high altitude meteorology

7/31/2019 High Altitude Meteorology

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High-Altitude Meteorology

Warm Temperatures Aloft Are Bad News

Warm Temperatures Aloft Decrease Performance

Temperatures warmer than standard significantly reduce aircraft climb and cruise performance:

At 10 degrees C warmer than standard, you can expect a moderate decrease in performance

At 20 degrees C warmer than standard, you can expect a significant decrease in performanceCant Climb or Cruise As Well

When temperatures are warmer than standard: You may not be able to climb to cruise altitudes that you could reach easily with cooler

temperatures

You can also expect a reduction in cruise speed by as much as 20 to 40 knotsCompare Actual Temperatures Aloft to Standard Temperatures

When you are getting a weather briefing, compare the actual temperatures aloft with the standardtemperature for the altitudes at which you will be cruising.

You do this to determine: Your ability to climb to a planned cruise altitude at the start of the flight

Your cruise performance at altitude

How to Compare Standard and Actual Temperatures Aloft

The International Standard Atmosphere (ISA)

The standard temperature and pressure at any particular altitude are part of what is referred to asthe International Standard Atmosphere (ISA).

The International Standard Atmosphere values at sea level are: 15 degrees Celsius

29.92 inches of mercury

There are standard values for every altitude.

Temperature Aloft in the Standard Atmosphere

Here is how the temperature changes with altitude in the International Standard Atmosphere: At sea levelthe temperature is 15 degrees C Each 1,000 ft higherthe temperature is 2 degrees C cooler until about 36,000 feet

36,000 feet and higherthe temperature stays at -56.5 degrees C

There are terms used to describe the atmospherebased on the way the temperature behaves withincreasing altitude:

Tropospherethe lower part of the atmospherewhere the temperature decreases with altitudeTropopausethe boundary between thetroposphere and the stratosphereStratospherethe upper part of the flyableatmosphere where the temperature stays thesame as altitude increases (the temperature issaid to be isothermal)

Tropopause Height Varies With Latitude and Season

As opposed to the standard atmosphere, the actual height of the tropopause varies.

Here is where you can normally expect to find the tropopause:


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Over the polesabout 20,000 feet Over the equatorabout 65,000 feet

Higher in the summer Lower in the winter Higher in the daytime Lower at nightSignificance of the Tropopause

Tropopause characteristics:

Slopes down from the equator towards the poles Often has abrupt steps in the slope (this is where the jet streams form) Tends to cause the topping out of most thunderstorms

To Calculate Standard Temperature Aloft, Start With 15 Degrees C and Subtract 2 Degrees Per 1,000Feet

The easy way to calculate standard temperature at a specific altitude:1. Multiply the altitude in thousands times 22. Subtract 153. Convert the temperature to a negative number

Examples

18,000 feet: 18 x 2 = 36 - 15 = 21 (-21 degrees C)24,000 feet: 24 x 2 = 48 - 15 = 33 (-33 degrees C)30,000 feet: 30 x 2 = 60 - 15 = 45 (-45 degrees C)

Above 24,000 Feet They Leave Off the Minus Sign on Temperatures

On winds and temperature aloft forecasts: The first two digits represent the wind direction in tens of degrees, in regard to true north

The second two digits represent the wind speed in knots The final two digits are the temperature in degrees Celsius. Since the temperatures are

always below zero above 24,000 feet, they leave off the minus signIn the example at ONT at 30,000 feet, the winds are from 270 degrees true at 69 knots. Thetemperature is minus 45 degrees Celsius.

FT 3000 6000 9000 12000 18000 24000 30000 34000 39000

ONT 2509 2408-01 2720-05 2825-09 2827-23 2853-33 276945 287051 286354

If Winds Are More Than 100 Knots, They Add 50 to the First Two Digits

Since there are only four digits available to express the wind direction and speed, they have touse a code when the winds are 100 knots or more. In this case they add 50 to the wind direction.

For example, at PHX at 30,000 feet the winds are from 260 degrees and the wind strength is 111knots.

FT 3000 6000 9000 12000 18000 24000 30000 34000 39000

PHX 2309 2310+04 2517-03 2523-10 2538-22 2782-30 761130 761253 269056

To encode this they add 50 to the first two digits:

26 + 50 = 76

So the wind direction is expressed as 76.

Since you cant have a wind from 760 degrees (more than 360 degrees), you know to subtract 50

from the first two digits to read the wind direction:

76 50 = 26


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The wind is actually from 260 degrees.

The fact that you had to subtract 50 from the first two digits tells you that you need to add 100 to

the wind speed:

The wind speed is given as 11.

11 + 100 = 111

The wind speed is 111.

The Maximum Wind Speed That Can Be Shown on the Forecast Is 199 Knots

Since only four digits are available to express the wind speed and direction using this system, the

maximum wind speed that can be shown is 199 knots.

In the rare case when the wind is stronger than 199 knots, it is shown on the winds aloft forecast

as 199 knots.

