chapter 14 cabin atmosphere control

8/21/2019 CHAPTER 14 Cabin Atmosphere Control

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CABIN ATMOSPHERECONTROL

INTRODUCTIONThe crew and passengers of modern, high-performance aircraft are physically unable to survive the extreme envi-ronment in which these airplanes fly without some sort of conditioning of the air within the cabin and cockpit.Primarily because of the various altitudes at which an aircraft operates, the cabin atmosphere must be controlledto increase the comfort of the occupants or even to sustain their lives. This chapter will discuss the physiology ofthe human body that determines the atmospheric conditions required for life, how oxygen and cabin altitude arecontrolled to provide a livable atmosphere for the aircraft occupants, and how the comfort needs of the passen-

gers and crew are met.


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FLIGHT PHYSIOLOGY

In order to understand the reasons for controllingthe cabin atmosphere or environment, it is necessaryto understand both the characteristics of the atmos-

phere and the physiological needs of the persons fly-ing within that atmosphere. Each type of aircraft willhave specific requirements according to the alti-tudes and speeds at which the aircraft is flown.

THE ATMOSPHEREThe atmosphere envelops the earth and extendsupward for more than 20 miles, but because air hasmass and is compressible, the gravity of the earth

pulls on it and causes the air at the lower levels tobe more dense than the air above it. This accountsfor the fact that more than one-half of the mass of theair surrounding the earth is below about 18,000 feet.

The atmosphere is a physical mixture of gases.Nitrogen makes up approximately 78% of the air, andoxygen makes up 21% of the total mixture. Theremainder is composed of water vapor, carbon diox-ide and inert gases. Oxygen is extremely importantfor both animal and plant life. It is so important foranimals that if they are deprived of oxygen for even afew seconds, permanent damage to the brain or evendeath may result. Water vapor and carbon dioxide arealso extremely important compounds. The othergases in the air, such as argon, neon, and krypton arerelatively unimportant elements physiologically.

The density of air refers to the number of airmolecules within a given volume of the atmosphere.As air pressure decreases, the density of the air also

decreases. Conversely, as temperature increases thedensity of the air decreases. This change in air den-sity has a tremendous effect on the operations ofhigh altitude aircraft as well as physiological effectson humans. [Figure 14-1]

Turbine engine-powered aircraft are efficient at highaltitudes, but the human body is unable to exist in

this cold and oxygen-deficient air, so some provi-sion must be made to provide an artificialenvironment to sustain life.

Standard conditions have been established for all ofthe important parameters of the earth's atmosphere.The pressure exerted by the blanket of air is consid-ered to be 29.92 inches, or 1013.2 hectoPascals (mil-libars), which are the same as 14.69 pounds persquare inch at sea level, and decreases with altitude

as seen in figure 14-1. The standard temperature ofthe air at sea level is 15Celsius, or 59Fahrenheit.The temperature also decreases with altitude, asillustrated in figure 14-1. Above 36,000 feet, thetemperature of the air stabilizes, remaining at-55C (-69.7F).

HUMAN RESPIRATIONAND CIRCULATIONThe human body is made up of living cells thatmust be continually supplied with food and oxygenand must have their waste carried away andremoved from the body. Blood, circulated throughthe body by the heart, carries food and oxygen to thecells and carries away waste products.

When people inhale, or take in air, the lungsexpand and the atmospheric pressure forces air into fill them. This air fills millions of tiny air sacscalled alveoli, and the oxygen in the air diffusesthrough the extremely thin membrane walls ofthese sacs into blood vessels called arteries.

Nitrogen is not able to pass through these walls.The blood circulates through the body in thearteries and then into extremely thin capillaries to

the cells, where the oxygen is used to convert thefood in the blood into chemicals that are usableby the cells. The waste product, carbon dioxide, isthen picked up by the blood and carried back intothe lungs through blood vessels called veins. Thecarbon dioxide is able to diffusethrough the


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Cabin Atmosphere Control 14-3

FEET IN. OFHG. MM OFHG. PSI C po

0 29.92 760.0 14.69 15.0 59.0

2,000 27.82 706.7 13.66 11.0 51.9

4,000 25.84 656.3 12.69 7.1 44.7

6,000 23.98 609.1 11.77 3.1 37.6

8,000 22.23 564.6 10.91 -0 .8 30.5

10,000 20.58 522.7 10.10 -4 .8 23.4

12,000 19.03 483.4 9.34 -8 .8 16.2

14,000 17.58 446.5 8.63 -12.7 9.1

16,000 16.22 412.0 7.96 -16.7 1.9

18,000 14.95 379.7 7.34 -20.7 -5.1

20,000 13.76 349.5 6.76 -24.6 -12.3

22,000 12.65 321.3 6.21 -28.6 19.4

24,000 11.61 294.9 5.70 -32.5 -26.5

26,000 10.64 270.3 5.22 -36.5 -33.6

28,000 9.74 237.4 4.78 -40.5 -40.7

30,000 8.90 226.1 4.37 -44.4 -47.8

32,000 8.12 206.3 3.99 -48.4 -54.9

34,000 7.40 188.0 3.63 -52.4 -62.0

36,000 6.73 171.0 3.30 -55.0 -69.7

38,000 6.12 155.5 3.00 -55.0 -69.7

40,000 5.56 141.2 2.73 -55.0 -69.7

42,000 5.05 128.3 2.48 -55.0 -69.7

44,000 4.59 116.6 2.25 -55.0 -69.7

46,000 4.17 105.9 2.05 -55.0 - 69.7

48,000 3.79 96.3 1.86 -55.0 -69.7

50,000 3.44 87.4 1.70 -55.0 -69.7

55,000 2.71 68.8 1.33

60,000 2.14 54.4 1.05

64,000 1.76 44.7 .86

70,000 1.32 33.5 PSF TEMPERATURE

113.2 REMAINS CONSTANT

74,000 1.09 27.7 77.3

80,000 .82 20.9 58.1

84,000 .68 17.3 47.9

90,000 .51 13.0 35.9

94,000 .43 10.9 29.7

100,000 .33 8.0 22.3

Figure 14-1. This chart illustrates that as altitude increases from sea level, the pressure decreases. It also shows that to a point,temperature decreases before finally leveling off.


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74-4 Cabin Atmosphere Control

membrane walls into the alveoli, where it isexpelled during exhalation. [Figure 14-2]

There are two important considerations in provid-ing sufficient oxygen for the body. There must beenough oxygen in the air to supply the body withthe amount needed, and it must have sufficient

pressure to enter the blood by passing through themembrane walls of the alveoli in the lungs.

Oxygen makes up approximately 21% of the mass ofthe air, and so 21% of the pressure of the air iscaused by the oxygen. This percentage remainsalmost constant as the altitude changes, and iscalled the partial pressure of the oxygen. It is the

partial pressure of the oxygen in the lungs that

forces it through the alveoli walls and into theblood. At higher altitudes there is so little total pres-sure that there is not enough partial pressure of theoxygen to force it into the blood. This lack of oxy-gen in the blood is called hypoxia.

HYPOXIAAny time the body is deprived of the requiredamount of oxygen, it will develop hypoxia. Ashypoxia becomes more severe, a person's time ofuseful consciousness decreases. Time of useful con-sciousness is defined as the time a person has to

take corrective action before becoming so severely

impaired that they cannot help themselves. One ofthe worst things about hypoxia is the subtle way itattacks. When the brain is deprived of the needed

oxygen, the first thing people lose is their judgment.The effect is similar to intoxication; people areunable to recognize how badly their performanceand judgment are impaired. Fortunately, hypoxiaaffects every individual the same way each time it isencountered. If a person can experience hypoxiasymptoms in an altitude chamber under controlledconditions, they are more likely to recognize thesymptoms during subsequent encounters.

Two of the more common first indications ofhypoxia occur at about ten thousand feet altitude.These are an increased breathing rate and aheadache. Some other signs of hypoxia arelight-headedness, dizziness with a tingling in thefingers, vision impairment, and sleepiness.Coordination and judgment will also be impaired,

but normally this is difficult to recognize. Because itis difficult to recognize hypoxia in its early stages,many pressurized aircraft have alarm systems towarn of a loss of pressurization.

CARBON MONOXIDE POISONINGCarbon monoxide is the product of incomplete com-

bustion of fuels which contain carbon and is found

in varying amounts in the smoke and fumes from

Figure 14-2. The cardiovascular sy stem is made up of the heart, lungs, arteries, and veins. This system transports food and oxy-

gen to the cells of the body and transports waste in the form of carbon dioxide from the cells back out of the body.


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burning aviation fuel and lubricants. Carbonmonoxide is colorless, odorless and tasteless, butsince it is normally combined with other gases in

the engine exhaust, you can expect it to be presentwhen exhaust gases are detected.

When carbon monoxide is taken into the lungs, itcombines with the hemoglobin in the blood. It is thehemoglobin that carries oxygen from the lungs to thevarious organs of the body. Since hemoglobin has afar greater attraction for carbon monoxide than it hasfor oxygen, it will load up with carbon monoxideuntil it cannot carry the much-needed oxygen. Thisresults in oxygen starvation, and when the brain isdeprived of oxygen the ability to reason and makedecisions is greatly impaired. Exposure to even asmall amount of carbon monoxide over an extended

period of time will reduce the ability to operate theaircraft safely. The effect of carbon monoxide iscumulative, so exposure to a small concentrationover a long period of time is just as bad as exposureto a heavy concentration for a short time.

