19 landing gear

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B1


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ACKNOWLEDGEMENTSWith special thanks to:AIRBUS INDUSTRIEfor permission to reproduce drawingac.,


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WHAT THIS BOOK COVERSThis book contains information on the following:-* Landing gear structure.* Nosewheel units.* Power steering and feed back systems.* Indicator systems.* Mainwheel units.* Shock absorbers.It describes the systems, components, and their maintenance.For further information on associated systems/components see books in thisseries "WheelsTyres and Brakes" and "Hydraulics".


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-,, . . .

AUTHORITYIt is IMPORTANTto note that the information in this book is forstudy / training purposes only.When carrying out a procedure/work on aircraft/aircraft equipment youMUST always refer to the relevant aircraft maintenance manual or equipmentmanufacturer's handbook.You should also follow the requirements of your national regulatory authority(the-C1\i\tnthe UK) and laid down company policy as-regards localprocedures, recording, report writing, documentation etc.For health and safety in the.workplace.you.should.Iollow the .. -~-regulations/guidelines as specified by the equipment manufacturer, yourcompany, national safety authorities and national governments.


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HOW TO TACKLE THIS BOOKYou should read and understand the information inthis book and relate this toyour own aircraft/ experience where this is appropriate.You should be able to describe (in writing and verbally) the operation, servicingand logical fault finding of the following:-* The construction and retraction/extension of an undercarriage unit.* The purpose and general arrangement of landing gear.* Caste ring, self centring, and shimmy damping of a nose wheel unit.* Power steering.* Feedback - mechanical - electrical - electronic.* Indicator systems.* Warning systems.* Locking systems.* Emergency lowering systems.* Shock absorbers - principles - operation - servicing.


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LANDING GEAR

The functions of landing gear are:-(a) Toprovide a means ofmanoeuvring the aircraft on the ground.(b) Tosupport the aircraft at a convenient height to give clearance for

propellers and flaps etc., and to facilitate loading.(c) Toabsorb the kinetic energy of landing and provide a means of

controlling deceleration (vertical and horizontal).Once airborne, the landing gear serves no useful purpose and is so much deadweight. Itwould be ideal to replace it with some ground based equipment but,while in the case of (a)and (b)this is possible, no satisfactory alternative existsfor (c). For this reason research has gone into the design of undercarriageunits in order to reduce their weight and stowed volume when retracted.The geometrical arrangement of the undercarriage units is not standard,conforming only to the designers requirements. However,on the majority ofaircraft the tricycle layout is used. This consists of twomain undercarriageunits just aft of the C of Gwhich supports up to 90% of the aircraft's weightand all initial landing shocks, with a nose wheel unit to keep the aircraft level.For many years, the tail wheel layout was standard but now nose wheelconfiguration has replaced it almost entirely. Its advantages over the tail wheellayout are:-(a) Increase ground stability due to:-

(i) Reduces nose over tendency.(ii) Reduced ground looping tendency.

(b) Aircraft adapts flyingposition on the ground which leads to:-(i) Better pilot-vision.(ii) Reduced drag on take-off,(iii) Easier loading.(iv) Jet blast clear of the ground.

Another layout in limited use is the bicycle or tandem arrangement wherebythe main units are mounted one behind the other on the fuselage centre linewith small outrigger units in the wings for lateral steadiness, as used on theHarrier and certain American, French and Russian aircraft. The 747 has fourmain units and a nose unit while aircraft like the DC10 have three main units(two in the wings and one on the centre line of the fuselage), and one nose unit.


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JAR 25Jar 25 lays down the requirements that an undercarriage has to meet with, aseparate main paragraph covering the following topless-1. General.2. Shock absorber tests.3. Limit drop tests.4. Reserve energy absorption drop tests.5. Retraction mechanism.6. Wheels.7. Tyres.8. Brakes.9. Nosewheel steering.(you are advised to read this publication and get, at least, a general feel forwhat it contains)

Undercarriage UnitsUndercarriage units comprise basically:-(a) Aleg, pin jointed to the aircraft structure (ifretractable).(b) One or more wheel and tyre assemblies.(c) Ameans of absorbing landing shocks and controlling recoil.(d) Ameans of controlling deceleration of the aircraft.It is not usual for a brake unit to be incorporated in a nose or tailundercarriage unit, but some aircraft (Boeing 727) have a brake unit fitted onthe nose leg. The nose or tail undercarriage unit must castor, and, in manycases, be steerable to allow the aircraft to be manoeuvred on the ground by thepilot. ' -retractable unit must be capable of folding into as small a space as possibleduring flight and be covered by fairing doors to reduce drag.

Loads Sustained by UndercarriageAn undercarriage unit has.to withstand varying load.ip.gs during its life. Theseloads are, of necessity, trarisrnltted tome-mountings in ttreaircrart-strueeere,so these too must be very strong. The loads sustained are.-(a ) Compressive - static and on touch down.(b) Rearwards bending - on touch down and braking.(c) Side - during cross wind landings, take offs and taxying.(d) Torsional - during taxying and when turning.


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Types ofUndercarriage (Fig. 1)Thewheels are mounted on axles attached to the base of the leg so that theytransmit compressive loads to the shock absorber. There are two basicmethods ofmounting the shock absorber:-(a) Direct Acting

The shock absorber forms an integral part of the legwith the axlemounted directly on to it. This forms an uncomplicated unit, but sinceall the compression loads must be accommodated inside the leg, it maybe lengthy which tends to increase stowage problems and tends tomagnify structural loadings. Since the shock absorber is integral, it issubject to all the types of load sustained by the undercarriage. Torquelinks are required to prevent rotation of the shock absorber.

(b ) Levered or Articulated LegThe shock absorber is mounted outside the leg and loads aretransmitted to it by a system of levers so that the wheel movement ismuch greater than the shock absorber movement. The shock absorberis subjected only to direct compressive loads and this may make itshorter and lighter, more compact and robust.

L E V' ! ? S U SP E NS IO N D I I l' C T A C T IN G

Fig. 1 TYPES OF UNDERCARRiAGE ~

--CamberThis is defined as the inclination of the wheel with respect to the vertical planewhen viewed from the front (or rear) of the aircraft. Most large aircraft wheelsare not cambered but many small aircraft wheels are.Positive camber is when the wheel is inclined away from the aircraft. Negativecamber is when-the wheel is inclined towards the aircraft.

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/

fIJI},7,)} ,

,~Tll/II)I,)I>

Fig. 2 CAMBER

NOSE UNDERCARRIAGES-A noseIThdercarriage unit is usually a lighter structure than a main unit since .._-_.it carries less weight and is usually subject only to direct compression loads. itdoes, however,carry the attachment for the towing equipment and-so mustwithstand shear and bending loads as well.Question: Can you list four or fivedesign features that are different on

a nose gear unit compared to a main gear unit?(Takeabout 5 mins].


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--.-

-

Answer: A nose gear unit will usually have:1. The ability to caster.2. A self centring system.3. A steering system - which is powered for all largeaircraft.4. An anti shimmy device.5. No braking system (except some Boeing aircraft).

CasteringTo enable the aircraft to be manoeuvred on the ground the nose wheel mustcastor freely though subjected to compression and shear loading, whichpresents a problem in the bearing design.

Self CentringAutomatic self centring is essential as soon as the aircraft's weight is removedfrom the wheel to ensure the correct positioning of the wheel prior toretraction. Centring is achieved by either a spring loaded cam or a hydraulicdashpot or by the steering mechanism. Levered suspension noseundercarriages tend to centre because of the natural trail of the wheel.With the spring loaded cam system at the top of the swivelling portion of the legthere is a cam in which there is a V shaped slot. In this slot rides a springloaded roller or peg. As the wheel is rotated from the in-line position, so thecam rotates forcing the peg or roller to ride up the side of the cam, compressingthe spring. As the weight of the aircraft is removed from the wheel the springforces the roller or peg down to the base of the V in the cam thereby centringthe wheel.SteeringSome small aircraft do not have a steerable nose (or tail) wheel and are steeredusing differential braking, Others have a nose wheel that is mechanicallyconnected to the rudder pedals so as the pedals are moved so theWhe:ft'el"w'""*IDJ------steer - in the air as well as on the ground.Large aircraftusually have powered-nose vlheel steering..this is because thewheels would be impossible to move manually.Question: How much weight is usually felt through the nose wheels?(2 minutes).


