in the instrument.Since the pressure in the induction system is belowambient pressure at idle or low power settings,the use of absolute pressure eliminates the con-fusion of having both positive and negative numberson the gauge. At idle, the reading on the MAP gaugewill be about 10 in. Hg. At full throttle with anunsupercharged engine at sea level, the readingwill be about 28 in. Hg. With an unsuperchargedengine the full throttle reading will always be belowambient pressure because of friction and pressure

Figure 2-51. An illustration of the basic operating prin-ciples of a torque pressure gauge. Theplanetary reduction gears have a ring gearwhich is prevented from rotating by the pis-tons in the oil filled cylinders. The torqueforce on the ring gear causes pressure to beapplied to the oil in the cylinders. This pres-sure is a measurement of torque.

82


REDUCTION GEARBOX

PROPELLER DRIVE SHAFT

TORQUEPRESSURE

GAUGE

Figure 2-52. The torquemeter system for a turboprop engine. The ring gear of the planetary gear set is prevented fromrotating by the helical splines on the outer circumference that mesh with splines in the outer housing.Torque force causes the ring gear to more rearward and push against the small piston. The piston appliespressure to the oil in the cylinder and this pressure is a measure of the torque produced.

83


loss in the induction system. A supercharged engine

will have a redline on the MAP gauge to indicatethe maximum permissible manifold pressure. Theredline might range from 35-75 in. Hg dependingon the type of engine.

J. Primary PowerSetting Instruments

The specific powerplant instruments installed in aparticular aircraft will vary considerably dependingon the type of powerplants it has and what kindsof information the pilot needs to operate the enginesproperly. The most important kinds of powerplantinstruments have been described and some infor-mation has been given concerning what types ofpowerplants would use each kind of instrument. Inorder to gain a better understanding of the application

of powerplant instruments to different powerplants,

the primary power setting instruments used with

various types of engines will be described.

A reciprocating engine with a fixed-pitch propelleruses the engine tachometer as the primary powersetting instrument. This is normally the only in-strument available on aircraft with this engine andpropeller combination that can be used to deter-mine the power setting. The fixed-pitch blade anglefor the propeller is chosen so that full throttlecan be used for takeoff without over-speeding theengine.

When a constant-speed propeller is fitted on areciprocating engine, the tachometer alone cannotbe used to accurately set engine power. The con-stant-speed propeller will automatically vary theblade angle to maintain a selected RPM.

Figure 2-53. The engine pressure ratio system for a jet engine.

84


EPRTRANSDUCER

Pt7PROBEPt2

PROBEWIRING CONNECTIONTO COCKPIT GAUGE1

CARBURETORAIR CONTROL

CARBURETOR AIRTEMPERATURE BULB

CARBURETOR

THROTTLE VALVECARB. TEMP.

INTAKE PIPE MANIFOLDPRESSURE

SUPERCHARGERIMPELLER

INTAKE AIR DUCT

CARBURETORHEAT VALVE

THROTTLE

Fairly large movements of the throttle lever canbe made without affecting the engine RPM, so thatthe tachometer by itself cannot be used to set enginepower. The primary power setting instrument forthis type of aircraft is the manifold pressure gauge.A cruise power setting would be made by first settingthe desired engine RPM with the propeller controland then adjusting the manifold pressure gauge

to the desired power setting. Information is availablein the Pilot's Operating Handbook so the pilot candetermine what settings will produce a given percentof power for cruise. Common cruise settings wouldbe 55, 65 or 75 percent of maximum engine power.Supercharged reciprocating engines also use themanifold pressure gauge as the primary power set-ting instrument.

Figure 2-54. The EPR gauge measures engine pressure ratio which is the ratio of Pt 7 / Pt2. It provides indications of thethrust being produced by a turbojet or turbofan engine.

Figure 2-55. The measurement point for manifold pressure is downstream of the carburetor and downstream of thesupercharger in this radial engine.

85


PRESS.RELIEFVALVE

AIR FILTER

WASTE GATE

TO FUELDISCHARGE

NOZZLES

TO FUELFLOW GAGE

r)

i TO FUELPUMP

TO MAN. 1,-PRESS

T1 GAGE

WASTE GATECONTROLLER

ENGINE-DRIVENOIL PUMP

OIL RETURN

THROTTLE

WASTE GATEACTUATOR

FLUSH AIRSCOOP ,,t20.

TURBINE

COMPRESSOR

EXHAUST

I

t OVERBOARD

INTAKE AIR FROM COMPRESSOR)

The primary power setting instrument for aircraft

with turboshaft and turboprop engines is the tor-quemeter. Engine RPM is not a good measure of enginepower because of the way these engines operate.The gas temperature gauge is a very important in-strument for these types of engines. The pilot needsto monitor the gas temperature reading to preventdamage to the engine by excessive heat. Turboshaftand turboprop engines are often given a flat ratingfor the maximum permissible engine power. For ex-ample, an engine might be rated at 575 SHP (shafthorsepower) from sea level to 20,000 ft. This is ineffect a derating of the engine at lower altitudes.The maximum power the engine can safely produceat low altitudes is limited by the strength of thereduction gearbox. At higher altitudes, the engineis less efficient and must work harder to producehorsepower. This results in higher engine tempera-tures; therefore, at higher altitudes, the maximumsafe throttle setting is determined by the redlineon the gas temperature gauge. The torquemeter isthe primary power setting instrument because ithas a direct relationship to engine horsepower. The

maximum power setting that can be safely usedis determined by the strength of the gearbox atlow altitudes and the gas temperature at higheraltitudes.

The primary power setting instrument for turbojetand turbofan engines depends on who manufac-tured the engine. Some of these engines use theEPR gauge and others use the N i tachometer asthe primary power setting instrument. There aresome advantages and disadvantages to each methodand the choice of which one to use is based onconventional usage.

Turbojet and turbofan engines made by Pratt &Whitney and Rolls Royce use the EPR gauge asthe primary power setting instrument. Those en-gines made by GE and a few other companies usethe Ni tach as the primary power setting instrument.There is a standard placement of primary engineinstruments for virtually all air carrier jet airplanes.

The most important engine instruments are in-stalled in the center of the instrument panel, towardthe top. The primary power setting instrument will

Figure 2-56. Manifold pressure is measured downstream of the supercharger and downstream of the throttle plate inthe fuel-air control unit for a turbo-supercharged, fuel-injected reciprocating engine. (Courtesy CessnaAircraft Co.)

86


be the one at the top of this stack of engine in-struments. The EPR gauge will be at the top ofthe stack on some air carrier jets, and the N I tachon other airplanes. Figure 2-57 shows the primaryengine instruments for an air carrier jet with Pratt& Whitney engines. Figure 2-58 shows the primaryengine instruments for an air carrier jet with GEengines.

Vibration IndicatorsAircraft with turbine engines often have a vibrationindicator system which monitors vibration from theengines. Any significant imbalance in a turbine en-gine can cause serious damage due to the veryhigh rotational speeds of the rotors. Vibrationcaused by an imbalance or other factors is indicatedto the crew in the cockpit so that they can takeappropriate action. The vibration indicator systemshown in figure 2-59 is typical of the type foundon modern jet airplanes. The sensor is a piezoelectriccrystal which produces an electrical signal whenit is vibrated. This signal is sent to a signal con-ditioner and then to the cockpit instrument to pro-vide an indication of the amount of vibration atthe sensor location on the side of the engine.

Logic Circuits andDigital Systems

Logic circuits and microprocessors have made pos-sible some of the very sophisticated electronic in-struments and similar systems on modern aircraft.They are used for many different applications fromrelatively simple switching functions to complexcomputer systems. Certain basic principles are in-volved in these advanced systems, and these willbe covered first.

1. Binary NumbersThe binary number system and binary codes arethe method used by logic gates to transmit andprocess information. The word digital refers to theuse of binary numbers and codes. A simple exampleof the concept of digital and analog type signalsis shown in figure 2-60. The values of voltage andcurrent in the circuits represent the digital andanalog systems. In a circuit with a variable resistor,there are an infinite number of different valuesfor current flow. Another example of this wouldbe a simple fuel quantity system that uses a floatoperated variable resistor. The resistor moves invery small increments so that there are an infinitenumber of different current flows that can occurin the circuit. The circuit with the switch illustratesthe concept of digital values. The switch is either

on or off with no in-between settings. This followsthe binary or digital signal system because onlytwo different values are used. The two conditionsin a binary or digital circuit are called 1 and 0,or high and low, or on and off.

The binary number system is a base 2 numbersystem. The decimal number system that we aremore familiar with is a base 10 system. The decimalsystem uses 10 different digits to make up numbers.The 10 numbers are 0 through 9. By showing someexamples of converting numbers from binary todecimal and vice versa, the binary system will bebetter understood.

The procedure for converting a number from binaryto decimal is illustrated in figure 2-61. We startby writing down the binary number as shown byrow (A). Then just above each binary digit, the base2 equivalent for that digit is written as shown byrow (B). In row (C), the decimal equivalents areentered starting at the right and working back tothe left. Notice that the decimal equivalents startwith 1 at the right and are doubled each digit asyou move toward the left. Next you look at the binarydigits in row (A) and wherever there is a binary1 you bring down the decimal equivalent as shownin row (D). Where there is a binary zero in row(A), you do not bring down any decimal equivalentsince binary zero and decimal zero have the samevalue. Finally, the decimal equivalents are addedup to produce the decimal equivalent of 77.

The procedure for converting a decimal numberto binary will be illustrated by converting the decimalnumber 77 to its binary equivalent. The techniquefor converting from decimal to binary consists ofa series of divisions by the number two. We startby setting up a table with three columns as shownin figure 2-62. The first step is to divide 77 by2 and enter the quotient and remainder in theproper column. The quotient is then brought downeach time and divided by 2 and the values recorded.The process is repeated until the quotient is zero.The digits in the remainder column are read fromthe bottom up and this will be the binary equivalent.

There are some terms used with binary numbersthat refer to how the binary digits are grouped. Forexample, in the binary number 101 110 I, thereare two sets of three digits each. In digital or binaryterminology, we would say that there are three bitsin each byte. A bit is a binary digit, while a byteis a group of bits together. As an analogy, we mightcompare them to letters and words. The previousexample would be like a group of words with threeletters in each word. This is what is meant by binarycodes, how the binary digits are grouped.

87


Figure 2-57. Powerplant instruments for an air carrier jet with Pratt and Whitney engines that utilize an EPR gauge.

88


40 60\ 1 /

0\ %RPM /80

40 60\ I /

20\ %RPM /80

N2

40 60 \\ I /

0 \ %RPM / 80..... cfe.--:

10075.2

\ N1 /

Figure 2-58. Powerplant instrument layout for an air carrier jet with G.E. engines and the N1 tachs at the top of the stack.

89


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Figure 2-60. An example of the difference between analog and digital. The circuit with the switch represents digitalbecause it has two conditions: on and off. The circuit with the variable resistor is analog because it canhave an infinite number of different current flows between maximum and minimum.

(b)

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Figure 2-61. An example of the method for converting anumber from binary to decimal notation.

Two common types of binary codes that are usedwith logic circuits are Binary Coded Decimal (BCD)and Octal. An example of BCD coding would be1001 1101 1110 J. The rule for BCD is that thereare four bits in each byte. An example of Octal codingis 100 111 101 J. This is the same as an earlierexample. There are three bits in each byte. The reasonthat standard coding systems such as these are usedis that it is more efficient than having a variablenumber of bits in each byte. The Octal code getsits name from the fact that there are eight differentdecimal equivalent numbers that can be encoded ina three bit byte. For example, 000 is equal to decimal0 and 111 is equal to decimal 7.

2. Logic GatesLogic gates can be thought of as the most simpleand basic building blocks for digital control systems.

Figure 2-62. An example of converting a number fromdecimal to binary notation.

A logic gate uses binary signals as its inputsand outputs. It is a semi-conductor device whichis manufactured using techniques similar to thoseused to produce diodes and transistors. Since logicgates are found in all types of systems on modernaircraft, it is useful to know something about themso that their function in a circuit can be understood.There are six basic kinds of logic gates that canbe identified by their standard schematic symbols.All of them have certain things in common: theymay have different numbers of inputs, but eachlogic gate has just one output. Logic gates can

91


be thought of as special types of electronic switches,in fact, they are often used to perform switchingfunctions. For simplicity, logic gates with no morethan two inputs will be described here.

AND Gate

The AND gate is shown in figure 2-63 along withits truth table. The truth table shows all the possiblecombinations of inputs and the output that willbe produced for each set of inputs. The truth tablecan be explained with a statement: the AND gateproduces a binary 1 output only when all inputsare binary 1. The name of this logic gate can makeit easier to remember the truth table.

OR Gate

The OR gate is shown in figure 2-64. The statementwhich describes the truth table is: the OR gateproduces a 1 output when any input is 1. Noticethat in the truth table for the OR gate, the inputcombinations are listed in the same order as theywere for the AND gate. This makes it easier toremember the truth tables. All logic gates with twoinputs have the inputs listed in the same order.

c. INVERT Gate

The Invert gate is shown in figure 2-65. This isa very simple logic gate, it only has one input andthe output is always the opposite of the input.The Invert gate simply inverts any signal that isapplied to it. Notice the small open circle on theoutput side of the Invert gate symbol. This willbe used in combination with the basic shapes al-ready covered to identify other kinds of logic gates.

NAND Gate

The NAND gate is shown in figure 2-66. Noticethe small open circle on the output side. This dis-tinguishes it from the AND gate and also tells howit works. The NAND gate is just an AND gate withthe ouputs inverted. This can be stated as: theNAND gate produces a 0 output only when all theinputs are 1.

NOR Gate

The NOR gate is shown in figure 2-67. The NORgate is an OR gate with the outputs inverted. Thetruth table can be described as: the NOR gateproduces a 0 output when any input is 1. Noticethe small open circle on the output side whichdistinguishes it from the OR gate.

f. EXCLUSIVE OR Gate

The Exclusive OR gate is shown in figure 2-68.The truth table can be explained by the statement:the Exclusive OR gate produces a 1 output wheneverthe inputs are dissimilar. It is the only logic gatewith two inputs that has equal numbers of onesand zeros in the outputs column.

An aircraft schematic using logic gates is shownin figure 2-75. It is evident that in order to

A B

0

0

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AC CBA B

0

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0

Figure 2-66. The NAND gate and its truth table.

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Figure 2-64. The OR gate and its truth table.

92

0

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A

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(A)

understand and troubleshoot this aircraft system,the technician should be familiar with logic gatesand their truth tables.

Amplifier or BufferThe buffer or amplifier is shown in figure 2-69.It produces no change in the logic state of theinput signal. It is used to amplify or increase signalstrength. It can also be used to isolate one partof a circuit from another part of the same circuitor from some other unit.

Positive and Negative LogicLogic gates use different values of voltage to representbinary ones and zeros. The terms positive and negativelogic refer to whether the voltage value for a binary1 is more positive or more negative than the voltagevalue for binary 0. Figure 2-70(A) shows examplesof positive logic. The voltage value for binary 1 ismore positive (or less negative) than the value forbinary 0. Figure 2-70(B) shows examples of negativelogic which is the opposite situation.

3. Integrated CircuitsThe evolution of electronic circuit technology isshown in figure 2-71(A) and figure 2-71(B). For many

Figure 2-68. The EXCLUSIVE OR gate and its truth table.

Figure 2-69. The AMPLIFIER and its truth table.

Figure 2-70.Example of positive logic.Example of negative logic.

years, the vacuum tube represented the state ofthe art in delicate control of electron flow for radiosand early computers. The development of the tran-sistor in the 1960s allowed much smaller and moreefficient electronic components to be made.

The transistor was made from a semi-conductormaterial like silicon or germanium. In the 1970s,the integrated circuit was invented. An integratedcircuit is a small chip of semi-conductor materialwith the equivalent of many transistors on it. Itwas no longer necessary to manufacture transistorsone at a time in individual units. Since the inventionof the integrated circuit, the technology has ad-vanced rapidly by crowding more and more tran-sistors onto this small chip of silicon or germanium.

Figure 2-71In the 1960s, transistors began to replacevacuum tubes as a means to provide precisecontrol of electrical signals for amplifiers andother electronic devices including computers.An integrated circuit contains the equivalent ofmany transistors on a small chip of silicon. Veryfine wires connect the chip to the outer pins.

93


The term microprocessor refers to an integrated

circuit that has the equivalent of thousands of tran-sistors on one tiny chip of silicon. The microproces-sor has made possible the widespread use ofcomputer controls for cars, radios, and in airplanesas well. The logic gates that we have been discussingare not manufactured one at a time. Logic gatesare contained within integrated circuits. Integratedcircuits vary considerably in complexity. Simple in-tegrated circuits that contain 6 or 8 logic gatescan be purchased for about 25 cents. The mostcomplex integrated circuits are those calledmicroprocessors, which would have the equivalentof thousands of logic gates on one chip. A schematicrepresentation of the logic gates in a simple in-tegrated circuit is shown in figure 2-72.

Integrated circuits are manufactured from siliconthat has been specially processed. The circuit thatwill be placed onto the silicon is drawn with greataccuracy. The circuit drawing is then shrunk toa small size using techniques similar to those forreducing a photograph. The circuit tracings aretransferred to the chip of silicon by etching theminto the chip. The small chip of silicon is thensealed in a housing of plastic or ceramic. The in-tegrated circuit has connector pins along the sidesto carry signals to and from the small chip of siliconwithin. Figure 2-71(B) shows these features of anintegrated circuit with very fine wires connectingthe chip to the outer contact pins along the sides.

One of the goals when designing integrated cir-cuits is to make the chip as small as possible.On a very high speed microprocessor, the time it

Figure 2-72. An example of a relatively simple integratedcircuit (IC) showing logic gate functions andpin connections.

takes electrons to move the width of the chip limits

the speed of computation.

Integrated circuits are divided into categoriesbased on their complexity. The four categories nor-mally referred to are:

Small scale integration (SSI)

Medium scale integration (MSI)

Large scale integration (LSI)

Very Large scale integration (VLSI)

If a digital system has many different integratedcircuits that are connected to each other, they mustbe compatible with each other. Integrated circuitsthat use the same values of voltage for binary onesand zeros and operate at similar speeds are saidto be in the same family. Two of the more commonfamilies are TTL (transistor-transistor logic) andCMOS (complimentary metal oxide semi-conductor). The TTL family, for instance, uses positive logicwith binary 1 +5V and binary 0 = OV.

Dip StandardsIntegrated circuits (ICs) are manufactured in stand-ard sizes and shapes. This means that ICs madeby different companies can be installed in the sameway and simplifies replacement. The DIP (dual inlinepackage) standard concerns the numbering of theconnector pins and the shape and size of the in-tegrated circuit. A typical DIP integrated circuit isshown in figure 2-72. With the notch at the top,the connector pins are numbered down the left sideand then up the right side. The spacing of the pinsfits the standard pattern. The total number of pinsvaries from about 8 to more than 40 dependingon the complexity of the integrated circuit. Logicgates can be used for many different purposes. Thesmaller ones can be used as high speed switches.Adders and subtractors are used in a computerto perform mathematical calculations. Clocks arelogic gates that supply a set frequency to synchronizethe operation of different units. Latches and flip-flopsare used for memory functions. A typical computercontains many integrated circuits that perform avariety of functions such as those mentioned here.

ARINC 429 Digital StandardsThe latest air carrier jets have many different com-puters and digital systems on board the aircraft.These many digital systems must have a rapid andefficient way to communicate using binary codedsignals. ARINC 429 is the standard for Digital In-formation Transfer Systems that is used on theseairplanes. This standard specifies the use of a dualdigital data bus where the various units connectedwill receive messages on both sections of the bus

94


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but only transmit on one of them. The type of mes-sage transmission is serial which means differentmessages are sent in a series one after the other.The messages are separated by time intervals. Thisindustry standard also specifies what coding sys-tems will be used such as binary, binary codeddecimal and discrete. The standard length of anARINC 429 word is 32 bits as shown in figure2-73. The use of ARINC 429 by the equipmentmanufacturers permits the airlines and other usersto purchase electronic equipment from differentsources and know that the different units will becompatible with each other.

6. Digital Aircraft SystemsThe trend in the design of modern aircraft andaircraft systems is to make greater use of digitalcircuits and digital signals because of the ad-vantages they have over older style analog systems.This follows another trend which is the use of moreautomated monitoring and control systems.

Some of the fundamental concepts involved inthe design of electronic systems for current produc-tion aircraft are:

The use of proximity sensors to replacemicroswitches.

The use of transistors to replace potentiom-eters, relays etc.

Designing equipment to fit in standard sizeLRUs (line replaceable units).

Increased use of BITE (built-in test equipment).

Computer controls for navigation, engines, sys-tems instruments, etc.

Electronic instruments (CRTs) to replace elec-tro-mechanical instruments and displays.

7. LEDs and LCDs employed for lighted displaysthat replace older style displays and controls.

The use of computer monitoring systems like EICAShas permitted the elimination of a flight crew memberon many aircraft. The flight engineer is replaced bythe sophisticated computer monitoring systems andautomatic control systems to create what is calledthe two-man cockpit in aircraft such as the Boeing747-400 and McDonnell Douglas MD-11.

The use of ARINC 600 standard LRUs permitsmore efficient installation and maintenance ofaircraft equipment. Many of the LRUs contain BITEsystems which can be used to troubleshootproblems and identify faulty LRUs for replacement.Maintenance technicians still need to be veryfamiliar with the aircraft systems so that they cando a better job of troubleshooting and repair, andso that the number of false removals of good LRUscan be reduced.

M. Takeoff Warning SystemsA takeoff warning system is designed to sounda warning if the flight crew tries to takeoff inan airplane with the flaps or other important sys-tems in an incorrect position for a safe takeoff.Air carrier jets will have takeoff warning systemswhile most simpler aircraft will not. The criticalitems that are monitored by the takeoff warningsystem are:

Pitch trim or stabilizer trim

Speed brake

Leading edge flaps and slats

Trailing edge flaps

A schematic of a typical takeoff warning systemis shown in figure 2-74. The two series switchesare on the throttles and landing gear. When thereis weight on the wheels and the throttles are ad-vanced for takeoff, both of these switches will be

BYTE 4 BYTE 3

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ORDER OF TRANSMISSION

>-1-Ea.

Figure 2-73. ARINC 429 is the industry standard for digital information transmission systems. An ARINC 429 standardword contains 32 bits.

95


e•PITCH TRIM

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closed. If any of the parallel switches is closed,

a takeoff warning will be initiated.

A more detailed schematic of a takeoff warning sys-tem is shown in figure 2-75. Notice the use of logicgate symbols on this relatively simple aircraft system.

There have been aircraft accidents caused by thefailure of the takeoff warning system and anattempted takeoff with one of the critical systemsset improperly. It is an important system that should

be inspected regularly and maintained properly to

give warnings when needed.

N. Angle of Attack IndicatorsThe angle of attack indicator is a system that givesan instrument indication in the cockpit of the angleof attack for the wings of the airplane. Angle of attackis the angle between the chord line of an airfoil andthe relative wind. The angle of attack instrument

Figure 2-74. A simplified schematic of a takeoff warning system.

Figure 2-75. The takeoff warning system for an air carrier jet airplane. Logic gate symbols are often found on aircraftschematics.

96


LATERALACCELEROMETERSNO.1 NO.2

STALL PROTECTIONCOMPUTER

RIGHT AOA VANE

STICK PUSHER

CONTROLCOLUMN

LEFT AOA VANE

STICK SHAKER

can be very useful in critical flight conditions wheremaximum performance must be employed. Duringlanding and takeoff from short runways and in anencounter with wind shear, the angle of attack in-strument can allow more precise control of theairplane. There are two common types of sensorsused for AOA systems. The first type is a small vaneon a pivot that is installed on the forward fuselageof the airplane. This type can be seen as a part

of the stall warning system shown in figure 2-76.Angle of attack sensors can be used with a stallwarning system, an AOA gauge, or both. The vanerotates to align itself with the relative wind and thisinformation is transmitted to the cockpit with asynchro system where it moves a pointer on the AOAinstrument. The second type uses a sensor whichis a tube with two slots that projects out into theairstream, normally from the forward fuselage. This

ALTITUDETRANSDUCERSNO.1 OUTBOARDS

Figure 2-76. The stall warning system for a corporate jet airplane that employs vane-type angle-of-attack sensors.(Courtesy Canadair Group, Bombardier Inc.)

97


POTENTIOMETERSHAFTPADDLE

PADDLECHAMBER

AIRFLOW THROUGH THE SLOTS IN THE PROBEMOVE THE PADDLE WHICH ROTATES THEPOTENTIOMETER SHAFT TO CHANGE THE

RESISTANCE AS THE ANGLE OF ATTACK CHANGES.

ANGLE OF ATTACK PROBE.

ANGLE OF ATTACK INDICATOR.(C)

type of probe, along with the AOA indicator, is shownin figure 2-77. The two slots in the probe are connectedto two chambers. As the angle of attack changes,the relative pressure in the two slots changes alongwith the pressures in the air chambers. The differencein air pressure rotates a vane in the unit inside the

Figure 2-77 An angle-of-attack indicating system thatuses the air pressure type of AOA probe.

fuselage which is connected by synchro to the cockpitinstrument. The units displayed on the cockpit in-strument face can be a percent or decimal numbersthat indicate the angle of attack. The reading of 100%or 1.0 would indicate a stall angle of attack, so thepilot can readily determine if a safe margin is beingmaintained from the stall angle-of-attack.

0. Stall Warning SystemsA stall warning system is required for all modernairplanes. Several of the common types will bedescribed, but they all rely on some measurementof angle of attack to activate the stall warning sys-tem. The airspeed of the airplane cannot be usedto operate a stall warning system because theairplane can stall at many different airspeeds. Anairplane will stall at a higher airspeed in a steepbank than it will in level flight because some ofthe wing lift is being used to make the airplaneturn. The airplane will stall at the same angle ofattack in both straight and turning flight, so AOAis a better indicator for stall warning than airspeed.

The stall warning system found on many olderairplanes used a vibrating reed and required no out-side power source. Figure 2-78 shows this type ofsystem. A small hole in the leading edge of the wingis connected by tubing to a reed and horn near thecockpit. At high angles of attack close to a stall,a low pressure or suction is produced at the openingin the leading edge and this pulls air through thereed to make it vibrate and produce a noise.

On later small airplanes, a vane operated switchis located on the leading edge and this switch isclosed by the upward movement of the vane athigh angles of attack. This system uses DC powerto operate the stall warning horn. Figure 2-79 showsthis type of stall warning sensor.

Modern high performance airplanes use an AOA probeas previously described which is connected to a stallwarning circuit. These airplanes usually have severalother sensors connected to the stall warning circuitor computer. Sensors for flap and slat position areused to give an accurate stall warning for any flightcondition. Most jet airplanes have a stick shaker aspart of the stall warning system. This device actuallyshakes or vibrates the control column to warn thepilot of an approaching stall. The stick shaker canbe seen at the left side of figure 2-76.

P. AnnunciatorsOther than simple single-engine airplanes, mostaircraft have an annunciator panel which groupstogether a number of different indicator lights fora variety of aircraft systems and equipment. There

98


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7. ADJUSTABLE PLATE

are usually three categories of annunciator indicationsbased on how important the information is to theflight crew. Warnings are the most serious and nor-mally activate a red light. For very serious conditionslike fires, a sound warning is also given. Cautionannunciators are usually amber in color. Status orspecial purpose annunciators may be white or someother color lights. If the airplane has a large numberof annunciators and the panel must be located over-head or some other location which is not in frontof the crew, master caution and warning lights areemployed. With this system, there is one red lightand one amber light in plain view in front of thepilot and/or copilot. When any warning light on themain panel is illuminated, the master warning lightalso comes on. The crew is alerted to look up atthe main annunciator panel to find the source ofthe warning. The master warning light is then resetso it will be available to give any additional warnings

that might be needed. The master caution light worksin a similar manner. Some of the typical indicationsfor an annunciator panel for a small twin-engineairplane are shown in figure 2-80

Q. FARs for Warning Systemsand Annunciators

Some of the FAA requirements for warning systemsand annunciators are given below to familiarize thereader with the kinds of rules that apply to thesesystems.

FAR 23.207Stall Warning

A. There must be a clear and distinctive stallwarning with flaps and landing gear in anynormal position in straight and turning flight.

Figure 2-78. The components in a vibrating reed stall warning system. (Courtesy Cessna Aircraft Co.)

99


STALL WARNING UNIT

STAGNATION POINT

THE VANE IS PLACED AT THE STAGNATION POINTON THE LEADING EDGE OF THE WING.

THE VANE ACTUATES A PRECISION SWITCHINSIDE THE HOUSING.

The stall warning may be inherent buffet ora device. If a device is used, visual indicationsby themselves that require the pilot's attentionto be directed in the cockpit are not acceptable.

The stall warning must begin not less than5 knots above stall speed; but not more than10 knots or 15% above stall speed, whicheveris greater. The stall warning must continueto the stall.

FAR 23.729Retractable Landing Gear

There must be a position indicating systemfor extended and retracted.

Warning system.

There must be a warning if one or morethrottles are retarded and the landing gearis not down arid locked.

There must be a warning when the flapsare extended to or beyond the approachflap setting and the landing gear is notdown and locked.

FAR 23.1203Fire Detector SystemFor multi-engine, turbine-powered airplanes,

multi-engine reciprocating engine airplanes incor-porating turbosuperchargers and all commutercategory airplanes:

A. There must be a means which ensures theprompt detection of a fire in an engine com-partment.

FAR 23.1303Flight and Navigation InstrumentsThe following are required flight and navigation

instruments:

E. A speed warning device for:

Turbine engine powered airplanes

Other airplanes for which Vmo/Mmo andVd /Md are established under FAR 23 ifVmo/Mmo is greater than 0.8 Vd / Md.

FAR 23.1353Storage Battery Design and Installationg. Nickel cadmium battery installations capable

of being used to start an engine or auxiliarypower unit must have:

A system to control the charging rate ofthe battery automatically so as to preventbattery overheating.

A battery temperature sensing and over-temperature warning system with a meansfor disconnecting the battery from its charg-ing source in the event of an over-tem-perature condition.

3. A battery failure sensing and warning sys-tem with a means for disconnecting thebattery from its charging source in the eventof a battery failure.

FAR 91.219Altitude Alerting System or DeviceTurbojet-Powered Civil Airplanes:1. No person may operate a turbojet powered

civil airplane unless it is equipped with analtitude alerting system or device that:

Figure 2-79. The electrically powered stall warning sys-tem for a small airplane that uses a vaneoperated switch.