The decrease in pressure as altitude increasesmakes it more difficult to get the proper amount ofoxygen. If there is carbon monoxide in the cabin, orif a person is smoking tobacco while flying, it willintensify the problem and even further deprive the

brain of the oxygen it needs.

Most small single-engine airplanes are heated withexhaust-type heaters in which the cabin ventilatingair passes between a sheet metal shroud and theengine exhaust pipes or muffler. If a crack or even a

pinhole size leak should exist in any of the exhaustcomponents, carbon monoxide can enter the cabin.The possibility of this type of poisoning is mostlikely in the winter months when heat is mostneeded and when the windows and vents are usu-ally closed to keep out cold air. Combustion heatersthat burn fuel from the aircraft tanks to produce heat

can also be a source of carbon monoxide. This typeof heater is found on many small and medium-sizedtwin engine general aviation aircraft as well as onolder airliners.

Early symptoms of carbon monoxide poisoning aresimilar to those of other forms of oxygen depriva-tion; sluggishness, a feeling of being too warm, and

a tight feeling across the forehead. These earlysymptoms may then be followed by a headache anda throbbing in the temples and ringing in the ears.Finally, there may be severe headaches, dizziness,dimming of the vision and if something is not donesoon, this can continue until unconsciousness anddeath.

If carbon monoxide poisoning is suspected, theheater should be shut off and all possible ventsopened. If the aircraft is equipped with oxygen,100% oxygen should be breathed until the symp-toms disappear, or until landing. Carbon monoxidedetectors are available that can be installed on theinstrument panel. These are simply small contain-ers of a chemical that changes color, generally to adarker color, when carbon monoxide is present. Asan example, light yellow ones will turn dark greenand white ones will turn dark brown or black. Ifthere is any indication of carbon monoxide in thecabin, every part of the exhaust system should bechecked to find and repair the leak before the air-craft is returned to service. [Figure 14-3]

Figure 14-3. One type of carbon monoxide detector consistsof a tablet that changes color when exposed to carbonmonoxide.


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OXYGEN AND PRESSURIZAT!ON SYSTEMS

As an aircraft climbs from sea level to increasinglyhigh altitudes, the crew and passengers move fur-ther and further from an ideal physiological condi-tion. In order to compensate for an atmosphere that

becomes thinner as altitude increases, two differentapproaches have been developed. One of these is to

provide pure oxygen to supplement theever-decreasing amount of oxygen available in the

atmosphere. The other is to pressurize the aircraft tocreate an atmosphere that is similar to that experi-enced naturally at lower altitudes. For aircraft thatfly at extremely high altitude, a combination of

pressurization and supplementary oxygen for emer-gencies is required.

OXYGEN SYSTEMSAt higher altitudes (generally above 10,000 feet) theair is thin enough to require supplemental oxygenfor humans to function normally. Modern aircraftwith the capability to fly at,high altitudes usually

have oxygen systems installed for the use of crewand/or passengers.

CHARACTERISTICS OF OXYGENOxygen is colorless, odorless and tasteless, and it isextremely active chemically. It will combine withalmost all other elements and with many com-

pounds. When any fuel burns, it unites with oxygento produce heat, and in the human body, the tissuesare continually being oxidized which causes theheat produced by the body. This is the reason anample supply of oxygen must be available at alltimes to support life.

Oxygen is produced commercially by liquefying air,and then allowing nitrogen to boil off, leavingrelatively pure oxygen. Gaseous oxygen may also be

produced by the electrolysis of water. When electri-cal current is passed through water (H2O), it will

break down in to it s two elements, hydr ogenand oxygen.

Oxygen will not burn, but it does support combus-tion so well that special care must be taken whenhandling. It should not be used anywhere there is

any fire, hot material or petroleum products. If pureoxygen is allowed to come in contact with oil,

grease or any other petroleum product, it will com-bine violently and generate enough heat to ignitethe material.

Commercial oxygen is used in great quantities forwelding and cutting and for medical use in hospi-tals and ambulances. Aviator's breathing oxygen issimilar to that used for commercial purposes,except that it is additionally processed to removealmost all of the water. Water in aviation oxygencould freeze in the valves and orifices and stop theflow of oxygen when an aircraft is flying in cold

conditions found at high altitude. Because of theadditional purity required, aircraft oxygen systemsmust never be serviced with any oxygen that doesnot meet the specifications for aviator's breathingoxygen. This is usually military specificationMIL-O-27210. These specifications require theoxygen to have no more than two milliliters ofwater per liter of gas.

SOURCES OF SUPPLEMENTAL OXYGENAircraft oxygen systems employ several differentsources of breathing oxygen. Among the more com-mon ones are gaseous oxygen stored in steel cylin-

ders, liquid oxygen stored in specially constructedcontainers called Dewars, and oxygen generated bycertain chemicals that give off oxygen when heated.A Dewar, sometimes called a Dewar flask, is a spe-cial type of thermos bottle designed to holdextremely cold liquids. Recently, a system usingmicroscopic filters to separate oxygen from othergases in the air has been developed for medicaluses, and is being investigated for use in aircraft.

GASEOUS OXYGEN

Most of the aircraft in the general aviation fleet use

gaseous oxygen stored in steel cylinders under apressure of between 1,800 and 2,400 psi. The mainreason for using gaseous oxygen is its ease ofhandling and the fact that it is available at most ofthe airports used by these aircraft. It does have allthe disadvantages of dealing with high-pressuregases,


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and there is a weight penalty because of the heavystorage cylinders. [Figure 14-4]

Figure 14-4. Most general-aviation aircraft store oxygen insteel, high-pressure cylinders.

LIQUID OXYGEN

Most military aircraft now carry their oxygen in aliquid state. Liquid oxygen is a pale blue,transparent liquid that will remain in its liquid stateas long as it is stored at a temperature of below181. This is done in aircraft installations bykeeping it in a Dewar flask that resembles adouble-wall sphere having a vacuum between thewalls. The vacuum prevents heat transferring intothe inner container.

Liquid oxygen installations are extremelyeconomical of space and weight and there is nohigh pressure involved in the system. They do havethe disadvantage, however, of the dangers involvedin handling the liquid at its extremely lowtemperature, and even when the oxygen system isnot used, it requires periodic replenishing because

of losses from the venting system. [Figure 14-5]

CHEMICAL, OR SOLID, OXYGEN

A convenient method of carrying oxygen for emer-gency uses and for aircraft that require it only

occasionally is the solid oxygen candle. Many largetransport aircraft use solid oxygen generators as asupplemental source of oxygen to be used in theevent of cabin depressurization.

Essentially, a solid oxygen generator consists of ashaped block of a chemical such as sodium chlorateencased in a protective steel case. When ignited,large quantities of gaseous oxygen are released as acombustion by-product. They are ignited eitherelectrically or by a mechanical igniter. Once theystart burning, they cannot be extinguished and willcontinue to burn until they are exhausted. Solidoxygen candles have an almost unlimited shelf lifeand do not require any special storage conditions.There are specific procedures required for shippingthese generators and they may not be shipped ascargo aboard passenger carrying aircraft. They can

be shipped aboard cargo only aircraft and must beproperly packaged, made safe from inadvertent acti-vation, and identified properly for shipment. Theyare safe to use and store because no high pressure isinvolved and the oxygen presents no fire hazard.They are relatively inexpensive and lightweight. Onthe negative side, they cannot be tested without

actually being used, and there is enough heat gener-ated when they are used that they must be installedso that the heat can be dissipated without anydamage to the aircraft structure. [Figure 14-6]

Figure 14-6. Solid oxygen generators, called candles, are

used in many large aircraft to provide supplemental oxygenfor the passengers in case of depressurization. They arealso found in some smaller business aircraft.Figure 14-5. Military aircraft usually use liquid

oxygen, stored in special insulated containers called


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MECHANICALLY-SEPARATED OXYGEN

A new procedure for producing oxygen is itsextraction from the air by a mechanical separation

process. Air is drawn through a patented material

nitrogen and other gases are trapped in the sieveand only the oxygen passes through. Part of theoxygen is breathed, and the rest is used to purge thenitrogen from the sieve and prepare it for anothercycle of filtering. This method of producing oxygenis currently being used in some medical facilitiesand military aircraft. It appears to have the possi-

bility of replacing all other types of oxygen becauseof the economy of weight and space, and the factthat the aircraft is no longer dependent uponground facilities for oxygen supply replenishment.

OXYGEN SYSTEMS AND COMPONENTSThe aviation maintenance technician will encounteroxygen systems during the course of servicing andrepairing aircraft. Actual servicing or repair of theoxygen system itself must be accomplished in accor-dance with the manufacturer's instructions, but ageneral knowledge of gaseous, liquid, and chemicaloxygen systems and how they operate will enablethe technician to better prepare the aircraft for flight.

GASEOUS OXYGEN SYSTEMS

Gaseous oxygen systems consist of the tanks the

oxygen is stored in, regulators to reduce the pres-sure from the high pressure in the tanks to the rela-tively low pressure required for breathing, plumb-ing to connect the system components, and masks todeliver the oxygen to the crewmember or passenger.