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Answer: This varies with the aircraft of course, but it is usually about10%of the aircraft total weight. Taking a reasonably "small"commercial aircraft of say 100 tons (or tonnes - there is just2% difference between a tonne [or metric ton] and a ton)then that would mean a weight of 10 tons through the nosewheels. Many cars of less than a ton overall have powersteering.

Early methods involved the use of differential braking, i.e. braking the wheel oninside of the tum only. This method destroys the forward momentum of theaircraft and necessitates opening up the engines to a high thrust inorder to getthe aircraft rolling again. Italso increases brake and tyre wear, and increasesheat.Powered steering using hydraulic steering motors (jacks], now common allowsthe engines to be set at the minimum thrust for taxying, thereby saving fuel.This method of steering is more accurate and also reduces tyre and brake wear.Toallowthe nose unit to caster freely when towing or on push back the powersteering must be capable of being isolated or being switched off.This may be done in several ways:-(a) By a by-pass valve in the hydraulic system supplying the power steeringjack. When the steering is switched off the valve is opened allowing fluidto pass freely from one side of the steering jack to the other when thejack is being moved by the towing arm. When steering is selected the

by-pass valve is closed - by hydraulic pressure (Ref.Fig. 6).(b) By disconnecting the torque links during towing. This allows the noseunit to caster freelywithout moving the steering jack.(c) By the use of an "isolating pin" which is inserted into a special hole in

the nose unit by the push back crew - this effectively isolates the nosegear steering mechanism. The pin is removed after push back and theusual procedure is to show it to the pilot, before he/she taxies theaircraft away .Powered steering systems are normally operated hydraulically using eitherr-v=(a) A single jack with an equal area both sides, or(b) Twounequal area jacks (Fig. 3).

------.~- ..-----


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. . ..

S IN GLE E aU AL A RE A J AC K

P LA N V IEW O F NOS E WHE EL S TE ER IN G MECHA NIS M

W he n s tee rin g is s ele cted in th e n ig htd ec k p re ss ure n uid is s up plie d to th elarg e v olu me e nd o f o ne jac k w hile theo th e r j ac k is connected to r et urn . T o s te erthe other way th e s ele ct io ns a re r ev er se d

P LA N V IEW O F DOU BL E JA CK A RR ANGEME NT

Fig. 3 JACK ARRANGEMENT

Question: Why do you think the single jack has to have an equalarea both sides? (2 mins.)Answer: If the areas were not equal the jack would move faster oneway and slower the other.Question: With reference to the drawing below. If the jack was of theunequal area type which way would it move the fastest? Be

careful! (5minutes)IN - .... OUT

Answer: It would move in the quickest. There is less volume to fillbecause of the jack ram. Remember, it is fluid flow rate (orvolume fill rate) that determines speed. It is pressure (andarea) that determines what load can be moved .

.. ControlSteering is controlled, depending on the type of aircraft, by:-(a) A separate steering wheel on the flight deck instrument panel.(b) Operation of rudder pedals.(c) Change-over switch to aileron control wheel.


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Incorporated in the steering motor may be:-(a) Self centring jack.(b) Shimmy damper.

FeedbackPower steering is similar to P.F.C.U.s in that "feedback" is required.Question: There are two types of "feedback" - can you say what theyare? (2mins)Answer: Positive and negative feedback.Question: P.F.C.U.s and power steering use negative feedback. Canyou describe negative feedback? (10 mins)Answer: When an input is put into a system, the output from thesystem tries to cancel the input. Your answer should besimilar to that.Feedback on a power steering system can be achieved in three ways:-(a) Mechanical - a cable system as fitted to some Boeing aircraft.(b) Electrical - using a Wheatstone Bridge.(c) Electronic - using a computer as in the A320.

Shimmy DamperDue to the flexibility of tyre side walls and the fact that the nose unit is allowedto caster, an unstable swivelling oscillation (a form ofvibration) known as .:shimmy can be induced into the nose undercarriage. Excessive shimmyespecially at high speeds, can set up vibrations throughout the aircraft and canbe dangerous. Wear in the wheel bearings, low tyre pressures, wear inundercarriage linkage and mountings can all increase the tendency to shiIll~I!l_Y._~~Shimmy is damped in several ways:-

-------~~- Provisiori in t h e steenng motor.Hydraulic damper.Heavy self centring springs.Double nose wheels.

(a )(b )(c )(d)

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The hydraulic damper is connected to the nose wheels in such a way that whensteering or castering takes place the piston is caused to move through the oilfilled cylinder - with the oil passing from one side of the piston to the other.When shimmy occurs the nose unit tries to oscillate, about the vertical axisand tries to push the piston back and forth through the fluid.This back and forth motion is of relatively high frequency and low amplitude.Because of the inertia and viscosity of the fluid it takes a finite amount of timeto make up its mind to pass through the holes (with the piston moving in onedirection during half of the vibration cycle).When the fluid is just starting to move through the piston holes that half of thevibration cycle is over and the piston is starting to try and move in the oppositedirection. So in this way the damper appears to "lock up" when shimmy occursbut allows steady movement in either direction during steering/castering.(Figure 4)Question: What does viscosity mean? (2 mins).Answer: It is a measure of the "resistance to flow of a fluid".(Note: Try not to use the word "thickness").

OR IF I C E S E A L S P I S T O N

O I L-FI L LE D C YL I ND ERFig. 4 HYDRAULIC DAMPER

TWIn wheels help-to prevent shilnnzy by making the neseunit.more.stahle, _Each wheel unit will have its own natural vibration frequency (associated withit's R.P.M. This frequency tends to be different in the two wheels, so that theynever coincide.

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POWER STEERING SYSTEMS

A nose wheel power steering system ensures ease of manoeuvring an aircraftwhilst on the ground.The steering system is normally hydraulically operated and mechanically,electrically or electronically controlled. It is part of the aircraft hydraulicsystem and linked directly with the hydraulic, mechanical and electricalfunctions of the nose leg retraction system.The heart of the system is the steering motor (or jack) attached directly to thenose undercarriage structure. It comprises one or two hydraulic jacksanchored to the rotating portion of the leg.Movement of the pilots control opens a selector valve to divert fluid pressure tothe required end of the steering motor which rotates the nose wheels. Whensteering is not selected the nose wheel is free to castor. Immediately prior toretraction the steering system is automatically switched 'off and the wheelscentred.

The Single Jack Type Steering System (Fig. 5)In this system the steering motor is a unit housing a steering jack and acentring jack in an integral shell. Both ends of the steering jack ram areanchored to the nose undercarriage structure, fluid being supplied up thecentre of the ram from both ends. The integral jack body, connected to thesteering crank and nose wheels moves along the ram as pressure is applied inthe jack and so rotates the wheels to port or starboard as required. An electro-mechanical follow-up system using a drum switch ensures that the amount ofnose wheel rotation equals that signalled by the pilot.

Centring JackOn retraction it is essential that the nose wheel centres automaticallyotherwise structural.damage would ensue. This is the function of the centringjack.On 'UP' selectiQJ;lthe steering system is isolated electrically and at the same-tlliienydraulic fluid from the Ieb action system-is- fed to -the centring jack whichmoves the steering jack body to the central position thus centralising the nosewheels. On some aircraft the steering system is isolated as soon as the noseleg extends on 'take-off.The centring jack comprises a piston rod anchored to the undercarriage leg, afloating piston and a jack cylinder integral with the steering jack body. Fluid isonly directed to thecentring jack on 'UP' selection.


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S T E E RI N G MO T O R

A XL E FI T T I N G

Fig. 5 STEERING JACK ARRANGEMENT


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Hydro-Mechanical System (Figure 6)In this system the steering motor is of the "single jack" type and hydraulicselection is made by a mechanically operated selector valve. Selection is madeby the pilots' steering wheel via the steering cables to the steering drum. Thefeedback mechanism to cancel the signal is via the follow-up cables, e.g.1. Pilot steers to the right.2. Steering drum rotates anti-clockwise.3. Left hand side of the beam pulled down.4. The valve is caused to select by the linkage to supply fluid to the bottomof the jack.5. The nose wheels steer to the right.6. This puts tension into the right hand follow-up cable and releases thetension in the left hand follow-up cable.7. This neutralises the beam and the selector valve, and motion stops.If the pilot continues to move his steering wheel then the selection will be ~maintained and the wheels will continue to steer. As soon as he/she holds thesteering wheel steady then the follow up cable will cause the system to "catchup" and select the valve to neutral thus stopping the steering in that positionand holding the jack in a hydraulic lock.NOTES:1. The control valve is shown disproportionately large.2. The drawing is simplified by removing a gear system from behind thesteering wheel but the principle is similar to that fitted to Boeingaircraft.The throttling valve gives a constant rate of fluid supply.Question: Why would the flow vary from the hyde system? (10 mins).Answer: For 99.9% of aircraft, the hydraulic circuits (flaps - gearselection - brakes, etc.) are arranged in parallel, i.e. they allhave the same pressure fed to them (unless there is a

reducing valve fitted) but-they SHARE the fluid flow from thesupply circuit. If one of the other circuits is selected - sayflaps - when power steering is operated, the fluid flow rate tothe power steering (and inCidelltally - the flaps) will bereduced. Under-these conditions the thri}ttling valve will bemore open, thus ensuring that the rate of movement of thesteering jack is constant.