100


2. Will alert the pilot upon approaching a pre-selected altitude in either ascent or descent,by a sequence of both aural and visual signals

in sufficient time to establish level flight atthe preselected altitude.

The control and operation of a typical altitudealerting system are shown in figure 2-81.

5

6

7

8

AUTOPILOT OFF LIGHT (AMBER) — Indicates the autopilot has disengaged.

DOOR OPEN LIGHT (RED) — Indicates that the forward baggage compartment door, the cabinentry doors and /or the emergency exit door are not secured safely for flight.

HEATER OVERHEAT LIGHT (AMBER) — Indicates an abnormally high temperature has occurredin the combustion heater and it has been automatically shut off. Once the light illuminates,the heater will not operate until the overheat switch in the right forward nose section (accessiblein nosewheel well) has been reset.

LEFT ALTERNATOR OFF LIGHT (AMBER) — Indicates the left alternator is not supplying electricalcurrent.

LOW VOLTAGE LIGHT (RED) — Indicates electrical system bus voltage is less than 24.5 volts.

WING AND STABILIZER DEICE SYSTEM PRESSURE LIGHT (GREEN) — Indicates pressureis being applied to the surface deice boots to inflate them.

RIGHT ALTERNATOR OFF LIGHT (AMBER) — Indicates the right alternator is not supplyingelectrical current.

WINDSHIELD ANTI-ICE SYSTEM LIGHT (GREEN) — Indicates that heating elements in thewindshield anti-ice system are operating.

RIGHT ENGINE FIRE LIGHT (RED) — Indicates an excessive temperature condition or possiblefire has occurred in the right engine compartment.

RIGHT LOW FUEL LIGHT (AMBER) — Indicates fuel quantity in right main fuel tank is 60lbs. or less.

LEFT LOW FUEL LIGHT (AMBER) -- Indicaties fuel quantity in left main fuel tank is 60 lbs.or less.

LEFT ENGINE FIRE LIGHT (RED) —. Indicates an excessive temperature condition or possiblefire has occurred in the left engine compartment.

DAY/NIGHT SWITCH — Sets brightness level of annunciator panel indicator lamps for eitherday or night operation.

TEST SWITCH — Tests operation of annunciator panel lamps, landing gear system positionindicator lights, and aural warning tones of landing gear, fire detection and stall warning systems.Also, switch can be used to silence an activated engine fire detection warning tone.

Note: For Illustration only. Not to be used for operational purposes.

Figure 2-80. (Courtesy Cessna Aircraft Co.)The annunciator panel for a small twin-engine airplane.The meanings for the various annunciator lights

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102


CHAPTER III

Communication and Navigation Systems

The avionics and radio equipment found in aircrafthas seen more change and advancement in recentyears than any other part of the aircraft. Most ofthese changes have resulted from the same tech-nology that has produced personal computers anda world wide telecommunications industry. Al-though early radios were in use prior to World WarI, it is only since the 1940s that extensive usehas been made of radios for communication andnavigation in all types of aircraft. The basic VHFcommunications and navigation systems that areused in aviation were developed in the 1940s, butthe introduction of newer systems has been in-creasing dramatically in recent years. This chapterwill cover the types of avionics equipment that arealready in widespread use and the latest develop-ments that are only beginning to be installed onairplanes. Most of the systems described utilize radiowaves, so we will begin with the fundamentals ofradio systems.

A. Radio FundamentalsThe basic components found in radios and theoperating principles involved will be explained hereso that the specific aircraft avionics systemsdescribed later can be understood more easily.

1. Types of Aircraft Radio SystemsThe first radios installed in airplanes were usedfor communications and it was only much laterthat navigational radio systems were developed.Radio systems for other purposes have also beendeveloped, especially in the last 20 to 30 years.Some of the uses for radio in modern aircraft canbe categorized as follows.

Communications — Both voice and data.

Navigation — Many different systems are inuse today.

ATC Radar — The Air Traffic Control systemrelies on radar.

Weather Avoidance — Used to avoid areas ofadverse weather.

Approach Aids — A specialized type of nav-igation to guide an aircraft down to the runwayin bad weather.

Altitude Measurement — Gives precise alti-tude above ground level.

7. Airborne Collision Avoidance — Warns thepilot of nearby aircraft.

All of these examples use radio waves, but thereare other avionics systems that do not rely on theuse of radio waves.

2. Electromagnetic Wavesa. Frequency Bands

The range and diversity of electromagnetic waves,or EM waves, is very broad. The entire spectrumof EM waves includes not only radio waves but visiblelight, gamma rays, infrared, etc. The range of radiowave frequencies that we will be involved with isillustrated in figure 3-1. The frequencies are dividedinto bands which, starting at the low end, are: verylow frequency (VLF), low frequency (LF), medium fre-quency (MF), high frequency (HF), very high frequency(VHF), ultra high frequency (UHF), super high fre-quency (SHF) and extremely high frequency (EHF).Common aircraft radio systems are included on theright side in figure 3-1. Notice that there is an aviationapplication for all of the bands except EHF. The twocommon radio broadcast bands are also listed forpurposes of comparison.

All EM waves consist of two different invisibleenergy fields that travel through space. The electricfield and the magnetic field are at right angles toeach other and to the direction of propagation ortravel. Figure 3-2 shows the two fields and thedirection of propagation. Radio waves are producedwhen a radio frequency electrical signal is sentdown a conductor to an antenna. The antenna trans-forms the electrical signal into EM waves whichpropagate outward from the antenna through space.The EM waves travel through space at the velocityof light, which is 186,284 miles per second or300,000,000 meters per second. When the EMwaves strike a receive antenna, they induce voltageand current to produce an electrical signal thatmatches the one applied to the transmit antenna,but of course the signal is much weaker. The basicoperating frequency of a radio is called the carrierfrequency, because this is the signal that carriesthe data or information that needs to be transmittedfrom one place to another.

The wavelength of EM waves is often used in describ-ing antennas and other radio components. The

103


VERY LOWFREQUENCY

(VLF)

3 KHz

OMEGA 10-14 KHz

30 KHz

LORAN C 100 KHzLOW

FREQUENCY(LF)

.•.•.•.-

NDBs 190-500 KHz

300 KHz

AM BROADCAST 550-1800 KHzMEDIUM

FREQUENCY(MF)

HIGHFREQUENCY

(HF)

3 MHz —

HF COMM 2-30 MHz

:•:. MARKER BEACONS 75 MHz 30 FM BROADCAST 88-108 MHzMHz

.•.•.•..VHF NAV (VOR) 108-118 MHz

VERY HIGHFREQUENCY VHF COMM 118-137 MHz

(VHF)

300 MHz GLIDESLOPE 328-336 MHz

ULTRA HIGHFREQUENCY

(UHF)

3 GHz

SUPER HIGHFREQUENCY

(SHF)

DME 960-1215 MHz

TRANSPONDER 1030 AND 1090 MHz

RADAR ALTIMETER 4.3 GHz

MLS 5.0-5.1 GHz

DOPPLER NAV 8.8 GHz

WEATHER RADAR 9.375 GHz

30 GHz

EXTREMELY HIGHFREQUENCY

(EHF)

300 GHz

Figure 3-1. Radio frequency chart showing the operating frequencies of common aircraft systems.

104


VARYINGELECTRIC

FIELD

VARYINGMAGNETIC

FIELDDOT DASHDOT DOT

TRANSMITTER RECEIVER

MICROPHONE

HEADSETRECEIVINGANTENNA

TRANSMITTINGANTENNA

LOUDSPEAKER

wavelength of an EM wave is the distance from peakto peak for the invisible waves in the electric andmagnetic fields. Wavelength is measured in meters

and it is inversely proportional to the frequency. Thewavelength in meters can be found by dividing theconstant 300,000,000 by the frequency in hertz. Athigher frequencies, it is easier to use 300 dividedby the frequency in MHz to obtain the wavelength.

b. ModulationIf we design a radio transmitter that sends out asteady carrier wave, we would not be able to transmit

Figure 3-2. An electromagnetic wave with the electricfield and magnetic field at right angles toeach other and the direction of propagation.

any data or information. The receiver would onlyproduce a steady hum or nothing at all dependingon the design of the receiver. In order to transmitany data or intelligence, it is necessary to vary oralter the carrier wave in some way. The processof changing or varying the carrier wave is calledmodulation. When the Italian inventor GuglielmoMarconi was developing the first practical radio sys-tem in 1896, there was no way to modulate theradio wave in order to transmit voice, so he useda method of switching the transmitter on and offto transmit Morse code signals. This simplest formof modulation is called CW or radio telegraphy sinceit borrowed the Morse code from the telegraph in-dustry. Figure 3-3 shows a radio carrier wave whichuses CW to send Morse code dots and dashes. Withinten years, new inventions permitted voice and musicto be transmitted by radio using improved typesof modulation. Figure 3-4 illustrates the principles

Figure 3-3. The simplest form of transmitting data withradio waves is with Morse code dots anddashes or CW.

Figure 3-4. Simplified voice radio system.

105


of a simple voice radio system. A microphone con-verts sound waves to electrical signals that are com-bined with the carrier wave using AM or FMmodulation. The receiver picks up the modulatedcarrier wave and then separates out the audio fre-quency using a demodulator. The audio output ofthe demodulator drives a speaker or headset toreproduce the sound waves that were picked upby the microphone in the transmitter. Current radiosystems use a number of different types of modula-tion, the two most common being amplitude modula-tion (AM) and frequency modulation (FM). Whenamplitude modulation is used, the intensity oramplitude of the carrier wave signal is varied up

and down as shown in figure 3-5. The frequency

of the carrier wave is changed when frequency

modulation is used. Figure 3-6 shows a carrier wavewith this type of modulation. The two commercialbroadcast radio bands are a good example here sinceone uses AM and the other FM. One of the advantagesof FM is that it is less affected by atmospheric noisefrom thunderstorms and other disturbances.

c. Audio and Radio Frequencies

Two terms that are often used in discussions aboutradio systems are audio frequency (AF) and radiofrequency (RF). Audio frequencies are those of20,000 Hz or less. They are called audio because

111,6,411.411,41,

THE CARRIER IS A HIGH-FREQUENCY THE AMPLITUDE MODULATED CARRIER IS

ALTERNATING CURRENT. PICKED UP BY THE RECEIVER IN THIS FORM.(A) (D)

0 THE SIGNAL IS PASSED THROUGH A DETECTORWHERE ONE-HALF IF IT IS REMOVED.

(E)

DC VARYING AT AN AUDIO FREQUENCY ISPRODUCED IN THE MODULATOR.

011111reirelnTHE CARRIER IS MODULATED, OR CHANGED, IN

AMPLITUDE TO COINCIDE WITH THE AUDIO FREQUENCY.THE SIGNAL IS TRANSMITTED IN THIS FORM.

A DEMODULATOR REMOVES THE CARRIERAND LEAVES ONLY PULSATING DC.

THE PULSATING DC IS FED INTO AN AMPLIFIER WHEREIT BECOMES AC, JUST LIKE THAT PRODUCED

BY THE MICROPHONE.

Figure 3-5. An example of the use of amplitude modulation (AM).

106


these are approximately the frequencies of soundwaves that can be heard by the human ear. Radiofrequencies are those above 20,000 Hz. These termsare most often used when discussing the electricalsignals that are produced and used by radioreceivers and transmitters.

d. Ground, Sky and Space Waves

The behavior of radio waves as they travel throughthe earth's atmosphere and beyond are classifiedby the terms ground, space and sky waves. Thepropagation characteristics of ground waves, skywaves and space waves are illustrated in figure3-7. Radio waves at frequencies below the HF band(below 3 MHz) are called ground waves becausethey will follow the curvature of the earth and bend.Radio waves that operate in the HF band from3 MHz to 30 MHz are called sky waves. They tendto travel in straight lines and will not follow thecurvature of the earth. Sky waves will bounce orrefract off the ionosphere. The ionosphere is madeup of layers of ionized particles from about 60-200miles high. When sky waves strike a layer of theionosphere in the right way, they will be refractedso that they will come back to earth hundreds ofmiles away. This characteristic of sky waves canbe used to achieve long range transmission of radiosignals. Space waves are the result of transmissionsof radio waves at frequencies above 30 MHz orabove the HF band. Space waves travel in straightlines, but they will not bounce off the ionosphere.The radio signals used to communicate with orbitingsatellites are above 30 MHz. A good example canbe given here for those who are familiar with AM

and FM radio receivers. The commercial broadcaststations known as AM broadcast in the MF bandso that the EM waves behave as ground waves.At night, it is not unusual to pick up an AM stationthat is hundreds of miles away because the signalhas followed the curvature of the earth. CommercialFM stations operate in the VHF band and the recep-tion range is limited to line-of-sight or straight linesbecause they are space waves. The maximum recep-tion range for these stations is usually about 60miles because they are blocked by the earth's sur-face at greater distances.

3. Basic Radio Componentsa. Amplifiers

An amplifier is a device that increases the strengthof a signal. They are found in both transmittersand receivers. A transmitter must increase thestrength of the signal sent to the antenna so thatthe EM waves will travel a useful distance outwardfrom the antenna. A receiver needs amplifiers be-cause the strength of the signal from the antennais very low and must be increased to enable thesignal to be heard. Up until the 1960s, mostamplifiers relied on vacuum tubes to increase thestrength of signals. The transistor has replaced thevacuum tube for most amplifier applications.Amplifiers can be categorized as Class A, ClassB and Class C. The difference between these isthe shape of the output waveform. Figure 3-8 showsthe output of a Class A amplifier, it is completesine waves just like the input. The Class B amplifierhas an output which shows only half of each sinewave as shown in figure 3-8. The Class C amplifier

Figure 3-6. An example of the use of frequency modulation (FM).

107


... l p. NOSPHER • • ".•.:•:•:•:•:...:•:.:•:•:. • •

RADIOSTATION

...A. 0,,

••••••••••:.:-.-. •ry

OF

IT

RADIOSTATION

RADIOSTATION

has an output waveform which is less than half

of the sine wave as shown at the bottom in figure3-8. The Class C amplifier is often used as a poweramplifier because of its higher efficiency. The outputof the Class C amplifier can be sent through anLC circuit or other device to restore the completesine wave shape.

b. Oscillators

An oscillator is a device that produces the frequen-cies needed by both receivers and transmitters. Asimple oscillator is an LC circuit or tank circuitmade up of a capacitor and inductor in parallel.The LC circuit will have a resonant frequency whichmatches the desired frequency. An LC circuit byitself will not continue to oscillate because of resis-tance in the components and wires. Figure 3-9shows an LC tank circuit which can be connectedto a battery to produce oscillations. If the switchin figure 3-9 is moved to position A, the capacitorwill be charged by the battery. If the switch isthen moved to position C, the tank circuit will startto oscillate as energy is transferred rapidly backand forth between the capacitor and inductor. Theoscillations will become weaker and die out becauseof the resistance in the circuit. In order to maintainoscillations, some energy must be fed back into

the tank circuit. In earlier radios a vacuum tube

was used to supply the needed feedback. A tran-

sistor is used instead of a vacuum tube in newerradio designs as shown in figure 3-10. The resonantfrequency or oscillation frequency is determinedby the values of capacitance and inductance inthe tank circuit. The LC circuit will not be stableover a period of time and may drift off the correctfrequency. A common technique to stabilize theoscillator and produce a more accurate frequencyis to use a crystal as shown in figure 3-11. Thepiezoelectric effect of the crystal will produce a moreaccurate and consistent output frequency from theoscillator.

c. Modulators and Demodulators

We will use a voice communications radio as anexample of the purpose and function of modulatorsand demodulators. In the radio transmitter, a deviceis needed which will combine the AF signal withthe RF carrier wave signal before it is sent to theantenna. This is the function of a modulator, itcombines the AF and RF signals so that informationcan be transmitted. The output of the modulatoris called modulated RF. The signal produced bythe antenna in a receiver will be modulated RF.In order to hear the voice as an output of the

GROUND WAVE

SKY WAVE(A)

(B)

::::IONOSPHERE•

DOES NOT FOLLOWEARTH'S CURVATURE

SPACE WAVE(C)

Figure 3-7. The propagation characteristics of ground waves, sky waves and space waves.

108


CLASS AAMPLIFIER

CLASS BAMPLIFIER

CLASS CAMPLIFIER

(C)

• A 411.w ...nill III w

DAMPED OSCILLATION CAUSEDBY RESISTANCE IN THE CIRCUIT

Cl

7.-

ACC

CRYSTAL

IC I

OUTPUTFigure 3-8. An example of the output waveform for Class

A, B and C amplifiers.

Figure 3-9. A parallel LC tank circuit. Without feedback,the oscillations will be weakened by resis-tance in the circuit and die out.

receiver, the AF component must be separated out.The demodulator removes the RF component of themodulated RF signal and produces an AF output.

When the AF and RF signals are combined inthe modulator, they must have the proper relative

Figure 3-10. An oscillator circuit with feedback suppliedby a transistor.

Figure 3-11. A crystal controlled oscillator.

109


0

strengths for maximum efficiency. The amount ofmodulation is called the modulation rate. If theAF signal is too weak compared to the RF signal,the modulation rate will be low and the efficiency

will also be low. If the modulation rate is over 100%,

there will be distortion in the signal due to thegaps created. Figure 3-12 gives examples of 50%,100% and more than 100% modulation rates with

1V

1.5V —

2V -

1

L s-1

(A)

1

1V -

2V

J J \

(B)

+ 2V -

+ 1V -

0

1V -

2V -

n.n

\ -0

•-•

(C)

Figure 3-12. Examples of different modulation rates.50%100%

(C) Over 100%

110


2

FOR MAXIMUM EFFICIENCY, THE LENGTH OFTHE ANTENNA SHOULD BE EXACTLY ONE-HALF

OF A WAVE LENGTH.

WHEN THE VOLTAGE IS MAXIMUM AT THE ENDSOF THE ANTENNA.

AMPS

THE CURRENT WILL BE MAXIMUM AT THE CENTEROF THE ANTENNA.

(C)

AM modulation. Most radio transmitters are ad-justed to about 90-95% modulation to provide alittle margin to prevent distortion. Shouting intoa microphone when using a radio can cause over-modulation and should be avoided.

Filters

A filter is used in a radio circuit to remove or filterout unwanted frequencies. The signals that areprocessed by the circuits in a radio often have ad-ditional frequencies present that are not needed.If the proper filter is installed, it will filter out thefrequency or frequencies that are not wanted. Afilter is usually made up of an arrangement of in-ductors and capacitors as shown in figure 3-13.A low pass filter will remove all frequencies abovea certain value and pass the low ones. A high passfilter does the opposite. If a range of frequenciesmust be blocked, a band reject filter will be used.A bandpass filter will allow a certain band of fre-quencies to go through and block frequencies eitherabove or below that range.

Antennas

An antenna is a device that transforms electricalsignals into EM waves in the case of a transmitantenna, or transforms EM waves into electricalsignals in the case of a receive antenna. Dependingon the particular radio system involved, an antennamay be used for transmit only, receive only, orboth. The maintenance, inspection and installationof antennas is usually the responsibility of theairframe technician since they are attached to thestructure or skin of the aircraft. Antennas oftenhave general names that describe some of their

Figure 3-13. Radio frequency filters are combinations ofinductors and capacitors.

basic characteristics. Two of the more common typesare the Hertz dipole antenna and the Marconimonopole antenna. The Hertz dipole antenna hastwo metal conductors in a straight line with theconnection in the middle. It is called a half-waveantenna because the overall length is equal to onehalf the wavelength of the EM wave it is designedto be used with. Figure 3-14 is an example of aHertz dipole antenna. The Marconi antenna is asingle metal conductor with a length of 1/4

wavelength as illustrated in figure 3-15. In orderto work properly, the Marconi antenna must havemetal surrounding the mounting base. The metalat the base is needed for efficient operation of theantenna. The necessary metal at the base is calledthe groundplane or counterpoise. In figure 3-15,the groundplane is the four metal rods at the baseof the antenna; the metal skin of an aircraft isused as a groundplane for most aircraft antennas.Most antennas must be installed with the correct

Figure 3-14. The Hertz dipole antenna is a half-waveantenna.

111


ANTENNA

A

GROUND PLANEA

4

A

4

MINIMUM

RADIATION

MAXIMUMRADIATION

SOLIDCENTERCONDUCTOR

INNER INSULATOR OUTERJACKET

BRAID

polarization. Polarization refers to the orientationof the electric field relative to the earth. If the electricfield is vertical, it has vertical polarization.

The Marconi antenna will produce a verticallypolarized radiation pattern as shown in figure 3-16.Horizontal polarization means that the electric fieldwill be parallel to the earth's surface. The hor-izontally polarized pattern of a Hertz dipole antennais shown in figure 3-17. Another way to understandpolarization is that the polarization of the aircraftantenna should normally match the polarizationof the ground based antenna and the EM wavesit will utilize. An aircraft VHF communications an-tenna is an example of a Marconi antenna withvertical polarization. The common example of a Hertzantenna is the VHF navigation antenna found onsmall airplanes. It is a V-shaped dipole antennawith horizontal polarization.

Antennas must not only be matched to theproper radio, but the conductor that connectsthe radio and antenna is very critical. A specialtype of conductor used to connect radios and

antennas is called a coaxial cable or coax. Figure3-18 shows the basic parts of a coaxial cabledesigned to carry RF signals. It consists of a centerconductor covered by a special kind of insulationand an outer conductor around the insulation.Plain wires cannot be used for radio frequencysignals because the energy loss would be too greatat these frequencies. The antennas and coaxialcables must be maintained in good condition toensure proper performance from the radio system.

The connection between an antenna and a radionormally requires a coupler in order to give thebest transfer of energy between the two of them.Two common types of antenna couplers are theLC circuit and the transformer types. A transformertype of antenna coupler is shown in the antennaconnection in figure 3-19. The use of an LC circuitas a coupler between the coax and antenna is shownin figure 3-20. Most aircraft antennas are speed-rated. For example, an antenna rated at 250 mphshould not be installed on an aircraft with a higherVne speed.

Figure 3-15. The Marconi antenna is a 1/4-wave monopoleantenna that requires a groundplane.

Figure 3-16. A vertically polarized marconi antenna willproduce this type of radiation pattern.

Figure 3-17. A horizontally polarized Hertz antenna willproduce this type of radiation pattern.

Figure 3-18. Coaxial cable is used to carry radio fre-quency electrical signals between radiosand antennas.

112


IC(

4111,

I II Ft-

111111111I+B

Tuning Circuits

An antenna will intercept many different EM wavesof different frequencies so some method must beused to separate out the desired frequency. Thetuning circuit performs this function. A simpletuning circuit is shown in figure 3-21, it consistsof a variable capacitor and an inductor in parallel.As the tuning knob is rotated on the radio, it movesthe variable capacitor until the resonant frequencyof the circuit matches the frequency of the desiredstation. This signal is passed into the radio andthe other frequencies are blocked out. A better typeof tuner which is found on most modern radiosuses a frequency synthesizer which contains a num-ber of crystals that can be combined to match thedesired frequency. The basic operation of a fre-quency synthesizer is shown in figure 3-22. Eachcrystal has a particular frequency and by usingswitches the crystals can be combined to producemany additional frequencies. When two frequenciesare combined, two new frequencies are created thatare equal to the sum and the difference of thetwo frequencies. By using this technique, hundredsof frequencies can be created using a relatively smallnumber of fixed frequency crystals.

Transmitters

The components that have been described will becombined in a block diagram to see how they worktogether. A voice radio transmitter is shown in figure3-23. The microphone changes the sound wavesof a human voice into AF signals that are amplifiedand then sent to the modulator. The oscillator in

this radio operates at one half the carrier frequency,so its output is amplified and then doubled. Themodulator combines the AF and RF signals whichare then amplified in the power amp before beingsent down the coax to the antenna.

h. Receivers

In the 1920s, a new type of radio receiver wasinvented that produced better sound quality. It wascalled the superheterodyne or superhet radio. Theonly major difference between the superhet andearlier radios was that it reduced the modulatedRF signal from the antenna to an AF signal inmore than one jump or stage. Since modern radiocomponents are much smaller and more efficientthan in the 1920s, virtually all modern radioreceivers are superhet. Figure 3-24 shows a blockdiagram of a VHF superhet aircraft receiver. TheRF signal from the antenna is combined with alocal oscillator frequency to produce a lower IF fre-quency. The intermediate frequencies found in asuperhet radio are abbreviated as IF. The basicprinciple of the mixer is that when two differentfrequencies are combined, two new frequencies arecreated; the sum and the difference of the two com-bined frequencies. In this example, the output ofthe mixer is the difference between the RF frequencyand the local oscillator frequency. The IF signalis amplified and then sent to the detector anddemodulator. The detector chops off half of eachsine wave to produce a varying DC signal froman AC signal. The AF signal is amplified and usedto drive the speaker.

Figure 3-19. An isolation transformer can be used as an antenna coupling device.

113


p

111

CRYSTALS

Speakers and MicrophonesAircraft radios often supply an audio output forthe pilot and voice transmitters require an audioinput from a microphone. A speaker is a devicethat transforms electrical signals into sound waves.A dynamic speaker is shown in figure 3-25.

When the audio frequency signal is applied tothe windings in the speaker, it sets up a magneticfield that expands and contracts at an audio rate.This field causes the metal diaphragm to vibrateat a corresponding rate to produce the movementof air that generates sound waves. Dynamic mi-crophones are available which operate in the op-posite way. Many newer and more efficient typesof microphones are now being manufactured, butthey all work by transforming the vibrations ofsound waves into varying electrical signals.

Audio Control PanelsWhen an aircraft has more than one radio, an efficientmeans of switching the microphone and speakerconnections from one radio to another is needed.

Figure 3-20. An LC circuit can be used as an antennacoupler.

Figure 3-21. This simple radio receiver uses a variablecapacitor to tune in different frequencies bychanging the resonant frequency of the tankcircuit.

The audio control panel performs this function. Atypical audio control panel is shown in figure 3-26.

An audio control panel is not a radio because itonly uses audio frequencies, but it is associated withthe radios in the aircraft. This audio control panelhas a row of toggle switches that can be used toconnect the audio output of the various radios tothe speaker or headphones. It also has a rotary selectorswitch to connect the microphone audio output tothe different radio transmitters and intercom systemsavailable for the aircraft. The audio control panelillustrated also has three lights that are the indicatorsfor the marker beacon system on the aircraft. Themarker beacon system will be described later.

B. Regulations and Standardsfor Radios

Aircraft avionics equipment might have to comply witha number of different regulations and standards de-pending on the type of equipment and the type ofaircraft in which it is installed. Regulations from theFAA and the FCC apply to the manufacture and useof most types of equipment and carry the force of law.FAA standards for equipment are usually in the formof TSO (Technical Standard Order) approvals. FCCrules generally apply to equipment which produces radiowaves. An FCC Station License is required for aircraft

Figure 3-22. The frequency synthesizer is a crystal con-trolled tuning device found on many mod-ern radios and other electronic units.

114


ANTENNA

POWERAMP

SPEECHAMP

MICROPHONE

1/2 RFFREQUENCY

OSCILLATORFREQUENCY

BUFFERAMP

FREQUENCYDOUBLER MODULATOR

SPEAKER500 OHMS

ANTENNA

11.4 MHz 11.4 MHz

RFAMP MIXER IF

AMPIF

AMP

AFAMP

LOCALOSCILLATOR

DETECTOR &DEMODULATOR

that have radio transmitters other than ELT. Eachdifferent type of transmitter must be listed on thelicense that is displayed in the cockpit. The roleof ARINC in established standards has been de-scribed earlier. These standards apply to the equip-ment in air carrier jets and bizjets primarily. Inaddition to the rules for the equipment itself, thereare some FAA Regulations concerning the use ofradio equipment in flight. Some of these FAA rules

are given here. Others will be covered later whenthe specific types of equipment to which the rulesapply are discussed.

FAR 91.130 (c)

No person may operate an aircraft in an AirportRadar Service Area (ARSA) unless two-way radiocommunication is established and maintained with

Figure 3-23. Simplified block diagram of a VHF voice radio transmitter.

Figure 3-24. Simplified block diagram of a superheterodyne VHF voice radio receiver.

115


PERMANENTMAGNET CORE

AUDIOFREQUENCY

AC

THIN STEELDIAPHRAGM

/1‘

PHONE

Hi C2 AUXA I 1 COM 2 1 NAV 2 MKR ADF DME AUX IONIC; Li EU

* * •0TEST

M A, )30

•

ATC. A transponder with Mode C automatic report-

ing of aircraft pressure altitude is also required

in all ARSAs.

minutes or 100 nautical miles from the nearest

shoreline, must have:

1. Radio communication equipment appropriateto the ground facilities.

FAR 91.131 (c) and (d)No person may operate an aircraft in a Terminal

Control Area (TCA) unless it has:

A two-way radio with appropriate frequenciesavailable.

An operable transponder with Mode C altitudereporting.

FAR 91.205 (d) and (e)Minimum Equipment Requirements for IFR

Two-way radio communications and naviga-tion equipment appropriate to the ground fa-cilities that will be used.

At and above 24,000 ft. MSL; approved DME(distance measuring equipment).

FAR 91.511Large and turbine-powered, multi-engine

airplanes, if operating over water more than 30

Figure 3-25. A dynamic speaker produces a magneticfield that varies at an audio rate, causing thediaphragm to vibrate and produce soundwaves.

Two transmitters.

Two microphones.

Two headsets (or headset and speaker).

Two independent receivers.

If needed, one HF transceiver.

C. Intercom andInterphone Systems

Intercom and interphone systems are not radio sys-tems, they use audio signals to permit communicationbetween various points in and around the aircraft.The two systems operate in a similar manner, thedifference is who uses the systems and where thephone jacks are located. The intercom system is usedfor voice communications from one point to anotherwithin the aircraft. Large aircraft have intercom sys-tems so that the cockpit crew can communicate withthe cabin crew and vice versa. On small airplanes,the intercom is used to communicate within the cock-pit area and is needed because of noise in the cockpitarea. The interphone system permits conversationbetween the cockpit and someone outside the aircraft,usually maintenance or service personnel. The opera-tion of intercom and interphone systems is the same.Phone jacks are available at different locations wherea handset or headset can be connected. The handsetor headset contains a microphone, a small speakerand a push-to-talk switch (PTT). The phone jacksand wiring are connected to an audio amplifier sothat the volume can be controlled. Switches are avail-able to select the desired system and a ringing systemlike that of a telephone is used for alerting the otherparty. On larger aircraft, a PA (passenger address)system is included so that announcements can bemade to the passengers by the flight crew or cabincrew. Figure 3-27 shows the interphone system for

6.