Practice at Comparing Actual Temperature Aloft to Standard

FT 3000 6000 9000 12000 18000 24000 30000 34000 39000

PHX 2309 2310+04 2517-03 2523-10 2538-22 2782-30 761130 761253 269056

The temperature at PHX at 30,000 feet is -30 C (they left off the minus sign, so you have tosupply it).

The standard temperature at 30,000 feet is -45 C (30 X 2 = 60, 60 - 15 = 45).

Temperature at PHX at 30,000 feet = -30

Standard temperature at 30,000 feet = -45Temperature compared to ISA =

(remember -30 is warmer than -45)ISA +15

ISA + 15 means your aircraft will not climb or cruise as well as normal.

Performance Charts Show Performance at Temperatures Compared to ISA

Performance charts often show jet aircraft performance at standard temperature (ISA) and ISA

plus or minus so many degrees.

Performance numbers are often given for: ISA +10 ISA ISA -10


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Jet Streams

River of Air With a Core Speed of 50 Knots or More

A jet stream is:

A river of air aloft With wind speeds of at least 50 knots, up to 200 knots or more Concentrated into a core at the tropopause

Usually located at one of the steps where the tropopause changes height abruptly

Where to Find Jet Streams

Knowing about the location of jet streams can be helpful: There can be several jet streams in the U.S. at one time They can combine and/or separate In the winter they tend to be at lower altitudes, stronger, and located closer to the equatorA Jet Stream Can Mean Bummer Headwinds and Turbulence

As a pilot you care about jet streams because: They can contain winds of up to 200 knots or more (great as a tailwind, a bummer as a

headwind)

Clear Air Turbulence (CAT), when not associated with thunderstorms, is almost alwaysassociated with a jet stream

It Can Mean Moderate to Severe Turbulence Over Great Distances

The speed of the wind associated with a jet stream can change dramatically as you move in andout of the core. This wind shear can cause moderate to even severe turbulence over greatdistances.

AIRMETs Are Issued for Moderate CAT, and SIGMETs for Severe CAT

When moderate turbulence aloft is forecast: An AIRMET is issued

AIRMETs for turbulence are labeled with the identifier TANGOWhen severe turbulence aloft is forecast:

A SIGMET is issued

The U.S. High-Level Significant Weather Prognostic Chart

This Prog Is Your One-Stop Source for High Altitude Weather Information

The U.S. High-Level Significant Weather Prognostic Chart:Forecasts significant weather at altitudes above 24,000 feet to 60,000 feetIs handily available on www.duat.comIs updated on the same schedule as TAFs (00Z, 06Z, 12Z, and 18Z)

It Shows Jet Streams, Turbulence, Thunderstorms and More

The U.S. High-Level Significant Weather Prognostic Chart also shows:The location and altitude of the core of maximum winds of jet streamsAreas of moderate or greater turbulence

Embedded thunderstorm clouds (CBs) or thunderstorm clouds with little or no space betweenthem


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How Altitudes Are Shown


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Altitudes on the chart are shown as a three-digit number:To read the altitudes on the chart, add two zeros as you would when reading cloud heights onMETARs and TAFsAltitudes below the base of the chart (24,000 feet) are shown with XXX instead of analtitude number

How Jet Stream Locations and Speeds Are Shown

Jet streams with a maximum speed of more than 80 knots are shown with:

Bold lines along the core of the maximum windsArrowheads to tell you the wind directionWind speeds depicted by 50-knot pennants and 10-knot barbsThe altitude of the core in a block underneath the wind speed indicator

How Far to Climb or Descend to Get Out of a Jet Stream

When the speed of a jet stream core is 120 knots or more:3-digit numbers below the altitude tell you how far you would have to climb or descend fromthe core of maximum winds to reach an altitude where the winds are down to 80 knotsThe numbers are separated by a slash with a plus for a climb and a minus for a descent

Jet Stream Speed Changes

Wind speed changes are shown:

In increments of 20 knotsWith double hatched lines along the core lineUsing wind speed indicators on either side of the double hatched lines to show

whether the change is an increase or decrease in wind speedTropopause Heights

Tropopause heights are shown in boxes:Rectangular boxes show specific tropopause heightsHome-plate shaped boxes with an H or an L show area centers of tropopause highor low heights

Thunderstorms

Scalloped lines show:Areas of embedded thunderstorms (CBs)CBs that have little space between them that would be difficult to

fly around visually

The tops and bases of the thunderstorms are shown along with an abbreviation indicating isolated(ISOL), occasional (OCNL), frequent (FRQ), or embedded (EMBD) CBs.

Turbulence

Bold dashed lines show:Areas of moderate or greater turbulenceNot associated with thunderstorms

The tops and bases of the turbulence are also shown.


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Intensity of the turbulence is shown by:Moderate turbulencea mountain symbol

Severe turbulencemountain symbol with a cap cloud

You will notice that the areas of turbulence shown on the chart are usually located along the bold

lines showing the jet stream locations.