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The by-pass valvemay be actuated hydraulically or electrically, and when openallows fluid to pass fromone side of the jack to the other duringtowing/pushback.On retraction, the system must be centred using a centring linkage or acentring jack.

T HRO TT LI NG V A LV E V A L V E

R ET UR N;=::::::;:;:=/- C ON TRO L L IN KA GE

BY-PASSWHEEL

,/" S TE ERI NG C ABL E--~STEERING DRUM

) FO L L O W U P C A BL E

JAC}~1)euBtE P RE S S U RE RE L I E F V A L V E

Fig. 6 MECHANICAL CONTROL STEERING SYSTEM

Electrical SystemThis is similar to the mechanical system except that the selector valve iselectrically operated froman unbalanced wheatstone bridge - this being causedby the pilot's input. Negativefeedback is provided by an input from themovement of the nosewheel, which re-balances the bridge, thus selecting thecontrol valveto neutral. There are no steering and follow-up cables.


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Figure 7 shows the principle of operation of an electrical method of control. Itconsists of a Wheatstone bridge, one element of which is controlled by avariable resistor which is moved by the pilot.When the pilot moves!his/her handwheel the bridge becomes unbalanced andthe resulting output signal is amplified to operate one solenoid of an electricallyoperated hydraulic selector valve.

' 2 I ! N de SUPPLY

PILorsSTEERINGCONTROL

NOSELEG

FEEDBACKLINKAGE

HYDRAULICSELECTORVALVE

+

AMPLIFIER

Fig. 7 ELECTRICAL CONTROLSTEERING SYSTEM

This valve will send fluid to one side of the steering jack and open the otherside to return.As the nose wheel steers so it moves a followup resistor in the bridge to catchup the value of the input resistor - and when it does so the bridge becomesbalanced; the output returns to zero; the solenoid in the selector valve shutsand the valve holds the steering jack at that position with a hydraulic lock.When the pilot steers in the opposite direction the bridge is unbalanced theother way and the opposite solenoid is energised in the selector valve to causethe nose wheel to steer in the opposite direction.

Example:1. Pilot steers right.2. The input resistor increases in value.3. There is a higher voltage on the right hand side of the bridge whichcauses the right hand wire to the amplifier to go positive.

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4. The amplifier causes the left hand solenoid of the selector valve tooperate.

5. The selector valve selects to cause the hydraulic jack to steer the nosewheel to the right.

6. This causes the followup resistor to increase in value to catch up theinput resistor. When this happens the bridge becomes balanced;current to the selector valve ceases and steering stops.

NOTES:1. Ifthe pilot wishes to steer all the way (to the right) then he/she will

continue to move the input resistor and the followup resistor willcontinuously try to catch it up - until full movement is achieved.

2. For operation of electrically operated selector valves refer to the Book inthis series entitled 'Hydraulics'

Computerised SystemYouwill have to take my word for it that a digital computer is basically a verysimple machine. It has several things in its favour in that it is small; takeslittle power; and is quick, but it is SIMPLE. In fact it can only perform thefollowingfunctions:-(1) ADDITION.(2) STOREINFORMATION.(3) MOVEINFORMATIONAROUND.(4) COMPAREINFORMATION- and that's all.(There is no room here to discuss this further - but do contact your tutor if you feelyou cannot believe what has been said - A ten minute 'phone call would explainall.)

With reference to figure 8. When the pilot puts a steering input into thesystem, the input is sent to the computer (B.S.C.U.). This signal will be stored(2 above) in the computer and an analog signal (dc)will be sent to a solenoid inthe-steering senro valve.. _This will cause fluid pressure to go to one side or other of the steering jack andcause the nose wheels to st~r~As the nose wheel unit steers -soa feed back signal is send back to the.computer where it is compared (4 above) with the input signal - if it is not thesame then the system will carry on steering. When the feed back signal is thesame as the input signal then the computer will stop the steering action.

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oAUTO 'I LO T

C It [(I P O W 1 IrROW10S[ tE". DOO' lSeLOSII ' CIAcullU IMEN -00011$ NlE CLOSEDI

AI':.( IU ,lIosr ...1((1.~.~Clrr

,~ PO S TRANSM .I CTL I

Fig. 8 THE A320 POWERSTEERING CONTROLSYSTEM

Question: Study Figure 8. What other computers have an input intotl].~~~~~ringcomputer? (2mins),~~~-

Answer: The auto-pilotand the ELAC- a flying control computer.Question: Study Figure 8 again and state when can power steering beused. (5mins).


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Answer: Itcan be used when:-(a) It is switched onAND(b) Airspeed is less than 80 knotsAND(c) At least one engine is runningAND(d) Towinglever is in normal positionAND(e) The weight on wheels switch is made.

UNDERCARRIAGE SYSTEMSThe purpose of these systems is to:-(a) Raise and lower the undercarriage and doors.(b) Reliably lock the undercarriage and doors in the up and down positions.(c) Indicate to the pilot the position of the undercarriage.(d) Interconnect the undercarriage with other systems for low speedwarning, etc.

Retraction and LoweringSystemsThe most common form of retraction system is hydraulic (refer to the Book inthis series "Hydraulics")but some aircraft may use electrical actuators or apneumatic system.Emergency loweringmay be accomplished by:-,. The use of a standby pump.,. The use of a duplicate system.* The use of compressed nitrogen blow down.'* ' Gas operated release locks and free fallwith spring assisted down locks.,. Awind downmechanism.Locking Methods

-,-JHAHR~25_states that "There-must-be a positive means to keep the landing gear _extended in flight and on the ground ..... ,; and to keep it in the correctretraction position ".Question: Can you name 3 undercarriage locking systems?(3mins).


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Answer: (a )(b )(c )

Mechanical.Hydraulic.Geometric.

Mechanical locks are the most positive and reliable so are always used. Theymay be hook or plunger types and may be spring or hydraulically operated.Ahydraulic lock is often also used - but it is not positive or reliable so is onlyused asback up. .Question: Can you define hydraulic lock? (5mins)Answer: Atrapped column of fluid between a jack and a selectorvalve and/or a N.R.V.Ageometric lock may also be used - as a back up. It is not positive because itcould be shaken out of its locked position. Ageometric lock is a lock caused bythe arrangement of a set of links (Refer Figure 9).

/ , ' / , / ; ' ! / / /! I I / / / I I III/t::~~~~~~/ I ~ . 1 j jS ID E S TA Y 1 / / / 1 / IfI/I '/J

- - -A IN LE G The se l in ks a re g eome tr ic al lylo ck ed as g rav ity w ill k ee pthem In their present position.A h ydr au li c jack o r s om e o th erd ev ic e w il l be r eq ui re d t o 'b re ak 'the lo c k .

= H ING E POIN TSUNDERCARRIAGE

MOVt: :MENT

Fig. 9 GEOMETRIC LOCK

Selection of the undercarriage may be mechanical or electrical and signallingmay be electrical orelectrontc with a standby mechanical back-up.Sequencing between leg and fairing doors may be mechanlcaChydraulic orelectrical.

Indicating and Warning Circuit (Figure 10)Micro switches or proximity switches are operated by the up and down locks,and are wired into an indicator lamp circuit. There is usually one lamp perindication per undercarriage and on some aircraft, indication may be shown ona C.R.T. screen. Where lamps are used, a standby bulb may be provided.Lamp indications areas followsr-


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GreenRedNo lightsLocked down.Unlocked.Locked up.