Figure 3-26. An audio control panel performs the switching functions between radios and the microphones andspeakers. (Courtesy Terra Corp.)

116


a bizjet airplane. External interphone jacks are locatedin the nosewheel area, avionics equipment bay areaand in the aft fuselage near the auxiliary power unit(APU). These external jacks permit communicationbetween the cockpit and maintenance personnel atthese locations outside the aircraft.

D. Communications RadiosThere are a number of different radio communica-tions systems available for aircraft use. They differprimarily in the frequencies used and the type ofcommunication involved. The most important useof communications radios is for Air Traffic Controlsince the controllers need to be in contact withthe pilots to give necessary instructions. The generaltrend since the 1930s has been the use of higherfrequencies and the development of specialized com-munications for other than ATC purposes.

1. HF Communications

Up until the 1940s, most aircraft radio communica-tions utilized frequencies in the LF, MF and HFbands because suitable equipment was not availableto use higher frequencies. Aircraft HF radios operateon frequencies between 2 and 30 MHz. The onlymodern aircraft that carry HF comm radios arethose that operate long distances over water orin the remote regions of the earth. Air carrier jetsand bizjets that routinely fly the Atlantic and Pacificoceans will have HF comm radios for ATC purposes.

The HF comm radios have a maximum receptionrange of about 1,500 to 2,000 miles compared toa maximum of about 250 miles for VHF comm.The reception range of VHF comm radios is restrictedto line-of-sight distances as shown in figure 3-28.The probe and flush mount antennas used for HFcomm require a special antenna tuning and couplingdevice. This is automatically repositioned each timea new frequency is selected in order to tune theantenna for that particular frequency. Smalleraircraft with HF comm will use a long wire antennathat usually extends from a wing tip up to thevertical fin. Up to the 1960s, many aircraft useda long wire trailing antenna which extended outthe aft fuselage of the airplane. This antenna couldbe run in and out to select the proper antennalength. It is not suitable for high speed aircraft,so it is little used today. HF comm radios utilizeground and sky waves to achieve their greater recep-tion range. Aircraft HF transmitters produce anoutput power of 80-200 watts which is much higherthan the output power typically found with VHFtransmitters. This is necessary to achieve long dis-tance communication. A disadvantage of HF is thatit is more affected by atmospheric interference than

VHF. Sometimes an aircraft in the middle of theocean will lose communication because ofthunderstorms or other disturbances.

2. VHF Communication

The use of frequencies in the VHF band for aircraftcommunication was developed in the 1940s. VHFprovides much clearer reception and is much lessaffected by atmospheric conditions. EM waves inthe VHF band are space waves so that the receptionrange is limited to line-of-sight distances. At 1000ft., the reception range is about 30-40 miles. Themaximum reception range using ground based sta-tions is about 250 miles at altitudes above 35,000ft. Much less power is required for VHF than forHF comm. Aircraft VHF transmitters have an outputpower of 5-20 watts. The standard radio com-munications system in the U.S.A. for ATC purposesis VHF. This is also true for most other countriesof the world. The International Civil Aviation Or-ganization (ICAO) has designated VHF as the stand-ard radio communication system for ATC purposesover land.

The range of frequencies used for VHF commis 118-137 MHz using AM modulation. In the 1950s,an aircraft VHF comm radio could tune only 90channels or different frequencies. Later the separa-tion between channels was reduced to produce 360channels. Modern VHF comm radios have 720 or760 channels available. The spacing between chan-nels is now 25 kHz, so that adjacent usable fre-quencies would be 120.15, 120.175, 120.20 etc.The latest models of aircraft VHF comm radios uselighted displays that employ LEDs (light emittingdiodes), LCDs (liquid crystal displays), or gas dis-charge tubes. Figure 3-29 shows a complete setof radios that might be found on a typical generalaviation airplane. The radios use lighted displaysfor the frequencies and other information that isneeded. The use of lighted displays and crystalcontrolled tuning has eliminated the complexswitching systems employed on earlier tuners. Themodern radio displays two different frequencies,the one on the left is the active frequency andthe one on the right is the standby frequency thatis held in memory. This is a very nice feature whichallows the pilot to switch the two numbers in thedisplay by simply pushing a transfer button. Thetransfer buttons can be seen in figure 3-29 onthe VHF comm and NAV radios. Technicians shouldbe familiar with the use of aircraft radios fortroubleshooting purposes and also because theymight have to taxi an airplane which may requirethe use of the radio. The antennas used with VHFcomm are Marconi 1/4 wave monopoles that use

117


ADF 2

r —GPWS t -

I I

r 1PA rn- - -

J

OBSERVER I INPUTS/OUTPUTSINTERPHONE PROVISIONED FOR

OXYGENMASK

NAV 2

MKR1110-

ND- AUDIO1111. FLIGHT DECK

ELECTRONICS SPEAKERS•ADF 1

UNIT

VHF 1 1

VHF 2

r - - — AURAL WARNINGI VHF 3 SYSTEM

- --- - -Ir

I HF 1 HIE NOSEWHEEL BAY

rHF 2 roe-

INTERPHONE

AVIONICS BAYINTERPHONE

I I

PILOTS

HEADPHONES& BOOM MIC.

OXYGENMASK

CO-PI LOTS

HEADPHONES& BOOM MIC

PILOTS AUDIO

CONTROL UNIT

CO-PILOTS AUDIO

CONTROL UNIT

LCO-PI LOTS

PTT SWITCH

DME 1

DME 2

VOR/LOC

NAV 1

MKR

A

APUINTERPHONE

PILOTSPTT SWITCH

VOR/LOC

J

Figure 3-27. The intercom and interphone system for a corporate jet. The external interphone jacks are shown in thelower right. (Courtesy Canadair Group, Bombardier Inc.)

1 1 8


AT LOWERALTITUDE CANNOT

COMMUNICATEWITH VHF

VHF LINE OF SIGHT PATHLOW FREQUENCY

GROUND WAVE

vertical polarization. There is a separate antennafor each VHF comm radio.

Radiotelephone

Aircraft often carry a radiotelephone system whichis somewhat similar to the portable cellular phoneavailable for cars. It employs radio signals to permittelephone calls to be made from the aircraft inflight. The frequencies used are 450-500 MHz inthe UHF band. The antenna used is a Marconiantenna of a slightly different shape and size com-pared to a VHF comm antenna.

SATCOMM

A very recent development in aircraft systems is Satel-lite Communications or SATCOMM. A UHF radio isinstalled in the aircraft to communicate with com-mercial satellites in orbit overhead. To date, it isbeing used primarily for telephone calls from bizjetsand air carrier jets. It is beginning to be used fordatalinks from an aircraft in flight to the airline com-puter system. This permits monitoring of the progressof the flight and the status of the aircraft systems.In the future, SATCOMM will be used to replace HFcomm for communications and ATC purposes foraircraft over the oceans or remote areas. The equip-ment currently available is very expensive, usuallycosting hundreds of thousands of dollars. The antennaused with SATCOMM is a special type that mustbe installed on the top of the aircraft.

5. Selcal

Selcal is an abbreviation of selective calling, a specialcommunications system for air carrier aircraft. Sel-cal is not a separate radio system, it is a piece

of equipment that is connected to the existing commradios on the aircraft. It is connected to the VHFand HF radios on the aircraft.

The system is used for communications betweenaircraft in flight and certain airline managers. Itis called selective calling because it works somewhatlike a telephone system. An example of a SELCALdecoder and the connections to the aircraft's VHFand HF radios is shown in figure 3-30. Each aircraftis assigned a code number which is a part of theSELCAL equipment. When the proper code isreceived, a tone is heard in the cockpit to tell thecrew that someone is calling them. They pick upa handset in the cockpit and talk to the personthat has called them. The code consists of four tonesthat are transmitted to the aircraft in series. Eachof the four tones has twelve possible frequencies,so that over 20,000 different codes are available.An example of a SELCAL communication will il-lustrate the operation of the system. The head dis-patcher for the airline is sitting in his office in Chicagoand needs to call the flight crew of one of the airline'saircraft to give them a message. According to theschedule, the airplane is somewhere between Bostonand Atlanta. The dispatcher picks up his telephoneand dials a special access code and the code forthat airplane. The signal is sent out over many dif-ferent ground transmitters and received by hundredsof airplanes in flight. The phone will ring only inthe cockpit of the airplane he is calling and hewill pass on the message when they answer. TheSELCAL system is a great help to the airline whenthey must reroute a flight or pass along importantinformation to the crew in flight.

Figure 3-28. Example of the line-of-sight restriction that applies to VHF and other space wave transmissions.

119


ITMEAMODE titi•mikv

oSS,,v0111

MOPE 711•NAV

OSSIVOR t IM E

E. Emergency LocatorTransmitters (ELTs)

The ELT is a self-contained transmitter that isdesigned to help locate an airplane after a crash.A typical ELT with its antenna and coaxial cableis shown in figure 3-31. It is required on most smallairplanes, but is not required on air carrier jets andbizjets. The ELT is battery powered and is automat-ically turned on by crash forces. It will transmit aspecial swept tone for 48 hours on two different emer-gency frequencies. The two frequencies are 121.5 MHzand 243.0 MHz; 121.5 is the civilian emergency fre-quency and 243.0 is the military emergency frequency.The transmitter is activated by an accelerationoperated switch when a rapid deceleration force isapplied along the longitudinal axis of the aircraft.The ELT must be installed as far aft as possiblebut in front of the tail surfaces since this area hasbeen shown to remain intact in most airplane crashes.The batteries in the ELT must be replaced or rechargedat specific intervals as required by the FARs. Thereare times when an aircraft technician may need totest an ELT so he should be familiar with the pro-cedure. If possible, the ELT should be tested with

the antenna disconnected or shielded to prevent thetransmission of emergency signals into the air. If this

cannot be done, it is still permissible to test theELT, but only during the first five minutes of anyhour and for three audio sweeps maximum. A VHFcomm radio is turned on and tuned to 121.5 MHz.The ELT is then switched on manually until the signalis heard on the receiver and then switched off again.

FAR 91.207Emergency Locator Transmitters

No person may operate a U.S. registered civilairplane unless it meets the applicable requirementslisted below for ELTs.

Each emergency locator transmitter must bein operable condition and meet the require-ments of TSO-C91 or TSO-C91A and it mustbe installed as far aft as practicable.

Batteries used in the ELT must be replacedor recharged as appropriate:

When the transmitter has been in use formore than one cumulative hour; or

When 50% of the useful life has expired.

Figure 3-29. A typical set of radios and associated equipment for a small airplane. The VHF corn and na y radios showboth active and standby frequencies. (Courtesy Terra Corp.)

120


The expiration date for the replacement orrecharge of the battery must be legibly markedon the outside of the transmitter and enteredin the aircraft maintenance record.

The requirements for ELT do not apply to:

A newly acquired aircraft that must be ferriedto a place where the ELT will be installed.

An aircraft with an inoperative ELT thatmust be ferried to a place for ELT repair.

c. Turbojet powered aircraft.

Scheduled air carrier flights.

Training flights conducted entirely within 50nautical miles of the airport of operations.

Design and test flights.

Delivery flights of new aircraft.

Aircraft engaged in aerial application ofchemicals for agricultural purposes.

Research and development aircraft.

j. Exhibition and air racing aircraft.

V.H.F1

Self test0

H.F.1

H.F.2

V.H.F.2

V H F3

Codeselect(4 x 4wires)

Self testReset(5 wires)

Lampdrive(5 wires)

Decoder

Interruptercircuit

V.H.F. 1

V.H.F. 2

V.H.F. 3

H.F. 1

H.F. 2

Lampswitches

Channelamps To

chimesChimeswitch

Supply

Figure 3-30. A typical SELCAL decoder unit showing the connections to the VHF and HF com radios.

121

0


k. Aircraft equipped to carry only one person.

I. An aircraft during any period in which theELT has been temporarily removed for in-spection, repair, modification or replace-ment, subject to the following:

A maintenance record entry must bemade that includes the date of removal,the serial number and the reason forremoval.

A placard must be placed in view of thepilot which states "ELT not installed".

3. The aircraft must not be operated morethan 90 days after initial ELT removal.

The flight data recorder has many more inputsthan the cockpit voice recorder. It has a recordingtime of 8 hours on smaller aircraft and about 24hours on larger aircraft. The CVR and FDR arelocated in the same area of the aft fuselage andhave similar protection from water, fire etc. Someof the typical types of data that are recorded onthe FDR are listed below.

Air carrier jets have been required to carry CVRsand FDRs for some years, but recently new reg-ulations have gone into effect that require thesedevices on smaller aircraft. Some of these new reg-ulations are summarized here.

F. Cockpit Voice Recordersand Flight Data Recorders

The cockpit voice recorder (CVR) and the flight datarecorder (FDR) are designed to automatically recordinformation in flight that can be used during an in-vestigation following an accident or serious incident.They are installed on all air carrier jets and somecommuter airliners and privately owned aircraft. Therecorders are installed in the aft fuselage as shownin figure 3-32 since this area is least likely to beseverely damaged in an accident.

The CVR is designed to record sounds in thecockpit and communications on the intercom andradio systems. It has a hot microphone in the cockpitwhich is always activated to record voices, warningsounds, engine noise etc. The CVR is also connectedto the intercom so that conversations between themembers of the crew can be recorded. It is alsoconnected to radios so that communications withATC are recorded. The CVR has a continuous re-cording system that holds approximately the last30 minutes of data. It is located in the aft fuselagefor better survival and it is waterproof and protectedagainst fire and impact forces.

Figure 3-31. An ELT transmitter for small airplanes withthe antenna and coaxial cable.

FAR 91.609Flight Recorders and Cockpit Voice Recorders

Multi-engine turbine powered airplanes or ro-torcraft with 10 passenger seats or more man-ufactured after October 11, 1991 must havea digital flight data recorder with 8 hours storage.

After October 11, 1991, multi-engine turbinepowered airplanes and rotorcraft with 6 pas-senger seats or more and with a required min-imum flight crew of 2 pilots must have anapproved cockpit voice recorder with minimumstorage of 15 minutes.

3. If an accident or incident occurs, the operatormust hold the data 90 days or longer if requested.

FAR 91 Appendix EFlight Recorder SpecificationsThe flight recorder required for certain aircraft

under FAR 91.609 must record the following items:

Airspeed.

Altitude.

Magnetic Heading.

Vertical Acceleration.

Longitudinal Acceleration.

Pitch Attitude.

Roll Attitude.

Pitch Trim Position.

N I , EPR or Prop RPM and Torque.

Vertical Speed.

Angle of Attack.

Autopilot Engagement.

TE Flap Position.

LE Flap Position.

Reverse Thrust.

Spoiler/Speedbrake Position.

122


CVR AND FDRRECORDERS

CONE OFSILENCE

CLEAR "A"SIGNAL

ON COURSESIGNAL

BI•SIGNALZONE

\ /

\ //

\ // /

/

/

/ / \ \/zz

//

/ / CLEAR "N" SIGNAL/

/

. \N

Figure 3-32. Large commuter aircraft and air carrier jets have a Cockpit Voice Recorder and Flight Data Recorderinstalled in the aft section of the fuselage.

G. Navigational SystemsThere is a much wider variety of navigational sys-tems available to aircraft than communications sys-tems. There is a wide range of capabilities for thevarious systems and some of them are very spe-cialized. Some of them have been around for 50years and others are much newer. We will startwith a brief description of a NAV system that isnow obsolete. A basic understanding of this oldersystem will make clearer the greater capabilitiesof more modern NAV systems.

1. Four-course Radio RangeThe four-course radio range was the first radionavigation system developed in the U.S. to guideaircraft in poor weather conditions. Before its in-vention, the standard navigation system consistedof powerful light beacons that the pilot followedduring either day or night flying. The four-courseradio range gets its name from the fact that onlyfour pathways or courses were usable with thissystem. The ground stations transmitted signalsin the LF band, so the signals could be very difficultto use when atmospheric conditions caused inter-ference. The antenna arrangement on the groundtransmitted four different directional radio beamsas shown in figure 3-33. The signals transmittednorth and south of the station were modulatedwith the Morse code letter "N". The signals trans-mitted east and west were modulated with the letter"A". If the pilot was flying in any one of these sectorsand listening to the signals, the Morse code keyingfor the letter A or N, as appropriate, would be heard.If the aircraft was exactly along a line where thesignals merged, a steady hum or tone would beheard in the headphones. The pilot navigated bylistening to the sounds and aligning the airplane

along the centerline of one of the four courses.As long as the aircraft was on course, the pilotwould hear the steady tone in the headphones.Since the radio signals were affected by atmosphericnoise, the pilot often had to fly for hours on endlistening to noise and static and trying to pick outthe correct signals. The system was not very easyto use and the last one in the U.S. was takenoff the air in the 1970s.

2. Automatic Direction Finder (ADF)The ADF system has been in use since the 1930sand even though it is not as accurate or easy touse as the more common VOR system, it is stillwidely used because it is inexpensive. Many smaller

Figure 3-33. The first radio navigation system was the 4course radio range which operated in the LFband.

123


PLANE OF LOOP PERPENDICULARTO DIRECTION OF WAVE TRAVEL

— r 1

MINIMUMPOSITION

MAXIMUM

airports that have no other radio aids to navigationwill have a transmitter that can be used with ADFequipment. The term automatic direction finder ap-plies to the aircraft equipment and the term non-directional beacon (NDB) refers to the associatedground based equipment. For our purposes, wecan use ADF and NDB to mean the same thingsince the overall system requires both airborne andground based equipment to operate. The ADFreceiver can receive signals transmitted in the rangeof 190 to 1,800 kHz. The signals are in both theLF and MF bands. The ADF equipment on theaircraft can receive two different types of transmittedsignals. The range 190-500 kHz is used by NDBtransmitters that are specifically designed foraircraft use. The range of 550-1,800 kHz is theband used by commercial AM broadcast stations.The broadcast stations can be used for navigation,but are not as good as the NDBs because the pilotdoes not always know exactly where the transmittersite is located. The NDB locations are shown onaeronautical charts so the location of the transmitantenna can be determined more accurately. Thesignals transmitted from the ground sites are om-nidirectional. The ADF equipment determines sta-tion direction through the use of a directionalantenna. The directional antenna is called a loopantenna. The older versions were actually shapedlike a loop as shown in figure 3-34. The strengthof the output signal from the antenna depends onthe angle between the plane of the loop and thedirection of travel of the EM wave. When the EMwave is at right angles to the plane of the loop,

the signal is minimum or a null. When the EMwave and the loop antenna are parallel, the signalstrength is at a maximum. As the loop antennais rotated, there will be a rise and fall in the signalstrength received. If the loop is rotated 360°, therewill be two peaks and two nulls in the signal strengthas shown in figure 3-35. The null is used to deter-mine station direction rather than the peak becausethere is a greater change in the signal strengthwhen the null is reached than when a peak isreached. With a loop antenna alone there wouldbe two nulls for each 360°, which means the stationcould be in one of two directions. This ambiguityproblem is removed by using a second antennacalled the sense antenna. Figure 3-36 shows howthe signals from both antennas are combined todetermine the direction of the transmit station.Older loop antennas were rotated by electric motors.The newer types of loop antenna do not rotate them-selves, they use an electronic system to cause rota-tion of the signal. In either type, the principle ofoperation is the same. The simple type of cockpitindicator used with an ADF is called a radio compassindicator or ADF indicator. This type of simple ADFindicator can be seen in figure 3-37. It has a compassrose with degree markings and a pointer whichpoints in the direction of the transmitter site. Whenthe pointer is straight up, it shows that the stationis directly in front of the aircraft. If the pilot keepsthe pointer at the top of the instrument, it willguide him to the location of the transmitter. Onnewer types of ADF equipment, the cockpit indicatoris called a radio magnetic indicator (RMI).

PLANE OF LOOP PARALLEL TODIRECTION OF WAVE TRAVEL

Figure 3-34. Example of the directional characteristics of a loop antenna.

124


FIELD PATTERN OF THE LOOP ANTENNA

FIELD PATTERN OF THE SENSE ANTENNA

COMBINED FIELD PATTERN OF THE LOOPAND SENSE ANTENNA

(C)

t I,Jr4

.1111

The appearance of this RMI is illustrated in figure3-38. The RMI has two pointers that can be operatedby two ADF signals, two VOR signals, or one ofeach. It also includes a compass card which is

90

180

270

360

*I

I • 3

Figure 3-35. If a loop antenna is rotated 360°, two peaksand two nulls will occur.

Figure 3-36. When the signals from the loop and senseantennas are combined, the ambiguityproblem is eliminated.

slaved to a remote DG and flux gate compass asdescribed in chapter 1. The primary use of ADFis for approaches to airports that don't have anybetter radio aids to navigation available.

3. Very High FrequencyOmnirange (VOR)

The VOR system is the standard IFR radio navigationsystem for cross-country flying in the U.S. and mostof the rest of the world. The VOR system wasdeveloped to overcome some of the problems withthe old four course radio range. The major ad-vantages of VOR over the older system are:

An infinite number of radials or courses areavailable, not just four.

Since it operates in the VHF band, the VORis much less affected by thunderstorms andatmospheric conditions.

The VOR is much more accurate.

The VOR is much easier to use, the pilot followsan indicator needle instead of listening toMorse code signals for navigation.

The first VOR was installed in Indianapolis in1939 and by the 1950s, coverage was almost com-plete over the entire U.S. Since the reception rangeof VOR is limited by the same line-of-sight con-siderations that apply to VHF comm, about 1,500VOR ground sites are required for nationwidecoverage. The range of frequencies used by VORsis 108-118 MHz. The ground sites transmit twokinds of signals, a reference signal and a rotatingsignal. The reference signal uses FM and the rotatingsignal uses AM. The two signals are aligned sothat they will be in phase when the receiver isstraight north of the ground site. The phase angle

Figure 3-37. An ADF radio receiver and its indicator in-strument.

125


for east is 90° and for south it is 180°. No matterwhere the aircraft is in relation to the ground site,the direction can be determined by measuring thephase angle between the two signals. The cockpitindicator for VOR is shown in figure 3-39. Thisindicator has three different parts: the course devia-tion indicator (CDI), the omni bearing selector (OBS)and the TO-FROM indicator.

OBS — The omni bearing selector is a knobthat the pilot rotates to select the desired radialfrom the station.

CDI — The course deviation indicator is the ver-tical needle or pointer. When it moves to theleft of center, the pilot must turn left to getback on the desired radial set with the OBS.

3. TO-FROM Indicator — The to-from indicatoris a small window where one of three wordsis displayed: TO, FROM or OFF. The wordTO means that the pilot is flying toward thestation. The word FROM means the pilot isflying away from it and the word OFF meansthat usable signals are not being received or

the indication is changing between the TO andFROM conditions.

The procedure for tuning in a station with theVOR is similar to that used for tuning an ADF.The pilot locates the station on an aeronautical chartor other reference and determines the frequency.

The pilot then tunes in the desired frequencyand listens for the Morse code identifier. Both VORsand ADFs transmit a two or three letter identifierin Morse code. When the station has been identified,the pilot is ready to use it for navigation by meansof the appropriate cockpit indicator instrument.

The latest types of VOR indicators use light barsinstead of a needle for the CDI. A VOR indicatorthat uses light bars can be seen in figure 3-29on the left side. On aircraft equipped with an HSIor EHSI, the VOR steering commands are displayedon the HSI instead of the more simple VOR indicator.The aircraft VOR equipment must be tested foraccuracy if it is used for IFR (this is covered inthe FAR section later on).

LUBBER OFF-WARNINGLINE

FLAG

SINGLE BAR POINTER(YELLOW)

COMPASS CARD

DOUBLE BAR POINTER(GREEN)

MODE SELECTSWITCHES

Figure 3-38. The RMI can display either ADF or VOR radio navigational information. (Courtesy Canadair Group,Bombardier Inc.)

126


Distance MeasuringEquipment (DME)

The military services have their own radio navigationsystem which operates on principles similar to thoseof VOR. The system is known as TACAN (TacticalAir Navigation) and it uses signals in the UHF band.An additional feature of TACAN that is not a partof the VOR system is the use of distance measuringequipment as an integral part of TACAN. The DMEportion of TACAN is used by civilian aircraft to aug-ment the information available from the VOR. TheVOR and TACAN transmitters are usually locatedat the same ground sites and referred to asVORTACs. The frequencies utilized by DME are inthe range of 960-1,215 MHz. The basic operationof DME is illustrated in figure 3-40. The airplaneDME transmitter sends out a pulse signal in alldirections. This is referred to as the interrogation.When a DME ground station receives a valid in-terrogation from an aircraft, it sends back a replyafter a fixed delay of 50 microseconds. The aircraftDME equipment measures the travel time for thesignals to be sent and received back, and calculatesthe distance in nautical miles. The distance infor-mation is displayed on an indicator in the cockpitfor the pilots. The distance measurement given byDME is a slant range distance so some error willresult from the altitude of the aircraft. The amountof difference between slant range distance andhorizontal or map distance is normally small andthe error can be ignored. If the aircraft is at a highaltitude and almost directly over the DME groundsite, the error will be at its greatest. For example,if the aircraft is directly over the DME site and18,000 ft. above it, the DME will indicate 3 nm(1 nm = 6,080 ft.). With the use of microprocessors,a modern DME can give other indications in additionto distance. If the DME distance is known, thengroundspeed and time to station can be foundthrough mathematical calculations. The pilot canselect which readout is needed; distance in nauticalmiles, groundspeed in knots or time-to-station inminutes. Due to the fact that most DME groundsites are located in the same place as a VOR, thetwo radios are tuned at the same time. When thepilot selects the proper frequency for the VOR thatis being used, the DME equipment is tuned auto-matically to the proper DME channel.

RNAV — Area NavigationThe RNAV equipment in aircraft contains a computerthat processes the signals received from VOR andDME ground sites. The main advantage of RNAV isthat it permits random direct routes of flight. Theuse of conventional VOR navigation along airways

requires that the aircraft be flown directly from oneVOR site to the next. Since the VOR sites seldomline up directly along the desired flight path, theaircraft ends up flying a zigzag course to get fromone place to another over long distances. The useof RNAV equipment permits the aircraft to fly directlyto the destination without having to fly straight toand from each of the VOR sites. This more directrouting is illustrated in figure 3-41. The RNAV com-puter processes signals from VOR and DME trans-mitters and displays steering information to the pilotto guide the aircraft along a direct route of flight.

Figure 3-39. The VOR indicator contains the CDI needle,the TO-FROM indicator and the OBS knobto select the desired radial.

127


NAUTICAL MILES /

Indicator

Control Panel

DISTANCE READOUT

DME ANTENNAS

PULSE PAIR GROUPS—Randomly Spaced

//fi STATION delays signal 50 microseconds then

retransmits 63MHz ABOVE or BELOW received frequency.

DMEGROUNDFACILITY

DME

1 2 3!51 ,,..•n•••••

O

Before takeoff the pilot will program the computerwith the desired waypoints that establish the desiredroute of flight. A waypoint is established as a directionand distance from a VOR and DME site. For example,the waypoint OMN 240/25 would indicate a pointthat is 25 nautical miles southwest of the OMN trans-mitter site. The pilot programs the RNAV computerby designating a number of waypoints along thedesired flight path. During flight, the RNAV computerperforms the calculations needed to display guidancecommands using a CDI or HSI that will guide theaircraft from one waypoint to the next. Even thoughthe RNAV equipment is designed to permit directroutes, the aircraft must be able to receive usablesignals from VORTAC sites. Waypoints cannot be usedif they would take the aircraft beyond the line-of-sightreception range. A limitation on the use of RNAVfor IFR flight is the ATC system. In congested areaswith a lot of air traffic, the direct routes of flightmay not be approved by air traffic controllers.

6. TranspondersThe transponder equipment found on aircraft isdesigned to make it easier for air traffic controllersto identify specific aircraft so that they can preventmid-air collisions and provide guidance to theaircraft. The transponder is a device which is relatedto radar, so we will begin with a short historyof the use of radar to identify aircraft. The useof radar to locate aircraft in flight dates back tothe 1930s. The principle used is called primaryradar or echo location radar. The radar transmittersends out a brief pulse of EM waves which traveloutward at the speed of light and bounce off themetal parts of an airplane. The reflected energyor echo is received back at the radar site whereit produces a spot of light on the radar scope.The problem with this primary radar is that allthe blips on the radar scope look the same. DuringWorld War II, a system was developed to makeit easier to distinguish the friendly aircraft from

Figure 3-40. The operation of the DME radio system using pulse signals.

128


ACTUAL FLIGHT PLANik

VOR DMESLT

VOR/DMEJFK

c41EWR

DESIRED FLIGHT PLAN

VOR DMESt T

0

VOR DMEOBK

DAD WAYPOINTS

VOR DMEJFK

EWA

the enemy aircraft. The system was known as IFF(identification friend or foe) and it is still calledthat by the military. This type of radar is alsocalled secondary radar. A small radar frequencyreceiver and transmitter unit is installed in eachairplane. When the radar pulse from the groundsite strikes the aircraft, the IFF equipment sendsa coded signal back to the ground site. The basic

operation of the primary and secondary radar sys-tems is shown in figure 3-42. The coded signalreceived at the radar site from the aircraft permitsit to be identified. In the years since World WarII, both primary and secondary radar have beenadapted for ATC purposes. Transponder is thename of the secondary radar equipment installedon aircraft.

WITHOUT AREA NAVIGATIONVOR DME

CRL

USING AREA NAVIGATION

VOR DMECRL

0

VOR DMEPMM

Figure 3-41. The use of RNAV equipment permits direct flights using way points.

129


ATC RADAR TRANSMITTERRECEIVER

TYPICAL ATC REPLY

NO ATC REPLY

PSR—Primary Surveillance Radar

SSR—Secondary Surveillance Radar (ATC)

ATC—Air Traffic Control

\\\A A \\N\\\ \-

A 1/ATC \\

A\‘\\A \\ SSR Interrogation

\\\ \\

cr===.- '

ICIIIIPSR Reflection

PSR

ATC RADARANTENNAS

Reply — Altitude or Code2 to 14 PULSES — 1090MHz

-Lit:--20.3 microseconds

IDENTIFICATION PULSE

Ground SurveillanceRadar Scope

Figure 3-42. Both primary and secondary radar are used for Air Traffic Control purposes.

