Questions - Aircraft Cabin Environmental Control Systems

Study Aid Questions

Fill in the Blanks

1. Oxygen is a _________________________, __________________________, and _________________________ gas at normal atmospheric temperatures and pressures.

2. The words ____________________________________________ should be marked clearly on any cylinder containing oxygen for aircraft use.

3. Oxygen cylinders are considered empty when the pressure inside drops below _______________ psi. This ensures that air containing ___________________________________ has not entered the cylinder.

4. When oxygen is delivered only as the user inhales, it is known as a ______________________________ system.

5. There are two types of individual oxygen demand regulators: they are:
_____________________________________________ _____________________________________________

6. The main cause of contamination in an oxygen system is _______________________________________.

7. The cabin pressurization system must be capable of maintaining a cabin pressure attitude of _______________. or lower regardless of _______________________________.

8. Metal fatigue from repeated ___________________________, and ____________________________ weakens the aircraft structure.

9. There are three typical sources of air used to pressurize reciprocating aircraft. They are:
______________________________ ______________________________ ______________________________

10. Aircraft equipped with turbine engines typically use ______________________________________ as a source for cabin pressurization.

11. Aircraft cabin pressurization systems work in two different modes of operation. They are:
_____________________________________________ ______________________________________________

12. The flight crew can select and control _____________________________, ____________________________, and ____________________________________ on the cabin pressure controller.

13. The cabin pressure controller controls the position of the ___________________________________ located normally at the rear of the aircraft.

14. Many transport category aircraft have an outflow valve that operates ___________________________ , using signals sent from a remotely located ________________________________ that act as the pressure regulator.

15. Pressurization safety relieve valves are used to prevent __________________________________ of the cabin.

16. ____________________________________ are used to ensure that air pressure outside the aircraft does not exceed cabin air pressure.

17. Aircraft with reciprocating engines often use a _________________________ cooling system, and turbine powered aircraft use a __________________________ cooling system.

18. The air cycle cooling system is supplied with air from either ___________________________ or ____________

19. The heart of the air cycle cooling system is the _______________________________________, also known as the ________________________________________

20. The duct temperature sensors used in the temperature control system are ____________________________. Their ______________________________________ changes as temperature changes.

21. The temperature at which a substance changes from a _____________________ into a ___________________ when heat is added is known as the boiling point.

22. R134a refrigerant is filtered and stored under pressure in a reservoir known as __________________________

23. Most evaporators are constructed of ___________________________ or _____________________________ tubing coiled into a compact unit.

24. A concern of exhaust shroud heat system is that __________________________ could contaminate the cabin.

25. Maintenance of combustion heaters consists of routine items such as _________________________, checking ____________________________________ wear, and ensuring inlets are not plugged.


1. Gaseous oxygen systems are used in commercial aircraft and LOX oxygen systems are primarily used in military aircraft. TRUE/FALSE

2. Most oxygen storage cylinders are painted blue, but yellow and white may be used as well. TRUE/FALSE

3. Demand‐flow systems are used most frequently by the crew on high performance and air transport category aircraft. TRUE/FALSE

4. A blown out green disk in the side of fuselage indicates that the oxygen supply is low and this should be investigated before flight. TRUE/FALSE

5. The solid chemical oxygen generators used in most airliners are activated automatically by depressurization or manually by a switch operated by the flight crew. TRUE/FALSE

6. Petroleum products such as grease and oil should never be used for oxygen system maintenance because an explosion could be the result. TRUE/FALSE

7. The cabin pressurization system must maintain the cabin pressure altitude below 10,000 ft. TRUE/FALSE

8. Metal fatigue can result from repeated pressurization cycles and can weaken the airframe structure. TRUE/FALSE

9. Superchargers are the common source of pressurization on modern reciprocating engine aircraft. TRUE/FALSE

10. Turbochargers are driven by a shaft or a drive belt. TRUE/FALSE

11. Large turbine powered aircraft use bleed air from the engine compressor section as a source of air for cabin pressurization. TRUE/FALSE

12. The isobaric control mode of a cabin pressurization system controls cabin pressure to maintain a constant pressure between the air pressure outside the cabin and the ambient air pressure. TRUE/FALSE

13. Small aircraft often use a pneumatically controlled outflow valve while transport category aircraft use electrically/electronically controlled outflow valves. TRUE/FALSE

14. Pressurization safety relieve valves are used to prevent negative cabin pressure. TRUE/FALSE

15. Many pack valves used in air conditioning systems are electrically controlled and pneumatically operated. TRUE/FALSE

16. The primary heat exchanger is cooled by an electric fan during stationary ground operations and ram air is used in flight. TRUE/FALSE

17. The water separator has a fiberglass bag (sock) that condenses and coalesces the mist into larger water droplets. Gravity will drain the water droplets from the water separator. TRUE/FALSE

18. The function of the refrigeration by‐pass valve is to regulate the temperature of the air cycle machine so it does not freeze when passing through the water separator. TRUE/FALSE