Fronts

Also shown on the chart is:Surface location of fronts with their direction and speed of movementNOT the location of the fronts at altitude

Cyclones

Tropical cyclones (hurricanes, typhoons, and tropical storms) are shownwith:Tropical storma six/nine like symbol with the center open

Hurricane/typhoona six/nine like symbol with the center of the symbolfilled in

Squall Lines

Squall lines are shown with:Long dashed lines separated by aV

Volcanic Eruptions

Volcanic eruption sites are shown by:A symbol that looks like a mountain with an explosion ontopThe name of the volcanoIts latitude and longitude

Sandstorms and Dust Storms

Widespread sandstorms and dust storms are shown with:A large "S" symbol and an arrow

No arrow indicates a severe sandstorm or dust haze

Avoid the North Side of a Trough

Turbulence generally is:Greater on the north side of a jet streamStrongest as the jet stream speeds up around thebend at the bottom of a trough


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Consider an Altitude ChangeTo get out of turbulence:

Consider an altitude change

Ask ATC where other pilots are finding a smooth rideTry Crossing at an AngleThe duration of the turbulence you encounter will usually be less if:

You cross the jet stream at an angle instead of flying along the jet stream

Techniques for Thunderstorm Avoidance

Preflight Planning for Thunderstorm AvoidanceThe simplest and best technique for thunderstorm avoidance is to plan a trip so as to completelyavoid areas of potential thunderstorms.

Helpful thunderstorm avoidance planning tools are:

High Level Significant Weather Prognostic Chart Severe Weather Outlook U.S. Radar Summary Chart

U.S. Doppler Radar National Radar MosaicUsing Your EyesOne of the very best tools you have to avoid thunderstorms in the air is the windows in theaircraft.

Recommendations are:Stay at least 20 miles from any thunderstormAvoid flight under an anvil, which may spit out hailat you even though you are flying in the clear wellaway from the main storm cloudAvoid like the plague any tall cloud that has adistinct cauliflower shapethis is exactly what themeanest thunderstorms look like

Airborne Radar, Your Best Tactical Tool

But there are times when you cannot stay completely clear of clouds. In this case you have to useother tools than your eyesight to avoid thunderstorms.

Airborne tools for avoiding thunderstorms include: Up-loaded NEXRAD displaying precipitation returns for the entire 48 contiguous states

Lightning detection devices, called sferics Airborne radar, which shows areas of precipitation up to about 160 milesNEXRAD:

Gives radar information for the entire 48 statesIs helpful in planning strategic doglegs to allow you to fly around areas of precipitationIs not useful as a tactical tool to fly around specific areas of precipitation, due to the delay inreceipt of the data and lack of detail

Airborne Radar, Your Best Tactical Tool

But there are times when you cannot stay completely clear of clouds. In this case you have to

use other tools than your eyesight to avoid thunderstorms.

Airborne tools for avoiding thunderstorms include:Up-loaded NEXRAD displaying precipitation returns for the entire 48 contiguous statesLightning detection devices, called sferics


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Airborne radar, which shows areas of precipitation up to about 160 milesNEXRAD:

Gives radar information for the entire 48 states

Is helpful in planning strategic doglegs to allow you to fly around areas of precipitationIs not useful as a tactical tool to fly around specific areas of precipitation, due to the delayin receipt of the data and lack of detail

Airborne lightning detection systems:Give you good information to help you avoid hazardous weather associated with lightning

Dont provide the detail and precision location that airborne radar doesAirborne radar:

Gives immediate information with detail and precise locationOnly gives reliable information out to about 160 milesRequires more operator knowledge and skill


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Airborne lightning detection systems:

Give you good information to help you avoid hazardous weather associated with lightning Dont provide the detail and precision location that airborne radar doesAirborne radar:

Gives immediate information with detail and precise location Only gives reliable information out to about 160 miles Requires more operator knowledge and skill

How Airborne Radar Works

There are week-long courses on how to do this, but here are some basics on the operation ofradar:

Radar sends a horizontally sweeping beam of radar signals like a light beam The beam is a cone shape that gets bigger in diameter the further away it is from the sending

antenna Most small jets, with a 12-inch antenna, will have a beam width and height of 8 degrees4

degrees from the center of the beam to the edge When this beam hits precipitation it reflects back to your radar antenna, and the position of

the precipitation along with your systems estimate of its intensity is displayed on your screen

Since thunderstorm turbulence is frequently associated with precipitation, the system gives youinformation to help you avoid the worst turbulence.

The worst turbulence is usually associated with radar

returns:That are strongerIn which intensity changes very quickly over a

short distancein other words, when the contoursare close togetherThat have anything other than smooth, rounded

edgesThat have unusual shapes such as hooks andbowsthese can be associated with tornadoes and

hail


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The Two Basic Controls

The two basic controls a pilot has over radar are:The range displayed on the screenThe vertical tilt angle of the horizontally sweepingbeam

Setting the Range Control

Most radar systems give useful returns fromprecipitation up to about 160 miles from the aircraft.Beyond that range, only more intense precipitation willpaint a return on your screen.