Storage Cylinders

Most military aircraft at one time used a low-pres-sure oxygen system in which the gaseous oxygenwas stored under a pressure of approximately 450

psi in large yellow-painted low pressure steel cylin-ders. These cylinders were so large for the amountof oxygen they carried that they never became pop-

ular in civilian aircraft, and even the military hasstopped using these systems.

Today, almost all gaseous oxygen is stored in greenpainted high-pressure steel cylinders under a pres-sure of between 1,800 and 2,400 psi. All cylindersapproved for installation in an aircraft must beapproved by the Department of Transportation(DOT) and are usually either the ICC/DOT 3AA 1800or the ICC/DOT 3HT 1850 type. Aluminum bottlesare also available, but are much less common.

Newer, light-weight "composite" bottles that complywith DOT-E-8162 are becoming more common.

These bottles are made of lighter, thinner metalscombined with a wrapping of composite material.

Cylinders must be hydrostatically tested to 5/3 of theirworking pressure, which means that the 3AA cylin-ders are tested with water pressure of 3,000 psi every

five years and stamped with the date of the test. 3HTyim"u:ersr~iinlsl"" "ucesceu ~Wuu a vtfaier pressurtr roi

3,083 psi every three years, and these cylinders mustbe taken out of service after 24 years, or after they havebeen filled 4,380 times, whichever comes first. E-8162cylinders are tested to the same standards as the 3HTcylinders, but must be taken out of service after 15years or 10,000 filling cycles, whichever occurs first.

All oxygen cylinders must be stamped near the fillerneck with the approval number, the date of manu-facture, and the dates of all of the hydrostatic tests.It is extremely important before servicing any oxy-

gen system that all cylinders are proper for theinstallation and that they have been inspectedwithin the appropriate time period.

Oxygen cylinders may be mounted permanently in theaircraft and connected to an installed oxygen plumbingsystem. For light aircraft where oxygen is needed onlyoccasionally, they may be carried as a part of a portableoxygen system. The cylinders for either type of systemmust meet the same requirements, and should be

painted green and identified with the words AVIA-TOR'S BREATHING OXYGEN written in white letterson the cylinder. Many high-pressure oxygen systems

use pressure-reducing valves between the supplycylinders and the flight deck or cabin equipment.These valves reduce the pressure down to 300-400 PSI.Most systems incorporate a pressure relief valve that

prevents high-pressure oxygen from entering the sys-tem if the pressure-reducing valve should fail.

On a hot day, the temperature inside a parked air-craft can cause the pressure in an oxygen cylinder torise to dangerous levels. Permanently mountedgaseous oxygen systems, especially in large aircraft,normally have some type of thermal relief system tovent oxygen to the atmosphere if the cylinder pres-

sure becomes too high. Venting systems may be tem-perature or pressure activated. To alert the crew thata thermal discharge has occurred, many systems usea "blow-out" disk as a thermal discharge indicator. Aflush-type fitting containing a green plastic diskabout 3/4 inch in diameter is mounted on the out-side of the aircraft near the location of the oxygen

bottles. If a thermal discharge occurs, the disk blowsout of the fitting, and leaves the vent port visible. Ifthe disk is found missing, there is no oxygen in thesystem and the aircraft must not be flown in condi-tions where supplemental oxygen might be required.

A thermal discharge requires maintenance on theoxygen system. The discharge mechanism must be


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reset or replaced, the indicator disk replaced andthe system serviced with oxygen to the correct pres-sure. Consult the maintenance manual for the par-

ticular aircraft to determine the proper procedures.

Regulators

There are two basic types of regulators in use, andeach type has variations. Low-demand systems, suchas are used in smaller piston-engine powered gen-eral aviation aircraft, generally use a continuousflow regulator. This type of regulator allows oxygento flow from the storage cylinder regardless ofwhether the user is inhaling or exhaling. Continuousflow systems do not use oxygen economically, buttheir simplicity and low cost make them desirablewhen the demands are low. The emergency oxygen

systems that drop masks to the passengers of large jettransport aircraft in the event of cabindepressuriza-tion are of the continuous flow type.

Continuous Flow Regulators are of either the man-ual or automatic type. Both of these are inefficientin that they do not meter the oxygen flow accordingto the individual's needs.

Manual Continuous Flow Regulators typically con-sist of two gauges and an adjustment knob. One typ-ical regulator has a gauge on the right that shows the

pressure of the oxygen in the system and indicates

indirectly the amount of oxygen available. Theother gauge is a flow indicator and is adjusted bythe knob in the lower center of the regulator. Theuser adjusts the knob so that the flow indicator nee-dle matches the altitude being flown. The regulatormeters the correct amount of oxygen for the selectedaltitude. If the flight altitude changes, the pilot mustremember to readjust the flow rate. [Figure 14-7]

Automatic Continuous Flow Regulators have abarometric control valve that automatically adjuststhe oxygen flow to correspond with the altitude.

The flight crew need only open the valve on thefront of the regulator, and the correct amount of oxy-gen will be metered into the system for the altitude

being flown. [Figure 14-8]

Figure 14-8. Automatic continuous flow regulators adjustoxygen flow automatically as altitude changes.

Oxygen is usually supplied to the flight crew of anaircraft by an efficient system that uses one of sev-

eral demand-type regulators. Demand regulatorsallow a flow of oxygen only when the user is inhal-ing. This type regulator is much more efficient thanthe continuous flow type. [Figure 14-9]

Figure 14-9. This demand-type regulator is fitted to aportable oxygen bottle and a full-face type mask. This type

of system is often used aboard cargo aircraft as a smokecombat unit to allow a crewmember to locate and extin-guish a cargo fire.

Figure 14-7. Manual continuous flow regulators must bereset as altitude changes.


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Diluter Demand Regulators are used by the flightcrews on most commercial jet aircraft. When thesupply lever is turned on, oxygen can flow from the

supply into the regulator. There is a pressurereducer at the inlet of the regulator that decreasesthe pressure to a value that is usable by the regula-tor. The demand valve shuts off all flow of oxygento the mask until the wearer inhales and decreasesthe pressure inside the regulator. This decreased

pressure moves the demand diaphragm and opensthe demand valve so oxygen can flow through theregulator to the mask. [Figure 14-10]

A diluter demand regulator dilutes the oxygen sup-plied to the mask with air from the cabin. This airenters the regulator through the inlet air valve and

passes around the air-metering valve. At low alti-tude, the air inlet passage is open and the passage tothe oxygen demand valve is restricted so the usergets mostly air from the cabin. As the aircraft goesup in altitude, the barometric control bellowsexpands and opens the oxygen passage while clos-ing off the air passage. At an altitude of around34,000 feet, the air passage is completely closed off,and every time the user inhales, pure oxygen ismetered to the mask.

If there is ever smoke in the cabin, or if for any rea-

son the user wants pure oxygen, the oxygen selector

on the face of the regulator can be moved from theNORMAL position to the 100% position. Thiscloses the outside air passage and opens a supple-

mental oxygen valve inside the regulator so pureoxygen can flow to the mask.

An additional safety feature is incorporated thatbypasses the regulator. When the emergency lever isplaced in the EMERGENCY position, the demandvalve is held open and oxygen flows continuouslyfrom the supply system to the mask as long as thesupply lever is in the ON position.

When a person breathes normally, the lungs expandand atmospheric pressure forces air into them. Butat altitudes above 40,000 feet not enough oxygen

can get into the lungs even with the regulator on100%. Operation of unpressurized aircraft at andabove 40,000 feet requires the use of pressuredemand regulators. These regulators have provi-sions to supply 100% oxygen to the mask at higherthan ambient pressure, thus forcing oxygen into theuser's lungs.

Pressure Demand Regulators operate in much thesame way as diluter demand regulators except atextremely high altitudes, where the oxygen is forcedinto the mask under a positive pressure. Breathing

at this high altitude requires a different technique

Figure 14-10. The flight crews of most commercial aircraft use diluter demand oxygen systems.


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from that required in breathing normally. The oxy-gen flows into the lungs without effort on the part ofthe user, but muscular effort is needed to force the

used air out of the lungs. This is exactly the oppo-site of normal breathing. [Figure 14-11]

OXYGEN REGULATORPRESSURE DEMAND

100%MERGENCY >( OXYGEN .

ON I

IY SUPPLYNORMAL n

Figure 14-11. This pressure demand regulator supplies oxy-gen under pressure for flights above 40,000 feet.

Masks

Masks are used to deliver the oxygen to the user.These are either of the continuous flow or demandtype.

Continuous flow masks are usually the rebreathertype and vary from a simple bag-type disposable

mask used with some of the portable systems to therubber bag-type mask used for some of the flightcrew systems. [Figure 14-12]

Figure 14-12. Rebreather type masks are used with contin-uous flow oxygen systems.

Oxygen enters a rebreather mask at the bottom of thebag, and the mask fits the face of the user veryloosely so air can escape around it. If the rebreather

bag is full of oxygen when the user inhales, thelungs fill with oxygen. Oxygen continues to flowinto the bag and fills it from the bottom at the sametime the user exhales used air into the bag at the top.When the bag fills, the air that was in the lungslongest will spill out of the bag into the outside air,

and when the user inhales, the first air to enter thelungs is that which was first exhaled and still hassome oxygen in it. This air is mixed with pure oxy-

gen, and so the wearer always breathes oxygen richair with this type of mask. More elaboraterebreather-type masks have a close-fitting cup overthe nose and mouth with a built-in check valve thatallows the air to escape, but prevents the user from

breathing air from the cabin.