19. A vapor cycle air conditioning system is an open system in which a refrigerant is circulated through tubing and a variety of components. TRUE/FALSE

20. Most modern vapor cycle cooling systems on aircraft use R12 refrigerant which has replaced the R134a refrigerant. TRUE/FALSE

21. During a feel test components and lines in the high side (from the compressor to the expansion valve) should warm or hot to the touch. TRUE/FALSE

22. During a feel test low side lines and the evaporator should be warm. TRUE/FALSE

23. The charging capacity of a vapor cycle air conditioning system is measured by weight. TRUE/FALSE

24. Most single‐engine light aircraft use a bleed air system to heat the cabin. TRUE/FALSE

25. The fuel for the combustion heater is drawn from a dedicated small fuel tank inside the cabin. TRUE/FALSE

Knowledge Application Questions

1. What component in the pressurization system is the principal control of a pressurization system?

2. What source of air supply is used by most turbine‐powered aircraft for pressurization?

3. What pressurization control unit will change the position of an outflow valve?

4. On reciprocating engine aircraft, what are some methods of supplying heat to the cabin?

5. What is the function of ventilating air in a combustion heater?

6. What are the typical components used in an air‐cycle cooling system?

7. What causes a temperature drop in an air‐cycle cooling system?

8. Describe how a water separator in an air cycle cooling system removes moisture from the air?

9. Describe the theory of operation of a vapor‐cycle cooling system illustrated in textbook figure 16‐74?

10. Describe the theory of operation of an air cycle cooling system illustrated in textbook figure 16‐63.

11. What are the main components in a vapor‐cycle cooling system?

12. How does a continuous‐flow oxygen system operate?

13. What must be accomplished if an oxygen system has been open to atmosphere for 2 hours or more?

14. What is used to purge oxygen lines of moisture?___________________________________________________________________

15. What type of oxygen must be used in aircraft?

16 Why is oil added to a vapor‐cycle Freon system?

17. What safety precautions should be observed when servicing oxygen systems?

18. What sources of air are used by pressurized aircraft with a reciprocating engine?

19. What is the main advantage of using LOX systems?

20. What kind of adjustments can be made on a pressure controller?

Multiple Choices Questions

1. Which section of a turbine engine provides high pressure bleed air to an air cycle machine for pressurization and air-conditioning?
a. C-D inlet compressor duct.
b. Inlet compressor.
c. Turbine compressor.

2. At which component in an air cycle cooling system does air undergo a pressure and temperature drop?
a. Expansion turbine.
b. Primary heat exchanger.
c. Refrigeration bypass valve.

3. In a freon vapor cycle cooling system, where is cooling air obtained for the condenser?
a. Pressurized cabin air.
b. Turbine engine compressor.
c. Ambient air.

4. What is ventilating air used for on a combustion heater?
a. Provides air required to support the flame.
b. Carries heat to the places where needed.
c. Provides combustion air for ground blower.

5. Turbine engine air used for air conditioning and pressurization is generally called
a. ram air.
b. compressed air.
c. bleed air.

6. In the combustion heater, combustion air system, what prevents too much air from entering the heaters as air pressure increases?
a. Only a differential pressure regulator can be used.
b. Either a combustion air relief valve or a differential pressure regulator.
c. Only a combustion air relief valve can be used.

7. The cabin pressure of an aircraft in flight is maintained at the selected altitude by
a. controlling the air inflow rate.
b. controlling the rate at which air leaves the cabin.
c. inflating door seals and recirculating conditioned cabin air.

8. What controls the operation of the cabin pressure regulator?
a. Cabin altitude.
b. Bleed air pressure.
c. Compression air pressure.

9. The basic air cycle cooling system consists of
a. heaters, coolers, and compressors.
b. a source of compressed air, heat exchangers, and a turbine.
c. ram air source, compressors, and engine bleeds.

10. The purpose of the dump valve in a pressurized aircraft is to relieve
a. a negative pressure differential.
b. pressure in excess of the maximum differential.
c. all positive pressure from the cabin.

11. What component might possibly be damaged if liquid refrigerant is introduced into the low side of a vapor cycle cooling system when the pressure is too high or the outside air temperature is too low?
a. Compressor.
b. Condenser.
c. Evaporator.

12. How can it be determined that a vapor cycle cooling system is charged with the proper amount of freon?
a. The compressor loads up and RPM decreases.
b. Air bubbles appear in the sight glass.
c. Air bubbles in the sight glass disappear.

13. When charging a vapor cycle cooling system after evacuation, the low pressure gauge fails to come out of a vacuum. What is indicated?
a. The expansion valve failed to close.
b. Blockage in the system.
c. The compressor is not engaging.

14. What component in a vapor cycle cooling system would most likely be at fault if a system would not take a Freon charge?
a. Expansion valve.
b. Condenser.
c. Receiver dryer.

15. Frost or ice buildup on a vapor cycle cooling system evaporator would most likely be caused by
a. moisture in the evaporator.
b. the mixing valve sticking closed.
c. inadequate airflow through the evaporator.