The aircraft transponder system uses only twodifferent frequencies, one to transmit and one toreceive. The transponder receives on 1030 MHz andtransmits on 1090 MHz (it is a UHF system). Theground radar site sends out a coded interrogationpulse which in effect asks the airborne equipmentto answer or reply. When the transponder receivesa valid interrogation, it sends back the proper replysignal. The coding used in the transponder signalsis digital or binary. Each interrogation and replysignal consists of a number of pulses in a pulsetrain as illustrated in figure 3-43. For each locationin the pulse train, a pulse can either be presentor absent. The cockpit controls for the transponderpermit the pilot to set one of 4096 different numeri-cal codes. The numbers set into the transponderrepresent an octal coding so there are no 8s or9s in the code setting window. The possible codesettings range from 0000 to 7777. The computerin the ground radar site can identify the aircraftby the code its transponder is sending out.

Some transponder codes are reserved for specialpurposes; 0000 is used by the military, 1200 is forVFR aircraft and 7500,7600 and 7700 are reservedfor specific types of aircraft emergency situations.

There are several different operating modes as-sociated with transponder equipment. They are:

Mode 3/A — This is the basic transponder modethat can utilize one of 4096 different codes.

Mode C — This mode includes the above ca-pabilities but adds a coded message giving

Figure 3-43. The reply from an aircraft transponder usesbinary coded pulses of very short duration.

130


the aircraft's pressure altitude using an al-titude encoder.

3. Mode S — This is the latest development intransponders and is not yet fully operational,it will have the capability of sending additionalmessages such as ATC instructions or weatherreports that can be viewed on a CRT or printedon paper in the cockpit. Mode S also increasesthe number of different identification codesfor the aircraft to over one million.

7. FARs for VOR and Transponder

The FAA regulations which apply to testing andoperation of VOR and transponder equipment aresummarized here.

FAR 91.171VOR Equipment Check for IFR Operations

(a) No person may operate a civil aircraft underIFR using the VOR system of radio navigationunless the VOR equipment of that aircraft:

1 Is maintained, checked and inspectedunder an approved procedure; or

2. Has been operationally checked within thepreceding 30 days and was found to be withinthe limits for bearing error set forth below.

(b) The check must use one of the following:

An approved FAA or Repair station groundtest signal —

Designated VOR checkpoint on the airportsurface — ±4°.

3. Designated airborne checkpoint — ±6°.

An airborne check using a VOR radial andprominent ground point that can be seenfrom the air as established by the persondoing the check — ±6°.

If two separate VOR receivers are installed,they can be checked against each other— ±4°.

(c) Maintenance record entry

Each person performing one of the abovechecks shall enter the date, place and bear-ing error in the aircraft log or other recordand sign it.

If a test signal from a repair station is used,the repair station certificate holder mustenter the bearing transmitted and date inthe aircraft log or other record.

FAR 91.215ATC Transponder and Altitude ReportingEquipment and Use

(a) TSO requirements

All aircraft transponders must meet theappropriate requirements of TSO-C74b,TSO-C74c or TSO-C112.

After July 1, 1991, all initial installationsof ATC transponders in aircraft must meetthe requirements of TSO-C112 (Mode S).

Note: Due to development delays this requirementhas been dropped.

(b) Airspace requirements — all aircraft operatingin the following airspace must have a 4096code transponder with Mode C altitude re-porting or a Mode S transponder.

Terminal Control Areas — TCAs

Airport Radar Service Areas — ARSAs

3. In all controlled airspace above 10,000 ft.MSL and over 2,500 AGL.

FAR 91.413ATC Transponder Tests and Inspections

No person may use an ATC transponder thatis required by the rules of this Chapter unlessit has been tested and inspected within thepreceding 24 calendar months and meets therequirement of Part 43 Appendix F.

Following any installation or maintenancewhich could have introduced errors, the in-tegrated system must be tested in accordancewith paragraph (c) of Appendix E, Part 43.

(c) The above tests and inspections must be con-ducted by:

An appropriately rated repair station; or

A holder of a continuous airworthinessprogram as provided in Part 121, 127,or 135; or

3. The manufacturer of the aircraft, if the tran-sponder was installed by that manufacturer.

H. Long Range Navigation SystemsThe navigation systems in this section all haveusable ranges that exceed those of the VOR, ADFand other systems already covered. Some of theselong range navigation systems do not rely on groundbased transmitters and some do not use radio sig-nals at all. All of them have been developed sincethe 1960s so they are relatively more modern thanmost of the systems previously described. The longrange navigation systems use geographical coor-dinates to establish aircraft position and waypointsalong a desired flight path. As shown in figure 3-44,geographical coordinates are based on the grid sys-tem of lines of latitude and longitude. Longitude

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is expressed either east or west of the prime meridianthat runs through Greenwich, England. The max-imum longitude is 180° which is the InternationalDate Line exactly half way around the earth fromthe prime meridian. Latitude is measured northand south of the equator with a maximum valueof 90° at the poles. Since 1° is equal to approximately60 nm, for accurate positions each degree is dividedinto 60 minutes and each minute into 60 seconds.An example of a lat/ long position is 29°, 10 minutes,51 seconds North latitude by 81°, 3 minutes, 22seconds West longitude. This coordinate positionlocates the airport at Daytona Beach, Florida.

1. Loran C

The first Loran system was developed in the 1940sby the U.S. Navy for use by ships. Modifications weremade to produce Loran A, B, C and D. Loran Cis the only one that has any large scale use by aircraft.All the early transmitter sites were located alongcoastlines since it was a system for ships. Startingabout 30 years ago, pilots of privately owned airplanesbegan modifying Loran C units from boats and shipsfor aircraft use. The system has been improved andnewer equipment is now available that make LoranC a very useful radio navigation system. Within thelast several years new ground sites have been installed

Figure 3-44. Long range navigation systems define aircraft position in terms of latitude and longitude.

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SLAVE "Y"

in the western U.S. to give nationwide coverage. Theprimary aircraft users of Loran C continue to begeneral aviation aircraft.

The Loran C system uses ground transmitter sitesthat all transmit on the same frequency-100 kHz.The EM waves produced are ground waves. Theoperation of this system involves transmitter chainsin which each chain consists of one master stationand two or more slave stations. For purposes ofexplanation, we will consider a chain with two slavestations like that shown in figure 3-45. Each trans-mitter uses a tower about 1,000 ft. tall and hasan output power of approximately 4,000,000 watts.

The transmissions are sequenced so that themaster transmits first and then the slaves transmit.The location of the master and slaves are hundreds

of miles apart. The signals received by the aircraftwill have a time separation that is determined bythe aircraft location relative to the transmitter sites.A computer in the Loran C receiver performs thecalculations that determine location. The locationdetermination has an accuracy on the order of 400-1,000 ft. in most cases. The Loran C equipmentin the aircraft does not have to be tuned sinceall signals are received on 100 kHz. Each chaincan be identified by the time delay between trans-mission pulses and the information is stored inthe computer memory of the aircraft receiver. Thereception range is 1,000 miles or more. An ad-vantage of this system is that signals can be receivedat any altitude, even with the aircraft on the ground.The use of microprocessors has made the modernLoran C unit a very powerful and versatile navigation

X

SLAV E

‘‘

X-2000

Is -3000

01111111111F,

COMMONMASTER

X -Z

tio00

Both slaves have a common master

Figure 3-45. A Loran C chain consists of one master station and two or more slave stations.

133


system. A Loran C receiver contains a large amountof memory which stores the location of every airportand VOR in the U.S. The memory often includesextra information for each airport such as radiofrequencies available, runway direction and length,fuel available etc. The memory can also store pilotdesignated waypoints to enable direct routes to beflown. The output of the Loran C includes a CDItype indicator which can be used to guide the aircraftalong the desired route of flight. A Loran C receiverand its digital lighted display can be seen at thebottom of the radio stack in figure 3-29.

2. OmegaThe Omega radio navigation system was developedby the U.S. Navy for use by ships and aircraft.There are only eight Omega transmitter sites scat-tered around the earth, but they provide worldwidecoverage. The maximum usable reception range isapproximately 10,000 miles. The signals are trans-mitted in the VLF band by powerful ground basedtransmitters. Each ground station transmits onseveral different frequencies between 10 and 14 kHzin a repeating pattern. The transmissions from theeight stations are sequenced so that two differentstations don't transmit on the same frequency atthe same time. The accurate timing required forthis is maintained by atomic clocks. When an aircraftOmega receiver is turned on, it automatically selectsthe strongest signals for navigational use. Theprocessing of signals is performed by computersand the display of position and guidance information

is similar to that used with Loran C. The U.S. Navyalso operates a VLF communications system thatutilizes seven sites around the world. Many aircraftnavigation receivers can pick up both Omega andVLF comm signals. The Omega signals are moreaccurate for navigation and are used as the primarysource, the VLF communications station signals areused as a back-up or secondary means of navigation.The locations for the eight Omega stations and theseven VLF stations are shown in figure 3-46.

3. Inertial Navigation System — INS

The inertial navigation system or INS is a long rangeNAV system that does not rely on the receptionof radio waves. The system is totally self-containedwithin the aircraft. The key to the operation ofINS is the very accurate measurement of accelera-tion forces. The accelerometer sensors measure ac-celeration in directions parallel to the earth'ssurface. The INS unit can calculate direction andvelocity of the aircraft by measuring accelerationforces, but it cannot determine where the aircraftis when the unit is first turned on. For this reason,the INS must be aligned and calibrated beforetakeoff. When the INS is first turned on beforeflight and before the aircraft is moved, the positionof the aircraft is entered on a keyboard so theunit can align and calibrate itself. During flightthe INS calculates direction and velocity, whichwhen applied to the beginning position gives thepresent position. During very long flights, the INSwill develop a cumulative error. Toward the end

OMEGA STATIONS

Letter No. Location Latitude Longitude

VLF COMMUNICATION STATIONSFrequency

No. Location Latitude Longitude (kHz) Pwr (KW)

A 1 Aldra, Norway 66°25'N 13°08'E 1 Maine 44°39'N 67°17'W 17.8 1026

B 2 Monrovia, Liberia 6°18'N 10°40'W 2 Japan 34°58'N 137°01'E 17.4 48

C 3 Haiku, Hawaii, USA 21°24'N 157°50'W 3 Washington 48°12'N 121°55'W 18.6 124

D 4 La Moure, North Dakota, USA 46°22'N 98°20'W 4 Hawaii 21°26'N 158°09'W 23.4 588

E 5 La Reunion 20°58'S 55°17'E 5 Maryland 38°60'N 76°27'W 21.4 588

F 6 Golfo Nuevo, Argentina 43°03'S 65°11'W 6 Australia 21°49'S 114°10'E 22.3 989

G 7 Australia 38°29'S 146°56'W 7 Great Britain 52°22'N 01°11'W 16.0 40

H 8 Tsushima, Japan 34°37'N 129°27'E

Each station transmits a specific frequency.

Each station transmits three basic frequencies: 10.2 kHz,11.33 kHz, and 13.6 kHz. To prevent signal interferencebetween stations, transmisisons are timed such that only onestation is transmitting a particular frequency at a time.

Figure 3-46. There are eight Omega transmitter sites that provide world-wide coverage. The seven VLF stations can beused as back-ups.

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TORQUER

>AMPLIFIER

TO INSCOMPUTER

SIGNALPICKOFF

NULLPOINT

ACCELERATIONFORCE

of a long flight the error might be 20 miles. Toeliminate this cumulative error, modern INS unitscan be updated using radio signals received fromground stations where they are available. The simpletype of INS sensors are small weights which reactto acceleration forces by movement about a hingepoint as shown in figure 3-47. In order to giveaccurate readings, the accelerometers must bemounted on a gyro stabilized platform so that theyonly measure horizontal forces. This stable platformand the control panel for an INS system is shownin figure 3-48(A) and figure 3-48(B). In effect, theaccelerometers measure north-south and east-westaccelerations in order to determine aircraft position.

The newest type of inertial navigation system doesnot use conventional spinning gyroscopes and doesnot need a gyro stabilized platform. A modern InertialReference Unit (IRU) for an INS system contains threeaccelerometers and three ring laser gyros (RLGs). Thethree accelerometers measure acceleration forcesalong the aircraft's three axes; vertical, lateral andlongitudinal. The RLG is a device with no movingparts that replaces a conventional gyro with a spinningrotor. As shown in figure 3-49, the laser gyro usesa triangular housing and two different laser beams.The mirrors at the corners direct the two laser beamsin opposite directions around the triangular course.Sensitive detectors measure the Doppler frequencyshift that occurs when the unit is rotated. Threeof these RLGs are needed to measure rotation aroundthe three axes of the aircraft. A computer processesthe signals from the three accelerometers and thethree laser gyro sensors to determine aircraft heading,position and groundspeed.

This modern type of IRU is referred to as a strap-down system because it does not require a gyrostabilized platform like that shown in figure 3-48(A).The corrections that are needed are calculated bythe computer. Like any inertial navigation system,the strapdown INS must be given the geographicalcoordinates for present position during the align-ment before takeoff.

Inertial navigation systems can be programmed withcomplete routes of flight and can be coupled to theaircraft autopilot to provide steering commands.

4. Doppler NavigationThe Doppler navigation system does not rely onthe reception of radio signals from ground basedtransmitter sites, but it does use radio waves.

The Doppler system uses radar beams that areprojected downward and received back at theaircraft after they have bounced off the surfaceof the earth. The frequency commonly used is 8.8

GHz. The arrangement of the radar beams is shownin figure 3-50. Notice that two beams are projectedforward and two to the rear. When the aircraft isin motion, the frequency of the received signalswill be shifted upward or downward compared tothe frequency that was transmitted. The changein frequency of a wave when there is relative motionbetween the source of the wave and the observeris the Doppler effect. Figure 3-51 illustrates theDoppler effect with sound waves where the observerhears a change in pitch or frequency of the soundwaves as the ambulance goes by his position. TheDoppler effect is the same for both sound wavesand radio waves. That is where this system getsits name. If the aircraft is traveling forward overthe surface of the earth and not drifting right orleft, the frequency of the two forward beams isshifted upward equally and the frequency of thetwo rearward beams is shifted downward equally.If the aircraft is drifting, there will be a differencein the received frequency between the right sideand left side beams. By measuring all four beams,the groundspeed and side drift of the aircraft canbe calculated accurately. This information can beused to make a continuous determination of posi-tion. Like INS, Doppler NAV systems must be alignedbefore takeoff and will develop cumulative errorsin flight. Doppler can be updated using availableground based radio signals as is done with INS.Doppler navigation units were common in the past,but have now been largely replaced by INS andother newer long range navigation systems.

5. Satellite NavigationThe latest development in long range navigationsystems is the use of satellites in earth orbit. Two

Figure 3-47. One simple type of INS accelerometer.

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INS ROLLAXIS Nr'''n

Z PLATFORMPITCH

Al SYNCHRO

PITCH211Pr AXIS

PITCHGIMBALOUTER ROLL

SYNCHRO

INNER ROLLRESOLVER

SYNCHRONOUSMOTOR

ROLL AXISALIGNMENTSURFACEOUTER ROLL

GIMBAL

PITCH TORQUEMOTOR Z ACCELEROMETER

Z GYRO

AZIMUTH TORQUEMOTOR

INNERROLLGIMBAL

YAWAXIS

X-Y PLATFORM

X-Y PLATFORMRESOLVER

CO-ORDINATERESOLVER - ROLL

+ PITCH

X GYRO

Y ACCELEROMETER

Illf X ACCELEROMETER

+ AZIMUTH

INNER ROLLTORQUE MOTOR

OUTER ROLLTORQUE MOTOR

Y GYRO

(A)

AZIMUTHSYNCHRO

CDU (B) HSI

Figure 3-48.The gyro stabilized platform of INS accelerometers.The keyboard and controls for the INS system and the HSI which can be used to display INS navigationalinformation.

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GAS DISCHARGEREGION

CORNERPRISM

ANODE

READOUT DETECTOR

LIGHTBEAMS

MIRROR(1 OF 3)

-41F-CATHODE

L

PIEZOELECTRICDITHER MOTOR

ANODE

systems began development in the 1970s, one bythe USSR and one by the U.S. The U.S. systemis called GPS (global positioning system) and theRussian system is called GLONASS.

Both systems are still incomplete although somesatellites are in orbit now and can be used. Thetwo systems are very similar in terms of the fre-quencies and principles of operation. The GPS sys-tem was designed to include 24 satellites, 21 primaryand 3 spares. Due to delays caused by the SpaceShuttle Challenger disaster and other factors only18 satellites were in place as of the middle of 1992.The GLONASS system was also designed to usea total of 24 satellites, but only about 15 werein place by the middle of 1992. The satellites ofboth systems orbit at an altitude of about 10,000nm with a period of 11 to 12 hours. The positionand altitude of the satellites is known with greatprecision. The aircraft with a satellite navigationsystem communicates with the satellites using fre-quencies in the 1.6 GHz range. For accurate naviga-tion, the aircraft must be able to communicate withat least four different satellites as seen in figure

3-52. Currently the coverage in most areas is over90%. Since the altitude of all the satellites is known,this system can provide altitude as well as positioninformation. The accuracy is on the order of 80ft. which makes the system potentially more ac-curate than the other systems so far described.The GPS system was developed and is operatedby the Department of Defense. A possible limitationon the use of GPS for IFR is the difficulty in monitor-ing the accuracy of the satellite signals. Satellitenavigation systems can be purchased now, but theyare primarily used for VFR navigation or as a secon-dary system. The use of this system as a sole sourcefor IFR navigation has not yet been approved. Ithas been predicted that within a few years satellitenavigation may become the dominant long rangeNAV system and may also be used for precisionapproach guidance to airport runways.

I. Instrument LandingSystem (ILS)

An instrument approach procedure is a methodused to guide an aircraft to an airport runway for

Figure 3-49. A laser beam !RU (inertial reference unit) which uses laser beams to replace conventional gyros for an INSnavigation system.

137


landing in bad weather conditions. The procedures

must be FAA-approved and are published in special

books for pilots to use. There are two basic typesof instrument approach procedures: precision ap-proaches and non-precision approaches. The dif-ference is that precision approaches give the pilotvertical or descent guidance while non-precisionapproaches do not. Signals from VORs and NDBscan be used for non-precision approaches. Thestandard type of precision approach system usedin the U.S. and most of the world for civilian aircraftis the ILS. The military still uses a form of precisionapproach radar, but that kind of system is no longeroperated by the FAA. Instrument approaches haveweather minimums which specify the minimum ceil-ing (cloud height) and visibility needed to success-fully complete the approach. The weather minimumsfor a Category I standard ILS are 200 ft. ceilingand 1/2 mile visibility. The minimums for a non-precision approach would be about twice as much.

The ground equipment needed for an ILS system

has four parts.

Localizer — A radio beam for lateral guidance.

Glideslope — A radio beam for vertical guid-ance.

Marker Beacons — Radio signals that give dis-tance to the runway data.

Runway and approach lights.

The lights will not be discussed further becausethey require no equipment on board the aircraft.

The first three parts will be described in order.The localizer layout is shown in figure 3-53 witha view from above to show the localizer beam.

The signals transmitted by the localizer are on fre-quencies between 108 and 112 MHz. A dual beamis transmitted outward from the far end of the runway.The right half of the signal is modulated at 150 Hzand the left half is modulated at 90 Hz. The aircraftreceiver measures the relative strength of the 90 and

Figure 3-50. The orientation of the four radio beams projected from the bottom of the aircraft by a Doppler navigationsystem.

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I I

MEDIAN SPACINGIF VEHICLE WERE

STATIONARY

SOUND WAVES SOUND WAVESEQUALLY AND EQUALLY BUT

DENSELY SPACED WIDELY SPACED

NI:11))»)i))111114

VEHICLEAPPROACHING

VEHICLEDEPARTING

150 Hz signals that are received. When they are equal,the aircraft is lined up with the centerline of therunway. The cockpit indicator is a vertical needlejust like a VOR indicator, in fact it is often the sameone. If the needle swings to the left, the pilot mustturn left to get back on course.

The glideslope uses a principle like that of thelocalizer, but it transmits on frequencies of 328

Figure 3-51. Example of the Doppler principle applied tosound waves rather than radio waves.

to 336 MHz in the UHF band. A side view of theglideslope and runway is shown in figure 3-54.

The glideslope signal uses 90 Hz modulation abovethe glidepath and 150 Hz modulation below theglidepath. The center of the glidepath would produceequal parts of 90 and 150 Hz signal in the receiver.The cockpit indicator for glideslope is a horizontalneedle as shown in figure 3-55 which shows a simpleILS indicator. A needle deflected upward meansthe pilot must fly up or decrease his rate of descent.The actual glidepath angle used in an ILS systemis about 2 1/2-3°. This angle permits both large andsmall aircraft to use the ILS. The glideslope andlocalizer frequencies for an ILS are paired togetherin set combinations. The glideslope receiver is usual-ly slaved to the localizer receiver so that when alocalizer frequency is tuned in, the correct glideslopefrequency is automatically set in the glideslopereceiver.

The marker beacons are low powered transmittersthat transmit a cone shaped pattern straight upinto the air. When the aircraft flies directly overthe marker beacon site, an indication is given inthe cockpit to show the pilot the distance to theapproach end of the runway. This is illustratedin figure 3-56 which gives the approximate distancesfor the outer, middle and inner markers. All marker

N

N

NN

NN

NN \ /

\ /

7/in

Figure 3-52. When using satellite navigation, the aircraft usually needs to communicate with four satellites for accurateinformation.

139


beacons transmit on 75 MHz so different modula-tions must be used to identify the inner, middle

and outer markers. The aircraft receiver does notneed to be tuned and in fact is often turned onautomatically with the electrical system. The outermarker is modulated with a frequency of 400 Hzand a Morse code sequence of dashes. It also causesa blue light to illuminate in the cockpit so it canbe identified by sound or with the blue light orboth.

The middle marker is modulated with a frequencyof 1300 Hz and a sequence of Morse code dotsand dashes.

The amber indicator light comes on over the mid-dle marker. The inner marker is not used withall ILS systems. It uses a modulation of 3000 Hzand a sequence of Morse code dots. The white lightcomes on over the inner marker. The three markerbeacon indicator lights can be seen on the left sideof the audio control panel in figure 3-26.

Some ILS systems place an. NDB type transmitterat the outer or middle marker locations. These makeit easier for the pilot to navigate to the proper loca-tion to begin the approach. These are examplesof what are called transitional navigational aids.When an NDB transmitter is associated with amarker beacon location, it is called a compasslocator. The pilot would tune it in on his ADF receiverand follow the indications as he would for any NDB.

The signals produced by the localizer system areprojected in opposite directions so that the localizeris usable from either direction as shown in figure3-57. The course that is used with the ILS is calledthe front course and the other is called the backcourse. The glideslope is projected in one direction

150 Hz90 Hz

Figure 3-53. The localizer course is modulated by 150 Hzon the right side and by 90 Hz on the left sideof the center line.

Figure 3-54. The glideslope signal is modulated by 90 Hzabove and 150 Hz below the middle of theglidepath.

only. If an airport has an ILS for runway 9, thefull ILS is only available for landings to the easton runway 9 (090°). The back course of the localizerwould be available for landings on runway 27 (theopposite end of the same runway). The back courseapproach is a localizer only approach which willhave higher minimums than the ILS because it doesnot have the glideslope. The sensing of the indicatorneedle is backwards on the back course approach.The pilot would have to use opposite correctionson the back course compared to the ILS.

Figure 3-55. The ILS indicator has two needles: a verticalneedle for localizer and a horizontal needlefor glideslope.

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INNER MIDDLE

OUTER

RUNWAY.3 .6 5

NM

NM

NM

Figure 3-56. The marker beacon transmitters send sig-nals upward which provide indications inthe cockpit of distance to the runway.

RUNWAY • • ::::: . : . : : '

BACK

FRONTCOURSE

COURSE

Figure 3-57. The localizer transmits signals that provideguidance on both the front course and theback course. Only the front course is usedfor an ILS.

J. Microwave LandingSystem (MLS)

The microwave landing system is a recentlydeveloped precision approach system that is inlimited use at this time with about eight systemsin service in the U.S. The MLS was designed toovercome some of the problems with ILS. A majorproblem with ILS is that not enough frequenciesare available to install the system in all the placesit might be needed. ILS is affected by some bendingof the beams by obstructions and can only haveone glidepath angle that all aircraft must use. TheMLS transmitters operate at frequencies between5.0 and 5.1 GHz. Many new frequencies are availablein this range and the signals do not suffer thesame kind of interference or bending that affectsILS signals. The principle of operation of the MLSis called a time referenced scanning beam system.

Two beams are used: one that scans side to sideand one that scans up and down. The aircraftreceiver measures the time difference betweenreception of the TO and FRO scans for the twobeams in order to determine lateral and verticalposition. The scanning beams used by the MLSare illustrated in figure 3-58. The cockpit displayworks in a way similar to the one used with ILS.Because of delays in development, only a few MLSapproach systems are in use. One of the first wasinstalled at a small airport in Colorado that is servedby a commuter airline. The airline operates STOL(short takeoff and landing) aircraft and the glides-lope that is used is steeper than normal. This allowsa steep approach which gives greater terrainclearance in mountainous areas. The glidepathangle for MLS is determined by the processing ofthe signals by the aircraft equipment. It is not deter-mined by the installation of the ground antennas.This means that with MLS a different glidepathangle could be used by aircraft with different flightcharacteristics. It will probably be many years beforeMLS has replaced ILS to any great extent, in fact,the FAA just recently ordered new ILS equipmentfor installation at a number of U.S. airports. Theuse of MLS requires different receivers and antennasthan ILS, but both types of equipment may be foundon some aircraft.

K. Radar AltimeterThe radio altimeter or radar altimeter is a systemwhich measures the aircraft's height above groundlevel (AGL) with an accuracy of about 5 ft. A con-ventional altimeter is not that accurate and itmeasures MSL not AGL altitude. The usable rangefor a radar altimeter extends up to 2,500 ft., butit is mainly used during instrument approaches inbad weather. The basic Category I ILS minimumsare 200 ft. ceiling and 1/2 mile visibility. There areother categories of ILS with lower minimums. A

Figure 3-58. The MLS system has two scanning beams. The lateral beam that scans side to side is shown here.

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Indicator

1.1nn•••nn

TRANSMITTER RECEIVER/MIXER

MODULATOR

BEAT FREQUENCY

COUNTER

Category II ILS has minimums of 100 ft. ceiling and

1/4 mile visibility. The minimums for Category III areeven lower. When the aircraft is flying below 200ft. in bad weather, a better method of measuringaltitude than a barometric altimeter is needed forsafety. This is the main use for a radar altimeter,as a precise way to measure AGL altitude duringIFR approaches. The radar altimeter uses antennasthat are installed on the belly of the aircraft. Thetransmitter sends out radio waves at 4.3 GHz whichstrike the earth and bounce back to the receive an-tenna. By measuring the travel time for the radiowaves, an accurate calculation can be made of AGLaltitude. The basic parts of a radio altimeter or radaraltimeter system and one type of cockpit indicatorare shown in figure 3-59. Another type of cockpitindicator for a radar altimeter that uses a rounddisplay is shown in figure 3-60. This instrument hasa bug that can be set at 200 ft. or some other altitude

to give a warning to the pilot during an instrumentapproach.

L. Ground Proximity WarningSystem (GPWS)

The ground proximity warning system is designedto provide warnings to the flight crew when theaircraft is in danger of striking the ground dueto excessive descent rate or rising terrain. Thisequipment is required on all air carrier jets andit is found on some bizjets and other aircraft aswell. The main component in a GPWS system isa computer which monitors numerous inputs andmakes calculations to determine if the aircraft isin danger of hitting the ground. Some of the inputsto the computer are barometric altitude, radar al-titude, rate of climb or descent, flap position andlanding gear position. The GPWS is one of the few

Low Range Radio Altimeter Receiver/Transmitter

Figure 3-59. The aircraft radar altimeter system is also called a radio altimeter.

142


systems on a civil aircraft that gives a spoken voicecommand to the flight crew. When it is determinedthat a warning must be given, a recorded voiceon tape is activated which tells the flight crew to"PULL UP, PULL UP" or a similar type of message.

M. Weather RadarA radar weather unit is another piece of equipment

which is required for all air carrier jets and is com-mon on many other types of aircraft. Aircraft weatherradar is a pulse radar that typically operates at9.375 GHz. The radar antenna is installed on thefront of the aircraft where it sends out brief pulsesof radar frequency EM waves in order to locateand avoid thunderstorms. There must be somethingpresent in a thunderstorm which will reflect theradar pulse. Clouds are invisible to radar, but ice,hail and especially rain will reflect the energy backto the aircraft radar antenna. The strength of thereturn is affected by the size of the raindrops andthe rainfall intensity. Color radars use different col-ors for different intensity levels. Green, yellow andred are often used with red indicating the highestintensity of rainfall.

Radar signals can also be reflected from theground and the radar system can be used to locatesurface features on the earth below. This mappingfeature is especially effective when used to pickup well defined coastlines.

Weather radar is called a pulse radar systembecause it transmits very brief pulses of energy.

Figure 3-60. Some radar altimeters use a round indicatorinstrument.

This is necessary in order to use the same antennafor transmit and receive and to produce a usablemaximum range. The transmitter sends out a pulsethat has a duration of about one microsecond. Thenthe antenna is switched to the receiver for a periodof about 2,500 microseconds. The receiver mustbe connected long enough for the pulse to travelout to the maximum range and back again. Theuse of pulse radar also makes the system moreefficient since the transmit energy is concentratedin brief pulses which permits much higher valuesof peak power than would otherwise be possible.The major components and their location for aweather radar system on a business jet airplaneare shown in figure 3-61.

The major components of a weather radar systemand their functions will now be listed:

Antenna — The antenna is a parabolic reflectoror a newer and more efficient flat plate an-tenna. It does not rotate 360° like ATC radar,but scans side to side through an arc of ap-proximately 120°.

Radar screen or display — This shows the re-turns picked up on the radar, usually in 3or 4 different colors.

Cavity magnetron — This is a special deviceused to produce the radar frequency EM wavesfor the radar system (figure 3-62).

Synchronizer — The antenna and the screenmust be synchronized in order to show thecorrect location of the returns.

Duplexer — This unit rapidly switches the an-tenna between the transmitter and receiver.

Stabilization System — The antenna unitneeds to be gyro stabilized so that pitch androll attitudes of the aircraft will not cause in-correct display presentations.