Setting the range closer:Enlarges the display of the returns on the screenbut gives no more informationEliminates the display of returns beyond the rangeyou have selectedLets you see more detail when you are making

tactical decisions to avoid precipitation that isrelatively close to you


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Using the Tilt Control to Get the Most Information

The tilt control adjusts the vertical angle of the beam sweep: On some radars, the tilt control adjusts the vertical angle of the beam sweep regarding the

nose of the aircraft More expensive radars have gyroscopic stabilization that keeps the radar at a selected angle

to the ground regardless of whether the nose of the aircraft is pitched up or down. Thesesystems also keep the beam sweep horizontal even when the aircraft banks for a turn

Radar tilt facts and techniques:

Most pilots like to use the tilt control while enroute to keep some returns from the ground(often referred to as ground clutter) at the far edge of the display. This assures you that theradar is still working

The usual 8-degree beam is 8 miles or 48,000 feet tall at a distance of 60 miles and will

sweep both high-level and low-level precipitation and will sweep both high-level and low-level precipitationWith ground clutter at the far edge of the display and with a range setting of 60 miles or

more, you will most likely paint any strong precipitation beyond 60 milesAs they get closer, returns from things on the ground (cities, and most terrain) and fromlower-level precipitation will disappear from your screen as they move under your beam

angle

While at jet cruising altitudes, anything that displays on the screen inside of 30 miles shoulddemand your attention

When you want to avoid precipitation closer than 60 miles, you will occasionally want to tiltthe beam up and down (as much as 10 degrees) to look for precipitation your beam might bemissingSome radar systems have a vertical profile mode that lets you select an azimuth and havethe beam sweep vertically in that direction instead of horizontally. This allows you to see the

vertical extent of the precipitation in the direction you have selected


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When you are climbing or descending you may want to tilt the beam so that it displays theweather in the 18,000 to 25,000 foot altitude range, where thunderstorms serve up thenastiest turbulence. Provided your radar tilt control is calibrated properly, if you tilt an 8-degree beam up 4 degrees, the bottom of the beam will be parallel to the ground and at arange of 30 miles will display precipitation from your altitude up to 24,000 feet above youraltitude. This will help you see whats higher than your altitude and minimizes the display ofground clutter, which can be confusing on departure and approach

Loss of Returns Beyond Heavy Precipitation

In some cases heavy precipitation can reflect back all of the radar signals, leaving no signals toreflect off of precipitation beyond the heavy area. This loss of returns is often referred to asattenuation:


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In this case you have no information beyond the

point at which all the signals have been returned These areas will show up on your radar screen ashaving no returns. This can be a trap since there may

be heavy weather in an area that shows no returns If you have a little ground clutter at the far edge ofyour display, the ground clutter disappears in this

situation. This is a clue that you cannot rely on the

information you are receiving in that area

Some radars have a setting that will display a bluecolor to tell you when you are receiving no returnsbeyond an area of heavy precipitation. Like theabsence of ground clutter, this warns you that you aregetting no information about precipitation in that area.

Coping With Icing Conditions

Icing Is a Bad Thing

The presence of icing on an aircraft can significantly:

Raise the stalling speed Degrade the flight and stall characteristics Increase the incidence of engine damage or failure due to ice forming on the engine inlets

then breaking off and going through the engineIcing is a possibility anytime you are flying:

In visible moisture At temperatures of freezing and below. The temperature that is relevant is not the SAT (the

temperature of static, or ambient, air), but the temperature that the aircraft feels, the TAT(static air temperature plus temperature rise)

If the Systems Are Used Properly, Jets Usually Cope


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Jets usually have anti-icing capability in the form of:Bleed air or electric heat for engine nacellesBleed air for the wings and tail

Some aircraft can have de-ice in the form of either: Boots A weeping wing system, which pumps deicing fluid through a very fine mesh on the leading

edge of the wingSome combination of these systems is used to allow most jets to legally fly in known icingconditions.

All manufacturers will state when anti-icing systems must be activated, usually at a temperature afew degrees warmer than freezing. Some manufacturers state a temperature below which anti-icing is no longer required.

Use of Anti-Icing Degrades Performance

There is usually a performance penalty from turning on anti-icing systems: Most systems use bleed air, which when diverted from the engine, can significantly reduce

aircraft performance

Turning on anti-icing can reduce your climb rate when you are in icing conditions, just whenyoud like to climb as fast as you can

Regardless, the wise thing to do is follow the manufacturers recommendations.

Mountain Waves

Mountain Waves Mean Updrafts and Downdrafts But Not Necessarily Turbulence

When stable air flows over mountains it creates:A wave of smooth up and down oscillationsThis can create updrafts and downdrafts forhundreds of miles downwind of the mountains

Since at cruise altitude in a jet you are normally far above any rotor clouds, a mountain wavedoes not usually mean turbulence.


