Questions - Aircraft Hydraulic and Pneumatic Power Systems

Study Aid Questions

Fill in the Blanks

1. Hydraulic operations are roughly ______% efficient, with ______ loss due to fluid friction.

2. The 3 principle categories of hydraulic fluids are: ______ , ______ , ______ and base.

3. Phosphate ester based fluids are used mostly in transport category aircraft which is commonly referred to as ______. Presently, type ______ and ______ are used exclusively.

4. Hydraulic systems require special accessories such as ______ , ______ , and ______ to be compatible with the fluid being used.

5. Filter ratings are given in terms of ______ , and is an indication of the ______ size that will be filtered out.

6. Differential pressure indicators are designed to prevent false indications due to ______ , ______, or ______ .

7. Power packs are driven by either a(n) ______ or ______ .

8. Reservoirs can be pressurized or ______ . Commercial transport aircraft have pressurized reservoirs and most of them use ______ to do this.

9. A filter is a(n) ______ or ______ device used to clean the hydraulic fluid, thus ______ foreign contaminates from remaining in the system.

10. Maintenance of filters is relatively easy. It consists mostly of ______ the filter and element or cleaning the filter and ______ the element.

11. ______ pressure provides a convenient means of monitoring the condition of installed filter elements. It is the ______ used in the differential‐pressure or ______‐filter indicators found on many filter assemblies.

12. Once extended, the differential pressure indicator must be ______ and provides a ______ warning of a loaded element.

13. All aircraft hydraulic systems have one or more ______ pumps and may have a(n) ______ pump as an additional unit when the ______ pump is inoperative.

14. Pumps may be classified in 1 of 2 categories: ______ or ______ displacement.

15. The ______ section of the drive coupling, located ______ of the two sets of splines, is ______ in diameter than the splines.

16. Most valves are ______ controlled by a(n) ______ and are sometimes called ______ valves.

17. A sequence valve ______ fluid to a second actuator or motor after a set ______ has been reached, to do work in another part of the system.

18. A(n) ______ valve will give precedence to the critical hydraulic sub systems over ______ systems when system pressure is ______ .

19. Fuses detect sudden increase in flow such as a(n) ______ downstream, which in turn, ______ hydraulic fluid for the rest of the system.

20. Pressure relief valves are necessary to prevent ______ of components or ______ of hydraulic lines under ______ pressure.

21. ______ relief valves are used to relieve excessive pressure that may exist due to ______ of the fluid.

22. Pressure ______ are devices used to manage the output of the ______ to maintain system operating pressure within a fixed range.

23. The main purpose of the ______ valve is to isolate the normal system from an alternate or emergency system. The sliding component of this valve is to ______ either one of the other inlet ports.

24. Shut off valves are used to shut off the flow of fluid to a particular ______ or ______ .

25. The initial charge of an operational accumulator is known as ______ .


1. Large transport category aircraft hydraulic systems produce around 500 PSI of pressure. TRUE/FALSE

2. Both a high flash point and a high fire point are desirable for hydraulic fluids. TRUE/FALSE

3. The only contaminants that really affect a hydraulic system are abrasive type contaminants (ex. sand, rust, and machining chips) TRUE/FALSE

4. If the system becomes contaminated, it must to be purged immediately. TRUE/FALSE

5. Basic hydraulic systems have one of the following: directional valve, check valve, pressure relief valve, or selector valve. TRUE/FALSE

6. A hydraulic power pack does not require a centralized hydraulic power supply or long hydraulic lines. TRUE/FALSE

7. Pressurized reservoirs use quantity transmitters to gauge fluid quantity in the system. TRUE/FALSE

8. The micron element is designed to prevent passage of solids greater than 1 micron in size. TRUE/FALSE

9. Thermal lockout devices incorporated into some button indicators prevent operation of the indicator below a certain temperature. TRUE/FALSE

10. Most modern power‐driven pumps are variable‐delivery, compressor‐controlled type. TRUE/FALSE

11. All aircraft axial‐piston pumps have an odd number of pistons (5, 7, 9, etc.). TRUE/FALSE

12. When the pump outlet pressure of a Vickers variable delivery pump is below 2850 psi, it is held at its maximum angle in relation with the drive shaft centerline by the force of the yoke return spring. TRUE/FALSE

13. There are two types of in‐line check valves: simple‐type and orifice‐type. TRUE/FALSE

14. Automatic resetting fuses are designed to allow a certain volume of fluid per minute to pass through. TRUE/FALSE

15. An accumulator is a steel sphere divided into two chambers by a rubber diaphragm. TRUE/FALSE

Knowledge Application Questions

1. What color is mineral base hydraulic fluid?

2. What could happen when Skydrol spills onto plastic resins, vinyl compositions, lacquers and oil base paints?

3. What feature is built into the drive shaft of a hydraulic pump to prevent engine damage in the event of pump seizure or overload?