On some aircraft, an amber light may show if the aircraft is on the ground andthe fairing door is moved from its normal position for servicing.To warn the pilot if the undercarriage is not locked down on approach, anadditional warning system is provided. This may be via the throttle microswitch as in the drawing, or it may be via a ground proximity warning systemor wired into a pitot pressure operated micro switch.On many aircraft, mechanical standby indicators are also fitted (Fig. 12)

Question: Can you describe the difference between a micro switchand a proximity switch. (15 mins)

Answer: A micro switch is a mechanical switch that opens or closeswhen contact is made with its operating button.Proximity switches may not have any moving parts -depending on type - and do not rely on contact. On onetype a magnet (say on the landing gear door) comes close toa magnetically sensitive proximity switch (i\fOSwitch)- thiswill cause it to operate. On the inductive type aferromagnetic material (target) is fixed to, say, the door andthe proximity switch is just a coil - fitted to the structure.When the target gets close to the coil its inductance(resistance) is increased and an electronic circuit will pickthis up.(Tutor: My answer maybe too "heavy" - but its all good stuffanyway).

Figure 10 shows the landing gear in the unlocked position with the red lampconnected to the 28V d.c. supply via both microswitches.If the gear is on the way up then the next thing to happen is the uplockmicroswitch is operated breaking the circuit to the red light and the light goesQuLIf the gear was 011 its way down the next thing to happen would be thedownlock microswitch operates to switch off the red light and switch on thegreen.

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T HRO TT LE MI CRO SWI TC H\28V d.c.BU S BA R\

D OWN LO CK MI CRO SWI TC H

U P L O C KMICROSWITCH

\ t----

G R E E NRE D

LAMP

FL I G HT D E C K I N D I C A T O RHORN S I G N A LC O N D I T I O N I N GU N I T (S.G.U.)

C .R.T . D IS PL AY

WA RN IN G D IS PL A Y

Fig. 10 INDICATORSYSTEM

Should the pilot try to make a landing approach with the landing gear NOTlocked down then a warning system will operate. In this drawing that warningsystem is connected via the throttle.

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If the landing gear is not locked down then the electrical circuit is as shown inthe drawing. When the pilot decreases speed by pulling his throttles back themicroswitch in the throttle quadrant is operated to connect:-

* Warning lamps.* Horns.* CRTdisplays.* Config. warnings.Most modern aircraft will have a configuration warning system (Config warning)to check that all the systems are set correctly for a particular operation e.g.

* Landing.* Take-off, etc.

Aulo brake panel andgear position Indicator(System 1 1

A l S l t I DIIOSEWMUL(i):

l D G G E A R

Tyre press. Psi(optional'Hottest brake

~NTI SKID ______ Auto-brakeindication

landing GearSelector Lever Brake pressureIndication(alternate syStem)NDSpoiler_--+ __ frO f iTinextension TRT19c I IC.w.60300.;

$Af dB 'c 23 M 56 C.C. 26 I ~~~~-~-- ---

WHEELSYSTEM PAGE LANDING ROLL

Fig. 11 THE A320 LANDINGGEAR CONTROLS AND INDICATIONS


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Typical of a modern flight deck on a civilairliner is the A320. This uses what iscalled an ECAMsystem (Electronic Centralised Aircraft Monitor).This is a computer operated system which will show coloured symbols orpictures of the position of the landing gear. To achieve this the microswitch /proximity switch outputs are sent to a symbol generator which then sends theappropriate signal to the screen. The ECAMhas to be selected to Landing GearPage for it to actually show on the screen (Figure 11).Pictures of other systems can be shown on the screen such as.-

* Engine (gauges).* Cabin condition.* Hydraulics, etc. - by selecting the appropriate page.

,_ Landing Gear Selection (Figure 13)The selector might be a simple lever that operates a push/pull rod system tooperate a hydraulic valve.Asafety interlock is provided to prevent the inadvertent UP selection of thelanding gear when the aircraft is on the ground. This may be over-ridden bythe pilot in an emergency by the use of a separate override lever or by operatingthe existing selector in a special way.In the simple circuit illustrated, the undercarriage is electrically controlled byUP and DOWNselector push switches which operate the electro-hydraulicselector valve. The valve controls the flowoffluid to the jacks, which raise andlower the undercarriage.The push switches are housed in the undercarriage control box. The switchesconsist of a spring-loaded interlock unit so that pressure on one switchreleases the other. The UP switch incorporates a safety lever lock pin, whichprevents UP selection until its solenoid is energised by the closing of thecontacts of the micro switch fitted to one or both of the main legs.With the squat switch in the open circuit position as shown, no current isgoing through the solenoid so the spring pushes it forward under the UPselector button. Thus the button cannot be pushed in.

- w n - e n - the aircraft takes-off the squat switeh-eleses and thesutisequentc11rrentin the solenoid causes it to retract from under the UP button so the pilot cannow select the landing gear up. In an emergency the pilotcanrotate the UPselector button through 90. This will allow the button to be pressed in as thetwo "legs"pass either side of the solenoid lock pin. The buttons areinterconnected so that as one is pushed in so the other is pushed out.

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NOTES:1. The squat switch may be called.-

(a) Weight on wheels switch (Airbus).(b) Ground sensor (Lockheed. Douglas).(c) Ground air sensor (Boeing).(d) Flight ground switch (K.L.M.).

2. There are various different types of ground safety interlocks.

u l c U P BU T T O N ~\N D ERC ARRI AG E C ON TRO L BO X28V d.c. BU S BA R

u l c D OWN BU TT ONS O L E N O I D

M A I N u l c L E G

E LE CT RI CA LL Y O PE RA TE DHYD RA U L I C T WO WA Y

_ ~S E L E C T O R V A L V E __ ~ ~

r .,I

\I

---.---.-----.~

S QU AT S WI TC H

-Fig. 13 SELECTOR CONTROL SYSTEM


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Figure 13shows the principle of how landing gear UPprotection can beachieved - with override. What it does not show is the design of the selectorlever it-self.Levers in the flight deck are so designed as to be tactile. In other words theycan be identified by feel.This allows the pilot to instinctively knowby feel thathe/ she has the right leverwhen carrying out a selection. It also allowsselection in zero visibility conditions eg, in some sort of an emergency.Flap selector levers are shaped like a flap and landing gear selector levers areshaped like a tyre.Atypical lever is shown in figure 13a. This belongs to a Boeing747-400. Notethe selector positions: it's shape, the override button, and the alternate geardown switches.

O f

UP

D N

L A N D I N G G EA R L EV ERU P . . r e l e a s e s d o w n l o c k s. . p r e s s u r i z e s u p s i d e o f g e a r a c t u a t o r sO FF " d e pr e s s ur iz es l a nd i n g g ea r h y dr au l i c

s y s t e .D N - r e l e a s e s u p l o c k s_ p r e s s u r i z e s d o w n s i d e o f g e a r a c t u a t o r s

_ - _ _ _ , I - - - - L E V E R L O C K C V E R R I D E S W I T C HP U S H - r e l e a s e s l e v e r l o c k

A LT E R N A T E G E AR E XT EN S I ON S W IT CH E S( a l t e r n a t e a c t i o n , g u a r d e d )A L T N . . r e l e a s e s g e a r d o o r s a n d g e a r

u p l o c k s

C E N T ER P A N E L

Fig.13a BOEING747 ..400 SELECTOR

- 24a-


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Retraction TestingRetraction tests may be required at regular intervals to check the correctmechanical, electrical and hydraulic functioning of the undercarriage system.They will also be necessary after the rectification of any reported defects orreplacement of components.Before any retraction may be commenced, the aircraft must be jacked andtrestle i.a.w. the manual and electrical and hydraulic systems checked forserviceability. The actual procedure is detailed in the aircraft manual.To prevent accidents, due to personnel walking under the aircraft during thetest, warning barriers should be erected and all non-essential personnelcleared from the vicinity of the aircraft.The general procedure for a retraction test is:-(a) Check aircraft maintenance manual.(b) Jack aircraft. Remove ground locks.(c) Ensure hydraulic and electrical/ electronic systems are serviceable.(d) Clear area and aircraft in the region of undercarriage and place warning

placards.(e) Connect external hydraulic and electrical power.(f ) If in doubt, use hand pump for first retraction.(g) Select undercarriage up under power and check:-

1. Time up, time down.2. Flight deck and standby indicators.3. Any warning devices / config systems.4. Correct sequencing.5. Correct locking and unlocking.6. Correct fitting of leg and doors.7. Correct interconnection of services, e.g. retraction andsteering systems.8. Smooth operation.9. Leaks.

(h) Lower undercarriage - check down locks are in 8.1l.(:l'1hreegreens' areshowing.(i) Check free fallIemergency lowering system (Note: If the system is blowndown by gas the pressure will have to be relieved and the system bled).(jr~eno-the retracttcrrtest.r-: ---_._-_.-(k) Ensure landing gear is down and locked and 'three greens' are

showing. Fit ground locks.(1) Disconnect external power.(m) Lower aircraft and place chocks.(n) Record the work done and sign appropriate aircraft documentation .

