Chapter 3 Aircraft Systems TDF

Absolute pressure.

Pressure measured from zero pressure, or a vacuum. It is expressed in terms of PSIA.

Absolute zero.

The point at which all molecular motion ceases. Absolute zero is-460 ° F and -273 ° C.

Air-cycle cooling system

A system for cooling the air in the cabin of a turbojet powered aircraft. Compressor bleed air passes through two heat exchangers where it gives up some of its heat; then it drives an expansion turbine where it loses still more of its heat energy as the turbine drives a compressor. When the air leaves the turbine it expands and its pressure and temperature are both low.

Ambient pressure.

The pressure of the air surrounding a person or an object.

Ambient temperature.

The temperature of the air surrounding a person or an object.

Aneroid

An evacuated and sealed metallic bellows that is used as a pressure measuring element for absolute pressure.

Aviators’ oxygen

Oxygen that has had all of the water and water vapor removed from it.

British thermal unit (Btu).

The amount of heat energy needed to raise the temperature of one pound of pure water 1°F.

Calorie

The amount of heat energy needed to raise the temperature of one gram of pure water 1 ° C.

Charging stand.

A handy and compact arrangement of air conditioning servicing equipment. A charging stand contains a vacuum pump, a manifold gage set, and a method of measuring and dispensing the refrigerant.

Chemical oxygen generator system

An oxygen system used for emergency or backup purposes. A candle, a solid block of a chemical that releases oxygen when it is burned, is installed in a special fireproof fixture. When oxygen is needed, the candle is ignited by a firing pin striking a percussion cap, and oxygen flows through flexible tubing to the masks.

Combustion heater

A type of cabin heater used in some aircraft. Gasoline from the aircraft fuel tanks is burned in the heater

Compound gage

A pressure gage used to measure the pressure in the low side of an air conditioning system. A compound gage is calibrated from zero to 30-inches of mercury vacuum, and from zero to about 150-pounds per square inch positive pressure.

Compressor

The component in a vapor cycle cooling system in which the low pressure refrigerant vapors, after they leave the evaporator, are compressed to increase both their temperature and pressure before they pass into the condenser

Condenser

The component in a vapor cycle cooling system in which the heat taken from the aircraft cabin is given up to the ambient air outside the aircraft.

Constant-differential mode

The mode of pressurization in which the cabin pressure is maintained a constant amount higher than the outside air pressure. The maximum differential pressure is determined by the structural strength of the aircraft cabin

Continuous-flow oxygen system.

A type of oxygen system that allows a metered amount of oxygen to continuously flow into the mask. A rebreather-type mask is used with a continuous-flow system.

Deep-vacuum pump.

A vacuum pump capable of removing almost all of the air from a refrigeration system.

Desiccant

A drying agent used in a refrigeration system to remove water from the refrigerant.

Differential pressure.

The difference between two pressures. It is expressed in terms of PSID.

Diluter-demand oxygen system.

A popular type of oxygen system in which the oxygen is metered to the mask where it is diluted with cabin air by an airflow-metering aneroid assembly which regulates the amount of air allowed to dilute the oxygen on the basis of cabin altitude

Evaporator

The component in a vapor cycle cooling system in which heat from the aircraft cabin is absorbed into the refrigerant. As the heat is absorbed, the refrigerant evaporates, or changes from a liquid into a vapor.

Freon

The registered trade name for a refrigerant used in a vapor-cycle air conditioning system.

Gage pressure

Pressure referenced from the existing atmospheric pressure. It is expressed in terms of psig.

Heat exchanger

A device used to exchange heat from one medium to another. Radiators, condensers, and evaporators are all examples of heat exchangers. Heat always moves from the object or medium having the greatest level of heat energy to a medium or object having a lower level.

Hydrostatic test

A pressure test used to determine the serviceability of high pressure oxygen cylinders. The cylinders are filled with water and pressurized to 5⁄3 of their working pressure.

Hypoxia

A physiological condition in which a person is deprived of the needed oxygen.