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Mountain Waves Can Cause Difficulty in Holding Altitude

Usually the problem with a mountain wave is difficulty in holding: Altitude, and/or

AirspeedThis can occur even well into the flight levels.

You should be aware that strong winds aloft, particularly in the wintertime, can mean difficulty inholding altitude on the lee side of mountain ranges.

As with jet stream turbulence, ATC will normally have a good handle on the location of anymountain wave activity.

Respiration Is Otherwise Known As Breathing

Its All About Getting Oxygen In and Waste Gases Out

Breathing gets oxygen into the circulatory system and waste gases out.

Its Oxygen Molecules Per Breath That Count

The amount of oxygen that gets into the circulatory system with each breath is determined by:

The number of oxygen molecules in a given volume of air, say a gallon of airThis number of oxygen molecules per gallon is referred to as the molecular concentration of theoxygen in the airDont Count Oxygen Molecules, Use Partial Pressure

Since molecules are hard to count, partial pressure is used:

Partial pressure is a handy way of expressing the molecular concentration of oxygen in eachbreath of air

The partial pressure of a particular gas is the pressure it would exert if it alone occupied thespace taken up by the mixture

We Use Millimeters of Mercury to Express Partial Pressure

Air pressure can be expressed in lots of ways including:

Pounds per square inch Millibars of pressure Inches of mercury Millimeters of mercuryThe convention when talking about air pressure in regard to the human body is to use millimetersof mercury (mm Hg).

The standard pressure at sea level is: 760 mm Hg

Partial Pressure of Oxygen Is About 21% of Air Pressure

Oxygen is about 21% of the air for all altitudes up to about 70,000 feet.

The partial pressure of oxygen (PO2) at sea level is:

.21 times 760 mm Hg About 160 mm HgAt Sea Level Hemoglobin Is 97% Saturated

By the time the oxygen works its way through the lungs to the blood: The partial pressure of oxygen in the arterial blood is reduced below 160 mm Hg It is now about 100 mm HgThis works well in a healthy person at sea level, since:

This results in about 97 percent saturation of the hemoglobin cells of your blood, whichtransport the oxygen in the circulatory system

About 87 to 97 percent hemoglobin saturation is required for proper functioning


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At Altitude Hemoglobin Saturation Decreases Dramatically

But lets take a look at what happens as you go to higher altitudes without: Supplemental oxygen

Pressurization

Altitude PO 2 In Air PO 2 In BloodHemoglobin Saturation

Sea Level 160 mm Hg 100 mm Hg 97%

10,000 Feet 110 mmHg 61 mm Hg 87%

15,000 Feet 90 mm Hg 44 mm Hg 79%20,000 Feet 73 mm Hg 34 mm Hg 67%

At About 10,000 Feet and Above You Need Help Breathing

Since the body needs at least 87% hemoglobin saturation to function normally, it is just a matter

of mathematics that as you take the body to altitudes above 10,000 feet without supplemental

oxygen or pressurization, the body doesnt do well.

Hypoxia Is Lack of Sufficient Oxygen to Tissues

The Brain and Eyes Need Help

The reason the body doesnt do so well at high altitudes without the use of oxygen orpressurization is hypoxia, lack of sufficient oxygen to the tissues, especially the brain and theeyes.

Hypoxia Sneaks Up On You

One major concern about hypoxia is that the onset of symptoms is subtle and insidious:

It takes awareness and thoughtfulness to realize you have hypoxia Those are the very characteristics that you tend to lose as a result of hypoxia

Your own intellectual impairment makes it difficult to recognize that you are impaired

Your Eyes Are Affected EarlyThe very earliest symptom of hypoxia can come, some sources say, at altitudes as low as 5,000feet.

The symptom is reduced dark adaptation of the eyes: Over time you can experience a gradual loss of sharpness in your vision

Sometimes if you take a breath of supplemental oxygen, it can feel like someone immediatelyturned up the lights

Later Symptoms Include Fatigue

As your hypoxic state progresses you may feel: Extremely fatigued Uncoordinated

Drowsy Headachy Dizzy

BreathlessYour skin can take on a bluish tint, especially under the fingernails.


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Swallowing Yawning

Tensing the muscles in your throat Sometimes even pinching your nose closed and attempting to blow out of your nostrilsBut With a Cold or Allergy You May Have Ear Block

If you have congestion from an infection or allergy it can be difficult or impossible to open theEustachian tube to relieve the pressure in the middle ear:

This very painful condition is called ear block

It is possible you could even rupture your ear drumOf course, the solution is: Dont fly when you are congested

Theres Possible Sinus Agony

Pressure changes can also result in severe pain in the sinus cavities. Normally the sinuses areequalized to cabin pressure with small openings that connect to the nasal passages.