16.. What test is used to determine the serviceability of an oxygen cylinder?
a. Pressure test with water.
b. Pressure test with nitrogen.
c. Pressure test with manometer.

17. How often should standard weight high pressure oxygen cylinders be hydrostatically tested?
a. Every 4 years.
b. Every 3 years.
c. Every 5 years.

18. To be eligible for recharging, a DOT 3HT oxygen cylinder must have been hydrostatically tested every three years and be retired from service after
a. 15 years or 10 000 filling cycles.
b. 24 years or 4380 filling cycles.
c. 10 years or 5000 filling cycles.

19. What type of oxygen system uses the rebreather bag-type mask?
a. Demand.
b. Diluter demand.
c. Continuous flow.

20. The altitude controller maintains cabin altitude by modulation of the
a. outflow valve.
b. safety valve.
c. safety and outflow valves.

21. Hot compressor bleed air operates the conditioned air system on some turbine aircraft, how is cold air supplied?
a. By the ram cycle cooling unit.
b. By the flow control unit.
c. By the air cycle machine turbine.

22. For use in pressurized aircraft, which is generally the least complicated and requires the least maintenance?
a. High-pressure oxygen systems.
b. Low-pressure oxygen systems.
c. Chemical oxygen generator systems.

23. The main cause of contamination in gaseous oxygen systems is
a. moisture.
b. dust and other airborne particulates.
c. other atmospheric gases.

24. Where does the last stage of cooling in an air cycle air conditioning system occur?
a. Secondary heat exchanger.
b. Refrigeration unit compressor.
c. Expansion turbine.

25. The point at which freon flowing through a vapor cycle cooling system gives up heat and changes from a gas to a liquid is the
a. expansion valve.
b. evaporator.
c. condenser.

26. The point at which freon flowing through a vapor cycle cooling system absorbs heat and changes from a liquid to a gas is the
a. condenser.
b. expansion valve.
c. evaporator.

27. How is the cabin pressure of a pressurized aircraft usually controlled?
a. By an automatic outflow valve that dumps all the pressure in excess of the amount for which it is set.
b. By a pressure sensitive valve that controls the output pressure of the pressurization pump.
c. By a pressure sensitive switch that causes the pressurization pump to turn on or off as required.

28. Which is considered a good practice concerning the inspection of heating and exhaust systems of aircraft utilizing a jacket around the engine exhaust as a heat source?
a. Supplement physical inspections with periodic operational carbon monoxide detection tests.
b. All exhaust system components should be removed periodically, and their condition determined by the magnetic particle inspection method.
c. All exhaust system components should be removed and replaced at each 100-hour inspection period.

29. On some cabin pressurization systems, pressurization on the ground is restricted by the
a. main landing gear operated switch.
b. negative pressure-relief valve.
c. cabin pressure regulator.

30. The cabin pressure control setting has a direct influence upon the
a. outflow valve opening.
b. pneumatic system pressure.
c. inflow valve opening.

31. The function of the evaporator in a freon cooling system is to
a. lower the temperature of the cabin air.
b. transfer heat from the freon gas to ambient air.
c. liquefy freon in the line between the compressor and the condenser.

32. What is the purpose of a mixing valve in a compressor bleed air air-conditioning system?
a. Distribute conditioned air evenly to all parts of the cabin.
b. Combine ram air with conditioned air.
c. Control the supply of hot, cool, and cold air.

33. What component of a pressurization system prevents the cabin altitude from becoming higher than airplane altitude?
a. Negative pressure relief valve.
b. Cabin rate of descent control.
c. Positive pressure relief valve.

34. If the liquid level gauge in a vapor cycle cooling system indicates a low freon charge, the system should
a. be operated and a pressure check performed.
b. not be operated until freon and oil have been added.
c. be operated for a period of time to reach a stable condition and then the freon level rechecked.

35. If the cabin rate of climb is too great, the control should be adjusted to cause the
a. cabin compressor speed to decrease.
b. outflow valve to close slower.
c. outflow valve to close faster.

36. The position of the thermostatic expansion valve in a vapor cycle cooling system is determined by temperature and pressure of the
a. freon entering the evaporator.
b. freon in the outlet of the evaporator.
c. air in the outlet of the condenser.

37. The function of the condenser in a freon cooling system is to
a. transfer heat from the freon gas to ambient air.
b. change liquid freon into a gas before it enters the compressor.
c. transfer heat from the cabin air to the liquid freon.

38. The function of an expansion valve in a freon cooling system is to act as a metering device and to
a. reduce the pressure of the gaseous freon.
b. increase the pressure of the liquid freon.
c. reduce the pressure of the liquid freon.

39. Which prevents a sudden loss of pressurization in the event that there is a loss of the pressurization source?
a. Firewall shutoff valve.
b. Cabin pressure outflow valve.
c. Delivery air duct check valve.

40. When servicing an air conditioning system that has lost all of its freon, it is necessary to
a. check oil and add as necessary, evacuate the system, relieve vacuum, and add freon.
b. check oil and add as necessary, evacuate the system, and add freon
c. check oil and add as necessary, and add freon.

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