. :-_2.5__- _


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NOTE. Some testing may be done by using the aircraft electrically operatedpumps.

Steering Testing1. Refer to aircraft maintenance manual.2. Raise the aircraft on jacks and place 2 steel plates with grease betweenthem under the nose wheels - lower the aircraft.3. Connect external hydraulic and electrical power.4. Check that nose wheel casters with power steering selected otT.5. Select power steering on.6. Operate the steering wheel over its full range and check that the nosewheel follows smoothly and stops at selected positions.7. Jack the aircraft clear of the ground.S. Set the nose wheel a few degrees to one side and select the landing gear

up, checking that the nose wheel centres before the down-lockdisengages.9. Lower landing gear and repeat operation (S)with the nose wheeldisplaced in the opposite direction.10. Carry out further retractions to check that the steering is only operativewhen the nose undercarriage is down.11. Check that the stand-by accumulator is correctly charged with airpressure and operate the test rig to pressurise the accumulator.12. Select stand-by steering and check that the nose wheel can be steeredsatisfactorily. This check may involve a specified number of turns beforethe accumulator is exhausted or the stand-by system low pressurewarning lights illuminate.13. Set the stand-by selector to off, and disconnect the test rig and externalelectrical power.14. Lower the aircraft.15. Record the work done and sign the appropriate aircraft documentation.

UNDERCARRIAGECONFIGURATION(Figure 14)The increase in size and all up weight (AUW)of modern aircraft has led to anincrease inwheel loading; this being defined as the static load on each wheelof the landing gear at aircraft take-otT weight. Since the main undercarriagecarries a large proportion of the aircraft weight main wheels are the greatestproblem.

Question: Can you explain how wheel loading can be calculated?(There is a very simple way) (5 mins)Answer: The wheel loading is the same as the tyre pressure.


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Wheel loading has a direct bearing on the type of surface from which theaircraft can operate and thus the role of the aircraft directly affects theundercarriage configuration. Itcan be seen that an aircraft with a high wheelloading would sink into a lowstrength runway.Since it is very expensive to strengthen the very long runways required formodern transport aircraft, undercarriages which confer lowwheel loading arein considerable use. These replace large single wheels using high pressuretyres with a number of small wheels using lower pressure tyres.The actual configuration chosen for the aircraft is decided by the problem ofstorage when retracted as well as its load spreading properties.

DOUBLE WHEELSINGLE WHEEL BOGIE (4, 8 or 12 wheels)

Fig. 14 WHEEL CONFIGURATION

Multi wheeled units however-nave-other advantages than just reductionofwheel loading. Theyare:-. _@ }_ . Easier to service - although the unit is more complex the components,especially tfie wheels, are smaIler-anClmerefoie easier to handle.---- .

(b) Easier to stow - since the wheels are small the stowed volume is reduced.Bogie units especially can be folded to reduce volume (Fig. 17 & 18).(c) Greater safety factor - in the event of a tyre burst there will be one ormore serviceable wheels remaining to carry the load.


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The great disadvantage with bogie units is due to their large footprint. Sincethe unit must crab when the aircraft is turning, the turning radius will be largeand tyre wear occurs due to scrubbing. This problem has resulted in the useof a unique four wheeled unit on the Trident which spreads the load but doesnot restrict the turning of the aircraft. The articulated leg carries one axle withtwo wheels on either side of the leg (Fig. 16).Bogie Undercarriage UnitsBogie units comprise a telescopic leg containing a shock absorber, and a bogiebeam, pin jointed at its centre to the shock absorber, which carries a front andrear stub axles for the four or eight wheels. Torque links between the leg andthe beam prevent rotating of the oleo relative to the leg.The bogie beam is arranged so that the rear wheels usually trail when theweight of the aircraft is off the undercarriage. This and the aircraft's landingattitude, ensure that these rear wheels contact the runway first. This iscountered by the fitment of a bogie damper strut mounted between the leg andthe bogie beam. This is controlled articulation.

BRAKECOMPNSATINGAO O

Fig. 15 TYPICAL BOGIE UNDERCARRIAGE


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Initially the undercarriage unit operates as a levered suspension type, in thatthe bogie damper takes the initial landing shocks, until all four wheels are incontact with the runway when it becomes the direct action type. The mainshock absorber taking all subsequent loads and the bogie damper merelyallows for uneven runways.

Fig. 16 THE TRIDENT MAIN GEAR

MAIN

NOSEFig. 17 RETRACTION OF THE NOSE &MAIN UNITS

- TRIDENT


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Fig. 18 RETRACTION OF THE MAINGEAR - TUPOLEVTU134

Fig. 19 THE 747 MAlNGBAR-WING&BODY


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rig. 20 THE A300 MAIN GEAR

SHOCK ABSORBERSPrinciplesToallow landing and taxying of the aircraft to be accomplished without damageto the aircraft, the alighting gear incorporates shock absorbing devicesconsisting of the pneumatic tyres and shock absorber struts which are fitted toeach undercarriage unit.iThe.shock absorbers are commonly known a~~l~_()s.The oleoworks on the principle of converting the downward kinetic energy of

_______~ aircraft into ~~essureenergy within the oleo.Question: Whathappens to the forward kinetic energy of the aircraft on

landing. (lO mins).

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Answer: Most of it is converted into heat energy within the brakes.Some is used up by the use of spoilers, and reverse thrustwill account for some. Avery small amount is taken up asairframe drag.The oleo has two main functions on landing:-

1. Absorb the shock (kinetic energy).2. Control recoil.There are three types of shock absorbers (apart from bungee cords on somesmall aircraft):-

1. The liquid spring type.2. The gas/oil type without separator.3. The gas/oil type with separator.In the liquid spring type the energy is absorbed by the increase in pressure ofthe fluid and the recoil is controlled by controlling the rate at which the fluid isallowed to pass through the piston.With the gas oil type of shock absorber the gas absorbs the shock of landingwhile the fluid is used to control the rate of recoil.Question: Why is it so important to control recoil rate? (5 mins)Answer: If recoil wasn't controlled the pent-up energy in the shock

absorber would cause the unit to extend too quickly andpush the aircraft back up into the air - not a good thing.Each oleo leg consists of an inner tubular or solid member, fitted with a piston,telescoping into an outer tubular member. When the aircraft is on the ground,the leg telescopes sufficiently to create a pressure which supports the weight ofthe aircraft.When the aircraft becomes airborne, its weight is transferred to the main-planes, and the oleo leg fully extends. When the aircraft lands, the weight of...---- the aircraft telescopes the oleo leg, w_l1i~_l1_~Qsorbshe shock of landing.

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1YPESOF OLEOSOil-CompressionGenerally termed liquid spring, this type of oleo leg contains oil only. Itconsists of a thick walled cylinder housing a piston fitted with a large diameterpiston rod. When under load, entry of the piston rod into the cylinder reducesthe internal volume and the oil is therefore compressed; thus, compression ofthe oil absorbs the shock of landing.Todampen the rebound, the flowof oil is restricted to a fewsmall holes in thepiston head when the leg extends.In general a fluid is considered to be incompressible at the lower pressures butas the pressure increases so it becomes more compressible until at very highpressures it behaves very similar to a gas. It is related to the fluids BulkModulus.In the case of the liquid spring shock absorber the initial charge pressure isabout 1700 PSI (but refer to the manual). This pressure is felt both sides of thepiston - but as one side has got a greater area than the other so the netresultant force will push it out (extend).When the aircraft lands the cylinders volume is reduced by the entry of thepiston rod and the pressure rise is considerable.

Gas/Oil OleoThis type of oleo leg contains nitrogen and oil. When under load, entry of thepiston rod reduces the internal volume thus further compressing the nitrogenwhich absorbs the shock of landing. Restriction of the flowof oil through thepiston head dampens the recoil.This type may be designed with a separator or have nitrogen/ oil interface.

OIL-CQMPI5ESSION (LIQUID- SPRING)OLEO (Figure 21)This unit is not very popular on civil aircraft because of its harsh ride.This shock absorber consists of a thick-walled cylinder into which slides a larg~_diameter piston rod, fitted with a piston assembly. Ahigh pressure glandassembly seals the cylinder to make an oil-tight joint with the piston rod.

