Answers - Hydraulic and Pneumatic Power Systems

Fill in the Blanks

1. 100, negligible
2. mineral, polyalphaolefin, phosphate ester
3. Skydrol, IV, V
4. seals, gaskets, hoses
5. micron, particle
6. cold start, pump ripples, shock loads
7. engine gearbox, electric motor
8. non‐pressurized, bleed air
9. screening straining, preventing
10. cleaning, replacing
11. differential, operating principle, loaded
12. manually reset, permanent
13. power‐driven, hand, engine driven
14. positive, non‐positive
15, shear, in the middle of, smaller
16. electrically, servo, servo
17. diverts, pressure
18. priority, non‐critical, low
19. burst, preserves
20. failure, rupture, excessive
21. thermal, thermal expansion
22. regulators, pump
23. shuttle, seal off
24. system, components
25. preload


1. false, 3000‐5000
2. true
3. false, all contaminants are bad
4. false, filters cleaned or replaced
5. false, all of the following
6. true
7. false, non‐pressurized
8. false, 5 microns
9. true
10. false, compensator controlled
11. true
12. true
13. true
14. true
15. true

Knowledge Application

1. Red
2. Material will be damaged, softened and/or peeled
3. A shear section in the pump‐drive shaft
4. All lines should be plugged or capped after disconnecting.
5. A clogged filter
6. Hand pumps, engine‐driven pumps, electrically driven pumps, and air driven (bleed air).
7. Cabin pressurization, bleed air, aspirator and venture‐tee, and hydraulic system pressure.
8. Limit amount of pressure preventing failure of components
9. To manage the output of the pump to maintain system pressure and allow the pump to turn without resistance.
10. An accumulator
11. To control the rate of airflow
12. To remove contamination and moisture
13. Make sure the air or preload has been discharged.

14. No. petroleum base and phosphate ester fluids will not mix. Neither are seals for one fluid useable with others.

15. In a closed center system the fluid is always pressurized when the pump runs and therefore it will be faster then the open center system that needs to be built up pressure. Since most aircraft applications require instantaneous operation, closed center systems are the most widely used.

16. The advantage of the powerpack is that there is no need for a centralized hydraulic power supply system and long stretches of hydraulic lines which reduces weight.

17. required because the reservoirs are often located in wheel wells or other non‐pressurized areas of the aircraft and at high altitude there is not enough atmospheric pressure to move the fluid to the pump inlet.

18. By‐pass relief valve opens if the filter clogs this will permit continued hydraulic flow and operation of aircraft systems.

19. Centrifugal and impeller pumps

20. The angle of the pump starts to decrease when the system pressure reaches about 2850 psi. The output of the pump will decrease to adjust to system demand.

21. System pressure will increase.

22. Some controlled by pressure, some mechanically, some by electric switches.

23. The PTU is able to transfer power but not fluid between two hydraulic systems. For instance if the engine driven pump in the #1 system is inoperative than system pressure in the number 2 can power the PTU to power the number 1 system.

24. EDV solenoid will move up creating a path for the outlet pressure (Ps) to reach the top of the compensator valve. The outlet pressure will push the compensator valve down and the output pressure will act on the yoke actuating piston and reduce the yoke angle and pump output is reduced to zero.

25. Pressure‐reducing valves are used in hydraulic systems where it is necessary to lower the normal system operating pressure a specified amount.

26. a] Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a preset level.
b] Aid or supplement the power pump when several units are operating at once by supplying extra power from its "accumulated" or stored power.
c] Store power for the limited operation of a hydraulic unit when the pump is not operating.
d] Supply fluid under pressure to compensate for small internal or external (not desired) leaks which would cause the system to cycle continuously by action of the pressure switches continually "kicking in."

27. To provide electrical and hydraulic power if the aircraft loses electrical or hydraulic power.

28. When pressurized nitrogen is not directly used to actuate the landing gear actuators or brake units, it will apply the pressurized nitrogen to move hydraulic fluid to the actuator.

29. The engine driven pump won’t be able to draw fluid from the tank, and only the AC driven pump can supply fluid to essential systems.

Multiple Choices

1. a
When a constant-delivery pump is used in a hydraulic system in which the pressure must be kept at a constant value, a pressure regulator is required. This pressure regulator discharges fluid from the pressure line into a reservoir return line when the pressure exceeds the predetermined maximum for which the valve is adjusted. This valve also unloads the pump when the pressure in the system is within normal operating parameters.

2. a
The function of an accumulator is to dampen pressure * A surges, to aid or supplement the power pump when several units are operating to store power for the limited operation of the system when the pump is not running, and to supply fluid under pressure to compensate for small internal or external leaks.

3. b
To continue the simplification of the hydraulic systems, many manufacturers use an electric motor to drive the hydraulic pump and incorporate it with the necessary valves and reservoir into a single unit. This unit is typically called a "power pack".

4. c
Aircraft flexible hose is marked with a yellow, red, or white stripe running the length of the hose, called a lay line. This lay line is not only for identification of the hose, but it serves to indicate whether or not the line was twisted when it was installed.

5. b
The part number is the only sure way of knowing that the correct O-ring is being used, but most rings are also marked with a series of colored dots or stripes to indicate the type of fluid with which they are compatible. A blue dot or stripes indicates compatibility with MIL-H-5606 hydraulic fluid.

6. a
Excessive fluctuation of hydraulic pressure is a sure sign of inadequate supply of fluid. Sometimes fluctuation can be due to air in the gauge lines.

7. c
One of the more efficient types of filters used in aircraft hydraulic systems is made of specially treated paper folded into pleats to increase its surface area. This pleated paper micronic element, as it is called, is wrapped around a spring steel wire coil to prevent its collapsing.

8. a
There are many instances in an aircraft hydraulic system when it is desirable to allow fluid to flow in one direction but prevent its flow in the opposite direction. This can be done by using check valves. An orifice check valve restricts flow in one direction and allows free flow in the other.

9. b
When installing an O-ring over a sharp edge or threads, you should cover the sharp portion with paper, aluminum foil, brass shim stock, or with a piece of plastic.

10. a
The most commonly used two way seal is the O-ring, which may be used as either a gasket or a packing.

  • All answers to MCQs, are available only to Patreon, thus preserving the value of the exam. Becoming a Patreon, you can get access to answers and more questions. Go to the Aeronautics Guide Patreon page, or
  • Read Aircraft Hydraulic Systems section in order to find the correct answers.{alertInfo}

Previous Post Next Post