The oxygen masks that automatically drop from theoverhead compartment of a jet transport aircraft inthe event of cabin depressurization are of therebreather type. The plastic cup that fits over themouth and nose has a check valve in it, and the

plastic bag attached to the cup is the rebreather bag.

With demand-type masks the regulator is set up tometer the proper amount of oxygen to the user, sooutside air would upset the required ratio of air tooxygen. Demand-type masks must fit tightly to theface so no outside air can enter. [Figure 14-13]

Figure 14-13. Demand-type masks deliver oxygen

only when the wearer inhales.

A full-face mask is available for use in case thecockpit should ever be filled with smoke. Thesemasks cover the eyes as well as the mouth and nose,and the positive pressure inside the mask preventsany smoke entering.

Most of the rigid plumbing lines that carryhigh-pressure oxygen are made of stainless steel,with the end fittings silver soldered to the tubing.Lines that carry low-pressure oxygen are made ofaluminum alloy and are terminated with the sametype fittings used for any other fluid-carrying line inthe aircraft. The fittings may be of either the flaredor flareless


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type. It is essential in any form of aircraft mainte-nance that only approved components be used. Thisis especially true of oxygen system components.

Only valves carrying the correct part number shouldbe used to replace any valve in an oxygen system.

Many of the valves used in oxygen systems are ofthe slow-opening type to prevent a rapid in-rush ofoxygen that could cause excessive heat and becomea fire hazard. Other valves have restrictors in themto limit the flow rate through a fully open valve.

Typical Installed Gaseous Oxygen Systems

If an aircraft has an installed oxygen system, it willbe one of three types: the continuous flow type, thediluter demand type or the pressure demand type.

Most single engine aircraft utilize a continuous flowoxygen system. The external filler valve is installedin a convenient location and is usually covered withan inspection door. It has an orifice that limits thefilling rate and is protected with a cap to preventcontamination when the charging line is not con-nected. The DOT approved storage cylinder isinstalled in the aircraft in a location that is mostappropriate for weight and balance considerations.The shutoff valve on the cylinder is of theslow-opening type and requires several turns of the

knob to open or close it. This prevents rapidchanges in the flow rate that could place excessivestrain on the system or could generate too muchheat. Some installations use a pressure-reducingvalve on the cylinder. When a reducer is used,the pressure gauge must be mounted on thecylinder side of the reducer to determine theamount of oxygen in the cylinder. [Figure 14-14]

The pressure gauge is used as an indication of theamount of oxygen in the cylinder. This is not, ofcourse, a direct indication of quantity, but withinthe limitations seen when discussing system servic-

ing, it can be used to indicate the amount of oxygenon board.

The pressure regulator reduces the pressure in thecylinder to a pressure that is usable by the masks.This regulator may be either a manual or an auto-matic type. There must be provision, one way oranother, to vary the amount of pressure supplied tothe masks as the altitude changes.

The mask couplings are fitted with restricting ori-fices to meter the amount of oxygen needed at eachmask. In figure 14-14 the pilot's coupling has an ori-fice considerably larger than that provided for the

passengers. The reason is that the pilot and other

flight crewmembers require more oxygen since theyare more active, and their alertness is of more vitalimportance than that of the passengers.

Some installations incorporate a therapeutic maskadapter. This is used for any passenger that has ahealth problem that would require additional oxy-gen. The flow rate through a therapeutic adapter isapproximately three times that through a normal

passenger mask adapter.

Each tube to the mask has a flow indicator built intoit. This is simply a colored indicator tfiat is visiblewhen no oxygen is flowing. When oxygen flows, it

pushes the indicator out of sight.

Pressurized aircraft do not normally have oxygenavailable for passengers all of the time, but FAR Part91 requires that under certain flight conditions, the

pilot operating the controls wear and use an oxygenmask. Because of this requirement, most executiveaircraft that operate at high altitude are equippedwith diluter demand or pressure demand oxygenregulators for the flight crew and a continuous flowsystem for the occupants of the cabin. Aircraft oper-ating at altitudes above 40,000 feet will usually have

pressure demand systems for the crew and passen-gers. [Figure 14-15]

The masks for the flight crew normally feature aquick-donning system. The mask is connected to aharness system that fits over the head. This systemis designed so that the mask can be put on with onehand and be firmly in place, delivering oxygen,within a few seconds.

LIQUID OXYGEN SYSTEMS

Civilian aircraft do not generally use liquid oxygen(LOX) systems because of the difficulty in handlingthis form of oxygen, and because it is not readilyavailable to the fixed-base operators who service

general aviation aircraft. The military, on the otherhand, uses liquid oxygen almost exclusively

because of the space and weight savings it makespossible. One liter of liquid oxygen will produceapproximately 860 liters of gaseous oxygen at the

pressure required for breathing.

The regulators and masks are the same as those usedfor gaseous oxygen systems, the difference in thesystems being in the supply. Liquid oxygen is heldin a spherical container and in normal operation the

buildup and vent valve is back-seated so some of theLOX can flow into the buildup coil where it absorbsenough heat to evaporate and pressurize the systemto the amount allowed by the container pressure


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Figure 14-14. The typical general aviation aircraft has an installed system similar to this one.


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Figure 14-15. Aircraft that do not require oxygen to be constantly available to passengers will have a diluter or pressure demand

regulator for the flight crew.

regulator, normally about 70 psi. This gaseous oxy-gen maintains a relatively constant pressure in thecontainer and supplies the oxygen to the regulator.[Figure 14-16]

When the supply valve on the regulator is turnedon, LOX flows from the container into the supplyevaporator coil where it absorbs heat and turns intogaseous oxygen. If, for any reason, excessive pres-sure should build up in the system, it will ventoverboard through one of the relief valves.

CHEMICAL OXYGEN SYSTEMS

Another source of oxygen is the chemical system.This system uses chemical oxygen generators also

called "oxygen candles" to produce breathing oxy-gen. The size and simplicity of the units, and mini-mal maintenance requirements make them ideal for

many applications. The chemical oxygen generatorrequires approximately one-third the space forequivalent amounts of oxygen as a bottled system.The canisters are inert below 400, even undersevere impact. Oxygen candles contain sodiumchlorate mixed with appropriate binders and a fuelformed into a block. When the candle is activated, itreleases oxygen. The shape and composition of thecandle determines the oxygen flow rate. As thesodium chlorate decomposes, it produces oxygen bya chemical action. [Figure 14-17]

An igniter, actuated either electrically or by aspring, starts the candle burning. The core of thecandle is insulated to retain the heat needed for the

chemical action and to prevent the housing fromgetting too hot. Filters are located at the outlet to

prevent any contaminants entering the system.


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Figure 14-16. Liquid oxygen systems require specialized plumbing to handle the conversion from liquid to gas and the venting of

excess pressures.

Figure 14-17. Chemical oxygen candles produce oxygen byheating sodium chlorate. The sodium chlorate is convertedto salt and oxygen.

The long shelf life of unused chemical oxygen gen-erators makes them an ideal source of oxygen foroccasional flights where oxygen is needed, and forthe emergency oxygen supply for pressurized air-craft where oxygen is required only as a standby incase cabin pressurization is lost.

The emergency oxygen systems for pressurized air-craft have the oxygen generators mounted in either

the overhead rack, in seat backs, or in bulkhead pan-els. The masks are located with these generators andare enclosed, hidden from view by a door that may

be opened electrically by one of the flight crewmembers or automatically by an aneroid valve inthe event of cabin depressurization. When the dooropens, the mask drops out where it is easily acces-sible to the user. Attached to the mask is a lanyardthat, when pulled, releases the lock pin from theflow initiation mechanism, so the striker can hit theigniter and start the candle burning. Once a chemi-cal oxygen candle is ignited, it cannot be shut off. It

must burn until it is exhausted, and the enclosuremust not be closed until the cycle has completed.[Figure 14-18]


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Figure 14-18. Pulling a lanyard on some chemical oxygencandles removes the safety pin to allow the spring to actu-ate the igniter.

OXYGEN SYSTEM SERVICINGCare and attention to detail is the mark of profes-sional aviation maintenance, and nowhere is thischaracteristic more important than when servicingaircraft oxygen systems. Compressed gaseousoxygen demands special attention because of bothits high storage pressure and its extremely activechemical nature.

When possible, all oxygen servicing should be doneoutdoors, or at least in a well-ventilated area of thehangar. Removable or portable supply cylindersshould be removed from the aircraft for servicing.When oxygen servicing is performed in the aircraft,suspend all electrical work. In all cases the manu-facturer's service information must be used while

performing service, maintenance or inspection ofaircraft oxygen systems.

SERVICING GASEOUS OXYGEN SYSTEMS

Some generic procedures are listed here as an ori-entation to the oxygen system servicing.