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L O C A T I N G RI N G

R I N G

C Y L I N D E R

BL E E D E R P L U G '

G L A N DA S S E M B LY

C HA RG I N G V A L V E

Fig. 21 LIQUIDSPRING ~

The piston assembly consists of a pistenhead, sCreWecronto the. end ofthepiston rod and locked bya pin and split pin, and is.fitted with a piston ring.Fluid leak holes are drilled through the head on an inner and outer diameter,circ1iplimits the liftof the valve.


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The gland assembly consists of an inner washer fitted with four pegs, twogauge washers, the mesh of one at 45 to the other, a gland ring of rubberisedfabric construction solidly bonded together, making a non-resilient seal andhaving an interference fit on its inner and outer diameters, four small and onelarge chamfered rings, and an outer washer with radial slots cut across thedowel holes. The assembly is retained in the cylinder by a gland nut, recessedfor a housing fitted with a wiper ring. A locking plate engages slots in thecylinder and the nut, and it is secured by two bolts.The assembly is so designed that an increase in pressure on the inner washerwill give a corresponding increase in pressure on the gland ring which willalways be greater than the fluid pressure.A charging valve and a bleed plug are screwed into the cylinder. A dust plugwhich is fitted with a retaining chain, is used to protect the charging valve.

Operation (Figure 22)No LoadThe oleo leg is filledwith oil, bled of air, and pressurised, therefore, when theaircraft is airborne the piston is fully extended.Closure

When the aircraft lands the piston is forced into the cylinder, thus reducing thevolume and compressing the fluid to absorb the shock. Fluid transfersthrough all the holes in the head by lifting the plate valve.RecollWhen all the shock has been absorbed the piston is at the end of thecompressing stroke, pressure is equal on each side of the piston head but, as _,the area below the head is greater than the area above it, the forces areunbalanced. The greater force below the piston head tends to eject the pistonat high speed, and with this extending movement, the plate valve is moved toclose the inner holes.rthus fluid transfer is confined to the outer holes.only and"Recoil"is controlled.

When the aircraft stops taxying and is stationary, the leg is slightly compresseduntil the internal resultant force tending to eject the piston is equal to theweight of the aircraft acting on the oleo leg.

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PISTON IM O Ya IE NT ,

PISTON to Y E A I E N Tf lUTTER P lATESTOP

flUTTER PlATE(a.oseDl

F\JJID PRESSURE--4-...I.._ N:REASIHG CONSIDEAAllYNJ COMPReSSING

W HE el E NO O F U N IT

A I RC RA FT L AN DI NG L E G E XT E N D I N G S L O WL YFig. 22 OPERATION

ServicingVariations in the design of different kinds of liquid spring oleo legs,necessitates a variation in servicing procedure. For the type of oil, andservicing instructions for specific oleo legs, consult the aircraft servicingschedule. However, the servicing is similar in principle, and for the oleo legdescribed is as follows:-Topping-UpIf when carrying out a static deflection check, i.e. the reduction in length whilstunder load, excessive deflection is found. Top up the shock absorber asfollows:-(a ) Jack up the aircraft to remove Uieload on theoteoteg,(b) Connect the flexible charging pipe to the universal lubricating gun, and--~p*n*'mHlie!leed.thecharging pipe with the specified oU.Remov~_th,--e-,,--=-du-=--=-st,,--plug from the charging valve and connect the primed pipe and gun tothe valve.(c) Charge the oleo leg to the correct pressure by operation of the charginggun pump. Release the pressure in the pipe by slackening the pipebleeder screw. Remove the gun and the pipe, replace the dust plug,wire lock to the bleeder plug, and lower the aircraft to the ground.


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LeakageExternal seepage from the oleo legwill indicate a faulty seal of the chargingvalve or gland assembly which must be renewed. If leakage occurs past thebleeder plug, it may be caused by wear, slackness or dirt. If cleaning andtightening is ineffective, the plug must be renewed.WARNING:Charged oleo legs are dangerous. Before attempting to dismantle

an oleo leg, the pressure must be released.

Fault DiagnosisSymptoms Probable Causes Remedy

Excessive Deflection Loss of pressure - loss of oil -leakage at charging valve or gland.

Check reasonfor loss ofpressure or oil,and rectify.

Slowresponse todeflection - smoothmovement.

Pressure too low. Rectify pressure.

Slowresponse todeflection - erraticmovement.

Bent piston rod. Renew unit.

Fast response todeflection - harshmovement.

Pressure too high - faultypiston head assembly.

Rectify pressure;if fault persists,renew unit.

GAS/OIL OLEOWITHSEPARATOR (Figure 23)The oleo leg illustrated consists of an outer cylinder in which slides a plungertube,Jltted with a piston head. The plunger tube is prevented from rotating bytorsion links which secure it to the cylinder. The pIston head contains a springloaded flutter plate which normally covers small holes drilled through the head;more holes, outside the diameter of the flutter plate, act as restriction holes .

.~-.~--~-- ..The oleo leg is charged with oil and compressed nitrogen. A floating separatorin the plunger tube isolates the oil from the nitrogen.

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The upper end of the cylinder is closed by an oil head, which is fitted with twooil filler plugs, and the lower end of the plunger tube is closed by an air head,which is fitted with an air charging valve. To make an oil-tight joint betweenthe cylinder and the plunger tube, the lower end of the cylinder is fitted with agland assembly.

CYLINDER

OIL HEADIL FILLER PLUGS

PISTON HEAD

FLUTTER PLATE

PLUNGER TUBE

GAS CHARGING

Fig. 23 GAS/OIL OLEO WITH SEPARATOR


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Operation (Fig. 24)When the aircraft is airborne, the gas pressure moves the separator up theplunger tube, thus forcing the oil into the cylinder and the oleo leg fullyextends. In this condition the oil is at little or no pressure and the gas is at say1200 PSI - but of course check your manual. When the aircraft lands, itsweight telescopes the oleo leg and oil in the cylinder lifts the flutter plate andpasses freely through ALL the holes into the plunger tube. Oil entering theplunger tube forces the separator downwards which further compresses thenitrogen and absorbs the shock of landing.

FLUTTER PLATEClOSEO

r------::===r-t- OIL FLOW THROUGHAl PORTS

FLUTTER PLATEOPEN OIL COMPRESSED BUT WITHN EA RL Y O RIG iN A l V O lU M ELIMITED RETURN

OIL FLOW

AIRCRAFTLANDINGRECOIL

L . : : : : : = = : : : : = J ATTACHMEN T TO WHEE L END OF UN IT

GA S C OMPRESS ED WITHt REDUCED VOLUMEL..::::::=::::J A I TACHMENT TO WHEEL ENO OF UNIT7ig.24 OPERAT';':ON'

At the end of the compressing stroke, the gas pressure, which is now greaterthan that required to support the weight of the aircraft, tends rapidly to extendthe oleo leg. This action is prevented by the separator moving upwards andforcing the oil, assisted by a spring, to close the flutter plate. The flowof oilfrom the piston headto thecylinder is now restricted to a few smalLholeswhich governs the speed of extension. Thus, the restricted flowof oil dampensthe recoil. When the aircraft stops taxying and is stationary, the oleo leg iscompressed until the lnternal resultant force causing extension is equal to theweighfofthe- arrcf8if.---- -.----.---.---.-- ----------

ServicingThese types of oleo legs vary in detail for different aircraft, therefore, consultthe servicing schedule for specific servicing instructions. The general servicingprocedure is as follows:-


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Topping-UpPeriodically the oleo legmay require topping-up with nitrogen, but it isimportant that the oil level is correct, and if there is any doubt, the oil levelmust be checked before topping-up with gas. If the aircraft is unsupported andits weight is on the oleo leg, the oil levelmay be checked as follows:-(a) Fully deflate the oleo leg by use of an inflation adapter - releasing all

nitrogen pressure.NOTE: Before deflating the oleo leg, ensure that there is nothing underthe fuselage or mainplanes, such as trestles, etc., to cause damage asthe aircraft settles down. Also check in the manual that you are allowedto deflate the legwith weight on wheels.

(b) Charge a universal lubricating gun with the specified oil, remove one ofthe oil filler plugs and with the gun extension screwed into the hole,force oil into the oleo leg until no more will enter. This operation forcesthe separator downwards to the lower limit of its travel.

(c) Remove the other oil filler plug, and continue to pump in oil from thegun until the oil flowingfrom the hole is free from bubbles. Thisensures that air is not trapped in the oil chamber.

(d) Unscrew the gun and extension piece, replace both oil filler plugs, andlock with wire.