However: If you have an infection or an allergy, congestion can block the openings This can cause excruciatingeven incapacitatingpain, usually during a descent

Tooth Pain Can HappenIn some rare cases pressure changes can cause toothache: This occurs when there are air spaces in the teeth from cavities, fillings or caps

The pain is usually relieved when you descendIf you want to use a fancy word for this toothache, it is barodontalgia. In other words, toothachefrom barometric pressure changes.

Even The Bends Is Possible

Plus, with large pressure changes there is the risk of what scuba divers call decompressionsickness or the bends. This is caused by the formation of nitrogen bubbles in the blood whenyou go to higher altitudes where there are lower pressures.

You can see the same effect when you open a soda can and bubbles form when the dissolved gascomes out of the liquid because of the reduced pressure.

The nitrogen gas bubbles in the bloodstream can: Interfere with circulation Cause symptoms and damage similar to a strokeThere Can Be Gas Pain Too

Another symptom that can result from large pressure changes is abdominal pain due to suddenexpansion of gas in the intestines.

Minimize or Prevent These Problems From Pressure Changes With Slow Climbs and Descents

Most of the problems associated with pressure changes, even the risk of the bends, can be

minimized or prevented by slow climbs and descents.

Using Oxygen Can Be a Pain

Reliance on Continuous Oxygen Use Is Problematic

There are a number of reasons why it may not be desirable to rely on continuous oxygen use.

Some reasons are physiological:


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Many people find the use of a mask

uncomfortable

Prolonged use can cause irritation and rednesswhere the mask contacts the skin

Prolonged use of high concentrations of oxygencan produce:

Bronchial irritation

Infection Aviation oxygen is very dry in order to prevent

freezing in the linesit is dehydrating to users

Other reasons are operational:

The difficulty in communicating due to muffledspeech from the mask can be annoying andfatiguing

Your oxygen quantity may be more limiting for a

flight than your fuel quantity: Oxygen duration charts can help you

figure out how long your oxygen supplywill last

When you calculate your oxygen

duration, remember that breathing ratesand oxygen consumption increase whenyou are under stress

You will need to refill the oxygen tank frequently:

Getting oxygen refills is time-consuming

FBOs do not always have the equipment

to do them

Pressurizing the Cabin Solves a Lot of Problems

Eliminates the Need to Use Oxygen

Pressurizing the cabin:

Eliminates the need to use supplemental oxygen except in emergencies

Provides far greater comfort and less fatigue to the pilots and passengers Minimizes the problems associated with pressure changes on the body, with cabin altitude

changes as little 8 to 10 thousand feet instead of 10s of thousands of feetHow It Works

Pumps In Air and Regulates Outflow


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Heres how pressurization works:A compressor from a turbine engine or asupercharger pumps a continuous flow of air into astructurally reinforced cabinPressurization is maintained by regulating the airthat leaves the cabin through the outflow valveA safety valve opens automatically in the event offailure of the outflow valve to prevent the

pressure differential from exceeding the maximumallowedA pressurization dump valve can be used toquickly depressurize the cabin to clear it of fumesor smokeA proximity switch on the landing gear (squatswitch) will open the outflow valve to de-pressurize the aircraft when the aircraft is on theground (opening the cabin door when the aircraftis pressurized could prove disastrous)

Keeps Cabin Altitude Lower Than Pressure Altitude

Pressurization terms: When the air pressure in the cabin is higher than the outside ambient air pressure, it is said

that the cabin altitude is lower than the pressure altitude The difference between the cabin air pressure and the outside air pressure is referred to as

the pressure differential

How You Operate a Pressurization System

Mismanagement of the System Is Usually Only Uncomfortable

Operation of the pressurization system is usually a matter of passenger comfort more thansafety.

Once you have assured a supply of pressurization air to the cabin, and verified the cabin ispressurizing after takeoff, failure to reset the controls for climb or descent:

May result in disconcerting pressure changes, but Should not result in a loss of pressurization


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Three Gauges Tell You What You Need to Know

There are three gauges that tell you the status of the pressurization system:Pressure differential is shown on a gauge in the cockpit that reads in pounds per square inch(A)A cabin altimeter tells you the cabin altitude in thousands of feet (B)A cabin vertical speed indicator tells you how fast the cabin altitude is climbing or descendingin feet per minute (C)

A B C

The Two Basic Controls

The two basic controls of a pressurization system are:The rate control (A)The altitude selector (B)

The Rate Control

You use the rate control to set the rate youd like the cabin to climb or descend, say 500 feet perminute.

The Altitude Selector

You use the altitude selector to set the cabin altitude.