Leak Testing Gaseous Oxygen Systems

Searches for leaks are made using a special leakdetector. This material is a form of non-oily soapsolution. This solution is spread over every fittingand at every place a leak could possibly occur, andthe presence of bubbles will indicate a leak. If a leakis found, the pressure is released from the system,and the fittings checked for proper torque. Flarelessfittings can leak from both under andovertighten-ing. If the fitting is properly torqued andstill leaks, remove the fitting and examine all of thesealing surfaces for indications of damage. It may beneces-

sary to replace the fitting and reflare the tube orinstall a new flareless fitting.

Draining the Oxygen SystemDraining of the oxygen system should normally bedone after the high-pressure bottle has beenremoved or isolated from the system. Either out-doors or in a well-ventilated hangar, the system's

pressure should be bled off by opening the appro-priate fitting. Normally a system will require purg-ing after the system has been drained. All thesafety precautions mentioned later in this chaptershould be followed during any oxygen draining

procedure.

Filling an Oxygen System

Fixed base operators who do a considerable amountof oxygen servicing will usually have an oxygen ser-vicing cart. Such carts usually consist of six largecylinders, each holding approximately 250 cubicfeet of aviator's breathing oxygen. A seventh cylin-der, facing the opposite direction and filled withcompressed nitrogen, is normally carried to chargehydraulic accumulators and landing gear struts.Fittings on the nitrogen cylinders are different fromthose on the oxygen cylinders to minimize the pos-sibility of using nitrogen to fill the oxygen system,or of servicing the other systems with oxygen.[Figure 14-19]

Figure 14-19. Oxygen service carts consist of a battery of O2bottles hooked to a common service manifold. Sometimesa nitrogen bottle is on the same cart, but with differenthose fittings to prevent inadvertently interchanging thetwo systems.

Each oxygen cylinder has its own individual shutoffvalve, and all of the cylinders are connected into acommon service manifold that has a pressure gauge.A flexible line with the appropriate fittings con-nects the charging manifold to the aircraft fillervalve.

Various manufacturers of oxygen equipment use dif-ferent types of connections between the supply and


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the aircraft, and a well-equipped service cart shouldhave the proper adapters. These adapters must bekept clean and protected from damage. Leakage dur-

ing the filling operation is not only costly, but it ishazardous as well. Before filling any aircraft oxygensystem, all of the cylinders being refilled must bechecked to ensure that they are of the approvedtype, and have been hydrostatically tested withinthe required time interval.

No oxygen system should be allowed to becomecompletely empty. When there is no pressure insidethe cylinder, air can enter, and most air containswater vapor. When the water vapor mixes with theoxygen the mixture expands as it is releasedthrough the small orifices in the system. Thisexpansion lowers the temperature and the water islikely to freeze and shut off the flow of oxygen to themasks. Water in a cylinder can also cause it to ruston the inside and weaken it so it could fail with cat-astrophic results. A system is considered to beempty when the pressure gets down to 50 to 100 psi.If the system is ever allowed to get completelyempty, the valve should be removed and the cylin-der cleaned and inspected by an FAA-approvedrepair station.

When an aircraft's oxygen system is being filled

from a large supply cart, the cylinder having thelowest pressure should be used first. (The pressurein each tank should have been recorded on the

container with chalk or in a record kept with thecart.) The valve on the cylinder should be openedslightly to allow some oxygen to purge all of the

moisture, dirt and air from the line; then the lineshould be connected to the aircraft filler valve andthe valve on the cylinder opened slowly. Mostfiller valves have restrictors that prevent an exces-sively high flow rate into the cylinder. When the

pressure in the aircraft system and that in thecylinder with the lowest pressure stabilizes andthere is no more flow, this new pressure should berecorded and the cylinder valve closed. The valveon the cylinder having the next lowest pressureshould be opened slowly and oxygen allowed toflow into the system until it again stabilizes.Continue this procedure until the aircraft systemhas been brought up to the required pressure.[Figure 14-20]

The ambient temperature determines the pressurethat should be put into the oxygen system, and achart should be used to determine the pressureneeded. For example, if the ambient temperature is90F and a stabilized pressure in the system of1,800 psi is desired, the oxygen should be allowedto flow until a pressure of 2,000 psi is indicated onthe system pressure gauge. When the oxygen in thesystem drops to the standard temperature of 70,

the pressure will stabilize at 1,800 psi. If the ambi-ent temperature is low, the filling of the systemmust be stopped at a lower pressure, because the

Figure 14-20. The design of the hose manifold for oxygen servicing allows each bottle to be connected in turn to the receiving sys-tem. Over time, this setup allows each cylinder to be expended down to its allowable limit. Normally a small amount of pressureis kept in each tank, even when "empty."


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oxygen will expand and the pressure will risewhen it warms up to its normal temperature.[Figure 14-21]

AMBIENT TEMP.DEGREES FAHRENHEIT

FILLING PRESSURE

FOR 1800PSI AT 70

FOR 1850PSI AT 70

0 1600 1650

10 1650 1700

20 1675 1725

30 1725 1775

40 1775 1825

50 1825 1875

60 1875 1925

70 1925 1975

80 1950 200090 2000 2050

100 1050 2100

110 2100 2150

120 2150 2200

130 2200 2250

Figure 14-21. Charts such as this one are used to determine

the proper filling pressure for various ambient temperatures.

Purging A Gaseous Oxygen System

If the oxygen system, has been opened for servicing,

it should be purged of any air that may be in thelines. To purge a continuous flow system, oxygenmasks are plugged into each of the outlets and theoxygen supply valve turned on. Oxygen should beallowed to flow through the system for about tenminutes. Diluter demand and pressure demand sys-tems may be purged by placing the regulators in theEMERGENCY position and allowing the oxygen toflow for about ten minutes. After the system has

been thoroughly purged, the cylinders should befilled to the required pressure.

FILLING A LIQUID OXYGEN SYSTEM

Service carts for liquid oxygen normally carry theLOX in 25- or 100-liter containers. Servicing systemsfrom these carts is similar to that described in the

previous section on gaseous oxygen systems.Protective clothing and eye protection must be wornsince liquid oxygen has such a low boiling point thatit would be sure to cause serious frostbite if spilledon the skin. Any empty LOX system or one thathasn't been in use for some time should be purgedfor a few hours with heated dry air, or nitrogen.

The service cart should be attached to the aircraftsystem and, after placing the buildup and vent valvein the vent position, the valve opened on the servicecart. As the LOX flows from the service cart into the

warm converter, it vaporizes rapidly and cools theentire system. Considerable gaseous oxygen isreleased during the filling procedure, and it vents to

the outside air through the buildup and vent valve.This venting of the gaseous oxygen will continueuntil liquid oxygen starts to flow out of the ventvalve. A steady stream of liquid indicates that thesystem is full.

The system should vent freely as it is being filled andfrost should form only on the outlet and the hoses. Ifany frost forms on the supply container, it could bean indication of an internal leak, and since the pres-sure can build up extremely high, any trace of a leakdemands that the equipment be shut down immedi-ately and the cause of the frosting determined.

When the liquid oxygen cart is attached to the air-craft system, the valve should be fully opened, thenclosed slightly. If it is not, it is possible that the oxy-gen flowing through the valve could cause the valveto freeze in the open position and be difficult orimpossible to close.

There are two ways LOX converters are serviced.Some are permanently installed in the aircraft andare serviced from an outside filler valve. The

buildup and vent valve is placed in the vent posi-tion, the service cart is attached to the filler valve,

and liquid oxygen is forced into the system untilliquid runs out of the vent line. When the systemis full, the buildup and vent valve is returned tothe buildup position to build up pressure in theconverter. Other installations have quick-discon-nect mounts for the converters so the empty con-verter can be removed from the aircraft andreplaced with a full one. Exchanging convertersallows oxygen servicing to be done much morequickly and safely than can be done by filling theconverter in the aircraft.

INSPECTING THE MASKS AND HOSES

Disposable masks such as those used with many ofthe portable systems should be replaced with newmasks after each use, but the permanent masks used

by crew members are normally retained by eachindividual crewmember. These masks are fitted tothe face to minimize leakage and are usually treatedas personal flight gear. They should be occasionallycleaned by washing them with a cloth wet with alukewarm detergent solution and then allowingthem to dry at room temperature. The face portionof the mask may be disinfected with a mildantiseptic.

The quick-donning masks for use by airliner flightcrews are part of the aircraft and not crew personal


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equipment. Most airlines require each crewmemberto don and test the mask as part of the required

pre-flight inspection. Alcohol swabs in small

sealed packets are provided to sterilize the maskbefore the crewmember dons the mask.

The masks and hoses should be checked for leaks,holes or rips, and replaced rather than repaired.When storing the mask in the airplane, it should be

protected from dust and dampness, and especiallyfrom any type of grease or oil.

REPLACING TUBING, VALVES AND FITTINGS

It is extremely important when installing any oxygenline in an aircraft that no petroleum product is usedas a thread lubricant, and that the lines are thor-oughly cleaned of any trace of oil that was used inthe flaring or presetting operation. Triclilorethyleneor some similar solvent may be used to clean the tub-ing and fittings. After they are thoroughly clean, theyshould be dried either with heat or by blowing themwith dry air or dry nitrogen.