7. Waveguide — because of the power and fre-quency of the EM waves, coaxial cables cannotbe used to connect the antenna with the R /Tunit. A hollow tube called waveguide is usedfor this purpose (figure 3-63).

The weather radar has a maximum range from200-300 miles on a typical installation. The controlsallow the pilot to select different ranges and differentsettings so that the best indications of thunder-storms can be produced depending on the con-ditions encountered.

A tilt control is included so that the antennacan be tiled up and down to gauge the verticalextent of the storm cell. The radar antenna isprotected by a plastic or fiberglass radome whichmust be carefully maintained to prevent adverseeffects on radar performance. The radome often

143


ANTENNA

11 \Tr-I- 088E1A-

WEATHER RADAR RECEIVER/TRANSMITTERWEATHER RADAR DIGITAL INDICATOR

Figure 3-61. The major components of a weather radar system. (Courtesy Canadair Group, Bombardier Inc.)

144


n

ELECTRONS

ANODE

HEATEDCATHODE

CAVITIES

has conducting strips fastened on the outside toconduct static charges and lightning strikes awayfrom the radome. A typical arrangement of lightningdiverter strips is shown in figure 3-64. The radomeshould only be painted with approved types of paintwhich will not interfere with the radar frequencysignals that must pass through the radome.

Personal safety is very important when workingon aircraft with radar systems. Some of the com-ponents in the receiver/ transmitter unit can holdvery high voltages and should be worked on onlyby personnel that are familiar with the necessarysafety precautions. The emissions from the radarantenna can be very hazardous to human beings.The radar should never be turned on while on theground unless special precautions are taken. Themanufacturers maintenance instructions usuallyinclude some information on the MPEL. The max-imum permissible exposure level gives safe distan-ces from aircraft radar antennas. The best procedureis to never walk in front of an aircraft when theradar might be turned on.

N. Stormscope®The Stormscope is a weather avoidance system thatuses completely different methods to locatethunderstorms than a radar system. The Storm-scope is designed to receive the radio frequencyEM waves produced by lightning discharges. It usesa directional antenna system similar to that usedby the ADF equipment. In fact in some cases itis possible to connect the Stormscope to the aircraft

Figure 3-62. A cavity magnetron produces the powerfulSHF band EM waves for a weather radartransmitter.

ADF antennas with special couplers. The directionof the lightning is determined using the directionalantenna and the relative intensity of the dischargeis measured. The intensity is used as pseudo rangeon the display. It is not actual range like that ob-tained from weather radar, but it does give usefulinformation to the pilot. The display instrumentin the cockpit is normally a small round LCD displaythat shows a light dot for each lightning strikethat is detected. From the patterns on the displaythe pilot can determine where the worst areas arelocated and avoid them. Figure 3-65 shows theappearance of a typical Stormscope display instru-ment. The purpose of all weather detection systemsis avoidance. A very strong thunderstorm cell hasthe capability of tearing apart even the strongestof aircraft.

Since the Stormscope and the weather radar reactto different aspects of thunderstorms, the bestweather avoidance system would be to have bothinstalled in the aircraft. Many corporate aircraftin fact do have both systems installed.

0. TCAS — Airborne CollisionAvoidance System

The full meaning of the abbreviation TCAS is trafficalert and collision avoidance system. The prevention

RECEIVER-TRANSMITTER UNIT

WAVEGUIDE

RADAR ANTENNA

Figure 3-63. Waveguide is used to carry the radar fre-quency energy between the R/T unit and theantenna in a weather radar system. (Cour-tesy Piper Aircraft Co.)

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of collisions between aircraft is the primary respon-sibility of the ATC system. This TCAS system wasdeveloped because both the FAA and the airlineswere interested in having a system which woulddisplay collision threats to the pilots. This typeof equipment is being installed at the present timein air carrier jets. About half of them have hadthe equipment installed already. The TCAS equip-ment uses some of the same equipment and prin-ciples as the transponders discussed earlier. TheTCAS equipment on the aircraft includes a com-puter, a display screen in the cockpit and a direc-tional antenna system. The unit sends outinterrogations in all directions around the TCASaircraft. Any transponder equipped aircraft withinrange will send back a reply and the TCAS cal-culates direction, range and altitude of the otheraircraft. Of course it can only determine altitudeif the other aircraft is Mode C equipped. If theother aircraft does not have a transponder, it willnot be detected at all. Each aircraft that has been

detected within a certain range will be displayedas a lighted symbol on the display screen. If theother aircraft gets closer and creates a threat, thesymbol will change color and shape. If the TCASequipment determines that a sufficient danger levelis present, it will display an avoidance maneuvercommand to the pilots. The avoidance maneuverwill be in the vertical plane only, the present equip-ment is not able to suggest turns as avoidancemaneuvers. The pilot will be told on the displayto climb or descend at a certain rate to avoid thethreat aircraft. The general appearance of the cock-pit indicator for a TCAS is shown in figure 3-66.The aircraft symbols are different shapes and colorswith an altitude and climb or descent arrow nextto the symbol. The position of the TCAS equippedaircraft is shown by the airplane symbol at thecenter of the range circle. The installation of TCASin an aircraft normally uses a Mode S transponderand a special type of directional antenna just forthe TCAS equipment.

Figure 3-64. Lightning diverter strips are installed on nose radomes to prevent damage due to lightning strikes andstatic electricity.

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-04

Figure 3-65. The type of display normally used with aStormscope.

Figure 3-66. Simplified example of a TCAS display. Thesymbols for threat aircraft use differentshapes and colors.

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148


CHAPTER IV

Aircraft Antennas and Autopilots

This chapter contains three major topic areas: recom-mendations for installing and inspecting avionicsequipment, additional information concerningaircraft antennas and aircraft autopilot systems.

A. Installation and Inspectionof Avionics

There are a number of factors which make theinstallation of avionics equipment more critical thanthe installation of other types of equipment. Radiosand avionics are very sensitive to electromagneticinterference which can be created by nearby wiringand other electrically operated devices. The instal-lation and maintenance of good bonding jumperconnections is important to ensure proper operationof avionics units. Avionics equipment is easilydamaged by excessive heat which requires thatprovisions be made for adequate air circulation.The most common cause for failures in avionicsequipment is probably overheating. Figure 4-1shows the avionics cooling arrangement for a smallairplane. The tubing and plenums in this type ofsystem must be maintained properly to ensure thatadequate cooling airflow is available to the equip-ment. Other types of avionics cooling systems useelectric motor driven fans, these must be maintainedin good condition to prevent overheating. A thoroughknowledge of these and other important considera-tions is necessary to ensure proper performancefrom installed avionics equipment in aircraft. Therepair of avionics equipment is accomplished byappropriately rated FAA repair stations, but theinstallation, inspection and routine maintenanceon these units is often performed by A&P tech-nicians. Some of the important considerations wheninstalling and inspecting avionics equipment willbe discussed next.

1. Cleaning of Electronic EquipmentCleaning of electronic equipment is important inorder to remove accumulations of dust, dirt andlint that can block cooling holes and cause over-heating. Dirt and lint which collects on open terminalstrips and other electrical connections can absorbmoisture and cause short circuits. Open terminalstrips like those shown in figure 4-2 should becleaned regularly to prevent the accumulation ofdirt and lint that can cause these types of problems.

Electrical connections should be kept clean andfree of corrosion and oxidation which can add un-wanted resistance. When a mild abrasive is neededto remove corrosion and oxidation on terminal stripsand mating surfaces, emery cloth is recommended.

Commutators and slip rings should be cleanedwith crocus cloth or very fine sandpaper.

Older electronic equipment made extensive useof rotary selector switches and similar devices withmany sets of contacts. Spray cans of a special clean-ing solvent are available for cleaning the contactsof these devices. Before using a spray can contactcleaner, you should ensure that it is compatiblewith any plastic or non-metallic parts of theseswitches.

Routing WiresWiring of all types should be routed above linesthat carry fluids and clamped securely to the aircraftstructure. The wires should be routed so as toprevent abrasive damage from control cables,mechanical linkages and other moving parts in theaircraft. Frequent clamps and ties should be usedto prevent excessive wire movement due to in-flightvibration and other factors. The proper installationof clamps to prevent excessive wire movement isillustrated in figure 4-3. Where wires terminate atpieces of equipment, enough slack should be leftafter the last clamp to allow for shock mount mo-tions. If wires are clamped tightly too close to thetermination point, normal aircraft motions andvibration will put bending loads on the wire con-nectors and cause premature failure in the wires.

Another factor to consider when routing wiringfor electronic equipment is electromagnetic inter-ference (EMI). Antenna leads and other wiring sen-sitive to EMI should be routed away from the wiresfor inverters, power supplies, strobe lights, motorsand other components that are known to causeinterference. When troubleshooting a noise or in-terference problem in aircraft radios and sensitiveelectronic equipment, it is often necessary to reroutewires away from the source of the EMI once ithas been located.

Switches and Circuit BreakersThe radios on aircraft are usually connected to anavionics master switch. This switch is separate from

149


the normal master switch as shown in figure 4-4,This is a useful feature which makes it easier forthe pilot to ensure that all the radios are turnedoff when starting and stopping the engine. Theradios should be turned off when the engine isstarted and stopped in order to prevent damage

caused by surge currents and spikes of high voltagethat can occur during engine starts and engineshut down. When installing switches in aircraft cir-cuits, the rating of the switch must be adequateto handle both the type and amount of currentand voltage for the circuit. Figure 4-5 shows a typical

Figure 4-1. Avionics cooling installation for a small airplane that uses vents on the sides of the fuselage to move airacross the radios.

150


[ el

1/2-INCH MAXIMUM WITHNORMAL HAND PRESSURE

Figure 4-2. Open terminal strips should be kept cleanand free of corrosion. Check for metal ob-jects that could fall across the terminals andcause shorts.

aircraft toggle switch. Whether the circuit is ACor DC makes a significant difference in the properselection of switches.

For example, there is a common aircraft qualityswitch that is rated for 10 amps at 125-250 voltsAC. The same switch is rated at 0.3 amps whenit is used in a DC circuit up to 125 volts DC.If this switch was installed in a 10 amp DC circuitthe points would quickly burn and fail. The reasonfor this difference in ratings is that the currentin an AC circuit drops to zero twice each cycle,this greatly reduces the problem of arcing as thepoints in the switch are opening. The proper ratingof a switch in terms of both current flow and AC

Figure 4-3. Wires and antenna leads should be supported with proper clamps and ties.

Figure 4-4. An avionics master switch supplies electrical power to the avionics bus. It should be off when starting andstopping the engine.

151


PUSH-TO-RESET PUSH-PULL-TYPE TOGGLE-TYPECIRCUIT BREAKER CIRCUIT BREAKER CIRCUIT BREAKER

(A) (B) (C)

versus DC rating is very important to ensure ade-

quate performance and service life. The conditionof switches can be checked during an inspectionby operating the switch and checking the "feel"during operation. Many switches have over-centermechanisms and other devices that produce a dis-tinct feel to the switch. When the switch is gettingworn and ready to fail, it often starts to feel sloppyin operation.

Circuit breakers for aircraft circuits should bethe "trip free" type. This means that the circuitbreaker cannot be overridden by holding it in theengaged position. It will open the circuit regardlessof the position of the control toggle or push button.Various types of circuit breakers are available asshown in figure 4-6 and the correct selection ofcircuit breaker ratings for the particular circuitis important to prevent dangerous overloads inthe aircraft's circuits. During inspections, theproper operation of the circuit breaker should be

Figure 4-5. Switches used in aircraft circuits shouldhave the appropriate AC or DC rating toprevent premature failures.

Figure 4-6. Circuit breakers should be the trip-free typeand inspected regularly for proper operation.

determined. Most types can be manually openedto interrupt current flow. Even a small generalaviation airplane may have a large number ofswitches and circuit breakers as shown in figure4-7. These should be inspected for proper operationand for any abnormal "feel" which could indicatean impending failure.

4. Bonding and Shielding

Radio reception can be completely blocked or severe-ly interfered with by improper bonding and shieldingin the aircraft. The source of the noise interferencethat affects aircraft radios is both inside and outsidethe aircraft. Outside interference comes fromprecipitation static and thunderstorms. Inside in-terference can be produced by current flow in othercircuits and EMI emitters like ignition systems. Theproper installation and maintenance of bondingjumpers is a key factor in preventing radio inter-ference. Both braided wire bonding jumpers andthin metal straps are used for bonding connections.An installation of a braided bonding jumper ona shock mount is shown in figure 4-8. All partsof the aircraft that could create noise problemsshould be bonded. Electrical equipment that isshock mounted should have adequate bondingjumpers to carry the ground path current withoutproducing excessive voltage drop.

When the bonding jumpers carry ground pathcurrents, always use more than one. If there isonly one and it breaks, the radio or other pieceof equipment will be inoperative. When attachingbonding jumpers all dirt, grease, paint and coatingssuch as anodizing should be removed to ensuregood electrical contact. A heavy bonding jumperfor installation on airframe parts is shown in figure4-9. The general rule is that the maximum resistancefor a bonding jumper connection should be .003ohms. The FAA in AC 65.15 does state that if abonding jumper is only used for static electricitypurposes and does not carry ground path currents,0.01 ohms maximum is acceptable. Bondingjumpers accomplish a number of different functionson aircraft, some of these are listed below.

Supply the ground path for current flow forelectrical equipment, especially shockmounted equipment.

Reduce radio interference.

Decrease possibility of lightning damage (atcontrol surface hinges, for example).

Allow static charges to equalize between dif-ferent parts of the airframe. This can reducethe fire hazard caused by arcing near fueltank vents, etc.

152


MAIN BUSISOLATIONCIRCUITBREAKERS

AVIONICSCIRCUITBREAKERS(TYPICAL)

MAIN BUS TIE CIRCUITBREAKER EMERGENCY

AVIONICSPOWERSWITCHES

LIGHTING EQUIPMENTSWITCHES

ICING EQUIPMENTSWITCHES

AUXILIARY PHONEAND MIKE JACKS

GENERALCIRCUITBREAKERS(TYPICAL)

AVIONICS POWERSWITCHES

STARTERSWITCHES

PRIMERSWITCH

EMERGENCYALTERNATORFIELD SWITCH

ALTERNATOROUTPUT CIRCUITBREAKER(60-AMP SYSTEM)

ALTERNATOROUTPUT CIRCUITBREAKERS(95-AMP SYSTEM)

Figure 4-7. Typical circuit breaker panel for a twin-engine airplane. (Courtesy Cessna Aircraft Co.)

153


A number of factors should be kept in mind when

installing and inspecting bonding jumpers. Somerecommendations concerning bonding jumpers are:

Bonding jumpers should be as short as pos-sible (however, allow for any necessary motionas with a control surface).

Do not solder bonding jumpers. It makes thembrittle and they break.

Do not paint bonding jumpers. This also makesthem brittle.

Ensure good contact by removing dirt, grease,paint and other coatings.

Use compatible hardware to prevent corrosion.

Use compatible bonding jumpers (aluminumalloy for aluminum alloy structures and copperor brass jumpers for parts made of steel, stain-less steel, brass or bronze).

Figure 4-8. Bonding jumpers on shock mounts mustallow freedom of movement on the shockmounts and should be inspected regularly todetect breakage or corrosion.

Figure 4-9. Heavy duty bonding straps are often re-quired for bonding of major airframe com-ponents.

Shielding is an important part of noise suppres-

sion for aircraft radios. Shielding can be applied

at the source of the noise or at the componentor circuit that is sensitive to EMI. Shielding consistsof a metal outer cover for a wire or component.Electromagnetic fields that could cause interferenceare captured in the metal cover and sent to ground.The ignition system of an aircraft engine can produceserious interference and so all parts of the ignitionsystem need to be shielded.

On a reciprocating engine, for example, the mag-neto, ignition wires, spark plugs and "P" lead needto be shielded as illustrated in figure 4-10. Themagneto and spark plugs are shielded by beingmade with a metal housing or outer cover. Theignition wires use an outer wire braid shielding.The primary or "P" lead is the wire that connectsthe magneto to the cockpit ignition switch. It shouldbe a shielded wire to prevent noise. If all partsof the ignition system have been shielded and ig-nition noise is still present, it may be necessaryto install a filter capacitor on the magneto. Thisis a condenser or capacitor of the correct size whichwill help to filter out noise at the source. Otheraircraft components may require filters also, suchas certain motors and power supplies.

Under certain circumstances noise and inter-ference can be caused by the shielding on electricalwiring. The use of shielded wires can sometimesresult in a phenomenon known as ground-loop in-terference. This ground-loop problem is illustratedin figure 4-11.

Circuit A in figure 4-11 uses a shielded wirewith the shielding grounded at both ends. CircuitB is a single-wire circuit with ground connectionsat both ends. There is nothing to prevent the groundpath currents for circuit B from flowing along thewire shielding of circuit A. Depending on the typesof electrical signals involved, groundloops can causeinterference between different circuits in theaircraft. The way to prevent groundloop problemsis to leave one end of the shielding "floating" orungrounded. If one of the grounds for the shieldingin circuit A was disconnected, currents for circuitB could not use the shielding as a current path.Special precautions are recommended in AC 43.13-2A for the installation of inverters to prevent thesekinds of problems. The recommended proceduresto prevent inverter interference are:

Install inverters in separate areas, away fromsensitive electronic circuits.

Separate the input and output wires of theinverter.

3. Properly bond the inverter case to the airframe.

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SHIELDEDIGNITIONHARNESS

SHIELDEDSPARK PLUGLEAD

SHIELDEDSPARK PLUG

MAGNETO

IGNITIONSWITCH

SHIELDEDP LEAD

CIRCUIT "A" WITH SHIELDED WIREGROUNDED AT BOTH ENDS

CIRCUIT "B" WITHAIRFRAME GROUNDS

Figure 4-10. All parts of the ignition system should be shielded to prevent radio noise.

Figure 4-11. Example of how "ground loop" interferencecan occur.

Figure 4-12. Static wicks are installed on the trailingedges of the flight control surfaces to helpremove static charges in flight in order toprevent noise in the radios.

4. Use shielded wires for inverter output wiresand ground the shielding at the inverter endonly.

A number of items of aircraft equipment can createspecial interference problems, examples include in-verters, motors, strobe lights, rotating beacon lightsetc. Sometimes trial and error is necessary toeliminate noise and interference problems. The useof shielded wires and physical separation are basictechniques that can be used to prevent or eliminatenoise and interference between different aircraftsystems and equipment.

5. Static DischargersA common cause of noise in aircraft radios andrelated equipment is P-static interference. Precipita-tion or P-static noise is caused by static electricitythat builds up on an aircraft in flight. The staticelectricity is produced by friction and can buildup to 80,000 volts or more under certain conditions.Friction between the metal skin of the aircraft andparticles in the atmosphere is a common causeof P-static. Flying through rain, snow, ice or evendust particles can result in a static charge on theairframe. The exhaust stream of a turbine engine

155


STABILIZER TIP WICK

can cause static electricity due to friction betweenparticles in the exhaust and the metal tailpipe.P-static is a greater problem for high speed aircraftbecause higher speeds produce more friction andhigher charges. High speed aircraft usually requiremore static dischargers to reduce the static chargeon the aircraft. Static dischargers are small devicesfastened to the extremities of the aircraft that aredesigned to discharge the aircraft to the atmosphere.They are commonly installed on all classes of aircraftthat operate IFR and require all weather radio

reception. Static dischargers are fastened to thetrailing edges of the primary flight control surfaces:

ailerons, elevators and rudders. A recommendedinstallation of static dischargers for a small airplaneis shown in figure 4-12. High speed aircraft mayhave additional static dischargers on the outboardtips of the wing and horizontal stabilizer. The staticdischargers reduce the threshold for discharge tothe atmosphere so that the voltage on the aircraftis reduced. The locations of the static dischargersfor a business jet are shown in figure 4-13.

STRAIGHT WICK

Figure 4-13. The installation of static dischargers for a corporate jet airplane. (Courtesy Canadair Group, Bombardier Inc.)

156


RETAINER

NULL FIELD TYPEDISCHARGES

BLADE

PINSBLADE

SET SCREW

WING AND EMPENNAGETIP STATIC DISCHARGER

TAIL SECTION TYPICALINSTALLATION POINTS

Corona is a term associated with P-static chargeson aircraft. Corona refers to the glow that is some-times visible on the extremities of the aircraft whenstatic electricity is discharging to the atmosphere.St. Elmo's Fire is an older term that means thesame thing. The installation of static dischargerswill eliminate or reduce corona by controlling thedischarges to the atmosphere.

There are three basic types of static dischargersused on aircraft: static wicks, wire braid dischargersand null field dischargers. The static wick is alsocalled a flexible static discharger and is found onlow speed aircraft. It consists of a plastic tube orouter covering with a fabric braid inside. The innerbraid can be cotton, nylon, or some other material.The inner braid extends beyond the plastic coveringwhere it is fanned out to produce the dischargepoints. The FAA recommends that one inch of theinner braid should extend beyond the outer cover.

When they become worn, they can be retrimmedto this dimension until they become too short andmust be replaced. The inner braid of a static dis-charger is designed to have some built-in resistanceto control the discharge current and further reducenoise. The wire braid static dischargers are alsocalled the semi-flexible type. This type is simply

a piece of wire braid made of stainless steel wiresas shown in figure 4-15. The wire braid does nothave any built-in resistance, so this kind is notas effective as the other two kinds of static dis-chargers. Jet airplanes normally use the null fielddischarger which is more rugged than the othersfor high speed aircraft use. The null field dischargerconsists of a rigid shaft made of fiberglass or com-posite materials with very sharp metal points atthe aft end. The metal points are sometimes madeof tungsten for longer life. Static wicks and nullfield dischargers are illustrated in figure 4-14. Asshown in figure 4-14, the metal points of the nullfield dischargers are at right angles to the directionof flight. This feature helps to further reduce noisecompared to the other kinds of static dischargers.

Static dischargers should be maintained properlyto ensure that they will perform their intended func-tion. The attachment to the aircraft must be tightand with good electrical contact. Any corrosion orlooseness at the attachment point can create noisein the radios. Damaged or badly worn static dis-chargers should be replaced with new ones of theapproved type. The noise produced by P-static affectsthe frequency bands of HF and below more thanthe higher frequency bands. If the pilot complains

TRAILING EDGESTATIC DISCHARGER

STATIC WICK TYPE DISCHARGER

Figure 4-14. The Null Field and Static Wick types of static dischargers.

157


DOUBLERSHOCKMOUNT

RADIORACK

FLOORING

MACHINESCREW

PLATENUT

of noise on a radio system that operates at HFor below, the static dischargers should be inspectedto determine if the noise is P-static related.

6. Installation Methods

The installation of electronic equipment and radioequipment follows some of the same basic practicesthat are used for other equipment, but special pro-cedures may be required to prevent interferenceor other problems that especially affect these typesof aircraft systems. The specific instructions of themanufacturer should always be followed when avail-able. Some general recommendations from AC43.13-1A and -2A will be described here along withsome precautions that should be observed for alltypes of installations.

a. General Precautions

If a standard location and mounting rack is availablefrom the aircraft manufacturer, it should be usedto install items of equipment. A standard type ofshock mounted installation for electronic equipmentis shown in figure 4-16. If this is not available,the installer will have to determine the best locationand means of mounting for the equipment. Someof the factors that should be considered whenmaking this type of determination are:

Sufficient air circulation to prevent overheat-ing. This might require a certain free air spacein some cases and the installation of a coolingfan in other cases.

Adequate clearance from high temperaturesand flammable materials (next to a combustionheater would not be good place to install aradio).

3. Protection from water, fumes, hydraulic fluid,etc.

Figure 4-15. The semi-flexible wire braid static dis-chargers do not have any built-in resistance.

Protection from damage by baggage or seatdeflection.

Sufficient clearance to prevent rubbing orstriking aircraft structures, control cables,movable parts, etc.

Preventing interference and noise. Separatesensitive electronic equipment from inverters,power supplies, strobe lights, motors, etc.

If shock mounts will be used, ensure thatthe equipment does not exceed the weight car-rying capability of the shock mounts and installadequate bonding jumpers or straps.

b. Static Loads

Whenever it is necessary for the installer to fabricatea mounting for aircraft equipment, the strengthof the mounting should be verified with a load test.An example of a fabricated mounting for aircraftequipment is illustrated in figure 4-17. The equip-ment installed in aircraft must be able to withstandthe acceleration forces or "G" loads that are ex-perienced in flight. In a steep turn, for example,the additional "G" load is felt by the equipmentin the aircraft as well as by the wings and otherstructures.

A simple example of how a static load test mightbe performed will be explained here. The load factorsfor the test can be obtained from AC 43.13-2Awhich has a table similar to figure 4-18.

We will use the example of a radio that weighs5 lbs. and will be installed in the baggage com-partment behind the rear seats of a Normal categoryairplane. The mounting that is fabricated to hold

Figure 4-16. A standard mounting rack for avionicsequipment that includes shock mounts.

158


2

3

1—BULB ANGLE

2—STIFFENING FLANGE OR ANGLEAT ENDS OF PLATFORM

3—REINFORCEMENT ANGLE FOR BULKHEAD

CERTIFICATION CATEGORY OF AIRCRAFT

DIRECTION OFFORCE APPLIED

NORMAL./UTILITY ACROBATIC ROTORCRAFT

SIDEWARD 1.5 Gs 1.5 Gs 2.0 Gs

UPWARD 3.0 Gs 4.5 Gs 1.5 Gs

FORWARD* 9.0 Gs 9.0 Gs 4.0 Gs

DOWNWARD 6.6 Gs 9.0 Gs 4.0 Gs

*When equipment mounting is located externally to one side, orforward of occupants, a forward load factor of 2.0 g is sufficient.

MACHINE SCREWS ANDSELF-LOCKING NUTS

REAR CASESUPPORT

RIVETS OR MACHINE SCREWSAND SELF-LOCKING NUTS

the radio would be tested by applying loads equalto the weight of the equipment multiplied by theappropriate load factor. The sideward test load wouldbe 7.5 lbs., upward load 15 lbs., forward load 45lbs. and the downward load 33 lbs. If a locationwas chosen that was in the nose section of theaircraft and forward of all occupants, a forward testload of 2.0 Gs or 10 lbs. would have been sufficient.The mountings for aircraft equipment must be ableto withstand the appropriate level of accelerationforces or load factors that might be experiencedin flight. Standard industry practices for rivets, bolts,screws, etc. would be followed to ensure that ade-quate levels of strength are provided in the fastenersand in mounting brackets and similar parts. The

Figure 4-17. A fabricated mounting unit for avionicsequipment.

Figure 4-18. Static test load factors that could be usedfor testing equipment mountings and at-tachments.

FAA in AC 43.13-2A recommends the use of machinescrews and anchor nuts for the removable fastenersto hold aircraft radios in place. Where possible, ex-isting plate nuts should be used or new ones in-stalled. If that is not practical, then machine screwsand self-locking nuts can be used.

When radios or other equipment are installedin an instrument panel and the item will extendsome distance behind the instrument panel, a braceor support should be installed to the side or backof the equipment to minimize the load on the in-strument panel itself. An example of a rear bracefor an item of equipment installed in an instrumentpanel is shown in figure 4-19.

B. Antenna InstallationsThe antennas found in aircraft radio installationsare critical to the proper operation of the radiosystem. Antennas must be carefully installed andmaintained in order to provide the efficiency thatis needed for good radio reception and transmission.There are many factors that can affect the efficiencyof aircraft antennas. An A&P technician should befamiliar with the basic factors that affect the properoperation of an antenna. The inspection and main-tenance of aircraft antennas is part of an A&P tech-nicians responsibility in most cases because theantennas are fastened to the skin or other structureof the aircraft. Some of the key concepts that affectantenna operation will be covered in this section.

Figure 4-19. Example of the installation of a rear brace orsupport for radio equipment installed in aninstrument panel.

159


FORWARD POWER

RADIOTRANSMITTER

RF OUTPUT

RADIOTRANSMITTER

RF OUTPUT

1. Standing Wave RatioThe standing wave ratio is a measure of the efficiencyof an antenna installation. The standing wave ratio(SWR) is also referred to as voltage standing waveratio (VSWR). In order to demonstrate the principleof VSWR, we will consider what would happen ifjust one sine wave was sent down a transmissionline from a radio transmitter. Figure 4-20(A) showsthis sine wave traveling from left to right. If the trans-mission line had an infinite length, the sine wavesignal would eventually be reduced to zero by lineresistance. In an actual installation, the transmissionline is of a limited length and terminates at the an-tenna. The purpose of the antenna is to transform

the sine wave signal into radio waves, but this cannever be accomplished with 100 percent efficiency.The result of this less than perfect efficiency is thatsome of the energy is reflected back toward the trans-mitter from the antenna end of the transmission line.This is illustrated by figure 4-20(B) which shows someenergy being reflected back toward the transmitterand moving from right to left. The output of thetransmitter is not just one sine wave at a time, buta continuous series of sine waves. The reflected waveswill combine with the transmitter output waves toproduce standing waves on the transmission line asindicated by figure 4-20(C). A calculation based onthe relationship between forward power and reflected

REFLECTED POWER

RESULTANT STANDING WAVE(C)

RADIOTRANSMITTER

RF OUTPUT

Figure 4-20. Illustration of the principle involved in the standing wave ratio for an antenna installation.Forward power.Reflected power.

(C) Resultant Standing Wave.

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SPECIFICATIONS

VSWR 2.0:1IMPEDANCE 50 ohmsPOWER 40 wattsWEIGHT 0 2 lbs.CONSTRUCTION WhipHEIGHT 14.0 in.ELEMENT Open

CENTERCONDUCTOROUTER

JACKET

SHIELD (OUTERCONDUCTOR)

DIELECTRIC

SINGLE SHIELD

power gives the voltage standing wave ratio, whichis a measure of efficiency. With a perfect antennainstallation, there would be zero reflected power andthe VSWR would be 1:1 or simply 1. In an actualaircraft antenna system, the lowest VSWR is the mostefficient. Typical values for VSWR of aircraft antennasare in the range of 1.1 to 5.0 for the various typesof antennas. Manufacturer's catalogs usually list theVSWR for antennas so that the relative efficiencyof different types can be compared when selectingan antenna. The listing of VSWR in the specificationsfor an aircraft antenna can be seen in figure 4-21.

The example given here was for the VSWR of atransmit antenna, but the manufacturer's data alsolists the VSWR for receive only antennas. If the trans-mission line or coaxial cable that connects the antennais in good condition and properly suited to the in-stallation, the VSWR is affected by the antenna itself.However, if there is a fault in the coaxial cable theVSWR will go up significantly, which reduces efficien-cy. Special types of wattmeters and VSWR meterscan be used to measure the VSWR of an aircraftantenna installation for troubleshooting purposes.