On older systems the altitude selector has two circular scales: The outer scale is for setting the cabin altitude that you want the aircraft to be pressurized to The inner scale is for setting the cruise altitude

On systems that work through the Flight Management System (FMS) there is no separate controlpanel for the pressurization system. For normal operations:

You set the departure airport and the destination airport in the FMS

o The database knows the elevations of the airfields You also set your cruise altitude in the FMS at the same time you enter your departure

and destination informationo The system sutomatically sets the desired cabin altitude and the cabin climb or

descent rate so there is no rate control knob In the event the FMS control malfunctions, there will be a backup control panel with rate

control and altitude selector knobs


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Preflight the System Before Takeoff

Your Aircraft Flight Manual will spell out the procedure you should follow to preflight yourpressurization and oxygen system before take-off. Typically the procedure will include:

Check that the pressurization source(s) is ON, and set properly (this ensures you have asource of air to pressurize the cabin)

Make sure the gauge for the emergency oxygen shows the required pressure in the system Test the flight crews oxygen mask(s) to confirm that you can get oxygen from them (on

some systems, the oxygen gauge tells you the bottle pressure but does not confirm there is

pressure in the lines to the oxygen masks)On Takeoff Set Cruise Altitude

On takeoff you set the indicator to your intended cruise altitude asshown on the inner scale.

Next to your cruise altitude on the inner scale the outer scale willshow you what the cabin altitude will be at maximum differentialwhen you reach cruise altitude.

The Cabin Climbs AutomaticallyWhen you take off:

The cabin will climb automatically at the selected rate, say a comfortable 500feet per minute (A)

The aircraft climbs at a much higher rate (B)

A B

Since the cabin only has to climb about 8,000 feet or less, while the aircraft climbs as much as40,000 feet or more, the selected cabin altitude and cruise altitude are usually reached about thesame time.

You Should Check to Make Sure Its Working

It is good procedure to always make sure that the pressurization

process has started by:

Checking the pressure differential soon aftertakeoff

There are many reasons why the aircraft may fail to pressurize, but by far the most probable is:


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Failure of the pilots to ensure that compressed air is being supplied to the pressurizationsystem

On Descent Set in the Landing Elevation

When you are descending:- Set the indicator to or slightly above the airportelevation of your destination

- The cabin will begin a slow descent- The aircraft descends much faster

Again, since the aircraft has much further to descend than the cabin does, everything works outsmoothly.

If you climb or descend at an unusual rate, you can always use the rate control to change the rateof climb or descent of the cabin.

Dealing With Depressurization

Be Prepared

When you are flying a pressurized aircraft there is one particular emergency you should be

prepared to handle:

A sudden loss of pressurization.There Are Many Possible Causes of Depressurization

You can lose pressurization for many different reasons. There can be failure of: The pressurization vessel including structural failure of the cabin, a door, or a window

The system that provides compressed air to the cabin as a result of a hose rupture or a clampcoming off

The squat switch which can erroneously signal that the aircraft has landed and open the

outflow valve The air conditioning system which may overheat and shut off the pressurization source while

the emergency pressurization source valve fails to activate

In cold, wet weather control valves can freeze open or closedThe Depressurization Rate Can Vary

An aircraft depressurization can, depending on the cause of the failure, range from:

A slow loss of pressure to Explosive decompression

You Must Perform the Emergency Procedure

Every depressurization requires immediate execution of emergency procedures.

With an Explosive Depressurization You Will Be Very Distracted

With an explosive decompression, you will have to deal with significant distraction while executing

the emergency procedures.


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During an explosive decompression you can expect: A loud popping noise

A wind in the cabin along with flying debris A sudden and dramatic drop in temperature Fogging due to condensation of the water in the airAdditionally, you may suffer all of the effects of pressure changes we talked about earlier:

Ear block Sinus pain

Tooth pain Abdominal gas expansionPerform the Procedure Anyway

Despite all of this very significant distraction, it is critically important that you promptly performthe emergency procedure as outlined in the emergency checklist for your aircraft.

Put on Oxygen First

In every checklist the checklist requires the flight crew to:Put on oxygen masks

Execute an emergency descent

The aircraft checklist will describe the procedure for an emergency descent in the particularaircraft you are flying.

Without Oxygen You Have Little Time of Useful Consciousness

This table gives you what is known as time of useful consciousness without supplementaloxygen at various altitudes.

This is the time available to an aircrew member to recognize they are suffering from hypoxia andto take appropriate action (put on an oxygen mask and/or descend the aircraft to an altitudewhere oxygen is not required).

Average Effective Performance Time for flying

personnel without supplemental oxygen:

45,000 feet ..............................9 to 15 seconds40,000 feet ............................15 to 20 seconds35,000 feet ............................30 to 60 seconds30,000 feet .................................1 to 2 minutes28,000 feet............................2 1/2 to 3 minutes

25,000 feet .................................3 to 5 minutes22,000 feet ...............................5 to 10 minutes15,000 to 18,000 feet ..........30 minutes or more

This makes the urgency of putting on an oxygen mask after a depressurization in the flight levelsobvious.

Be Prepared for Difficult Communications

Once you have an oxygen mask on you will have difficulty communicating with othercrewmembers and ATC:


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Even if you have a microphone in your mask and an intercom system, your voice and that ofothers on oxygen will be muffled

ATC will have difficulty understanding you and communications will be difficult with othercrewmembers

This means you have to be prepared to work as a crew without talking much.