Tapered pipe threads must never be lubricated witha thread lubricant that contains any form of petro-leum. Oxygen-compatible thread lubricant thatmeets specification MIL-G-27617 may be used, orthe male threads may be wrapped with Teflon tape

and the fittings screwed together.

Before any tubing or fitting is replaced in an oxygensystem, the part must be thoroughly cleaned andinspected. The part should be checked for evidenceof corrosion or damage, and degreased with a vapordegreaser or ultrasonic cleaner. The new line should

be flushed with stabilized trichlorethylene, acetone,or some similar solvent, and dried thoroughly withdry air or nitrogen. If neither dry air nor nitrogen areavailable, the part may be dried by baking it at a tem-

perature of about 250F until it is completely dry.When the parts are dry, close them with properly fit-

ting protective caps or plugs, but never use tape inany form to seal the lines or fittings, as small particlesof the tape are likely to remain when it is removed.

PREVENTION OF OXYGENFIRES OR EXPLOSIONSSafety precautions for oxygen servicing are similarto those required for fueling or defueling an aircraft.The airplane and service cart should be electricallygrounded and all vehicles should be kept a safe dis-tance away. There should be no smoking, openflame or items which may cause sparks within 50feet or more depending upon the ventilation of thearea during servicing operations. Since the clothingof a person involved in servicing an oxygen system

is likely to be permeated with oxygen, smokingshould be avoided for ten to fifteen minutes aftercompleting the oxygen servicing.

The most important consideration when servicingany type of oxygen system is the necessity forabsolute cleanliness. The oxygen should be storedin a well ventilated part of the hangar away fromany grease or oil, and all high pressure cylinders notmounted on a service cart should be stored upright,out of contact with the ground and away from ice,snow or direct rays of the sun.

Protective caps must always be in place to preventpossible damage to the shutoff valve. The storagearea for oxygen should be at least 50 feet away from

any combustible material or separated from suchmaterial by a fire resistant partition. When settingup an oxygen storage area, you should be sure thatit meets all insurance company and Federal/StateOccupational Safety and Health Act (OSHA)requirements.

Because of the extreme incompatibility of oxygenand any form of petroleum products, it is a goodidea to dedicate all necessary tools to be used exclu-sively with oxygen equipment. Any dirt, grease oroil that may be on the tools or on any of the hoses,adapters, cleaning rags, or even on clothing is a pos-

sible source of fire.

PRESSURIZATION SYSTEMSThe air that forms our atmosphere allows people tolive and breathe easily at low altitudes, but flight ismost efficient at high altitudes where the air is thinand the aerodynamic drag is low. In order forhumans to fly at these altitudes, the aircraft must be

pressurized and heated so that it is comfortable forthe aircraft occupants.

PRESSURIZATION PROBLEMSTurbine engines operate effectively at these high

altitudes, but piston engines (as well as humanoccupants of an aircraft) require a supply of addi-tional oxygen. Superchargers compress the air

before it enters the cylinders of a reciprocatingengine, and the occupants can be furnished supple-mental oxygen to maintain life at these high, butaerodynamically efficient, altitudes.

Oxygen was used by some flight crews as early asWorld War I, and its inconvenience was tolerated bythe crew as a necessary part of the flight. By themiddle 1930s, airplanes and engines had been builtthat could carry passengers to altitudes where sup-

plemental oxygen was needed, but the inconve-nience of requiring passengers to wearoxygen


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masks proved to be a real deterrent to high altitudepassenger flights.

In 1934 and 1935, the American aviator Wiley Postmade a series of flights in his Lockheed Vega, theWinnie Mae, to altitudes near 50,000 feet. When fly-ing at these altitudes, Post wore a rubberized pres-sure suit that resembled the suit worn by a deep-seadiver. As a result of his experiments, Post felt thatflights at altitudes up to 30,000 feet would be practi-cal if some method were possible to enable the pas-sengers to breathe. Pressurization was the answer,

but the technology of aircraft structures in the 1930sdid not allow for pressurization of the aircraft itself.

During World War II, the need for bombers to oper-ate at extremely high altitudes for long flightscaused some of the manufacturers, notably theBoeing Airplane Company, to develop a pressurizedstructure or pressure vessel, as it is called.This allows the occupants of the aircraft to operatein a cabin that is artificially held at an altitude far

below the flight altitude of the airplane. Thismeans the pressure inside the aircraft's cabin ismuch higher than the ambient pressure (outside

pressure) when the aircraft is at high altitude.

Many of the piston engine transport aircraft of the

1950s had pressurized cabins and were able to carrypassengers in comfort over the top of most badweather. This type of aircraft made flying truly prac-tical as a means of mass transportation.

Piston engines were limited to relatively low alti-tudes and did not require a high cabin pressure, sono great structural problems showed up with pres-surized piston engine airplanes. But, when the jettransport airplane began to fly in the early 1950s, thelarge pressure differential required for the altitudesthey flew created metal fatigue. Metal fatigue caused

by the repeated pressurization and depressurization

cycles caused several disastrous accidents.

Today, aircraft structural design has advanced to thepoint that pressurized aircraft are able to safely andcomfortably carry large loads of passengers at effi-ciently high altitudes for long distance flights.Flight crews must be aware of the structural limita-tions of the aircraft and not exceed the maximumallowable differential between the pressure insidethe structure and the pressure on the outside. Theamount of air pumped into the cabin is normally inexcess of that needed, and cabin pressure is con-trolled by varying the amount of air leaving thecabin through outflow valves controlled hy thecabin pressure controller.

Most cabin pressurization systems have two modesof operation; the isobaric mode in which the cabin ismaintained at a constant altitude (iso means same,

and baric means pressure), and the constant pressuredifferential mode. In the isobaric mode, the pressureregulator controls the outflow valve as the aircraftgoes up in altitude to maintain the same pressure inthe cabin. When the pressure differential betweenthat inside the cabin and that outside reaches themaximum structural pressure limitation, the pres-sure controller shifts to the constant differentialmode and maintains a constant pressure differential.As the flight altitude increases, so does the cabinaltitude, always maintaining the same differential

pressure between the inside and the outside.

SOURCES OF PRESSURIZING AIRThe pressurization of modern aircraft is achieved

by directing air into the cabin from either the com-pressor section of a jet engine, from aturbosuper-charger, or from an auxiliarycompressor.

RECIPROCATING ENGINE AIRCRAFT

When pressurization was first used, it was for largeaircraft such as the Lockheed Constellation and theDouglas DC-6. These large cabins required great vol-umes of compressed air, and this was provided by a

positive displacement Roots-type compressor or bya variable displacement centrifugal compressor dri-ven by one of the engines. Pressurization air forsmaller piston-engine aircraft is provided by bleedair from the engine turbochargers. [Figure 14-22]

Figure 14-22. Pressure from the turbocharger of some lightaircraft is used to provide cabin pressurization.


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Figure 14-23. Theturbocharger is driven by hot exhaust gases. The compressor portion of this system is turned by a shaft attacheddirectly to the turbine. The turbine is driven by bleed air from the turbine engine. The ram air is compressed by the compressorand then blended with bleed air to the correct pressure and temperature.

TURBINE ENGINE AIRCRAFTThe compressor in a turbine engine is a good sourceof air to pressurize the cabin, and since this air isquite hot it is used to provide heat as well as pres-surization. Engine power is required to compressthis air, and this power is subtracted from that avail-able to power the aircraft.

Compressor bleed air may be used directly, or it maybe used to drive a turbocompressor. Outside air istaken in and compressed, and then, before it entersthe cabin, it is mixed with the engine compressor

bleed air that has been used to drive the turbocom-pressor. [Figure 14-23]

A jet pump flow multiplier can provide cabin pres-

surization air without the complexity of theturbo-compressor. Compressor bleed air flowsthrough the nozzle of a jet pump at high velocityand produces a low pressure that draws air in fromthe outside of the aircraft. The bleed air and theoutside air mix and flow into the cabin to providethe air needed for pressurization. [Figure 14-24]

Figure 14-24. Thejet-pump type pressurization uses aerodynamic principals to eliminate most moving parts.


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Air Cycle Machines are used by many modern tur-bine-engine aircraft to provide both pressurizationand temperature control. Air Cycle Machines use a

clever application of the laws of physics to cool hotengine bleed air. Boeing calls these systems"packs," an acronym for pneumatic air conditioningkit, The theory and operation of air cycle machineswill be discussed in detail in Section C of thisChapter. Three identical packs are installed on each747 and any one can supply all the conditioned airneeded for pressurization, temperature control andventilation.

CONTROL OF CABIN PRESSUREIt would be impractical to build the pressure vesselof an aircraft that is airtight, so pressurization is

accomplished by flowing more air into the cabinthan is needed and allowing the excess air to leakout. There are two types of leakage in an aircraft

pressure vessel; controlled and uncontrolled. Theuncontrolled leakage is the air that escapes arounddoor and window seals, control cables and otheropenings in the sealed portion of the structure.Controlled leakage flows through the outflow valveand the safety valve. This controlled leakage is fargreater than the uncontrolled, and it determines theamount of pressure in the cabin. Pressurization con-trol systems can be of the pneumatic or electronic

type, with the electronic type incorporating electri-cally controlled outflow valves.