2. Coaxial Cables and Connectors

Coaxial cables are required for the antenna connec-tions on most aircraft radios because of the RF

Figure 4-21. Antenna manufacturers catalogs usuallylist the V.S.VV.R. for each antenna. (Courtesyof Dayton-Granger, Inc.)

frequencies that are used. A coaxial cable is shownin figure 4-22. The proper installation and main-tenance of coaxial cables is very important since largelosses can occur if a fault is present. Coaxial cablesshould be rejected if they have become dented orif kinks are found. Any distortion or crushing whichcauses the cable to be oval in shape or flattenedare also cause for rejection. If abrasion or rubbinghas exposed or damaged the wire braid, the cableshould be replaced. Coaxial cable should be supportedby clamps about every 2 ft. to help prevent damage.A good rule of thumb for coaxial cable bend radiusis to use a minimum bend radius of 10 times thecable diameter. This will help to reduce the possibilityof kinks from sharp bends. Special types of end con-nectors are used with coaxial cable and they comein a number of different styles. Some can be removedand reused and other types are crimped or swagedon and cannot be reused. When installing and remov-ing coaxial cable connections, care should be usedto prevent damage to the connectors. If corrosionis found on connectors, it is usually better to replacethem rather than try to clean them. Even smallamounts of corrosion or corrosion pits can causea signal loss. Figure 4-23 shows a reusable coaxialcable connector. When installing this type, the wirebraid should be carefully spread out over the braidclamp and breakage of the wires should be avoided.The connector should be assembled carefully to pro-vide tight connections with good electrical contactand to avoid distorting the coaxial cable or the con-nector itself. If it is necessary to solder a connectorpin onto the center conductor, only an approvedelectrical solder should be used—never use acid coresolder or acid flux on electrical connectors. Anacceptable solder is 60/40 rosin core solder. Greatcare must be used in soldering to prevent excessiveheat damage to the coaxial cable insulation materials.

Some antenna cables are matched to the radioand antenna and should not be shortened or spliced.

Figure 4-22. The parts of a coaxial cable for antennas.

161


L. -1 1 / 4

DO NOT BREAK STRANDS

I

DO NOT NICK CENTER CONDUCTOR

-1/8NUT WASHER GASKET CLAMP

TRIM STRANDS WITH SC SSORSFLUSH WITH END OF TAPER

L.-- 3/32CONTACT FLUSH WITH END OF INSULATOR

A Atir..1SOLDERCONNECTION

16 COPPERWELDANTENNA WIRE

TO RECEIVER

UNDER SURFACE OF AIRPLANE

6"MINIMUM

This is true for some ADF antenna leads, for example.On other installations the antenna coax should bekept as short as possible and routed as directly aspossible to reduce line loss.

The specific antenna or radio manufacturers in-stallation instructions should be followed carefullyin this area since there are many different proceduresthat may apply depending on the specific installation.

Figure 4-23. Installation procedure for a reusable coaxialcable connector.

3. Wire AntennasA wire antenna is a length of wire that is supportedby masts and attachments above or below the aircraftfuselage. They are found most often on smaller aircraftand older aircraft. Jet airplanes seldom use wire an-tennas because of the vibration and increased chanceof damage at high speeds. The type of wire usedis most often a copper coated steel wire that is asolid single strand. Wire with an outer covering ofinsulating material is superior to non-insulated wirein reducing noise caused by P-static.

A type of wire antenna that is seldom used todayis the trailing wire antenna. The trailing wire antennawas a roll of wire on a drum in the aft fuselagewhich could be extended out the back of the aircraftin flight. It was very common in the 1930s and1940s for HF communications radios. The advantagewas that 200 ft. or more of wire could be extendedout the back of the airplane for better radio per-formance. The disadvantages were the added com-plexity and weight of the mechanism to extend andretract the antenna. It is not suitable for high speedaircraft and is rarely used on modern aircraft.

The wire type marker beacon antenna is shownin figure 4-24. This type may still be found on smallairplanes. It is about 4 ft. long and fastened to standoffand support masts on the bottom of the aircraft.The minimum fuselage separation should be 6 inches.

A long wire antenna for HF communications isstill commonly used on general aviation aircraftthat have HF equipment. Figure 4-25 shows a typicalinstallation with the wire running from a wing tipto the vertical fin and then down to a feed-throughon the top of the fuselage. The long wire antennaincludes a tensioning device to maintain the propertension on the wire and insulators at the appropriatepoints. A long wire antenna normally employs aweak point at the aft end so it cannot wrap aroundthe aircraft if it breaks due to excessive tension.

Figure 4-24. A wire-type marker beacon antenna.

162


LEAD—THRU INSULATOR25ARM300-20-30.

WIRE 14407

The most common use of wire antennas onmodern aircraft is as an ADF sense antenna. Thesewill be described in the next section.

4. ADF Antennas

All aircraft ADF receivers require two antennas, theloop antenna and the sense antenna. The loop antennais the directional antenna and the sense antennais needed to eliminate the ambiguity caused by thetwo nulls in the reception pattern. Air carrier jetshave an ADF antenna that combines the loop andsense antennas in one housing that is a low profileor flush mount and it is installed on the top or bottomof the fuselage. The ADF antennas on general aviationaircraft come in a greater variety and are most oftenseparate loop and sense antennas.

The loop antenna that is rotated by an electricmotor is still used, but is being replaced by the typethat rotates the signal rather than the antenna itself.A motor driven loop antenna for installation insidea housing is shown in figure 4-26. The newer non-rotating types are usually contained in a teardropshaped streamlined housing that installs on the topor bottom of the aircraft as shown in figure 4-27.The sense antennas used with the dual antenna

installations are either the wire type or whip type.The whip type sense antenna is a metal rod about4 ft. long and installed on the top or bottom. It isstill found on some helicopters were there isn't enoughroom for a long wire sense antenna, this is shownin figure 4-28. The long wire sense antenna is about15-20 ft. long and most often installed using thevertical fin as the aft anchor point to gain more fuselageclearance. The recommended minimum clearancefrom the fuselage is 12". A top installation for a sensewire is shown in figure 4-29. The sense wire canbe installed on the bottom as shown in figure 4-30if adequate ground and fuselage clearance can beobtained. Like the long wire HF antenna, the ADFsense wire will use masts, tension units and weaklinks as part of the installation.

In order to give accurate navigational information,ADF antennas must be installed and calibratedcorrectly. The loop antenna normally needs to beinstalled close to the electrical center of the senseantenna to give accurate indications of station pas-sage. This relationship is illustrated in figure 4-31.

Both the loop and sense antennas can be installedon the top or the bottom, but they must have the

STRAIN INSULATOR10ARM300-13CN

I 5 TA RA S3I80\1 UNIT - 1

0WING TIP BRACKET 3254

V-TENSION UNIT 5ARM300 -3/13111111111111U— Nowest MbYrI

VERTICAL FIN ANCHOR KIT-3280

Figure 4-25. A long wire HF comm antenna installation. (Courtesy Dayton-Granger Inc)

163


INTERNAL LOOP

ENCLOSED LOOP

TRANSPONDER

VHF COMMNo. 2

VHF COMMNO. 1

GLIDESLOPE

ADF SENSE

MARKERBEACON

DME ADF LOOP

correct relationship to each other for accurate read-

ings to be obtained. Since the ADF antenna systemis highly directional, it must be calibrated to givethe correct indications of ground station direction.

Figure 4-26. A motor-driven ADF loop antenna for inter-nal installations.

Figure 4-27. A non-rotating, teardrop-shaped ADF loopantenna.

This is called the check for quadrantal error or the

calibration check. Whenever an antenna is installed

or any change is made which could affect the ac-curacy of the ADF, a check for quadrantal errorshould be performed. The checks can be made onthe ground, but should always be confirmed witha flight check. To perform the ground check a nearbyNDB of known location is tuned in and the bearingis checked and adjusted at least every 45° as theaircraft is turned on the ground. The flight checkinvolves locating geographical points on the groundwith known bearings from the NDB and flying theaircraft over those locations to confirm the accuracyof the ADF bearing information. This flight checkshould be performed at low altitude to reduce errorsin established the aircraft position accurately.

5. Groundplane ConsiderationsWhen a 1/4 wave, Marconi-type antenna is installedon an aircraft, an adequate groundplane or counter-poise is required for proper operation. The aircraftsystems that use 1/4 wave antennas are VHF com-munications, ATC transponder, DME and UHFradiotelephone. When these antennas are installedon metal skinned aircraft, the metal skin suppliesthe groundplane. If the antenna is installed too closeto fiberglass areas or windshields, the groundplanearea is reduced and may result in poor performance.A basic rule of thumb that is sometimes used isthat the groundplane should extend in all directionsoutward from the base of the antenna a distanceequal to the height of the antenna. A shorter antennadoes not need as much groundplane as a longerantenna. The groundplane cannot be too big, butit can be too small which has an adverse effect onsignal pattern and strength. For DME andtransponders, which use similar frequencies, thegroundplane should extend 8-12" in all directions

Figure 4-28. Helicopter antenna installations are difficult because of limited skin area and limited ground clearance.

164


VERTICAL FINANCHOR KIT-3208

V-TENSION UNIT5ARM300-3

ADFSENSE

WIRE

ADFTEARDROPLOOP

WHIPANTENNA

METAL FOIL UNDERFABRIC OR WOOD SKIN

NOTE: THE LENGTH OF EACH FOIL RADIALSHOULD BE AT LEAST EQUALTO THE ANTENNA LENGTH.

from the antenna base. For VHF communicationsantennas a groundplane that extends 24" in all direc-tions is desirable. These sizes would give agroundplane that is a little larger than if the lengthof the antenna was used as the dimension. It isnot always possible to supply a large enoughgroundplane when installing antennas on aircraft withlimited metal skin area such as small helicopters,but the groundplane area should always be consideredand provided for to the extent possible.

If it is necessary to install these types of antennason aircraft with non-metal skin, a groundplane mustbe provided by the installer. This usually meansinstalling metal foil strips or wire mesh fastenedon the inside of the aircraft covering. The samerules would apply as to desirable lengths. An exampleof the use of a foil strip groundplane is seen infigure 4-32.

Figure 4-29. Top-mounted ADF sense wire antenna.(Courtesy Dayton-Granger Inc.)

Figure 4-30. Bottom-mounted ADF sense wire antenna.(Courtesy Dayton-Granger Inc.)

When installing 1/4 wave antennas, it is recom-mended that all grease, dirt and paint be removedfrom the skin area under the base of the antenna.Some avionics experts recommend that a gasket notbe used so that the base of the antenna contactsthe skin of the aircraft. Whether or not a gasketis used, the skin should be cleaned and strippedand a sealant applied around the base of the antennaafter installation.

The installation of antennas to the skin of aircraftrequires that some additional reinforcement be given

Figure 4-31. The ADF loop antenna should normally beinstalled near the electrical center of thesense wire antenna.

Figure 4-32. When installing Marconi antennas on anaircraft with non-metal skin, a groundplanemust be provided.

165


ANTENNA

FUSELAGE SK IN

EXISTING STRINGERS

VIEW A-A

REINFORCING DOUBLER

ALCLAD 2024-13

1 1/2"EDGE DISTANCE MIN.

APPROXIMATELY ONEINCH SPACING OF1/8" MIN. DIA. RIVET

ELECTRICALFIELD

MAGNETICFIELD

to preserve the strength of the aircraft structure.The use of a doubler as shown in figure 4-33 will

reinforce the aircraft structure and provide the ad-ditional support needed for antenna drag loads.

6. Reducing Antenna InterferenceA very important factor in the proper performanceof aircraft antennas is the prevention of interferencebetween one system and another. Interference canalso occur between a radio system antenna and othercomponents of the aircraft. A basic consideration isthat antennas for systems that operate on similarfrequencies must be separated by a certain minimumdistance to prevent interference. The possible inter-actions that can adversely affect aircraft radio systemsare many and varied. The more common problemsthat can occur will be described here, but sometimesa particular interference problem may require trial

Figure 4-33. A reinforcing doubler should be installedinside the skin at the base of the antenna.

and error to eliminate the cause of the antennainteraction.

The important factors that affect mutual inter-ference are frequency and wavelength, polarizationand type of modulation. The operating frequenciesfor the various radio systems are listed in the fre-quency chart in chapter 3. The polarization of radiowaves is based on the orientation of the electricfield relative to the earth's surface. The field orien-tations for vertical and horizontal polarization canbe seen in figures 4-34 and 4-35. The antennainstalled on the aircraft needs to have the properpolarization relative to the ground based antennafor optimum performance—particularly at frequen-cies above HF. Figure 4-36 gives the polarizationfor the various types of aircraft radio systems.

From the information in figure 4-36, it can beseen that all the systems use vertical polarizationexcept for VOR and the three parts of the ILS in-strument approach system.

a. VHF Communications Antennas

Aircraft that are equipped for IFR operations com-monly have 2 or 3 separate VHF comm radios whichutilize separate antennas. The VHF comm antennasshould be separated from each other by at least 5ft. This is easily accomplished on an air carrier jetwhich has a lot of fuselage skin area available, butmay be difficult on small aircraft which have muchless available skin area. Figure 4-37 shows the an-tenna locations for a Boeing 767 with good separationbetween similar systems. The VHF comm antennasuse vertical polarization and require a suitablegroundplane. When two antennas are installed onsmall aircraft, the best coverage is usually obtainedwith one antenna on the top and the other on thebottom of the fuselage. This desired top and bottomseparation is shown in figure 4-38 on a twin-engineairplane. The ELT antenna can cause serious in-terference with VHF comm and should be separatedby at least 5 ft. from any VHF comm antenna. Radio

Figure 4-34. When an EM wave has vertical polarization,the electric field is in the vertical plane.

166


MAGNETICFIELD

ELECTRICALFIELD

RADIOSYSTEM

RECEIVE,TRANSMITOR BOTH

POLARIZATION

LORAN C

ADF

VHF COM

DME & TRANSPONDER

ELT

VOR & LOCALIZER

MARKER BEACONS

GLIDESLOPE

RECEIVE

RECEIVE

BOTH

BOTH

TRANSMIT

RECEIVE

RECEIVE

RECEIVE

VERTICAL

VERTICAL

VERTICAL

VERTICAL

VERTICAL

HORIZONTAL

HORIZONTAL

HORIZONTAL

interference can be caused by parts of the aircraftas well as by other antennas. The vertical fin of anaircraft can cause significant signal blockage to anyVHF comm antenna that is installed too close. Atop mounted VHF comm antenna that is installedcloser than 5 ft. to the vertical fin will result in blockageand poor radio reception and transmission to therear of the aircraft. The VHF comm antenna is a1/4 wave Marconi antenna which must have an ade-quate groundplane or counterpoise for proper opera-tion. A common mistake is the installation of a VHFcomm too far forward on the upper fuselage. If itis less than 24" from the top of the windshield, thesignal pattern can be distorted by the lack ofgroundplane in the forward direction.

DME and Transponder Antennas

These two antennas are treated as equals becausethey use similar frequencies, polarization and modula-tion. The antennas used for these two systems are1/4 wave Marconi antennas with vertical polarizationand they both transmit and receive. Since thewavelength is shorter at higher frequencies, the min-imum separation distance is less than that for VHFcomm antennas. The DME and transponder antennasshould be separated from each other by at least 2ft. and an adequate groundplane must be providedaround the base of the antenna. These antennasare normally installed on the bottom of the aircraftto prevent signal blockage by the fuselage. A topmounted antenna may be used on a narrow portionof the aircraft that will not cause significant blockage.The top of the tail boom on a helicopter can be anacceptable location.

VOR and Localizer

VOR antennas are most often installed on the verticalfin of the aircraft. This gives good reception

Figure 4-35. When an EM wave has horizontal polariza-tion, the electric field is in the horizontalplane.

characteristics from all directions on most aircraft.On small aircraft, the VOR antenna is sometimesmounted on the top of the fuselage. If the VOR antennais mounted too far forward, a propeller modulationproblem can occur. When signals are being receivedfrom the front of the aircraft, the radio wave is choppedby the propeller blades. At certain RPMS, this cancause serious propeller modulation interference. Thecure for this involves changing propeller RPM or relocat-ing the antenna. Small aircraft often use the sameantenna for both VOR and localizer reception. Thisis practical because the two systems operate on similarfrequencies. When the localizer is being used for aninstrument approach, the signals are always receivedfrom the front of the aircraft. On a large aircraft,it is not possible to use the tail mounted VOR antennafor localizer reception because of fuselage blockage.These aircraft will use a separate localizer antennaor antennas that are mounted in the nose sectioninside the radome for the weather radar.

The location of the VOR and localizer antennasusually provides sufficient separation that inter-ference from other antennas is not a problem. If aVHF comm or other antenna is mounted closer than5 ft. from the VOR, it can cause some interferencedepending on the type of VHF comm antenna used.

d. Glideslope Antennas

Like the localizer, the signals from the ground trans-mitters for the glideslope are always received fromthe front of the aircraft. Some small aircraft use theVOR antenna to receive glideslope signals as wellas localizer signals. The glideslope operates on fre-quencies that are the third harmonic of VOR fre-quencies. This means that the glideslope frequenciesare three times the frequencies for VOR. A specialantenna coupler is used so that the VOR antennacan supply two separate VOR and localizer receiversand also supply signals for the glideslope receiver.

Figure 4-36. The polarization for various types of aircraftradio systems.

167


VHF-2 VHF-3

VOR ANTENNA(BOTH SIDES)

HF COMM ANTENNA

VHF-1ADF

000000000000000 Jo 0 0000000 0 0 0 0 0 0 0 0 0 0 0 0

GLIDESLOPEANTENNA

RADARANTENNA

LOCALIZERANTENNA

RADOME

DME1&2

ATC1&2

The same fuselage blockage problems occur on large

aircraft for both localizer and glideslope reception.The glideslope antenna or antennas for air carrierjets are installed inside the radome on the nose ofthe aircraft. Aircraft that do not have a nose radomecan utilize a separate glideslope antenna that ismounted on the forward fuselage on either the topor bottom. Blockage of signals by the fuselage orother parts of the aircraft is the primary considerationin locating localizer and glideslope antennas. Interfer-ence from other antennas is not as great a problemwith these systems as it is for some other radio systems.

e. Loran C and Omega

Loran C and Omega system antennas are receiveonly antennas and they operate at frequencies thatare widely separated from those of most otheraircraft radios. The major sources of interferencefor these radio systems are P-static noise and noisefrom aircraft electrical systems. The Loran C and

Omega antennas can be mounted on the top orthe bottom of the aircraft. The best location forthese types of antennas is based on preventinginterference from aircraft motors, generators, powersupplies and similar systems. The proper instal-lation and maintenance of bonding jumpers andstatic dischargers is critical to ensure good per-formance from these lower frequency radio systems.

f. ADF Antennas

The primary consideration in locating ADF antennasis to obtain the proper relationship between the loopand sense antenna to ensure accurate indicationsof station direction. The ADF antennas can be installedwith both loop and sense antennas on the top ofthe fuselage, both on the bottom or one on the topand one on the bottom.

The most common installation on small aircraftis with a wire sense antenna on the top and theloop antenna on the bottom of the aircraft. In any

Figure 4-37. Antenna installations on modern air carrier jets often include localizer and glideslope antennas inside theradome and flush mount VOR and HF comm antennas in the vertical fin.

168


169


case, the loop antenna must be located in the electri-

cal center of the sense antenna for accurate read-ings. The ADF antenna system is a directionalantenna system and interference from parts of theaircraft can sometimes cause bearing errors. Thisis one reason that a check of quadrantal error shouldalways be performed when ADF antennas are in-stalled or relocated. Proper bonding jumper andstatic discharger installations are important toprevent P-static noise in ADF receivers. ADF an-tennas should be located to minimize interferencefrom aircraft generators and alternators. Filtercapacitors can be used to reduce interference fromalternators and similar devices.

7. Types of AntennasMany different types of antennas are used in aircraftradio systems. Aviation technicians should be familiarwith the common types of antennas so that theycan properly identify, inspect and maintain them.Some of the common types of aircraft antennas andtheir basic characteristics will be described in thissection. Aircraft antennas usually have a speed ratingand should only be installed on aircraft that operateat and below their rated speed.

a. VOR Antennas

There are two basic types of VOR antennas foundon aircraft: the half-wave dipole and the balanced

loop types. The half-wave dipole antenna is a "V"

shaped antenna that has a figure eight-shapedreception pattern. This kind of antenna is shownin figure 4-39. The antenna has two metal rodsin the shape of the letter "V" or a fiberglass coveredelement made of thin sheet metal. It is installedon the aircraft on the vertical fin or on top ofthe fuselage with the open end of the "V" pointedeither forward or aft. The figure 8 reception patternworks well for normal VOR airway flying becausethe station is either in front of or behind the aircraft.It does not work well for RNAV when the VORstation may be off the side of the aircraft. Thedipole VOR antenna requires a special impedancematching device called a "balun". The balun islocated at the antenna end of the coaxial cablefor more efficient transfer of energy from the an-tenna to the coax and receiver. A balun is illustratedin figure 4-40. The balanced loop VOR antennahas a circular reception pattern and is thereforethe better type of antenna for RNAV. There arethree types of balanced loop antenna: the openloop towel bar, the blade and the internal mount.The towel bar and blade types are shown in figure4-41. These antennas come in two halves that aremounted on opposite sides of the vertical fin onairplanes. On helicopters or in special cases theyare mounted on each side of the aft fuselage or

Figure 4-39. Hertz dipole "V" type antennas for VOR reception. (Courtesy Comant Industries Inc.)

170


ATTACH _____ArTO ANTENNA DIPOLES

TWISTED SHIELD

PROTECTIVE OUTERCOVERING

X / 4

WIRE WRAPPED ANDSOLDERED TO SHIELD

oK---AIRFRAME GROUND

CENTER CONDUCTOR OPEN

TO NAVIGATION RECEIVER

tail boom. The blade-type, balanced loop VOR an-tenna has a higher speed rating than the towelbar or V-type and is used on bizjets and similaraircraft. Air carrier jets use a VOR antenna thatis mounted inside the vertical fin with non-metallicflush covers on each side. This kind of antennais shown in figure 4-37.

Localizer

Small airplanes usually do not have a separatelocalizer antenna, the VOR antenna is used toreceive localizer signals. On air carrier jets andsimilar aircraft, the large fuselage can cause block-age of the localizer signals so a separate localizerantenna is installed. A type of separate localizerantenna is seen in figure 4-42. This antenna isinstalled inside the radome on the nose sectionof the aircraft.

Glideslope

The signals from glideslope transmitters can bereceived on a VOR antenna because they operateat a frequency that is approximately the third

Figure 4-40. Balun for a VOR antenna.

harmonic of the VOR frequency. Single-engineairplanes commonly use a signal splitter or couplerto supply the glideslope receiver from the VOR an-tenna. Other general aviation airplanes often use aV-shaped glideslope antenna like that shown in figure4-43 to receive glideslope signals. This antenna looksa lot like a V-shaped VOR antenna but it is onlyabout 1/3 the size because of the shorter wavelengthof glideslope signals. When a separate glideslope an-tenna is installed on the aircraft, it needs to be locatedon the front of the aircraft to prevent blockage. Theloop type glideslope antenna in figure 4-44 can be

Figure 4-41. Balanced loop antennas for VOR reception.(Courtesy Dorne & Margolin Inc.)

171


DESCRIPTIONS65-147-2: Constructed with high-strength aluminum tubing andextrusion, with fiberglass base housing.

GLIDESLOPEANTENNA

SPECIFICATIONS

V.S.W . R 5 0.1

IMPEDANCE 50 ohmsPOWER N/AWEIGHT 0 2 lbs.CONSTRUCTION FiberglassHEIGHT 3 4 in.ELEMENT Grounded

SPECIFICATIONS

3 0.1IMPEDANCE 50 ohmsPOWER N/AWEIGHT 0.1 lbs.CONSTRUCTION MetalHEIGHT 15.3 in.ELEMENT Grounded

installed either externally or internally on the forwardpart of an aircraft. The dipole glideslope antenna infigure 4-45 is designed to be installed inside a radomeas it is not a streamlined design.

d. Marker Beacon

The older style wire-type marker beacon antennahas been previously described under the heading

of wire antennas. All marker beacon antennas need

to be installed on the bottom of the aircraft becausethe signals are received when the aircraft is directlyover the transmitter site. Another type of markerbeacon antenna found on smaller aircraft is thesled type. This is a bent metal rod which is about3- 1/2 to 4 ft. long and uses a sliding clip for thelead-in connection. When the antenna is installedon the aircraft, the clip can be loosened and movedto tune the antenna. A newer type of marker beaconantenna is the boat type antenna that is illustratedin figure 4-46. This antenna is smaller and morestreamlined than the wire or sled type antennas.

Air carrier jets most often use a flush mountedmarker beacon antenna that is installed in the bellyof the airplane.

Figure 4-44. A loop-type glideslope antenna for internalor external mounting.

Figure 4-42.A localizer antenna for installation inside aradome. (Courtesy Sensor Systems)

Figure 4-43. A "V"-type glideslope antenna. (CourtesyDayton-Granger Inc.)

Figure 4-45. A glideslope antenna for internal installa-tion. (Courtesy Dayton-Granger Inc.)

172


HF NUMBER 1COUPLER

HF NUMBER 2COUPLER

00

HF Communication

The trailing wire and long wire HF antennas foundon older aircraft and slow speed aircraft have alreadybeen covered. Older air carrier jets used a probe-typeHF antenna similar to the vertical fin mounted an-tenna shown in figure 4-47. This antenna includesa special coupler/tuner that retunes the antennaeach time the frequency is changed on the HF radio.This kind of antenna can be mounted on the verticalfin as shown or on a wing tip. The later modelair carrier jets use a flush mounted HF comm an-tenna that is installed inside the vertical fin asseen in figure 4-37. This antenna also requiresa special tuning device that is installed at the an-tenna connection point.

VHF Communication

The VHF comm radios on aircraft use a separateantenna for each radio. These antennas are 1/4 wave,monopole antennas that can be mounted on thetop or bottom of the aircraft. Lower speed aircraftuse the thin whip type antennas while higher speedaircraft employ blade type antennas that create lessdrag. The antenna may either be straight or bent,the bent antennas having the advantages of lessdrag and less height for belly mountings. A varietyof VHF comm antennas is shown in figure 4-48of both whip and blade types. Some blade-typeVHF comm antennas have a stainless steel leadingedge to prevent damage, this feature can be seenon the antenna in figure 4-49.

g. DME/Transponder

The same type of antenna can be used for eitherDME or transponder systems on aircraft. This ispractical because they operate at similar frequenciesand have similar characteristics. These antennasare almost always installed on the bottom of theaircraft, but they can be located on the top of anarrow tail boom or other location that does notcause serious blockage. The two common types arethe spike and blade antennas as illustrated in figure

Figure 4-46. A boat-type marker beacon antenna. (Cour-tesy Dome & Margolin Inc.)

4-50. The spike is a short metal rod with a ballon the end. This type is cheaper and easier toinstall, but it is more easily damaged and createsmore vibration and drag. The blade type is themost common type on modern aircraft. This antennacan be distinguished from the VHF comm bladebecause it is much smaller, about 2-4" long. Theseantennas are all 1/4 wave monopoles with verticalpolarization so an adequate groundplane must beprovided during installation.

ELT Antennas

Figure 4-51 shows the common type of ELT antenna,it is a thin metal rod that is located close to theELT itself. The antenna is a Marconi 1/4 wave an-tenna that requires a groundplane. It should nor-mally be installed as close as possible to the ELTbecause of the low output power of ELT transmitters.A blade type of ELT antenna is also available forhigher speed aircraft.

Satellite Navigation

The signals from GPS and GLONASS satellites arereceived from above the aircraft so the antenna needsto be installed on the upper surfaces of the aircraft.A typical GPS antenna is shown in figure 4-52. Thissmall, round antenna creates very low drag and yethas a VSWR of 2:1 which provides good signal recep-tion for the GPS/GLONASS navigation system.

Figure 4-47. Some jet transports have an HF probe-typeantenna installed in the vertical fin. Twoantenna coupling and tuning devices arealso installed in the fin to retune the antennawhen different frequencies are selected.

173


Satellite Communications

The SATCOM antenna, like the SATNAV antenna,must be installed on the top of the aircraft to preventsignal blockage. A variety of different designs areproduced for this kind of antenna. The antennain figure 4-53 is just one of the kinds of antennasbeing produced for satellite communications sys-tems for aircraft.

Loran C

An ADF antenna can be used to receive Loran Cnavigational signals by utilizing a special antennacoupler. Specific antennas for Loran C are nowbeing produced and they often bear a resemblanceto VHF comm antennas as indicated in figure 4-54.These antennas can be installed on either the topor the bottom of the aircraft and still provide goodreception because of the frequencies involved. Theseantennas often include a special anti-static coatingto reduce P-static noise in the radio.

1. Omega

Aircraft antennas designed to receive Omega/VLFsignals are available in two basic types: the "E"field and "H" field types. The antenna shown infigure 4-55 is the "E" field kind. These antennascan be installed on either upper or lower surfacesof the aircraft. The most important considerationwhen choosing a location is to reduce noise in-terference from aircraft systems. A "skin noise map"is often required which consists of measuring theVLF noise on various parts of the aircraft to findthe best antenna location. The lowest noise is usual-ly found on the aft underbelly of most aircraft.

m. MLS

The MLS receive antenna seen in figure 4-56 isa low profile, vertically polarized antenna designedto receive the MLS signals that operate on frequenciesof 5.03 to 5.09 GHz. This kind of antenna shouldbe located on the nose section of the aircraft for

Figure 4-48. Various VHF comm antennas. (Courtesy Comant Industries Inc.)

174


DESCRIPTIONS65-8282: This broadband fixed tuned antenna operates in thefrequency range of 116-156 MHz.

best reception and minimum blockage. Some MLSsystems require two antennas to be installed onthe aircraft for proper signal reception.

n. TCAS

The Traffic Alert and Collision Avoidance systemfound on air carrier jets requires a special typeof directional antenna like that seen in figure 4-57.This TCAS I antenna is normally located on the

Figure 4-49. A blade-type VHF comm antenna with astainless steel guard on the leading edge.(Courtesy Sensor Systems)

top of the fuselage and has three connector portsfor connection to the aircraft's TCAS I equipment.

o. Radiotelephone

Radiotelephone antennas come in a wide varietyof shapes and sizes. These UHF antennas are nor-mally installed on the bottom of the aircraft sincethey operate in conjunction with ground based line-of-sight radio waves. A number of different kindsof radiotelephone antennas are shown in figure 4-58.A major consideration when installing this type ofantenna is preventing noise that can be causedby loose joints and poorly bonded surfaces on theaircraft.