4. What maintenance action is required to minimize contamination during replacement of a hydraulic unit?

5. What condition will cause the filter by‐pass on a hydraulic filter to open and allow in unfiltered fluid?

6. Name four types of hydraulic pumps?

7. What sources could pressurize a pressurized hydraulic reservoir?

8. What is the purpose of a pressure relief valve?

9. What is the purpose of a pressure regulator?

10. What is used in some hydraulic systems to supplement the power pump output during high demand?

11. What is the purpose of restrictors that are used in aircraft pneumatic systems?

12. Why should a pneumatic system be purged periodically?

13. What must be accomplished with an accumulator prior to disassembling?

14. Can different hydraulic fluid types be mixed? Why?

15. Why do most aircraft hydraulic installations require a closed center hydraulic system?

16. What is the advantage of a powerpack hydraulic system?

17. Why are hydraulic reservoirs of many transport type aircraft pressurized?

18. Explain the principle of operation of a filter by‐pass valve.

19. Name 2 examples of non positive displacement pumps.

Normal pumping mode hydraulic

20. Refer to the figure above. What happens when the control pressure (Pc) increases?

21. Refer to the figure above: What happens when the pressure adjustment screw will be turned clockwise?

22. How are sequence valves controlled?

23. What is the function of the Power transfer unit used in large aircraft hydraulic systems?


Depressurized mode hydraulic

24. Refer to the figure above: What will happen if the EDV solenoid will be energized?

25. What is the reason for using pressure reducing valves?

26. What are the functions of an accumulator?
a] _________________________________________________________________

b] _________________________________________________________________

c] _________________________________________________________________

d] _________________________________________________________________

27. What is the function of the Ram Air Turbine that is often installed in large transport type aircraft?

28. What is pneudraulics?

Right hydraulic system of a Boeing 777

29. Refer to the figure above. What will happen if the fluid level in the reservoir drops below the standpipe?

Multiple Choices Questions

1. What device in a hydraulic system with a constant delivery pump allows circulation of the fluid when no demands are on the system?
a. Pressure regulator.
b. Pressure relief valve.
c. Shuttle valve.

2. A fully charged hydraulic accumulator provides
a. positive fluid flow to the pump inlet.
b. a source for additional hydraulic power when heavy demands are placed on the system.
c. air pressure to the various hydraulic components.

3. A hydraulic system referred to as a "power pack" system will
a. have an engine driven pump for greater pressure.
b. have all hydraulic power components located in one unit.
c. have a pressurized reservoir.

4. A hydraulic hose identified as MIL-H-8794 will have a yellow stripe running the length of the hose. This stripe
a. identifies that the hose is constructed of synthetic rubber and may be suitable for a wide r&ge of applications.
b. identifies that the hose is for hydraulic fluid only.
c. is used to ensure that the hose is installed without excessive twisting.

5. An O-ring intended for use in a hydraulic system using MIL-H-5606 (mineral base) fluid will be marked with
a. one or more white dots.
b. a blue stripe or dot.
c. a white and yellow stripe.

6. What condition would most likely cause excessive fluctuation of the pressure gauge when the hydraulic pump is operating?
a. Inadequate supply of fluid.
b. Accumulator air pressure low.
c. System relief valve sticking closed.

7. A filter incorporating specially treated cellulose paper is identified as a
a. sediment trap.
b. cuno filter.
c. micronic filter.

8. The purpose of an orifice check valve is to
a. restrict flow in one direction and allow free flow in the other.
b. relieve pressure to a sensitive component.
c. relieve pressure in one direction and prevent flow in the other direction.

9. To protect packing rings or seals from damage when it is necessary to install them over or inside threaded sections, the
a. packings should be stretched during installation to avoid contact with the threads.
b. threaded section should be covered with a suitable sleeve.
c. threaded section should be coated with a heavy grease.