(e) Connect the adapter to the nitrogen charging valve and charge withnitrogen until the required height is reached - check this height on apressure extension graph which may be on a plate riveted to the oleoleg and will certainly be in the aircraft maintenance manual (Fig. 26).Fit dust cap.

(f] Check for leaks, record and sign for the work done.If the aircraft has to bejacked for oleo charging then proceed as follows:-(a) Jack the aircraft as peLth_e__manualFor aircraft jacking see the Book in

this series "Aircraft Handling").----------(b) Place a bottle jack under the oleo leg and while releasing the pressure

operate 'the battle jack to collapsetbe leg.(c) With the leg fullycoUapsed connect a hydraulic charging gun to one ofthe oil filter plugs.

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(d) Open the other filter plug and pump fluid in until an air free flowof fluidis emitted from the plug. If too much air/gas comes out then the reasonmust be investigated.

(e) While pumping close the filter plug and pressurise the leg to say 1000PSI (but check the manual). This ensures that the separator is at thebottom of its travel. While going this make sure that you do not lift theaircraft off its main Jack - this could be VERYDANGEROUS.

(I) Release the pressure, disconnect the charging gun, fit plug and wirelock.

(g) Connect adapter and nitrogen charging line to the nitrogen chargingvalve.

(h) Charge slowly with nitrogen ATTHE SAMETIMElowering the battlejack. This is MOSTIMPORTANTas otherwise you will lift the aircraft offthe main jacks.

QUESTION: What is the problem if the aircraft is lifted off one ofthe main jacks? (5 mins)ANSWER: The main jack will almost certainly come out of

alignment with the aircraft adapter - unless you arevery lucky and of course you will have a very unstableaircraft on for example: one main; one nose; and onebottle jack.

(i ) Charge the leg to a specific pressure (say 1200 PSI - BUTcheck themanual).

(j ) Remove the bottle jack. Lower the aircraft. Check for leaks and recordwork done. Sign the appropriate documents.

WARNING: Charged oleo legs are dangerous. Before attempting to dismantlean oleo leg, the pressure must bereleased,

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Fault DiagnosisVarious faults that may occur in gas/oil oleo legs, and some of their probablecauses and remedies are as follows:

SYMPTOMS PROBABLECAUSE REMEDY

Excessive deflection Loss of air pressure - leakage at Check reason forcharging valve oil-level plug or loss of airgland. pressure andrectify.

Insufficient Air pressure too high. Check airdeflection pressure andrectify.

Excessive sustaining Oil level too low (air pressure Inspect for leaks.ram travel, and correct). Check oil levelrolling when the and rectify.arrcnrlt turns on theground.Oleo leg harsh in Oil quantity too much. Check oil levelaction. and rectify.

Oleo leg sluggish inaction. Temporary adhesion of glandwasher to sustaining ram.Aircraft standing in one positionfor long period.

Should rectifyitself after taxyingbut if the faultpersists changeunit.

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COMPRESSION EXTENSION

, ' . . .:'"

CENTERRO D HO LE

, '. , . . " ,,-0 , . : , t , ' ,


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GAS/OIL OLEOWITHOUTSEPARATOR(Figure 25)This type of unit is popular with many civil aircraft manufacturers. This ispartly because there is no separator and the unit is simpler. Because there isno separator the gas is at the top with the oil underneath.On some units the centre rod may be tapered so that as the unit compressesless oil can squeeze through the gap between the rod and the restructure head,and it provides for a progressive deceleration of the unit.Figure 25 shows an A310 unit (which is typical). Itconsists of two slidingcylinders (one inside the other). Various valves are fitted to allow a reasonablyunrestricted flow of fluid (from the bottom to the top of the restructure head)during collapse, but on extension the valves close (assisted by gravity and oilflow) to restrict the rate of oil transfer, and hence control recoil.

OperationWith the weight of the aircraft off the leg, the gas pressure forces down on theoil which extends the unit fully. On landing, the unit is caused to telescopewhich forces fluid through the centre rod and valves into the upper chamber -thus compressing the nitrogen. The landing shock is absorbed.On recoil, the fluid is forced down into the lower chamber but can only getthrough via the centre rod and one (restricted) valve. The main valve is closedand therefore recoil is controlled.Its static condition is where internal pressure times the area of the unit base isequal to the weight on that unit.

ServicingThe shock absorber is of direct nitrogen fluid contact type and is filled with oiland nitrogen through one common filling and charging valve. This is locatedon the top of the oleo and is connected to an internal filler tube whichautomatically limits the amo_un_!_ofil during refilling.Nitrogen pressure and compression distance characteristics are given by achart attached to the ole()}~g (Figure 26) - and of course it is in the manual.

-_---- ------_---------- ---_--Ramp servicing normally consists of checking the gas inflation pressure.

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The oil level is checked with the unit collapsed. This may be carried out withthe aircraft weight on the wheels or with it jacked and the unit compressedusing a bottle jack. In both cases the nitrogen pressure is released.After the oil has been topped up to the bottom of the stack pipe the unit ischarged with nitrogen with a similar procedure as carried out with the gas/oiloleowith separator.

rpsi BARS, " ' "20 1 SERVICING CHART100DIAGRAMME

f l O O \ AMORTISSEUR\1 0 0 \ L I"00 \\ , \ \ \ 1 \ -

1100 \ \ \ \ H 1\ \ 1 \ ~ ~.\ \ 1 \ 1 \ ll000 \ \\ \ ~ 40"C(,'0 fl\ \ \ \ 20"C(., flt o o _,\\ \\\ O-C(lZFIs o _,

I \.~ ~ ~~ !--"20"t1l.FI

/100 /: 4a"C110'F'Ii ,~\.~ /'J O '00 i'. 200"-~ ~ c i o K ' H(_I'00 ,- . . .o ! Z , ! I I IIIII 7 I IOn121214 IftC ....

'_WITH AI"P\AN( wtlGt4T ON O(A. '_AMO"'ISSE!JR SOUSMUS\.M[ ST"UT f'tlUSU,,[ CHUGE S'A'IQU M(SU"[.WITH""[$SUa[ OAII SA rMSSOf

2_0UUMeC COMlCT OtMNSION 2_[N ODUIII LA lOMOUUII.~ rot ,.., ""SSUItI tRON " 'H"' S\III U OtAO.. "":SUVIONI CU'tV

r; .. NCUU." ADO 0.' -~NlCnSAtM: GONFlE" A"'aoGlN '0 OIlAIN CO'tJtCl lOAZOT sr.c f'QM"auT OIM( ..... "[, ...... l"tMF'ONCMN'

C~C' 0( lOAtdTtSKutt- -- -~~~---' "

Fig. 26 A310 NOSE GEAR OLEO _CHARGING CHART

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SERVICING EQUIPMENTThe aircraft maintenance manual will specify the type of equipment that isrequired. In some cases it might be "special to type" and supplied by theaircraft manufacturer. In other cases the equipment might be standardequipment and be usable on many different aircraft types. What follows is adescription of some "standard equipment".

Universal Lubricating Gun (Figure 27)The universal lubricating gun is used to charge gas/oil and liquid spring oleoswith oil. Itconsists of a connection block, fitted at one end with a non-returnvalve and a bleeder screw, and the union of a rubber pipe assembly is screwedover the other end. A banjo-type adapter is secured through the block by acirc1ipand a pressure gauge is screwed into the adapter. A second adapter, for

'" connection to the lubricating gun, is screwed to the outer end of the non-returnvalve.

PRESSURE GAUGE ...

SPECTAL BAN.IO l'NTON

Fig. 27 UNIVERSAL CHARGING GUN

----4Tr1h...~n...n=rennIIvalve consists-ora-body anda;-unionreach formed with apjpe___ ___ .connection and screwed together to house a ball valve assembly. Aspecialbanjo union with trapped seals is connected to the union at the outer end ofthe pipe assembly, and a special banjo bolt, located through the union, isretained by a screwed dust cap and a chain. For the charging operation, thecap is removed and the bolt is screwed into the charging valve of thecomponent.


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Instructions for UseCharge the universal lubricating gun with the specified oil, then proceed asfollows:-(a) Screw the connection block end of the flexiblecharging pipe on to the

flexible charging pipe.(b) Ensure that the bleeder screw is screwed down tight, then prime theflexible charging pipe.(c) Remove the dust cap from the charging valve of the component andconnect the flexible charging pipe by rotating the special banjo bolt.(d) Bleed the flexible pipe by pumping fluid through and at the same timetighten banjo union.(e) Slacken the charging valve (ifnot of the non-return type), then operatethe lubricating gun until the specified pressure is obtained on thegauge.(f ) When the charging of the component has been completed, tighten thecharging valve (i f not of the non-return type), slacken the bleeder screwto release the pressure in the flexible charging pipe, remove the pipe.