C. Autopilots and Flight DirectorsThe FAA classifies autopilots as aircraft instrumentsso A&P technicians cannot repair or alter autopilots.There are many tasks related to autopilots thatmight be performed by aircraft technicians suchas installation, inspection, troubleshooting etc. An

autopilot is an expensive and complicated device.It often has various components located in manydifferent areas of the aircraft and many intercon-nections. The autopilot is connected to the flightcontrol system of the aircraft and autopilot mal-functions can be very serious indeed.

An autopilot system must always be approvedby the FAA for the specific make and model ofaircraft in which it will be installed. A type ofautopilot may be approved for a number of different

Figure 4-50. Typical antennas used for DME and transponder. (Courtesy Comant Industries Inc.)

175


SPECIFICATIONS

V.S.W . R 2 0.1IMPEDANCE 50 ohmsPOWER 40 wattsWEIGHT 0 3 lbs.CONSTRUCTION WhipHEIGHT 18.3 in.ELEMENT Open

DESCRIPTIONS67-1575-14: Dual band L1/L2 GPS Antenna provides coverageat 1227.6 MHz and 1575.42 MHz with a VSWR of 2.0:1.

SPECIFICATIONS

V.S.W.R. 2 5.1IMPEDANCE 50 ohmsPOWER 1300 wattsWEIGHT 26 lbs.CONSTRUCTION FiberglassHEIGHT 10.5 in.ELEMENT Grounded

aircraft, but different torque settings and adjust-ments may have to be made for each application.The maintenance instructions that apply to thespecific autopilot installation should always be fol-lowed as there are many differences in adjustmentsand testing for the various aircraft installations.The basic principles of operation for aircraftautopilots will be described here along with somespecific examples of aircraft autopilot installations.

1. Types of AutopilotsAutopilot systems are categorized according to thenumber of aircraft axes of rotation they controland according to their complexity. The autopilotutilizes the same control surfaces that the humanpilot does. The three control axes of an airplaneare shown in figure 4-59. The rudder controlsaircraft rotation about or around the vertical oryaw axis. The elevators control rotation about thelateral or pitch axis. The ailerons control aircraftrotation about the longitudinal or roll axis.Autopilots can be described as single-axis, two-axisor three-axis types. The single-axis autopilot usuallyoperates the ailerons only and is often referred toas a wing leveler. The two-axis autopilot controls

Figure 4-51. A whip-type ELT antenna. (CourtesyDayton-Granger Inc.)

the ailerons and elevator to provide additional con-trol of the aircraft. A three-axis autopilot operatesall three types of control surfaces: ailerons, elevatorand rudder. There is a very large difference in thecapabilities of a three-axis autopilot found on asmall general aviation airplane and the three-axisautopilot found on air carrier jets and similar

Figure 4-52. GPS antenna for satellite nay. (CourtesySensor Systems)

Figure 4-53. An antenna for satellite communications.(Courtesy Dayton-Granger Inc.)

176


Figure 4-54. Antennas for Loran C nav receivers. (Courtesy Comant Industries Inc.)

aircraft. For this reason two other categories ofautopilot will be added to the three already men-tioned. Two common abbreviations for these ad-vanced autopilots will be used to distinguish themfrom the other types.

The term "Automatic Flight Control System"(AFCS) generally represents the state-of-the-art thatwas reached a few years ago. The autopilot in theLockheed L-1011 is an example of an AFCS.

This is a three axis autopilot that can controlthe aircraft during climbs, descents, cruise flightand during instrument approaches. It also has anauto-throttle system which will automatically controlengine power or thrust. Some AFCS autopilots haveauto-land capability where the autopilot can actuallyland the airplane on the runway. These types ofautopilots require many back-up systems and highlevels of redundancy. The AFCS includes a flight direc-tor function which will be explained later.

The latest types of autopilots are referred to asFlight Management Systems (FMS). These include

OG

SPECIFICATIONS

VSWR N/AIMPEDANCE N/APOWER N/AWEIGHT 1 4 lbs.CONSTRUCTION FiberglassHEIGHT 82 in.ELEMENT Open

Figure 4-55. An "E" field Omega antenna. (CourtesyDayton-Granger Inc.)

177


Figure 4-56. An antenna for MLS reception. (CourtesyDayton-Granger Inc.)

additional computers called Flight Management

Computers that permit an entire flight from justafter takeoff to landing to be programmed in thecomputers and automatically controlled. The Flight

Figure 4-57. A TCAS I antenna. (Courtesy Sensor Sys-tems)

-0

[Doc,cYr

PAINT'1-

09

SPECIFICATIONS

V.S.W.R 2 0.1IMPEDANCE 50 ohmsPOWER N/AWEIGHT 0 1 lbs.CONSTRUCTION FiberglassHEIGHT 0 3 in.ELEMENT N/AAPPROVALS TSO-C104

DESCRIPTIONS72-1744: Traffic Collision Avoidance System I.

Figure 4-58. A variety of radiotelephone antennas. (Courtesy Comant Industries Inc.)

178


AXIS OF ROLL (LONGITUDINAL)

Management Computer can be thought of as amaster computer which controls the autopilot andauto-throttle computers. The computers can storein their memory many different routes and flightprofiles and they can be used to provide a maximumeconomy in fuel consumption or other desired con-trolling factor. The standard Boeing 767 autopilotsystems will be used later as an example of thecapabilities of an FMS installation.

2. Basic Autopilot Operation

The FAA states in AC 65-15A that the purposeof an automatic pilot system is primarily to reducethe work, strain and fatigue of controlling the aircraftduring long flights. The capabilities of a modernautopilot go way beyond simply controlling theaircraft during cruise operations. A sophisticatedautopilot system can land the airplane in weatherconditions that are so bad that the human pilot

could not legally land the airplane. We would haveto say that the statement is true for simple autopilotsystems, but is obsolete or outdated in describinga sophisticated modern autopilot. In this section,the basic parts and operation of simple autopilotswill be described. Figure 4-60 shows the basic partsof the rudder control channel of an autopilot. Theaileron and elevator channels would work in asimilar fashion. The basic parts and their functionsare:

Sensors or Gyros — These detect a change inaircraft attitude using gyros or similar sensingdevices.

Amplifier or Computer — This component pro-cesses the signals from the sensors and sendssignals to the servos to correct the attitude.

3. Servos — The servos receive the signals fromthe computer and supply the physical forcenecessary to move the flight control surface.

AXIS OF YAW (VERTICAL)

AXIS OF PITCH (LATERAL)

Figure 4-59. The control axes for an airplane.

179


SERVO

AIRCRAFT ON COURSE

GYRO INPUT SIGNAL

AMPLIFIER

CONTROL SURFACEFEEDBACK SIGNAL

FEEDBACKCIRCUIT

Figure 4-60. The basic operation of an autopilot.

Figure 4-61. An autopilot controller.

4. Feedback — All but the simplest autopilotshave a feedback system that sends signalsback to the computer that indicate the motionof the flight control surface. Without feedbackthe control of the aircraft would not be smoothand precise.

5. Controller — Figure 4-61 shows a typical con-troller. This unit is located in the cockpit andcontains the actuating switches and the pitchand turn knobs. The pilot can move the pitchknob or turn knob to supply manual com-mands to the autopilot that change the pitchattitude or command a turn.

The operation of any autopilot follows these basicprinciples although different types of sensors, servos,etc. may be used. On a modern autopilot the com-puters are digital computers and there are often threedifferent computers for each of the three control axes.

3. SensorsThe gyroscopic sensors used with autopilots aresimilar to the gyro instruments described in chapter1. The pitch, roll and yaw of the aircraft are detectedby gyro sensors that send signals to the computer.The output signals of the sensors are most oftenelectrical signals. A common method of producingthe output signals is a special type of variable trans-former called an EI pick-off which detects the motionbetween the gyro rotor and its gimbals. The latesttypes of autopilots use a sensor that employs laserbeams instead of a spinning gyro rotor. Figure 4-62shows one of these laser sensors that are called

180


ANODE

READOUT DETECTOR

LIGHTBEAMS

-41-CATHODE

GAS DISCHARGEREGION

CORNERPRISM

PIEZOELECTRICDITHER MOTOR

ANODE

MIRROR(1 OF 3)

BRIDLECLAMP

SUCTIONFROM

CONTROLLER

VACUUMSERVO

CONTROL CABLE

Figure 4-62. Ring laser gyro sensor for an autopilot.

ring laser gyros or RLGs. The RLG has two laserbeams that travel in opposite directions arounda triangular course. Sensitive detectors measurethe Doppler shift or frequency change wheneverthe unit is rotated. One of these is needed for eachaxis that must be measured for the autopilot. TheseRLGs are much more expensive than an actualgyro, but they eliminate the moving parts that causea conventional gyro to gradually wear out.

4. Servos

The servos supply the force needed to move theflight control surfaces of the aircraft. There are fourbasic kinds which will be described here. Somesimple autopilots found on small airplanes usevacuum sources like those used to operate gyroinstruments. The vacuum is directed to pneumaticservos that are connected mechanically to the normalflight control system. As seen in figure 4-63, thepneumatic servo is an air tight housing which con-tains a movable diaphragm. When vacuum is appliedto the servo, the diaphragm is displaced which pullson the bridle cable that is connected to the maincontrol cable by a bridle clamp. Two of these servoswould be needed for each control axis.

Servos that utilize electric motors are shown inFigures 4-64 and 4-65.

The servo shown in figure 4-64 uses a reversibleDC motor and reduction gearing to supply the forceto move the control surface in both directions. Theservo in figure 4-65 has an electric motor that runscontinuously and uses magnetic clutches to engagethe mechanism and apply torque to the capstanand control cable. This type has the advantage thatthe inertia forces in starting and stopping the motorare eliminated. It can be engaged and disengagedmore rapidly and precisely.

Figure 4-63. Pneumatic servo for a small aircraftautopilot.

181


DC MOTOR

DC MOTORCLUTCH SIGNALFROM AMPLIFIER

CLUTCHES

CONTROLCABLE

CAPSTAN

Air carrier jets and some of the larger bizjetsuse hydraulically powered flight controls. The nor-mal flight control system employs mechanicallinkages that control hydraulic units called PowerControl Actuators or PCAs. The autopilot servoson these types of aircraft are electro-hydraulic servovalves that utilize electrical signals from theautopilot computers to direct hydraulic fluid underpressure to a hydraulic actuator. The actuator por-tion of the electro-hydraulic servo valve suppliesmechanical force to the normal linkage of the flightcontrol system. Figure 4-66 shows the electro-hydraulic servo valve for a typical large aircraftautopilot system. Figure 4-67 shows an autopilotservo for elevator control in the tail section of anair carrier jet. The mechanical force produced bythe autopilot servo is transmitted by a push-pulltube to the normal flight control linkage that ac-tivates the PCAs. The level of redundancy in thissystem is typical for this class of aircraft.

5. Small Aircraft AutopilotsA single-axis autopilot for a single engine airplaneis shown in figure 4-68. This simple autopilot usespneumatic servos to actuate the ailerons. The sourceof power is a dry air vacuum pump which is enginedriven. The sensor is a gyro turn coordinator whichcontrols the pneumatic power applied to the servos.Some of the torque settings and rigging instructionsfor the autopilot can be seen in this drawing. Thisis the type of autopilot which is often called a wingleveler since it controls only the aileron controlsurfaces.

Figure 4-64. Autopilot servo with reversible DC motor,reduction gears and bridle cables.

A three axis autopilot with electric motor servosis illustrated in figure 4-69. The sensors used with

this system include gyro sensors and an altitudesensor. The altitude sensor shows that this autopilotwould have an altitude hold capability.

Radio signals from the aircraft's navigation radioscan be used by the autopilot to steer the aircraftalong a desired VOR or localizer course. The pitch,roll and yaw servos receive electrical signals fromthe computer that activate the electric motors to movethe control surfaces. A pitch trim servo is includedso that the autopilot can apply nose up or nose downpitch trim as required. The aircraft can operate witha wide range of CG positions and the autopilot, likethe human pilot, uses pitch trim to reduce the elevatorcontrol force to an acceptable level. The autopilotcontroller has switches to engage the heading, radioNAV and altitude operating modes. It also containsan on /off switch, a pitch control indicator and theknobs for manual control of autopilot pitch and turns.It should be noted that these autopilot componentsare located in various parts of the aircraft and someof the minor components such as bridle cables arenot shown. This autopilot system has the ability toguide the aircraft on an ILS approach using bothlocalizer and glideslope signals. This feature is calledan approach coupler and is required for certain typesof instrument approaches as will be covered laterin the section on FARs.

6. Flight Management System (FMS)The Boeing 767 will be used as an example ofa flight management system or FMS. This systemhas the capability of automatically controlling theairplane from just after takeoff (above 400 ft. AGL)

Figure 4-65. Autopilot servo with a motor that runs con-tinuously and is engaged by magneticclutches.

182


AUTOPILOT ACTUATORTo COCKPIT CONTROL

AUTOPILOT LVDT

ks\ \\\\\V

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To CONTROL SURFACE

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through roll out on the runway after landing atthe destination airport. The human pilot must takeover to turn off the runway and taxi to the gate.This does not mean that all flights will use allthese capabilities, but the autopilot and flight direc-tor will be used for some portion of each flightunder normal circumstances.

a. Flight Management Computers

The Flight Management Computers (FMCs) providea number of advanced features and functions whichwere not found on earlier autopilot systems. Someof the functions of the Flight Management Com-puter are:

Flight Planning — The entire flight can be pro-grammed into the computer using a cockpitkeyboard.

Performance Management — The system canprovide optimum profiles for climb, cruise, de-scent and holding patterns. A minimum costflight can be flown automatically by using op-timum climb settings, cruise settings etc.

Navigation Calculations — The FMC can cal-culate great circle routes, climb and descentprofiles etc.

Auto Tune of VOR and DME — The FMC canautomatically tune the radios to the correctstation frequencies.

5. Autothrottle Speed Commands — These are dis-played on the EADI as FAST/SLOW indications.

The FMC is in effect a master computer whichintegrates the functions of the laser sensors, FlightControl Computers, Thrust Management Com-puters, Air Data Computers, navigation sensors andEICAS computers. The autopilot sensors are calledInertial Reference Units (IRUs) and they are thesame in basic operation as the Ring Laser Gyros(RLGs) previously described.

b. Flight Control ComputersThe flight control computers are the autopilot com-puters and there are three of them. A block diagramof the connections to the three Flight Control Com-puters is shown in figure 4 - 70. The three computers

Control Surface Actuator

Figure 4-66. An electrohydraulic servo valve is employed in an autopilot system for aircraft with hydraulically poweredflight controls. Linear variable differential transformers (LVDTs) provide feedback signals of the movementof the mechanical parts of the system.

183


CONTROL CABLESFROM COCKPIT

CONTROLS

1-

,

t f POWER CONTROLACTUATORS - PCAs

MECHANICAL LINKAGETORQUE TUBES AND

PUSH-PULL TUBES

ili

AUTOPILOTSERVO

are independent so that a failure in one will notaffect the other two. The computers are moderndigital computers that are more compact and fasterthan earlier types of computers.

c. Thrust Management Computer (TMC)

The purpose of the TMC is to automatically setthe proper thrust level for the engines. A diagramof the autothrottle system is shown in figure 4-71.The output servo moves the throttle linkage toset the level of engine power calculated by theTMC. The system includes sensors on the engineswhich monitor the important engine operatingparameters. The monitoring of engine parameters

is used to prevent exceeding any engine operatinglimitation for RPM, EPR, EGT, etc. The autothrottlesystem can be used to maintain a given climb rate,indicated airspeed, Mach number or descent rate.Since the 767 has autoland capabilities, theautothrottle system will automatically close thethrottles just prior to landing so that a smoothtouchdown can be made. The TMC system alsoprovides a minimum speed protection which willmaintain a safe margin above stall speed for theparticular flight configuration. The autopilot systemand the autothrottle system can be engagedseparately or together using the controls on theflight control panel.

Figure 4-67. The autopilot servos on a large jet airplane provide mechanical force to move the normal control linkageand activate the hydraulic PCAs that move the flight control surfaces.

184


THE OPTIONAL GYRO SYSTEM AND THEWING LEVELER SYSTEM OBTAIN VACUUMFROM THE SAME VACUUM SOURCE

............... ...1

.....................................

. .

. ..... .. ...........

... . .•••• ...• ........

..... •••••• ....

14

13

106.50 INCHES

12

11 2

NOTE

TORQUE HOSE MOUNTING NUTS (3) TO12-14 LB. INCHES AND CABLE CLAMP (6)TO 70-90 LB. INCHES WHEN INSTALLING

RIGHT AILERON VACUUM HOSELEFT AILERON VACUUM HOSENUTSERVO

5. BRACKET

CLAMPDIRECT CABLEBELLCRANKROLL-TRIM KNOB

10. TURN COORDINATOR

INVERTERVACUUM RELIEF VALVEFILTERSUCTION GAGE

15. ON-OFF CONTROL

Note: For illustration only. Not to be used for maintenance purposes.

Figure 4-68. A single-axis autopilot with pneumatic servos. (Courtesy Cessna Aircraft Corp.)

185


OUTPUT ELEMENTSSENSING ELEMENTS COMMAND ELEMENTS

DIRECTIONALGYRO

AUTOPILOT CONTROLLERELECTRICAL

POWER

AILERONSERVO

TURN•AND•SLIPRATE GYRO

ATTITUDEGYRO

COMPUTER

RUDDERSERVO

ELEVATORSERVO

TRIMSERVO

ALTITUDESENSOR

RADIONAVIGATION

SIGNALS

HEADINGSELECTOR

d. Flight Control Panel

The flight control panel contains the switches toactivate the various functions of the autopilot andto adjust the settings for the desired vertical speed,IAS, Mach number, etc. The indicator lights forthe different modes of operation are also included

in the flight control panel. This panel is locatedin the glareshield above the center instrument paneland it is illustrated in figure 4-72.

e. Control Wheel Steering (CWS)

Control wheel steering is an operating mode for theautopilot in addition to the command operating

Figure 4-69. Diagram of a 3-axis autopilot that can be coupled to radio navigation receivers.

186

N


CONTROLSURFACES

F7777.7771 MECHANICAL CONNECTIONS

= ELECTRICAL CONNECTIONS

8151=AUTOPILOTCOMPUTER

NUMBER ONEAUTOPILOTSENSORS SERVOS PCAs

88AUTOPILOTCOMPUTER

NUMBER TWOAUTOPILOTSENSORS SERVOS PCAs

AUTOPILOTCOMPUTER

NUMBER THREEAUTOPILOTSENSORS SERVOS PCAs

THRUSTMANAGEMENT

COMPUTER

COCKPIT CONTROLS

FLIGHT MANAGEMENTCOMPUTERS

ENGINE SENSORS

AIR DATACOMPUTERS

AU TOT HR 0 TT LESERVO

MECHANICALTHROTTLE

LINKAGE

mode. The command mode is the normal autopilotmode where the pilot does not touch the controlsbecause the autopilot is flying the airplane. In theCWS mode, the controls are moved by the pilotas in normal flight and the force that is appliedto the controls is measured and used as an inputsignal to the autopilot computers. In effect, thehuman pilot is flying the airplane, but the autopilotis helping to move the control surfaces. Figure 4-73shows the connections between the force transducerand the flight control computer. The operation ofa typical force transducer is illustrated by figure4-74. The three electrical windings and the armature

above them make up a special type of variable trans-former. The AC input signal is applied to the centerwinding and the outer windings produce the outputsignal. The housing of the force transducer is flexibleso that its length will change based on the forceapplied to it. When the housing changes in length,it causes relative motion between the armature andthe coils. This motion alters the magnetic couplingand therefore produces a change in the output signal.

f. Flight Director

A flight director is a system that uses some ofthe basic components of an autopilot, but not all

Figure 4-70. Block diagram of a three-channel autopilot for a large aircraft with hydraulically powered flight controls.

Figure 4-71. Block diagram of an autothrottle system with a thrust management computer.

187


HDG

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F/DON

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11+1213101011

ALT

1111710101011

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LOC

APP

AI P ENGAGE

CMDF/DON

OFF

CMD

DISENGAGE

COCKPITCONTROLS

FLIGHTCONTROL

COMPUTERS

FORCETRANSDUCER

AUTOPILOTSERVOS

MECHANICALCONTROLLINKAGE

HYDRAULICPCAs

FLIGHTCONTROLSURFACE

= MECHANICAL CONNECTIONS

= ELECTRICAL CONNECTIONS

of them. A flight director uses sensors and com-puters, but it does not have servos. The flight direc-tor computer uses the signals from sensors tocalculate a correction which is then displayed asa command for the pilot to follow. The commandsfrom the flight director are displayed to the piloton the EADI by the command bars. The operationof the command bars on the EADI is shown infigure 4-75. On the left, the command bar symbolis above the airplane symbol. The indication is thatthe pilot needs to raise the nose to satisfy thisflight director command. On the right, the noseof the airplane has been raised so that the airplanesymbol aligns with the command bar. During flightdirector operations the pilot maintains manual con-trol of the aircraft, but follows the steering com-mands indicated by the command bars. One ofthe primary uses for the flight director is during

an instrument approach. By using the flight director

the pilot can more accurately fly the airplane onan ILS approach because the computer is makingrapid calculations to predict the optimum headingand attitude for the approach. Corrections for winddrift are automatic, all the pilot has to do is followthe flight director commands.

Another condition when the flight director is help-ful is in setting the proper takeoff pitch attitude.

g. Additional Features

Some of the additional features of the Boeing 767Autopilot and Flight Director System that are typicalfor this class of aircraft will be described briefly.

The Stability Augmentation System (SAS) involvescertain functions of the yaw control system. One ofthe purposes of the SAS is to eliminate a potentialproblem known as Dutch Roll. Many large swept

Figure 4-72. The autopilot control panel for a sophisticated autopilot includes switches to control the autopilot, theflight director and the autothrottle systems.

Figure 4-73. Location of the force transducers and servos in the control system of an air carrier jet airplane.

188

N


OUTPUT

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wing airplanes display a peculiar type of instabilityat high altitudes under certain flight conditions. Thiscan result in a continuous pitching and rolling motionknown as Dutch Roll. The SAS will automaticallymake rapid and precise rudder movements to reduceDutch Roll motions of the airplane. The system isalso referred to as the yaw damper and it can beengaged separately from the rest of the autopilot.

The runway alignment feature of the Boeing 767is a part of the autoland system. It will automatically

align the longitudinal axis of the airplane with therunway prior to touchdown. This feature is im-portant during crosswind landings because it isdesigned to prevent the airplane from landing ata crab angle to the runway. This is illustrated infigure 4-76. The runway alignment feature is limitedto control surface deflections of 25° for the rudderand 2° for ailerons. A very strong crosswind at90° to the runway could not be completelycounteracted. This same feature will also provide

CONTROL COLUMN

Control Wheel Steering Force Transducer

Figure 4-74. Basic operation of one type of force transducer.

Figure 4-75. The command bars in an AUDI are positioned by signals from the flight director computer. The pilot followsthe commands by aligning the airplane symbol with the command bars.

189


RUNWAYALIGNMENT

CROSSWINDNo+

CRABANGLE

corrections if an engine fails during the landingapproach. It would supply the control correctionsto counteract the asymmetrical thrust situation.

7. FARs for Autopilots

Some of the Federal Aviation Regulations that applyto autopilots and related systems will be given here.Most of the references are to FAR Part 23 andFAR Part 91 which apply mainly to small airplanes.The requirements for transport category aircraftautopilots include some of these same requirements,but with many additional requirements that won'tbe discussed here.

FAR 23.1329Automatic Pilot System

A. An automatic pilot system must be designedso that:

It can be quickly and positively disengagedor

One pilot can overpower the autopilot tocontrol the airplane.

B. Unless there is automatic synchronization, ameans must be provided to indicate to thepilot the alignment of the actuating device inrelation to the control system it operates.

The controls must be readily accessible to thepilot and operate in the same plane and senseas the cockpit controls.

The autopilot must not produce hazardousloads on the airplane or produce hazardousdeviations in the flight path in the event ofmalfunctions.

Each system must be designed so that a singlemalfunction will not produce a hardover signalin more than one control axis.

There must be protection against adverse in-teraction of integrated components resultingfrom a malfunction.

G. If the automatic pilot system can be coupledto navigation equipment, a means must beprovided to indicate the current mode of op-eration. Selector switch position is not accept-able as a means of indication.

FAR 23.1335Flight Director Systems

A means must be provided to indicate the currentmode of operation. Selector switch position is notacceptable as a means of indication.

FAR 91 Appendix ACategory II Operations: Required Instrumentsand Equipment

Refer to figure 4-77 for an example of the weatherminimums associated with Category II and CategoryIII ILS approaches.

2A9 An automatic pilot approach coupler or a flightdirector system is required for Category IIILS operations.

Figure 4-76. The stability augmentation system (SAS) inan aircraft with autoland capability has aspecial operating mode called runway align-ment. At approximately 500 ft. the runwayalignment feature will eliminate the crabangle caused by a crosswind so that theaircraft will be pointed down the runway attouchdown.

190


2B1 For Category III ILS operations additionalequipment requirements are a radio altimeterand autothrottle system.

without excessive drag or interference from autopilotcomponents. The alignment of the autopilot to theaircraft should be checked. This normally involveschecking such things as cable tension, torque set-tings, dimensional adjustments etc.

FAR 135.105

Before engaging the autopilot for an operationalAutopilot in Place of Second Pilot check, allow the gyros to come up to speed. ThisThe certificate holder may use an autopilot in normally requires from 2-4 minutes. After engaging

place of a second pilot if:

the autopilot, the following checks should be made.

The autopilot and its operation are approved.

The Administrator issues an approval amend-ment.

The autopilot is a 3-axis type.

The certificate holder shows that operationscan be conducted safely.

E. Exceptions: A second pilot must be used:

For IFR operations.

For Category II approaches.

3. If required by the aircraft manufacturer.

8. Autopilot MaintenanceThe information in this section is not meant to relateto any particular aircraft autopilot system. The pro-cedures are general and could be applied to mostautopilots as appropriate. Maintenance of autopilotsconsists of visual inspections, replacement of com-ponents, cleaning, lubrication, troubleshooting andoperational checkouts of the system.

An operational check of the autopilot shouldbe performed whenever an autopilot is installed,when components are replaced and whenever amalfunction is suspected. Many things can beoperationally checked on the ground, but somesituations may require a test flight with an air-borne checkout of the autopilot. Some generalprocedures for a ground checkout of an autopilotare as follows.

With the autopilot disengaged, manipulate theflight controls to see if they function smoothly and

ILS APPROACHCATEGORY

RUNWAY VISUALRANGE (RVR)

DECISION HEIGHT(ALTITUDE)

Category I 2400 feet 200 feet

Category II 1200 feet 100 feet

Category IIIA 700 feet None

Category IIIB 150 feet None

Category IIIC None None

Figure 4-77. The airport weather minimums for thevarious ILS approach categories.

Rotate the turn knob on the controller to theleft. The rudder pedals and control columnshould move in the proper directions to indicatea left turn. The motion should be smooth andwithout excessive binding, jerking or hesitation.

Rotate the turn knob to the right and watchfor the proper operation and motion of thecontrols.

Rotate the pitch knob up and down and watchfor the correct motion of the control columnaft and forward.

If the autopilot has automatic pitch trim, checkthe proper motion of the trim control as thecontrol column moves fore and aft. When thecontrol column moves back, the system shouldapply nose up trim and vice versa.

With the autopilot engaged, try to overpowerit by grasping the controls and applying force.It should be possible to overpower the autopilotif it is adjusted properly.

Check all of the controls and switches forproper actuation and correct indications.

It may be desirable to taxi the aircraft in orderto check out some of the operating modes.If you engage the heading hold mode and makea taxi turn to the right, the controls shouldshow motion commanding a turn to the left.

Check the autopilot disconnect switches toensure that the autopilot disconnects rapidlyand positively. There may be several ways todisconnect the autopilot; check them all.

9. If the aircraft has a flight director, check forproper indications by the command bars inthe ADI or EADI. Check the autopilot modeindicators in the ADI or EADI if so equipped.

If the aircraft has both an autopilot and flightdirector, they can be checked against each otheras an aid in troubleshooting. The autopilot andflight director share some components while othersare only used by the autopilot. This can be usedto help locate the source of the problem when mal-functions are suspected. A ground checkout canoften help to locate the source of a problem bycomparing the indications of the flight director andautopilot. If the flight director is commanding anincorrect control movement and the autopilot is

191


moving the controls in the same incorrect direction,then the fault is most likely a component they sharesuch as the sensors or computer. If the flight directorshows a correct command for nose up pitch, butthe autopilot does not move the controls to agreewith this command then the problem is likely notin the sensors or computer.

Some complaints about autopilot malfunctionsare the result of faults in components other thanthe autopilot itself. If a pilot reports that theautopilot will not track a VOR radial, the problem

could be a wiring fault in the wires that carry radio

signals to the autopilot rather than a problem withthe autopilot itself. Conditions such as riggingproblems or binding of the main control cables inthe aircraft itself can adversely affect the operationof the autopilot. Because the interactions and in-terconnections associated with autopilots can bevery complex, a good system schematic and athorough knowledge of the autopilot are necessaryfor efficient troubleshooting and maintenance ofautopilot systems.

192


APPENDIX A Glossary

Accelerometer — A device or instrument whichmeasures acceleration forces. The unit of mea-sure is most often "Gs" based on the accelera-tion of gravity.

Aircraft — A machine that operates in the air. Ex-amples include airplanes, gliders, balloons, air-ships and rotorcraft.

Airplane — An engine-driven, fixed-wing aircraftthat is heavier than air and supported in flightby wings.

Altitude engine — A reciprocating aircraft enginethat employs a supercharger to maintain sealevel power at higher altitudes.

Annunciator — The indicating lights that are usedto alert crew members to operating conditionsthat they need to be aware of. As in warning,caution and status indicator lights.

Appliance — Any instrument, mechanism, equip-ment, part, apparatus, appurtenance or acces-sory, including communications equipment,that is used in operating or controlling an air-craft in flight and is not part of an airframe, en-gine or propeller.