Jet pump.

A special venturi in a line carrying air from certain areas in an aircraft that need an augmented flow of air through them. High-velocity compressor bleed air is blown into the throat of a venturi where it produces a low pressure that pulls air from the area to which it is connected.

Latent heat.

Heat that is added to a material that causes a change in its state without changing its temperature.

Micron (“micro meter”)

A unit of linear measurement equal to one millionth of a meter, or one thousandth of a millimeter.

MSL

Mean sea level. Altitude is measured from mean, or average, sea level.

Negative pressure relief valve

A valve that opens anytime the outside air pressure is greater than the cabin pressure. It prevents the cabin altitude ever becoming greater than the aircraft flight altitude.

Outflow valve

A valve in the cabin of a pressurized aircraft that controls the cabin pressure by opening to relieve all pressure above that for which the cabin pressure control is set.

Partial pressure

The percentage of the total pressure of a mixture of gases produced by each of the individual gases in the mixture

Pressure altitude.

The altitude in standard air at which the pressure is the same as the existing pressure.

Pressure reducing valve

A valve used in an oxygen system to change high cylinder pressure to low system pressure.

Pressure relief valve

A valve in an oxygen system that relieves the pressure if the pressure reducing valve should fail.

Pressure vessel.

The strengthened portion of an aircraft structure that is sealed and pressurized in flight.

Pressure-demand oxygen system

A type of oxygen system used by aircraft that fly at very high altitude. This system functions as a diluter-demand system ( See diluter demand oxygen system) until, at about 40,000 feet, the output to the mask is pressurized enough to force the needed oxygen into the lungs, rather than depending on the low pressure produced when the wearer of the mask inhales to pull in the oxygen.

Purge

To remove all of the moisture and air from a cooling system by flushing the system with a dry gaseous refrigerant.

Rebreather oxygen mask.

A type of oxygen mask used with a continuous-flow system. Oxygen continuously flows into the bottom of the loose-fitting rebreather bag on the mask. The wearer of the mask exhales into the top of the bag. The first air exhaled contains some oxygen, and this air goes into the bag first. The last air to leave the lungs contains little oxygen, and it is forced out of the bag as the bag is filled with fresh oxygen. Each time the wearer of the mask inhales, the air first exhaled, along with fresh oxygen, is taken into the lungs.

Receiver-dryer

The component in a vapor-cycle cooling system that serves as a reservoir for the liquid refrigerant.

Reed valve

A thin, leaf-type valve mounted in the valve plate of an air conditioning compressor to control the flow of refrigerant gases into and out of the compressor cylinders.

Roots-type air compressor.

A positive displacement air pump that uses two intermeshing figure-8-shaped rotors to move the air

Schrader valve.

A type of service valve used in an air conditioning system. This is a spring-loaded valve much like the valve used to put air into a tire

Sensible heat.

Heat that is added to a liquid that causes a change in its temperature but not its physical state

Sight glass

A small window in the high side of a vapor-cycle cooling system. Liquid refrigerant flows past the sight glass, and if the charge of refrigerant is low, bubbles will be seen. A fully charged system has no bubbles in the refrigerant.

Sonic venturi.

A venturi in a line between a turbine engine or turbocharger and a pressurization system. When the air flowing through the venturi reaches the speed of sound, a shock wave forms across the throat of the venturi and limits the flow.

Specific heat.

The number of Btu’s of heat energy needed to change the temperature of one pound of a substance 1 ° F.

Superheat

Heat energy that is added to a refrigerant after it changes from a liquid to a vapor.

Therapeutic mask adapter.

A calibrated orifice in the mask adapter for a continuous-flow oxygen system that increases the flow of oxygen to a mask being used by a passenger who is known to have a heart or respiratory problem.

Thermostatic expansion valve (TEV)

The component in a vapor-cycle cooling system that meters the refrigerant into

the evaporator.

Volatile liquid

A liquid that easily changes into a vapor.