Fit the dust cap to the banjo and the dust cap to the componentcharging valve; replace any locking device removed during theoperation.

Inflation AdapterThe inflation adapter is used when inflating, deflating, or when checking the air --pressure in oleo pneumatic shock absorbers. It will usually contain a pressuregauge an a pressure release valve.

High Pressure Nitrogen Charging Trolley.._This~~ent is used for inflating gasj oil shock absorbers with nitrogen.---._ ..---- . --.--~------The trolley may consist ofone or more bottles with controls and reducingvalves. Pressure gauges on the trolley allow for reading the bottle pressure (say4500 PSI)and for reading the reduced line pressure. This second readingshould correspond with the gauge on the inflation adapter.

~.47 -


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The controls can be set to reduce the bottle pressure to the desired linepressure.A point of caution here. If you are charging and using 2 or more gauges tocheck the same line pressure (i.e. reduced pressure gauge on the trolley andthe adapter gauge on the oleo) then the readings should both be similar. Ifthey are not stop the charging immediately and change the equipment.

" " " " " " " " " " " " " " " " " " " " " " " " "

----_._--- .. __ .._----

48 -


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THE AIRBUS A320The following pages gives general information on the A320 landing geararrangement. It is a good example of a modem aircraft system.You should be able to understand how the system works and relate it to whatyou have already learnt in the remainder of the book.

-------.~~~

~~~-~-----


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~A320 Landing gear - ArrangementNLG MLG

NL G steering actuator

r-- ----:.~.~:-. ---

~A320 Landing gear - Main features- Conventional tricycle LI G with twin wheels and direct action shockabsorbers. .- MLG retracting laterally and NLG forward into the fuselage- Electrically controlled by two Landing Gear Control IInterface Units(LGCIU).- Hydraulically actuated with alternative free fall / spring down lockmode .

.d~Alternatinguse of both channels for-eaeh retraction/ extension cycle.- In the event of one channel failure, resetting of the L/ G control leverresults in change over to the other channel.__..._Eltmination of-micros-w~t~hesyUseo f trouble~tydetec-~~.--tors for position sensing.

- Landing gear computers are connected to the CFDIU to easemaintenance.- Radial tyres standard - MLG : 45x 16 R20, NLG : 30x8.8 R15.


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~A320 Land!ing gear - Braking system Carbon disc brakes are basic. Normal system (green hydro syst. supply) :- Electrically signalled through anti-skid valves- Individual wheel anti-skid control- Autobrake function- Automatic changeover to standby system in event of green hydr.supply failure. Alternate system (yellow hydro syst. supply) :- Hydraulically controlled through metering valve- Individual wheel anti-skid control

- No autobrake function . Emergency system (yellow hydro syst. supply or yellow brake poweraccumulator) :- Hydraulically controlled by pedals with brake press. indicationon gauges.- No anti-skid control. Parking brake (yellow hydro syst. supply or yellow brake poweraccumulator) :- Electrically signalled- Hydraulically controlled with brake press, indication on gauges .

Use of Braking & Steering Computer Unit (BSCU) BSCU is a fully digital dual channel computer controlling the followingfunctions:- Nose wheel steering command processing- Normal braking system controlAnti-skid control (normal and standby). Speed reference fromADIRS (nosewheel tacho, not required)- Auto brake function is basic (LO, MED, MAX). Values for LO &....MEl1by memory programming

- Brake temperature signal processing- Monitoring of all these functions_.__ ~_Brake temperature indication is basic


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~A320 Landing gear - NLG steering principle

/DISCONNECTIONPED AL - N OSE W HEEL S TEERIN G

ELAC ,E lE C. S U I'P lY as.c.u,N GIN E O il PRES S. PO S TRANSM . r C Tl Il /GSOUAT ISWITCHES aRAKINC .. SHERINC COM PU TER UNIT

P O S. T R AN SM ., Mo I Ii I .1

= = = = = = PRESSURE RETURN

The following options are offered :- TPI- BCF- WBS

Tyre Pressure indicating SystemBrake Cooling FansWeight and Balance System

-~Cross-ply and alternative tyre size options---- ---

~--------

- 3 -


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~A320 Landing gear - Auto. braking principle

$A320 Landing gear - Warning test buttonWarning test button functions:Pressing the Warn Test button on the L I G selector panel for a systemcheck will activate simulation of the aircraft configuration parameters ~to:

a) Illuminate the RED master warning lightb) Illuminate the REt) down arrow on the L I G selectorc) Display the ECAM L I G page on DU2.d) Sound a continuous aural warning.


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THE AIRBUS SHOCK ABSORBER

MaintenanceThe oleo type shock absorber is a special development and is different frommost other wide body shock absorbers in as much as the absorber itself is aseparate assembly contained within the shock strut.The absorber assembly comprises twomain components: a fixed cylinder anda sliding tube.The fixed cylinder is attached to the shock strut by an attachment rod, whilethe sliding tube can slide freely inside the shock strut.The tube is supported at the bottom of the strut by a self aligning bearing andin the middle by a plain bearing. The sealing rings of the absorber are insidethe shock strut, where they are protected against dust and sand.The grease sealing rings at the lower end of the strut and the sealing rings ofthe shock absorber are not line replaceable units.The shock absorber assembly is of direct nitrogen fluid contact type and isfilledwith oiland nitrogen through one common filling and charging valve.This is located on the front of the shock strut and is connected to an internalfiller tube which automatically limits the amount of oil during refilling,The oil is distributed inside the absorber into two cylindrical and one annularchamber. The three chambers are interconnected through orifices, integratedin the base plate of the fixed cylinder to cushion the landing impact.Avertical centre tube is used to brake the end of absorber expansion after takeoff. The absorber sliding tube travel is 450mm (17.72 in).Nitrogen pressure and compression distance characteristics are given by a

---~hart attached to the lower end of the-shock-strut.


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FIXEDCYLINDER-~~SEALINGRINGS

~

SHOCKSTRUT~ IATTACHMENT~RODPLAINBEARING

GREASESEALING RINGSPHERICALBEARING

oSHOCKABSORBER ASSEMBLY


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Servicing - Aircraft on GroundThe shock absorber oil refillingprocedure is divided into the followingsteps:-1. DEFLATE- Connect a selector valve and a pressure hose to the filling

and charging valve. Open the valve and release the nitrogen pressureuntil the absorber is against the inner stop. Check the remaining deadtravel of34mm.2. OILCHARGING- Connect an oil pump to the selector and charge the

absorber with oil until the shock strut is raised by 20mm. Check thenew travel of 54mm. NOTE: Use only oil indicated on the absorber dataplate, located beside the valve.3. BLEED- Wait one minute. Open the selector and bleed the absorber

until it reaches the inner stop again. Check fluid for bubbles. Ifnecessary repeat step 2.

4. PRESSURISATION- Connect a nitrogen pressure source to theselector. Pressurise the absorber to 5 bar. Check that there is nomovement on the shock strut.5. BLEED - Open the selector to bleed the fluid. The 5 bar (72.5 PSI)

nitrogen pressure acts inside the fixed cylinder against the fluid leveland forces out the surplus fluid until the lower end of the filler tube isreached when the bleed flowstops, the correct fluid level is obtained.NITROGENCHARGING- Gradually charge with nitrogen to obtain the correctpressure corresponding to "D"given by the servicing chart, attached to theshock strut."D"is measured between the base of the strut and the upper surface of thetorsion link attachment.


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1. DEFLATE

2. OIL CHARGING. .

- -------


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3. BLEED

4.PRESSURIZEc... _...........5 BARS

S f

~~---. ~~--- _-


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Pressure / Extension Check - Aircraft on GroundAtypical shock absorber check is performed as follows:-Checking of the correct absorber extension. Ambient temperature is + 100C.1. Connect a pressure gauge to the fillingand charging valve.2. The gauge indicates 100 bars (1450 PSI).3. Pressure versus DIMENSION"0" with the lOoe compensating curvegives 100mm (3.9in).4. Check "0" measured between the base of the shock strut and the uppersurface of the torsion link attachment. It should be lOOmm(3.9in).5. Ifnecessary, add nitrogen.

A:I20ClUiA

2.

3.

19 landing gear

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