Artificial horizon — An aircraft instrument usedto indicate pitch and roll attitudes. A gyro in-strument used for IFR and also known as a gyrohorizon, attitude gyro, bank and pitch indicatoretc.

Autosyn — A synchro system that transmits posi-tional information electrically from one place toanother. The Autosyn type uses AC power andelectromagnet rotors in both the sending and re-ceiving units.

Avionics — Aircraft electronic equipment. Mostoften refers to aircraft radios and similar compo-nents.

Balun — A special type of impedance matching de-vice used with V-shaped aircraft VOR antennas.

Bellows — A mechanical device for measuringpressure. It is a thin metal container with accor-dion shaped folds on the sides. Often separatedinto two chambers for use as a differential pres-sure measuring sensor.

Bonding jumper — A metal conductor used toelectrically connect two parts of an aircraft.Most often a wire braid or thin metal strap.

Bourdon tube — A curved, hollow metal tube usedas a pressure measuring sensor. Used for rela-tively higher pressures than a bellows or dia-phragm.

Calibrated airspeed — Indicated airspeed that hasbeen corrected for airspeed system errors anderrors caused by the location and alignment ofthe pitot and static ports or sensors.

Category II operation — An ILS instrument ap-proach using the specified procedures andmeeting the requirements for operations inweather conditions below those required for aCategory I approach.

Category III operation — An ILS instrument ap-proach using the additional procedures and re-quirements for operations in weather conditionsbelow those required for a Category II approach.

Class B Airspace — This is the same as a TCA(termimal control area), the airspace around amajor airport where special equipment andflight restrictions apply.

Class C Airspace — this is the same as an ARSA(airport radar service area), the airspace arounda busy airport where special equipment andflight restrictions apply.

Coaxial cable — A special conductor having a cen-ter conductor, a layer of dielectric insulationand an outer conductor. Designed to carry radiofrequency electrical signals as in an antennalead.

Compass dip — The tilting of the float in a mag-netic compass due to the curvature of the earth.Compass dip becomes greater as the distancefrom the equator increases. Magnetic compassacceleration error and north turning error are aresult of compass dip.

Compensator — A device to adjust or compensatefor some error in an aircraft instrument or radiosystem. The compensator magnets in an aircraftcompass installation.

Critical engine — The engine whose failure wouldmost adversely affect the peformance or han-dling qualities of an aircraft.

Dashpot — A small acceleration operated airpump used in an aircraft IVSI to decrease thelag in the indication.

193


Deviation — An error in a magnetic compasscaused by magnetic fields in the aircraft. Thiserror is minimized and recorded in a procedurecalled swinging the compass.

Diaphragm — A small lens shaped metal containerthat is used as a sensor to measure pressures.Used in altimeters and airspeed indicators be-cause of its sensitivity.

Doppler effect — The apparent change in fre-quency of a sound wave or radio wave whenthere is relative motion between the observerand the source of the waves. Used in the Dopp-ler navigation system by bouncing radar wavesoff the surface of the earth. The Doppler princi-ple is also used on the latest types of weatherradar systems on aircraft.

Drip stick — A fuel quantity measuring systemthat uses a tube or stick that is extended belowthe wing of the airplane. It is a mechanical mea-suring system that requires no outside power.

Encoding altimeter — An altimeter or sensor thatsupplies electrical outputs related to aircraft al-titude. The output is supplied to the aircrafttransponder which transmits the data to aground station. Also referred to as Mode C.

Erector mechanism — A device or mechanism inan artificial horizon that maintains the verticalspin axis of the gyro to ensure accurate readings.

Flight level — A level of constant atmosphericpressure related to a reference level of 29.92 in.Hg. Flight levels are indicated in hundreds offeet as in FL310, FL330, FL350 etc. Aircraftmust use flight levels rather than MSL altitudesat and above 18,000 ft.

Flux valve — The sensor for a flux gate compasssystem. The flux valve produces an electricaloutput from the earth's magnetic field to operateremote compass systems and other devices.

Gyroscope — A small wheel or rotor that is spunat high speed to operate aircraft intruments.Also used to stabilize certain components suchas radar antennas and INS accelerometers.

Heading indicator — A gyro instrument that indi-cates the magnetic heading of the aircraft. Alsocalled the DG, gyro compass, heading gyro etc.

IFR conditions — Weather conditions below theminimum for VFR flight.

Inclinometer — An aircraft instrument that indi-cates slips and skids. Also called the slip-skidindicator. Usually a small curved glass tubewith a ball in it.

Indicated airspeed — The speed of an aircraft asshown on its pitot-static airspeed indicator cali-brated to reflect standard atmospheric adiabaticcompressible flow at sea level and uncorrectedfor airspeed system errors.

Instrument — A device using an internal mecha-nism to show visually or aurally the altitude,attitude, or operation of an aircraft or aircraftpart. It includes electronic devices for automati-cally controlling an aircraft in flight (autopilots).

Knots — A velocity in units of nautical miles perhour. To convert from knots to MPH, multiply by1.151.

Kollsman window — The small window on theface of an altimeter that indicates the baromet-ric pressure for which the altimeter has been setwith the setting knob. Unit of measure is inchesof mercury or millibars.

Large aircraft — Aircraft of more than 12,500 lbs.maximum certificated takeoff weight.

Logic gate — A semi-conductor device that pro-duces an output from one or more inputs. Ituses digital signals and is a basic component inmany aircraft electronic systems.

Loran C — A radio navigation system that usessignals transmitted from ground stations on afrequency of 100 KHz. Most often used by gen-eral aviation aircraft.

Lubber line — The line on a magnetic compassthat is used to take readings. The numbers onthe compass card are read against the lubberline.

Mach number — The ratio of true airspeed to thespeed of sound for the specified flight conditionand altitude.

Magnesyn — A synchro system used on aircraft.The Magnesyn system uses AC power and per-manent magnet rotors in the sending and re-ceiving units.

Manifold pressure — The absolute pressure mea-sured at the appropriate point in the inductionsystem of a reciprocating aircraft engine and ex-pressed in inches of Mercury.

Octal — A binary code system used with inte-grated circuits and logic gates. The octal codeuses three bit bytes.

Omega -- A radio navigation system that employsground stations that transmit on frequenciesfrom 10-14 KHz. It provides world wide cover-age. VLF ground sites are also used by aircraftOmega systems.

194


Piezoelectric — A device that employs a crystalsensor that produces electrical output signalswhen it is squeezed or vibrated. An example isthe piezoelectric sensor for an engine vibrationindicator.

Polarization — The orientation of the electric fieldportion of an electro-magnetic radio wave. Verti-cal polarization would mean that the electricfield is vertical relative to the earth's surface.

Precession — The displacement of a gyro rotor byan outside force. The gyro will precess as if theforce was applied 90° ahead in the direction ofrotation from the actual point of application ofthe disturbing force.

Pressure Altitude — The altitude shown on anaircraft altimeter when 29.92 is set into the bar-ometric pressure setting window. This repre-sents the altitude in a Standard Atmosphere.

Proximity sensor — A sensor or transducer usedto replace microswitches in an aircraft positionindicating system. It is an electronic device withno moving parts and is considered to be morereliable than microswitches.

Quadrantal error — The error in indication for anaircraft ADF system. A check of quadrantal errorshould be made whenever an antenna is in-stalled or other maintenance is performed whichcould affect the accuracy of the ADF system.

Radar — Radio detection and ranging. Used in avi-ation for ATC purposes, weather avoidance sys-tems, navigation and precision altitudemeasurement.

Ratiometer — An electrical circuit used to operatean aircraft instrument. It is called a ratiometerbecause the pointer is positoned by the ratio ofthe field strength of two electromagnetic fields.

Rigidity — The characteristic of a gyro rotor thatcauses it to try to maintain its spin axis fixed inspace.

Sea level engine — A reciprocating aircraft enginehaving a rated takeoff power that is producibleonly at sea level. An unsupercharged engine.

Selsyn — A synchro system used on aircraft. TheSelsyn uses DC power with a variable resistor inthe sending unit and a permanent magnet rotorand three section coil in the receiving unit.

Slip -skid indicator — The same as inclinometer.See above.

Small aircraft — Aircraft of 12,500 lbs. or lessmaximum certificated takeoff weight.

Thermocouple — A device which uses two differ-ent metals to produce a DC output at the coldjunction when the hot junction is heated.Commonly used for CHT, EGT and other rela-tively high temperature measurements in anaircraft.

Torquemeter — An instrument system that mea-sures torque delivered to a shaft, usually by theaircraft engine. Common on large recip engines,turboprop engines and turboshaft engines.

True Airspeed — True airspeed is calibrated air-speed that has been corrected for altitude andtemperature effects. An airspeed indicator is de-signed to be accurate for the standard pressureand temperature at sea level. At higher alti-tudes, indicated or calibrated airspeed is lessthan true airspeed.

Variation — The apparent error in the indication ofa magnetic compass caused by the fact that thenorth geographic pole and the north magneticpole of the earth are not in the same location.

Venturi — A tube with curved inner walls thatproduces a reduction in pressure in accordancewith Bernoulli's Principle. Used in aircraft toproduce suction to operate gyros and as a jetpump.

Waveguide — A hollow tube used as a conductorfor radar frequency EM waves. Usually rectan-gular in cross-section and found in aircraftweather radar systems.

Wheatstone bridge — A bridge circuit with threefixed resistors and a variable resistor. It is usedto operate a meter movement that rotates apointer in an aircraft instrument for tempera-ture measurements.

195


196

40


APPENDIX B Abbreviations

ADC — Air data computer: A computer whichprocesses inputs from pitot tubes, static portsand TAT probes and provides outputs to thetypical pitot-static instruments as well as otheraircraft systems.

ADF — Automatic direction finder: A radio naviga-tion system using signals in the LF and MFbands.

ADI — Attitude director indicator: An instrumentthat combines pitch and roll data with the com-mand bars of a flight director.

AF — Audio frequency: 20,000 Hz and below.

AFCS — Automatic flight control system: Usuallyrefers to more advanced autopilots that includefeatures such as auto-throttle and stored flightplan routes.

AFM — Aircraft flight manual: An FAA-approvedmanual that lists operating requirements. Use-ful to mechanics as well as to pilots.

AGL — Altitude above ground level.AM — Amplitude modulated: This refers to a radio

wave that has been modulated in such a waythat the amplitude of the signal varies up anddown to match the modulating signal.

AOA — Angle-of-attack: The angle between thechord line of an airfoil and the relative wind.Also AOA instrument presentation.

ARINC — Aeronautical Radio Incorporated: This isan organization made up of airlines andmanufacturers which establishes standards foraircraft equipment. For example, ARINC 429 fordigital data.

ARSA — Airport radar service area: An area ofairspace around an airport with radar air trafficcontrol (ATC) in which special restrictions areplaced on aircraft flight operations.

ATC — Air traffic control.BCD — Binary coded decimal: A binary code sys-

tem that uses four bit bytes.

BIT — Each individual binary digit or number in adigital word or message.

BYTE — A group of binary bits which are treatedtogether as in a binary word with 32 bits. In thisexample, one byte = 32 bits.

CADC — Central air data computer: Same as ADC.

CAS — Calibrated airspeed.CAT II — Category II ILS instrument approaches.

CAT III — Category III ILS instrument approaches.

CFM - Cubic feet per minute: A measure of flow rateused with air-operated gyros.

CHT — Cylinder head temperature: An instrumentfound on many aircraft with air-cooled reciprocat-ing engines, usually a thermocouple system.

CIT — Compressor inlet temperature: Also calledTt2, it refers to the temperature of the air enter-ing the inlet of a turbine engine.

CPU — Central processing unit: One of the majorcomponents of a computer, the CPU containsthe ALU, control and memory functions.

CRT — Cathode ray tube: This is the display forelectronic aircraft instruments. It looks like a TVscreen, but is specially designed to be readablein the bright conditions of the cockpit.

CVR — Cockpit voice recorder: Records cockpitsounds and conversations from radio and inter-com systems.

DG — Directional gyro: An instrument which givesinformation concerning aircraft rotation aboutthe vertical axis. Usually referenced to magneticheadings.

DME — Distance measuring equipment: A two-wayradio system for determining aircraft distance innautical miles from a ground site.

EADI — Electronic attitude director indicator instru-ment: A display which combines pitch and rolldata along with indications from the flight direc-tor in the form of command bar movements.Other data such as radio navigation displays arealso included. A CRT instrument.

EGT — Exhaust gas temperature: An instrumentwhich displays the temperature of the engineexhaust gasses. Found on both reciprocatingand turbine engines.

EHSI — Electronic horizontal situation indicator: Adisplay which combines gyro stabilized magneticheading information along with radio naviga-tional information using a deviation bar in-dicator. A CRT instrument.

197


EICAS — Engine indication and crew alerting sys-tem: An electronic instrument system thatprovides indications for powerplant and aircraftsystem instruments, and provides alert, cautionand status messages for the crew. EICAS typi-cally uses two CRTs.

ELT — Emergency locator transmitter: A small self-

contained radio transmitter for crash locationpurposes found on most small aircraft.

EM WAVES — Electro-magnetic waves: Most oftenused to mean radio waves.

EPR — Engine pressure ratio: An instrument whichindicates the power being produced by certainturbojet and turbofan engines. EPR is the ratioof total outlet pressure divided by total inletpressure.

FCC — Federal Communications Commission: Thisgovernment agency establishes rules for manytypes of electronic equipment including theradio equipment on aircraft.

FDR — Flight data recorder: A system which recordsmany different operating parameters such as al-titude, airspeed, engine power, G loadings, flapsettings, etc. Used for accident investigation.

FM — Frequency modulation: A radio carrier waveuses FM when the carrier wave frequency isvaried up and down by the modulating signal.

FMS — Flight management system: A sophisticatedautopilot system that includes advanced fea-tures for managing virtually the entire flight.Uses an FMC (flight management computer).

FPM - Feet per minute: The standard unit of meas-urement for aircraft rate of climb indicators andsimilar devices.

GPH - Gallons per hour: A standard unit of meas-urement for fuel flow or fuel consumption foraircraft. Usually used for reciprocating engines.

GPS — Global positioning system: A satellitenavigation system being developed for themilitary, but available for use by civilianaircraft.

GPWS — Ground proximity warning system:Designed to give a warning to the flight crew toavoid ground impact due to excessive rates ofdescent, rising terrain, etc.

HSI — Horizontal situation indicator: An integratedaircraft instrument which displays magneticheading, radio navigation steering informationand sometimes additional information. Itreplaces the simpler DG instrument.

IAS — Indicated airspeed.

IC — Integrated circuit: A semi-conductor devicethat incorporates a number of logic gates in onecompact unit.

ICAO — International Civil Aviation Organization.

IFR — Instrument flight rules: Aircraft must operateIFR if weather conditions are below the mini-mums for visual reference flying.

ILS — Instrument landing system: A precision ap-proach using radio guidance signals to guide anaircraft to a landing runway.

INS — Inertial navigation system: A navigationalsystem that uses very accurate measurementsof acceleration to calculate aircraft position,course and speed.

IRU — Inertial reference unit: Most often refers tothe laser device which is the sensor for INS andother aircraft systems.

ITT — Inter-turbine temperature: This refers to agas temperature measurement on a turbine en-gine where the probes are located in betweentwo different sections of the turbine.

IVSI — Instantaneous vertical speed indicator: Aninstrument that eliminates the lag of a conven-tional VSI through the use of accelerationoperated dashpots.

KIAS — Knots indicated airspeed: We also findKCAS and KTAS.

LMM — Compass locator transmitter at the middlemarker.

LCD — Liquid crystal display: A common type ofdevice used in aircraft instruments and radiosthat have a lighted display.

LED — Light emitting diode: A common type ofdevice used in lighted displays on radios andother equipment.

LOM — Compass locator transmitter at the outermarker.

LRU — Line replaceable unit: Modern aircraft havemost electronic equipment installed in the formof LRUs which are standard size boxes that con-tain the equipment and which make replace-ment and maintenance simpler and moreefficient.

MLS — Microwave landing system: A new type ofprecision approach aid that may eventuallyreplace ILS. In limited use at this time.

MM — ILS middle marker.

MSL — Altitude in terms of mean sea level.

N i — A tachometer indication of the low pressurecompressor speed in a turbine engine.

198


N2 - A tachometer indication of the high pressurecompressor speed in a turbine engine.

Ng — A tachometer indication of the gas producerspeed in a turbine engine. Usually turboprop orturboshaft.

N P — A tachometer indication of the power sectionspeed in a turbine engine. Usually turboprop orturboshaft.

Nr — A tachometer indication of the rotor speed fora helicopter. Usually the main rotor.

NDB — Non-directional radio beacon: The groundbased radio transmitter that sends the signalswhich are received by the aircraft ADF radionavigation receiver.

OM — ILS outer marker.

POH — Pilot's operating handbook: This is an FAA-approved document which gives operating infor-mation for that particular aircraft. Used by bothpilots and mechanics. Same as AFM.

PPH - Pounds per hour: A unit of measurement foraircraft fuel flow and fuel consumption. Mostoften used with turbine engines.

PSIA - Pounds per square inch absolute: A mea-surement of pressure compared to a perfect vac-uum.

PSID - Pounds per square inch differential: A mea-surement of the differential pressure betweentwo pressures measured at different points.

PSIG - Pounds per square inch gauge: A measure-ment of pressure compared to ambient condi-tions, usually ambient atmospheric pressure.

P-static — Precipitation static: The static electricitycharge on an aircraft produced by friction withice, snow, rain, sand, dust etc. It can causenoise in the radios and other problems.

RAM — Random access memory: The memory in acomputer that can be affected by operator inputand is lost when the computer is turned off.

RF — Radio frequency: Refers to frequencies aboveaudio frequencies, or those frequencies above20,000 Hz (20 KHz).

RLG — Ring laser gyro: A laser beam device thatcan be used to replace spinning gyroscopes tooperate aircraft instruments and other aircraftsystems.

RNAV — Area navigation: The use of a computer toprocess signals from VOR and DME transmit-ters, permitting random direct routes to beflown using waypoints designated as a VOR ra-dial and distance such as OMN 243/24.

ROC — Rate of climb: An aircraft instrument thatgives readings in FPM of the aircraft rate ofclimb or descent.

ROM — Read only memory: Sometimes calledhard-wired, this is the memory in a computerwhich cannot be changed by the operator and isnot lost when the computer is turned off.

RVR — Runway visual range: A measured visibilityalong a runway, stated in feet. RVR 2400 = 1/2mile visibility.

SAS — Stability augmentation system: This systemis often associated with an autopilot and it isdesigned to provide additional stability to theaircraft for certain flight conditions. On sweptwing jets, for example, it helps to reduce Dutchroll.

SELCAL — Selective calling: A communicationssystem which allows the person on the groundto dial a code to contact a specific airplane inflight. Used by the airlines to contact their air-craft for operational reasons.

SWR — Standing wave ratio: A measure of the effi-ciency of an antenna, it is based on forwardpower and reflected power measurements.

TACAN — Tactical air navigation: a radio naviga-tion system in the UHF band designed primarilyfor military aircraft.

TAS — True Airspeed.

TCA — Terminal control area: An area of airspacearound a busy airport where special restrictionsare placed on aircraft flight operations.

TCAS — Traffic alert and collision avoidance sys-tem: This system is installed in some larger air-craft where it gives warnings to the pilots toprevent mid-air collisions. The TCAS uses tran-sponder principles of operation.

TIT — Turbine inlet temperature: Refers to the mea-surement of gas temperature for a turbine en-gine where the probes are located justdownstream of the combustion chambers or justin front of the first turbine stage. Can also referto a turbosupercharger temperature.

TOT — Turbine outlet temperature: Refers to themeasurement of gas temperature for a turbineengine where the probes are located down-stream of all the turbine sections. Also calledEGT.

V-speeds — Designated airspeeds related to a spe-cific aircraft certification requirement or operat-ing airspeed.

V — Velocity.

199


Va — Design maneuvering speed.

Vd — Design dive speed.

Vfe — Maximum speed with the flaps extended.

Vle — Maximum speed with the landing gearextended.

Vlo — Maximum landing gear operating speed.

Vmc — Minimum control speed: The lowest speedat which directional control of the aircraft canbe maintained with the critical engine failed andthe remaining engines at maximum continuouspower.

Vmo/Mmo — Maximum operating speed: In termsof airspeed and in terms of Mach number.

Vne — Never exceed speed: the maximum permiss-able speed under any circumstances.

Vno — Maximum structural cruising speed.

Vs i — The stalling speed or minimum steady flightspeed with the landing gear and flaps retracted.

Vso — The stalling speed or minimum steady flightspeed with the landing gear and flaps extended.

Vx — The speed for best angle of climb.

Vy — The speed for best rate of climb.

Vyse — The speed for best rate of climb, single en-gine operations with one engine inoperative.

VFR — Visual flight rules: for VFR the pilot mustbe able to control the aircraft by visual outsidereferences.

VHF — Very high frequency.

VOR — VHF omnidirectional radio range: a radionavigation system.

VORTAC — A combined VOR and TACAN transmit-ter site.

VSI — Vertical speed indicator: an aircraft instru-ment that indicates aircraft rate of change of al-titude in FPM.

VSWR — Voltage standing wave ratio: Same asSWR (see above).

200


AAbsolute pressure 14

Acceleration error 42

Accelerometer 23

Accelerometer operated dashpots 22

Air data computer systems 28

Airspeed

calibrated 17

indicated 17true 17

Airspeed indicator markings 19

Airspeed placards 12

Altimeter

encoding 16

non-sensitive 14

radar 141

sensitive 15Altimeter system tests and

inspections 24, 25

Altitude alerting system 100

Amplifiers 107

Amplitude Modulation (AM) 106

Angle of attack indicators 96

Annunciators 98

Antenna couplers 112

Antenna interference 166

Antenna separation, VHF comm ..166

Antennas, types of

DME/transponder

ELT

glideslope

GPS

HF comm

localizer

Loran C

marker beacon

MLS

Omega

Radiotelephone

SATCOM

TCAS I

VHF comm

VOR

ARINC 429

ARINC 429 digital standards

Artificial horizon

166,

170

173

173

171

173

173

171

174

172

174

174

175

174

175

173

170

54

94

31

INDEX

Attitude Director Indicator (ADI) 33

Audio control panels 114

Audio Frequency (AF) 106

Automatic Direction Finder (ADF) 123

Automatic flight control system 177

Critical engine

Crystal

Cylinder Head Temperature

(CHT) gauge

19

108

67

Autopilot maintenance 191 DAutopilot sensors 180 Deviation error 42Autopilots, types of 176 Diaphragm 13Autosyn synchro 63 Differential pressure 14Autothrottle 184 Dip standards 94Avionics master switch 149 Directional gyro 29

Distance Measuring EquipmentB (DME) 127

Bellows 13 Doppler navigation 135

Bimetallic temperature system 65 Drag cup 73, 74, 75Binary Coded Decimal (BCD) 91 Drip stick .56Binary numbers 87 Dutch roll 188BITE systems 53

Bonding jumpers 4, 152 EBourdon tube 13 EADI 50Bridle cable 181 EHSI 50

EICAS 50C Electromagnetic waves 103Caging knob 29 wavelength of 103

Calibrated airspeed 17 Electronic equipment, cleaning ....149

Capacitance quantity indicators 58 Electronic instruments 49Cavity magnetron 143 Electronic tachometers 75

CDI 126 Emergency Locator Transmitters

CMOS 94 (ELTs) 120

Coaxial cables 161 Engine pressure ratio indicators ....79

Cockpit voice recorders 122 Erector mechanism 32Command bars 33 Exhaust gas temperature

Compass dip 42 (EGT) gauge 67

Compass error

acceleration 42 Fdeviation 42 FAR 91.411 Altimeter system testsnorth turning 42 and inspections 25variation 42 Filters 111

Compass, flux gate 44 Flight data recorders 122Computerized fuel system 63 Flight director 187Computers Flight management computers 183

basic parts of 52 Flight management systemin aircraft 52 (FMS) 177, 182

Control Wheel Steering (CWS) 186 Float-type mechanical gauge 55Corona 157 Flux gate compass 44Counterpoise 111 Four-course radio range 123

201


Frequency modulation (FM) 106

Frequency synthesizer 113

Fuel-injected engine flowmeters 58

GGauge

float-type mechanical 55

magnetic direct reading 55

sight glass 55

Gauge pressure 14

Gimbals 29

Glass cockpit 49

Glipeslope 139

GPS (global positioning system) 137

Ground proximity warning

system 142

Ground waves 107

Ground-loop interference 154

Groundplane 111

considerations 164

HHeads up displays 51

Heads up Guidance System (HGS) 51

Hertz dipole antenna 111

HF comm radio 117

Horizontal polarization 112

Horizontal Situation Indicator

(HSI) 30

IIFF 129

Inclinometer 34

Indicated airspeed 17

Inertial Navigation System (INS)... 134

Inertial Reference Unit (IRU) 135

Instantaneous vertical speed

indicator 22

Instrument and equipment

requirements 12

Instrument categories 2

Instrument flight rules 12

Instrument Landing System (ILS) 137

Instrument lighting systems 5

Instruments

instrument flight rules 12

methods to install 3

visual flight rules day 12visual flight rules night 12

Integrated circuits 93

Intercom systems 116

Interphone systems 116

Inverters 154

ITT 68

K

LLatitude 131

Line replaceable unit 53

Localizer 138

Logic gates 91

Long wire sense antenna 163

Longitude 131

Loop antenna 124

Loran C 132

LRU 53, 95

Mach number 21

Machmeter 21

Magnesyn synchro 63

Magnetic compass 41

Magnetic directing reading gauge 55

Maintenance of gyro systems 40

Manifold pressure gauges 79

Marconi antenna 111

Marker beacons 139

Mass flowmeters 63

Master caution lights 99

Master warning lights 99

Maximum allowable airspeed

indicator 21

Mechanical bulb temperature

gauge 66

Mechanical tachometers 72

Microphones 1 14

Microwave landing system (MLS) 141

Mode 3/A 130

Mode C 130

Mode S 131

Modulation rate 110

Modulators and demodulators 108

NNorth turning error 42

Null field discharger 157

OOBS 126

Octal 91

Oil pressure indicators 75

Omega 134

Oscillators 108

P-static 6, 155

Pitot tube 23

Polarization

horizontal

vertical

112,

112,

166

166

Position indicating systems 69

Positive and negative logic 93

Powerplants, types of 7

Precession 29

Pressure

absolute 14

differential 14

gauge 14

Primary power setting instruments 84

Proximity sensor 71

QQuadrantal error 164

RRadar altimeter 141

Radio

HF comm 117

VHF comm 117

Radio Frequency (RF) 106

Radio frequency chart 104

Radiotelephone 119

Ratiometer tempereature gauge 67

Required instruments

flight and navigation 8

powerplant 8

Rigidity in space 29

RLG 181

RMI (radio magnetic indicator) 45

RNAV 127

Runway alignment 189

SSATCOMM 119

Satellite navigation 135

M

Kollsman window 16

202


Selcal 119 Stick shaker 98

Selsyn synchro 62 Stormscope 145

Sense antenna 124 Superheterodyne 113

Sensors, autopilot 180 Swinging the compass 42

Servos, autopilot 181 Synchro systems 61

Shielding 154

Shock mounts 4 TSight glass gauge 55 Tachometer generators 75Silicon 93 Takeoff warning systems 95Sky waves 107 TCAS 145Slip-skid indicator 34 Temperature measuring systems.... 65Space waves 107 bimetallic 65Speakers 114 mechanical bulb 66Speed of sound 21 ratiometer 67St. Elmo's Fire 157 thermocouples 67Stability Augmentation System Wheatstone bridge 66

(SAS) 188 Thermocouples 67Stall warning systems 98 Thrust Management ComputerStandard "T" configuration 11 (TMC) 184Standard Atmosphere table 15 TIT 68Standard rate turn 35 TO-FROM indicator 126Standard sizes for round Torquemeters 78

instruments 2 TOT 68Standing wave ratio 160 Total Air Temperature (TAT) 28Static discharagers 155 Transponders 128Static loads 158 Trip free circuit breakers 152Static ports 23 True airspeed 17Static wicks 157 TTL 94

Turn and bank 34

Turn coordinator 36

VVacuum pump 38

Vane-type flowmeters 60

Variation error 42

Variometer 22

Venturi for gyros 37

Vertical card compass 43

Vertical polarization 112

Vertical speed indicator 21Very high frequency omnirange

(VOR) 125

VHF comm radio 117

Vibration indicators 87

Visual flight rules day 12

Visual flight rules night 12

Vmc 19VSWR 160

wWaveguide 143

Weather radar 143

Wet pump for air driven gyros 38

Wheatstone bridge temperature

gauge 66

Wire antennas 162

203


SBN 0-89100--422--X

0042269 780891

ABOUT THE BOOKAircraft Instruments and Avionics is intended to be used in the instruction of studentsin an aviation maintenance technician training program.

This textbook includes:

Basic Instruments: Why study instruments, aircraft instrument requirements, pitot-staticsystem instruments, gyro instruments, compass systems, electronic instruments, andcomputers in aircraft

Powerplant Instruments and Logic Gates: Liquid quantity measuring systems, fuel flowindicators, temperature measuring systems, position indicating systems, tachometers, oilpressure indicators, torquemeters, engine pressure ratio indicators, manifold pressuregauges, primary power setting instruments, vibration indicators, logic circuits and digitalsystems, takeoff warning systems, angle of attack indicators, stall warning systems, andannunciators

Communication and Navigation Systems: Radio fundamentals, regulations andstandards for radios, intercom and interphone systems, communications radios,emergency locator transmitters (ELTs), cockpit voice recorders, flight data recorders,navigation systems, instrument landing system (ILS), radar altimeter, ground proximitywarning system (GPWS), weather radar, Stormscope®, and airborne collision avoidancesystem (TCAS)

Aircraft Antennas and Autopilots: Installation and inspection of avionics, antennainstallations, autopilots, and flight directors

ABOUT THE AUTHORMax Henderson taught Aviation Maintenance Technology subjects at Embry-RiddleAeronautical University. Prior to that he was an Electronics Technician in the U.S. AirForce. He holds a commercial pilot as well as a mechanic certificate with an airframe andpowerplant rating. He has also worked as a control tower operator.

JS312666 0090000

Jeppesen Sanderson Inc.

www.jeppesen.com55 Inverness Drive East

Englewood, Colorado 80112-5498

aircraft instruments and avionics - max f. henderson

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