Questions and Answers - Aircraft Hydraulics 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 below. What will happen if the fluid level in the reservoir drops below the standpipe?

Multiple Choices Questions

1. What should be checked when a shock strut bottoms during a landing?
a. Air pressure
b. Packing seals for correct installation
c. Fluid level

2. Overinflated aircraft tires may cause damage to the:
a. brake linings
b. wheel hub
c. wheel flange

3. What is the function of a cam incorporated in a nose gear shock strut?
a. Provides an internal shimmy damper
b. Straightens the nosewheel
c. Provides steering of aircraft during ground operation

4. Debooster cylinders are used in brake systems primarily to:
a. reduce brake pressure and maintain static pressure
b. relieve excessive fluid and ensure a positive release
c. reduce the pressure to the brake and increase the volume of fluid flow

5. The purpose of a relief valve in a brake system is to:
a. reduce pressure for brake application
b. prevent the tire from skidding
c. compensate for thermal expansion

6. A high speed aircraft tire with a sound cord body and bead may be recapped:
a. a maximum of three times
b. only by the tire manufacturer
c. an indefinite number of times

7. The fusible plugs installed in some aircraft wheels will:
a. indicate tire tread separation
b. prevent over inflation
c. melt at a specified elevated temperature

8. Instructions concerning the type of fluid and amount of air pressure to be put in a shock strut are found:
a. on the airplane data plate
b. in the aircraft operations limitations
c. in the aircraft manufacturer's service manual

9. The primary purpose for balancing aircraft wheel assemblies is to:
a. prevent heavy spots and reduce vibration
b. distribute the aircraft weight properly
c. reduce excessive wear and turbulence

10. The pressure source for power brakes is:
a. the main hydraulic system
b. the power brake reservoir
c. a master cylinder

11 What is the purpose of the torque links attached to the cylinder and piston of a landing gear oleo strut?
a. Limit compression stroke
b. Hold the strut in place
c. Maintain correct wheel alignment

12 When an air/oil type of landing gear shock strut is used, the initial shock of landing is cushioned by:
a. compression of the air charge
b. the fluid being forced through a metered opening
c. compression of the fluid

13 The purpose of a sequence valve in a hydraulic retractable landing gear system is to:
a. prevent heavy landing gear from falling too rapidly upon extension
b. provide a means of disconnecting the normal source of hydraulic power and connecting the emergency source of power
c. ensure operation of the landing gear and gear doors in the proper order

14. Which statement is true with respect to an aircraft equipped with hydraulically operated multiple disk type brake assemblies?
a. There are no minimum or maximum disk clearance checks required due to the use of self compensating cylinder assemblies
b. Do not set parking brake when brakes are hot
c. No parking brake provisions are possible for this type of brake assembly

15. Nose gear centering cams are used in many retractable landing gear systems. The primary purpose of the centering device is to:
a. align the nosewheel prior to touchdown
b. engage the nosewheel steering
c. center the nosewheel before it enters the wheel well

16. How long should you wait after a flight before checking tire pressure?
a. At least 3 hours
b. At least 2 hours
c. At least 30 minutes

17. An electric motor used to raise and lower a landing gear would most likely be a
a. shunt field series wound motor
b. split field shunt wound motor
c. split field series wound motor

18. A landing gear position and warning system will provide a warning in the cockpit when the throttle is
a. retarded and gear is not down and locked
b. advanced and gear is down and locked
c. retarded and gear is down and locked

19. Excessive wear in the center of the tread of an aircraft tire is an indication of:
a. incorrect camber
b. excessive toe out
c. over inflation

20. On most aircraft, the oil level of an air and oil shock strut is checked by:
a. removing the oil filler plug and inserting a gauge
b. measuring the length of the strut extension with a certain air pressure in the strut
c. releasing the air and seeing that the oil is to the level of the filler plug

21. How can it be determined that all air has been purged from a master cylinder brake system?
a. By operating a hydraulic unit and watching the system pressure gauge for smooth, full scale deflection
b. By noting whether the brake is firm or spongy
c. By noting the amount of fluid return to the master cylinder upon brake release

22. The best safeguards against heat buildup in aircraft tires are:
a. proper tire inflation, minimum braking, and ground rolls into the wind
b. short ground rolls, slow taxi speeds, minimum braking, and proper tire inflation
c. minimum braking, proper tire inflation, and long ground rolls

23 In brake service work, the term 'bleeding brakes' is the process of:
a. withdrawing air only from the system
b. withdrawing fluid from the system for the purpose of removing air that has entered the system
c. replacing small amounts of fluid in reservoir

24 Aircraft brakes requiring a large volume of fluid to operate the brakes generally:
a. use independent master cylinder systems
b. do not use brake system accumulators
c. use power brake control valves

25. To prevent a very rapid extension of an oleo shock strut after initial compression resulting from landing impact:
a. various types of valves or orifices are used which restrict the reverse fluid flow
b. the metering pin gradually reduces the size of the orifice as the shock strut extends
c. the air is forced through a restricted orifice in the reverse direction

26. The metering pins in oleo shock struts serve to:
a. lock the struts in the DOWN position
b. retard the flow of oil as the struts are compressed
c. meter the proper amount of air in the struts

27. After performing maintenance on an aircraft's landing gear system which may have affected the system's operation, it is usually necessary to:
a. conduct a flight test
b. re‐inspect the area after the first flight
c. make an operational check with the aircraft on jacks

28. Why do tire and wheel manufacturers often recommend that the tires on split rim wheels be deflated before removing the wheel from the axle?
a. to relieve the strain on the wheel retaining nut and axle threads
b. as a safety precaution in case the bolts that hold the wheel halves together have been damaged or weakened
c. to remove the static load imposed upon the wheel bearings by the inflated tire

29. Exposure to and/or storage near which of the following is considered harmful to aircraft tires?
1. Low humidity    2. Fuel                                3. Oil                        4. Ozone 

5. Helium              6. Electrical equipment      7. Hydraulic fluid      8. Solvents

a. 2, 3, 4, 5, 6, 7, 8
b. 1, 2, 3, 5, 7, 8
c. 2, 3, 4, 6, 7, 8

30. On all aircraft equipped with retractable landing gear, some means must be provided to:
a. retract and extend the landing gear if the normal operating mechanism fails
b. extend the landing gear if the normal operating mechanism fails
c. prevent the throttle from being reduced below a safe power setting while the landing gear is retracted

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