10. To prevent external and internal leakage in aircraft hydraulic units, the most commonly used type of seal is the
a. O ring seal.
b. gasket seal.
c. chevron seal.

11. Which allows free fluid flow in one direction and no fluid flow in the other direction?
a. Check valve.
b. Metering piston.
c. Shutoff valve.

12. Select the valve used in a hydraulic system that directs pressurized fluid to one end of an actuating cylinder and simultaneously directs return fluid to the reservoir from the other end.
a. Sequence.
b. Shuttle.
c. Selector.

13. What function does the absolute pressure regulator perform in the pneumatic power system?
a. Regulates the compressor outlet air pressure to stabilize the system pressure.
b. Regulates the compressor inlet air to provide a stabilized source of air for the compressor.
c. Regulates the pneumatic system pressure to protect the moisture separator from internal explosion.

14. (1) Relief valves are used in pneumatic systems as damage preventing units.
(2) Check valves are used in both hydraulic and pneumatic systems.

Regarding the above statements,
a. only No. 1 is true.
b. neither No. 1 nor No. 2 is true.
c. both No. 1 and No. 2 are true.

15. One of the distinguishing characteristics of an open center selector valve used in a hydraulic system is that

a. fluid flows through the valve in the OFF position.
b. a limited amount of fluid flows in one direction and no fluid flows in the opposite direction.
c. fluid flows in three directions in the ON position.

16. What type of packings should be used in hydraulic components to be installed in a system containing Skydrol?
a. AN packings made of neoprene.
b. Packing materials made for ester base fluids.
c. AN packings made of natural rubber.

17. Relief valves are used in pneumatic systems
a. as damage preventing units.
b. for one direction flow control.
c. to reduce the rate of airflow.

18. An aircraft pneumatic system, which incorporates an engine driven multistage reciprocating compressor, also requires
a. a surge chamber.
b. an oil separator.
c. a moisture separator.

19. The removal of air from an aircraft hydraulic system is generally accomplished
a. through automatic bleed valves on individual components during system operation.
b. by allowing the system to remain inoperative for several hours.
c. by operating the various hydraulic components through several cycles.

20. Pneumatic systems utilize
a. return lines.
b. relief valves.
c. diluter valves.

21. The component in the hydraulic system that is used to direct the flow of fluid is the
a. check valve.
b. orifice check valve.
c. selector valve.

22. What type of selector valve is one of the most commonly used in hydraulic systems to provide for simultaneous flow of fluid into and out of a connected actuating unit?
a. Three port, four way valve.
b. Four port, closed center valve.
c. Two port, open center valve.

23. What is the purpose of using backup rings with O rings in hydraulic systems above 1,500 PSI?
a. Prevent high pressure from extruding the seal between the moving and stationary part.
b. Provide a seal between two parts of a unit which move in relation to each other.
c. Prevent internal and external leakage of all moving parts within a hydraulic system.

24. The purpose of the pressure regulator in a hydraulic system is to
a. prevent failure of components or rupture of hydraulic lines under excessive pressure.
b. regulate the amount of fluid flow to the actuating cylinders within the system.
c. maintain system operating pressure within a predetermined range and to unload the pump.

25. A flexible sealing element subject to motion is a
a. gasket.
b. compound.
c. packing.

26. Which characteristics apply to aircraft hydraulic systems?
1. Minimum maintenance requirements.
2. Lightweight.
3. About 80 percent operating efficiency (20 percent loss due to fluid friction).
4. Simple to inspect.

a. 1,2,3,4
b. 1,2,4
c. 1,3,4

27. If a rigid tube is too short for the flare to reach its seat before tightening, pulling it into place by tightening
a. may distort the flare.
b. is acceptable.
c. may distort the cone.

28. The installation of a new metal hydraulic line should be made with
a. a straight tube to withstand the shocks and vibration to which it will be subjected.
b. enough bends to allow the tube to expand and contract with temperature changes and to absorb vibration.
c. a straight tube to permit proper alignment of the fitting and thereby reduce fluid loss through leakage.

29. Extrusion of an 0 ring seal is prevented in a high pressure system by the use of a
a. backup ring on the side of the O ring next to the pressure.
b. U ring on the side of the O ring away from the pressure.
c. backup ring on the side of the O ring away from the pressure.

30. What is one advantage of piston type hydraulic motors over electric motors?
a. They work satisfactorily over a wider temperature range.
b. There is no fire hazard if the motor is stalled.
c. They are considerably quieter in operation.