PRESSURIZATION COCKPIT CONTROLS

Most pressurization systems have cabin altitude,cabin rate-of-climb, and pressure differential indi-cators. The cabin altitude gauge measures the actualcabin altitude. The cabin altitude is almost alwaysmuch below that of the aircraft, except when the air-craft is on the ground. An example would be an air-craft cruising at 40,000 feet would normally have acabin altitude of about 8,000 feet. The cabinrate-of-climb indicator allows the pilot or flight

engineer to adjust the rate the cabin altitude isclimbing or descending to levels that arecomfortable for the passengers. Normal climb rate is500 feet per minute and normal descending rate is300 feet per minute. The cabin rate-of-climb can beautomatic or manual according to the type of aircraft.The differential pressure gauge reads the currentdifference in pressure between the aircraft's cabininterior and the outside air. The modes of operationof the pressurization system are generally automaticand manual control. In the manual control mode,the pilots can control the outflow valves directlythrough switches and indicators that are used to

position the outflow valves if the automatic modefails. If the cabin altitude exceeds 10,000 feet, onmost aircraft, an alarm

(intermittent horn) will sound, alerting the flightcrew to take action. [Figure 14-25]

CABIN AIR PRESSURE REGULATORAND OUTFLOW VALVE OPERATION

Cabin pressure regulators and outflow valves maybe pneumatically or electrically operated. Modernsystems are almost entirely electronically con-trolled. The outflow valve is controlled by the cabin

pressure regulator and can be closed, open or mod-ulated. This means that it is working at a positionsomewhere between the two extremes to maintainthe pressure called for by the controller. The cabin

pressure regulator contains an altitude selector anda rate controller.

Pneumatic Regulator and Outflow Valve OperationPneumatic regulators use variations in air pressureto activate the outflow and safety valves. The out-flow valve and the safety valve are normally locatedin the pressure bulkhead at the rear of the aircraftcabin. The safety valve is normally closed (excepton the ground) and is used primarily as a backup incase of a malfunction of the outflow valve. [Figure14-26]

When the aircraft is on the ground and prepared forflight, the cabin is closed and the safety valve isheld off its seat by vacuum acting on the diaphragm.The dump solenoid in the vacuum line is held open

because the circuit through the landing gear safetyswitch is completed when the weight of the aircraftis on the landing gear. As soon as the aircraft takesoff, the safety switch circuit opens and the dumpsolenoid shuts off the vacuum line to the safetyvalve, which allows the valve to close. If for any rea-son the pressure in the cabin should exceed a setlimit, the safety valve will open fully. This will pre-vent cabin over-pressurization that could cause thestructure of the aircraft to fail.

The outflow valve is closed until it receives a signalfrom the controller, and as soon as the safety valvecloses, the cabin begins pressurize at the rateallowed by the rate controller. This increase in pres-sure is sensed by the controller. When the cabinreaches the selected altitude, the diaphragm in thecontroller moves back and vacuum is sent into theoutflow valve to open it and allow some of the pres-surizing air to escape from the cabin. This modula-tion of the outflow valve will maintain the cabin

pressure at the altitude selected. As the flight alti-tude increases, the outside pressure decreases.When ambient pressure becomes low enough that

the cabin differential pressure nears the structurallimit, the upper diaphragm in the outflow valve


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Figure 14-25. The cockpit control panel for a typical transport-category aircraft pressurization system displays information on thecabin vertical speed, cabin altitude and differential pressure and provides controls for selecting automatic or manual mode, set-ting the desired cabin altitude and the reference barometric pressure.A means of manually controlling the outflow valve positionand system warning indications are also provided.

Figure 14-26. The cabin pressure is set at the control panel in the cockpit and controlled by the outflow valve. The safety valve issimilar to the outflow valve and functions as a backup for the outflow valve, and to dump pressurization when the wheels are onthe ground.


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Figure 14-27. The outflow valve maintains a set altitude until the pressure differential with outside air approaches the structurallimit of the aircraft. It then maintains a differential with the outside pressure.

moves up until the adjusting screw depresses thevalve and releases some of the reference pressure tothe outside air. This decrease in pressure allows the

outflow valve to open so it can maintain the cabinpressure at a constant amount above the outside airpressure. [Figure 14-27]

Electronic Regulator and Outflow Valve Operation.

Electronic regulators and electrically actuated out-flow valves perform the same function as pneumaticsystems, only the power source is different. Electricalsignals are sent to the cabin pressure controller fromthe cockpit control panel to set the mode of opera-tion, the desired cabin altitude and either standard orlocal barometric pressure. In automatic mode, thecabin pressure controller sends signals to the AC

motors, which modulate as required to maintain theselected cabin altitude. In manual mode, the con-troller uses the DC motors to operate the outflowvalves. Interlocks prevent both motors from operat-ing at the same time. All pressurized aircraft requiresome form of a negative pressure-relief-valve. Thisvalve opens when outside air pressure is greater thancabin pressure. The negative pressure-relief-valve

prevents accidentally obtaining altitude, which ishigher than the aircraft altitude. This possibilitywould exist during descent. The outflow valves auto-matically drive to the full-open position wheneverthe aircraft weight is on the wheels. Pneumaticallyoperated pressure relief valves open automatically ifthe cabin differential pressure becomes too great.

These valves are completely independent of the restof the pressurization system. [Figure 14-28]

Figure 14-28. The pressurization control system regulatesand maintains cabin pressure, and the rate of cabin pres-sure change, as a function of settings on the control panel.This is accomplished by regulating the flow of air ventedfrom the cabin through motor driven outflow valves.

AIR DISTRIBUTION

The air distribution system on most aircraft mixescold air from the air-conditioning packages (packs)and hot engine bleed air in the conditioned air man-ifold according to the temperature called for by theflight crew. This pressurized air passes through acombination check valve/shutoff valve on its way to

the delivery air ducts. This check valve prevents theair pressure from being lost through an inoperativecompressor. The pressurized air is then distributed


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to side wall or overhead vents in the cabin. Thecabin air is then drawn back into the conditioned airmanifold by recirculating fans, mixed with new

incoming air, then redistributed to the aircraftcabin. Each passenger can turn the conditioned air"on" or "off" by adjusting the air outlet control on

the gasper fan located in the overhead panel aboveeach seat. [Figure 14-29]

CABIN PRESSURIZATION TROUBLESHOOTINGIf a malfunction occurs in the pressurization sys-tem, the aircraft manufacturer's servicemanual

Figure 14-29. The Boeing 747 air distribution system is typical of systems found on large aircraft.


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should always be used to troubleshoot and repairthe system. Fault isolation systemsand trou-

bleshooting charts can be very helpful in isolatingthe defective system components. [Figure 14-30]

Figure 14-30. Most aircraft service manuals have troubleshooting charts to assist the technician in locating problems within thecabin pressurization system.


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CABIN CLIMATE CONTROL SYSTEMS

Aircraft fly in a wide variety of climatic conditions.Flights might begin on the ramp at 95Fahrenheit(35Celsius) and then climb to cruise at a tempera-ture of -40 Fahrenheit (-40 degrees Celsius).Climate control systems then must be able to pro-vide comfortable cabin temperatures, regardless ofthe outside air temperature. The quality of the airsupply is also important: it must be free of contami-nants, fumes, odors or other factors that might affectthe health or comfort of the passengers or crew.

VENTILATION SYSTEMSMost small general aviation aircraft have relativelysimple systems to supply unconditioned ambient

air to the cabin, primarily for cooling. The systemmay consist simply of a window that can be openedin flight or by any of several types of air vents thatdeliver ram air to the occupants. Occasionally, thesystem may include a fan to assist in moving airwhen the aircraft is on the ground.

Business jets and airliners generally have a systemthat supplies cool, conditioned air to individual airvents at each seat. The air vent system (sometimescalled the gasper system) consists of a gasper fan,ducts and the overhead ventilating air outlets abovethe passenger seats. Cooling air is blown over the

passengers, which is refreshing, but only 'when thepassenger opens the air outlet for that seat.

HEATING SYSTEMS

EXHAUST SHROUD HEATERSThe most common type of heater for smallsingle-engine aircraft is the exhaust-shroud heater.A sheet-metal shroud is installed around themuffler in the engine exhaust system. Cold air istaken into this shroud and heat that wouldotherwise be expelled out the exhaust is

transferred to the ambient air. This air is thenrouted into the cabin through a heater valve inthe firewall. When the heater is not on, this air isdirected overboard. This type of heater is quiteeconomical for small aircraft, as it utilizes heatenergy that would otherwise be wasted. .,-.

One of the problems with this type of heater is thepossibility of carbon monoxide poisoning if thereshould be a leak in the exhaust system. For this rea-son, it is very important that the shrouds beremoved and the exhaust pipes and mufflers care-fully inspected on the schedule recommended bythe aircraft manufacturer. Some leaks may be pre-sent but not large enough to show up clearly when

the metal is cold, so these components should betested with air pressure. It is possible to test some ofthem on the aircraft by connecting the output of avacuum cleaner to the exhaust stack and coveringthe muffler with a soapy water solution and watch-ing for bubbles. Some aircraft have AirworthinessDirectives that require the mufflers to be removed,submerged in water, and pressurized with air tosearch for leaks.