Why You Need to Descend

There will be a tendency to feel that once you have the oxygen masks on, you can take time totroubleshoot the problem before you start an emergency descent.

This would not be wise for any number of reasons: You and your passengers may not be receiving oxygen properly At altitudes above about 39,000 feet you cannot maintain full functionality without a

pressure-breathing system If you stay at high altitude, you and your other crewmembers may suffer the effects of

decompression sickness You dont know the condition of the aircraft or what other failures may be associated with the

one you know about

You will continue to have communications difficulties with your other crew and ATC so long as

you have to wear an oxygen maskUnless Terrain Is a Problem, Descend to Where You Dont Need Oxygen

The altitude you should descend to would be: 12,500 feet or less Unless terrain dictates otherwiseIf you are over the ocean and a descent could mean that you would not have fuel to reach a safelanding point, you could consider:

Stopping your descent at 25,000 feet

Ensuring that all occupants are receiving oxygen (25,000 feet is the maximum altitude towhich most passenger oxygen masks are certificated)

Requirements Based on Cabin Altitude

At Cabin Pressure Altitudes Above 12,500 Feet (MSL) Up to and Including 14,000 Feet (MSL)

The required minimum flight crew must use supplemental oxygen any time you are at cabinpressure altitudes:

Above 12,500 feet, and

Up to and including 14,000 feet for more than 30 minutesAt Cabin Pressure Altitudes Above 14,000 Feet (MSL)

The required minimum flight crew must use supplemental oxygen: Any time you are at cabin pressure altitudes above 14,000 feetAt Cabin Pressure Altitudes Above 15,000 Feet (MSL)

Each occupant of the aircraft (including passengers) must be provided with supplemental oxygen: At cabin pressure altitudes above 15,000 feet

Requirements Based on Flight Level

At Flight Altitudes Above Flight Level 250

At least a 10-minute supply of supplemental oxygen must be available for each occupant foruse in the event that a descent is required due to loss of cabin pressurization

This is in addition to any oxygen required based on cabin pressure altitudesAt Flight Altitudes Above Flight Level 350

Both pilots must have quick-donning oxygen masksA quick-donning mask is one that can be placed on your face with one hand within 5 secondsand be properly secured and sealed, and supplying oxygen.

Anytime there is only one pilot at the controls, that pilot must use oxygen


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Above Flight Level 410, one pilot must use oxygen at all timesWhen oxygen use is required, the mask must be secured and sealed and either:

Supplying oxygen at all times, or Automatically supplying oxygen whenever the cabin pressure exceeds 14,000 feet

Airspace

Flight Levels

At and above 18,000 feet MSL you set your altimeterto the standard pressure of 29.92 Hg.

You are now flying at flight levels, instead ofaltitudes:

The abbreviation for flight level is FL

Flight levels are referred to in 3-digit numbers

Example: 39,000 feet with 29.92 set in your

altimeter would be referred to as FL390

Class A Airspace

The airspace at and above 18,000 feet MSL in the 48 contiguous states and Alaska (but notHawaii), up to FL600, is Class A airspace.

This means that at and above 18,000 feet MSL:

No VFR flight is allowed

You must be:

Instrument rated

Current to fly instruments

Your aircraft must be:

Equipped for instrument flight

Current regarding the equipment tests and checks for instrument flight

Regardless of the weather conditions, you must have:

Filed an IFR flight plan Received a clearance from ATC

RVSM Airspace

Previously, because aircraft altimeters did not have adequate precision above FL290, the verticalseparation between aircraft above FL290 was 2,000 feet.

In order to make more flight altitudes available, Reduced Vertical Separation Minimum (RVSM)airspace has been introduced in most of the world (including the U.S.):

RVSM airspace goes from FL290 up to and including FL410

In RVSM airspace the vertical separation between aircraft is 1,000 feet

Above FL410 the vertical separation is still 2,000 feet


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To fly between FL290 and FL410:

Special pilot training, aircraft equipment, and maintenance are required

The operator of the aircraft must document that all of these requirements have been met andreceive RVSM operational approval from the local FAA Flight Standards District Office (FSDO)

Equipment Requirements

Transponder Requirement

An operable transponder with Mode C capability isrequired when flying:

In all airspace of the 48 contiguous states atand above 10,000 feet MSL

Excluding the airspace at andbelow 2,500 feet above thesurface

Specifically in Class A airspace

Flight Planning

When you do your flight planning for a high-altitude flight, there are some new things to thinkabout.

Using the High Altitude Enroute Charts

As you remember, the IFR Low Altitude EnrouteCharts are for use up to but not including 18,000

feet MSL.


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DME Requirement

At and above FL240, if VOR navigationequipment is required:

You must also have DME equipment

GPS can be used to substitute for DMEequipment


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High Altitude

high altitude meteorology

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