31. Generally, the first step in removing an accumulator from an aircraft is to
a. drain the reservoir.
b. relieve system pressure.
c. discharge the preload.

32. (Refer to figure 1)Which fitting is an AN flared tube fitting?
a. 1
b. 3
c. 2

Figure 1

33. (Refer to figure 2.) Which illustration(s) show(s) the correct spiral for teflon backup rings?
a. 1 and 2
b. 1 and 3
c. 3

Figure 2

34. If a hydraulic brake system uses neoprene rubber packing materials, the correct hydraulic fluid to service the system is
a. mineral base oil.
b. phosphate ester base oil.
c. vegetable base oil.

35. The internal resistance of a fluid which tends to prevent it from flowing is called
a. volatility.
b. viscosity.
c. acidity.

36. What is the viscosity of hydraulic fluid?
a. The internal resistance of a fluid which tends to prevent it from flowing.
b. The increase in volume of a fluid due to temperature change.
c. The fluid's ability to resist oxidation and deterioration for long periods.

37. Which is a characteristic of petroleum base hydraulic fluid?
a. Nonflammable under all conditions.
b. Flammable under normal conditions.
c. Compatible to natural rubber seals and packings.

38. (1) When servicing aircraft hydraulic systems, use the type fluid specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit.
(2) Hydraulic fluids for aircraft are dyed a specific color for each type of fluid.

Regarding the above statements,
a. only No. 1 is true.
b. only No. 2 is true.
c. both No. 1 and No. 2 are true.

39. Petroleum base hydraulic fluid is which color?
a. Purple.
b. Red.
c. Blue.

40. Which of the following is adversely affected by atmospheric humidity if left unprotected?
1. MIL-H-5606 hydraulic fluid.
2. Skydrol hydraulic fluid.
3. None of the above.

a. 2
b. 1 and 2
c. 3

41. Which is a characteristic of synthetic base hydraulic fluid?
a. Low flash point.
b. Low moisture retention.
c. High flash point.

42. Which statement about fluids is correct?
a. All fluids are considered to be highly compressible.
b. Any fluid will completely fill its container.
c. All fluids readily transmit pressure.

43. Three types of hydraulic fluids currently being used in civil aircraft are
a. mineral base, vegetable base, and phosphate ester base.
b. mineral base, phosphate ester base, and mixed mineral and phosphate ester base.
c. mineral base, phosphate ester base, and mixed vegetable and alcohol base.

44. Which of the following lists only desirable properties of a good hydraulic fluid?
a. High viscosity, low flash point, chemical stability, high fire point.
b. Low viscosity, chemical stability, high flash point, high fire point.
c. High flash point, low viscosity, chemical stability, low fire point.

45. Characteristics of MIL-H-8446 (Skydrol 500 A & B) hydraulic fluid are
a. blue color, phosphate ester base, fire resistant, butyl rubber seals.
b. light purple color, phosphate ester base, fire resistant, butyl rubber seals.
c. light green color, phosphate ester base, fire resistant, butyl rubber seals.

46. Where can information be obtained about the compatibility of fire resistant hydraulic fluid with aircraft materials?
a. Manufacturer's technical bulletins.
b. Aircraft manufacturer's specifications.
c. AC 43.13-1A.

47. Characteristics of MIL-H-5606 hydraulic fluid are
a. light purple color, phosphate ester base, fire resistant, butyl rubber seals.
b. blue color, vegetable base, will burn, natural rubber seals.
c. red color, petroleum base, will burn, synthetic rubber seals.

48. Characteristics of MIL-H-7644 hydraulic fluid are
a. blue color, vegetable base, will burn, natural rubber seals.
b. light purple color, phosphate ester base, fire resistant, butyl rubber seals.
c. red color, petroleum base, will burn, synthetic rubber seals.

49. If an aircraft hydraulic system requires mineral base hydraulic fluid, but phosphate ester base hydraulic fluid is used, what will be the effect on the system?
a. System will be contaminated, fluids will not blend, and the seals will fail.
b. No effect.
c. System will be contaminated, fluids will not blend, but there will be no seal problem.

50. What is used to flush a system normally serviced with MIL-H-5606 hydraulic fluid?
a. Naphtha or varsol.
b. Lacquer thinner or trichlorethylene.
c. Methyl ethyl ketone or kerosene.

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