The surface area of the muffler determines theamount of heat that is transferred to the air from themuffler. Some manufacturers have increased thisarea by using welded-on studs. This type of muffleris more efficient but it must be checked with specialcare as it is possible for minute cracks to start wherethe studs are welded onto the muffler. [Figure 14-31]

Figure 14-31. Some exhaust shroud heaters utilizewelded-on studs to increase the effective surface area forheat transfer.

ELECTRIC HEATING SYSTEMSElectric heating on aircraft is generally a supple-mental heating source. The heaters useheating


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elements that create heat through electrical resis-tance. Some aircraft use this type of heat when theaircraft is on the ground and the engines are not

running. A fan blows air over the heating coils toheat and circulate the air back into the cabin. Safetydevices are installed in these systems to preventthem from overheating if the ventilating fan should

become inoperative.

COMBUSTION HEATERSExhaust shroud heaters are used for smallsingle-engine aircraft, and compressor bleed airheating is primarily used on large turbine-poweredaircraft. Light and medium twin-engine aircraft areoften heated with combustion heaters. [Figure14-32]

Combustion heaters consist of two stainless steelcylinders, one inside the other. Air from outside theaircraft is directed into the inner cylinder, and avia-

tion gasoline drawn from the fuel tank is sprayedover a continually sparking igniter plug. The com-

bustion gases are exhausted overboard. Ventilatingair flows through the outer cylinder around thecombustion chamber, picks up the heat, and is dis-tributed throughout the cabin.

The hot air ducts are normally located where theywill blow warm air over the passengers' feet and thelower parts of their bodies. This type of heater has anumber of safety features that prevent it creating a firehazard in the event of a malfunction. [Figure 14-33]

Figure 14-32. Combustion heaters that utilize the same fuel as the engines are installed in many twin-engine aircraft.


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Figure 14-33. The combustion heater uses engine fuel to heat ram air, which heats the cockpit.

COMBUSTION AIR SYSTEM

A scoop on the outside of the aircraft picks up theair that used in the combustion process. The com-

bustion air blower forces this air into the combus-tion chamber when there is insufficient ram air. A

combustion-air-relief valve or a differential pres-sure regulator prevents too much air from enteringthe heaters as air pressure increases. The exhaust

gases are then vented overboard at a location wherethey cannot recirculate into the ventilation system.

FUEL SYSTEM

Fuel is taken from the aircraft fuel system and pres-

surized with a constant pressure pump, and passedthrough a fuel filter. Fuel flow is controlled by asolenoid valve that may be turned off

by the


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overheat switch, the limit switch, or by the pressureswitch. There is a second solenoid valve in the fuelline that is controlled by the cabin thermostat. It

shuts off the fuel at a point just before it enters thecombustion chamber.

VENTILATION AIR SYSTEM

Ram air enters the heater from outside the aircraft,and flows over the outside of the combustion cham-

ber, where it picks up heat and carries it inside theaircraft. There is a ventilating fan in the heater thatoperates when the aircraft is on the ground. Whenthe aircraft becomes airborne, a switch on the land-ing gear shuts off the ventilating fan and all airflowis provided by ram air. The ventilating air pressureis slightly higher than the pressure of the combus-

tion air, so in the event of a crack in the combustionchamber, ventilating air will flow into the combus-tion chamber rather than allowing the combustionair that contains carbon monoxide to mix with theventilating air.

CONTROLS

The only action required to start the combustionheater is to turn the cabin heater switch ON andadjust the cabin thermostat to the desired tempera-ture. When the cabin heater switch is turned on, thefuel pump starts, as well as the blowers for ventila-

tion air and combustion air. As soon as the combus-tion air blower moves the required amount of air, ittrips a pressure switch that starts the ignition coilsupplying current to the igniter plug. The fuel sup-

ply solenoid valve is opened and fuel can get to theheater. When the thermostat calls for heat, the sec-ond fuel solenoid valve opens and fuel sprays intothe combustion chamber and burns. As soon as thetemperature reaches the value for which the ther-mostat is set, the contacts inside the thermostatopen and de-energize the fuel solenoid valve, shut-ting off the fuel to the heater, and the fire goes out.The ventilating air cools the combustion chamber,

and the cool air causes the thermostat to call formore heat. The cycle then repeats itself.

SAFETY FEATURES

The duct limit switch is in the circuit to the mainfuel solenoid, and will shut off the fuel to the heaterif for any reason there is not enough air flow to carrythe heat out of the duct, or if the duct temperaturereaches the preset maximum value.

The overheat switch is the final switch in the sys-tem. It is set considerably higher than the duct limitswitch, but below a temperature that could cause a

fire hazard. If the temperature put out by the heaterreaches the limit allowed by this switch, the switch

will close the fuel supply solenoid valve and willalso shut off the combustion air flow and the igni-tion. A warning light will illuminate, alerting the

pilot that the heater has been shut down because ofan overheat condition. This switch, unlike the oth-ers, cannot be reset in flight, but can only be reset onthe ground at the heater itself.

MAINTENANCE AND INSPECTION

Combustion heaters are relatively trouble-free, butthey should be carefully inspected in accordancewith the recommendations of the aircraft manufac-turer and should be overhauled according to theschedule established by the heater manufacturer. Thefuel filter should be cleaned regularly and the spark

plug should be cleaned and gapped at the recom-

mended interval. The entire system should also bechecked for any indication of fuel or exhaust leakage.

COMPRESSOR BLEED AIR HEATERSTurbine engines have a large amount of hot air in theircompressors that is available for heating the cabin.The hot bleed air is mixed with cold ambient air to

provide air of the proper temperature to the cabin.This form of heating is usually combined with anair-cycle air-conditioning system. Theair-conditioning system of a large jet transportaircraft provides a means to cool or heat the

pressurizing air as required.

AIRCRAFT AIRCONDITIONING SYSTEMSAir conditioning is more than just the cooling of air.A complete air-conditioning system for an aircraftshould control both the temperature and humidityof the air, heating or cooling it as is necessary. Itshould provide adequate movement of the air forventilation, and there should be provision for theremoval of cabin odors.

AIR-CYCLE AIR CONDITIONINGIn a jet transport aircraft, hot compressor bleed air istaken from the engine compressors. An air-cyclemachine (ACM) applies several basic laws of

physics to cool this bleed air and then mix it withhot bleed air to provide air at the desired tempera-ture for ventilation and pressurization. The air-cyclemachine and its associated components are oftenreferred to as a "pack." [Figure 14-34]

SHUTOFF VALVE

The air-conditioning shutoff valve, often called thepack valve, is used to control the flow of air into the

system. It can either shut off the air flow or modu-late the flow of air to provide that which is neededto operate the air-conditioning package.


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Figure 14-34. The air cyc le system ut i l izes bleed air f rom the turbi ne engine(s) to heat and cool a ir f or cabin air cond it ioni ng.

PRIMARY HEAT EXCHANGER

The primary heat exchanger is a radiator throughwhich cold ram air passes to cool the hot bleed airfrom the engines. As the cold ram air passes overthe radiator's fin-like tubes, bleed air passingthrough the tubes is cooled. The flow of ram airthrough the heat exchangers is controlled bymove-able inlet and exit doors, which modulate inflight to provide the required cooling. On manyaircraft, the heat exchangers are sized to providemost, if not all, of the necessary cooling in flight. Onthe ground there is not enough air passing throughthe cooling doors, so fans called pack fans provideadequate airflow to cool the heat exchangers.

AIR CYCLE MACHINE BYPASS VALVE

When cooling requirements are low, some or all ofthe hot bleed air from the engines can be bypassedaround the ACM (the compressor and turbine) ifwarm air is needed in the cabin. There would be no

purpose in cooling all the air if warm air is calledfor by the temperature controls. This outlet air fromthe primary heat exchanger may be routed directlyto the inlet side of the secondary heat exchanger insome systems to provide additional cooling.

SECONDARY HEAT EXCHANGER

As cooling requirements increase, air exiting theprimary heat exchanger is routed to the compressor

side of the ACM. The compressor raises both the

pressure and temperature of the air passing throughit. The warmer, high pressure air is then directed tothe secondary heat exchanger. This heat exchanger

provides an additional stage for cooling the hotengine bleed air after it has passed through the pri-mary heat exchanger and the compressor of theACM. It operates in the same manner as the primaryheat exchanger.

REFRIGERATION BYPASS VALVE

Some systems use a refrigeration bypass valve tokeep the temperature of the air exiting the ACMfrom becoming too cold. Generally this air is kept at

about 35 F (2 C) by passing warm bleed airaround the ACM and mixing it with the output airof the ACM. The primary purpose of this valve is to

prevent water from freezing in the water separator.

REFRIGERATION TURBINE UNIT

Pressure and temperature, are interchangeableforms of energy. A turbine engine extracts energyfrom the burning fuel to turn the compressor, andthis energy raises both the pressure and the temper-ature of the engine inlet air. Compressed air withthis energy in it is taken from the engine and passed

through the primary heat exchanger, where some ofthe heat is transferred to ram air passing around thetubes in the radiator-like cooler. The high-pressure


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air, somewhat cooled, is then ducted into the aircycle machine where most of the remainder of itsenergy is extracted by the air cycle machine. It con-

sists of a centrifugal air compressor and an expan-sion turbine that drives the compressor. When thecompressor bleed air

chapter 14 cabin atmosphere control

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