18 Hydraulic & Pneumatic Systems Practice Questions & Answers
Every Hydraulic & Pneumatic Systems practice question from the FAA A&P Airframe (AMA) Practice Test, with the correct answer and a short explanation.
Start practice test →1. Which statement about MIL-H-5606 hydraulic fluid is true?
- A.It is dyed purple, is vegetable based, and uses natural rubber packings.
- B.It is dyed green, is a polyalphaolefin, and uses ethylene-propylene packings.
- C.It is dyed blue, is a phosphate ester, and uses butyl rubber packings.
- D.It is dyed red, is petroleum based, and uses neoprene or nitrile packings.✓ Answer
MIL-H-5606 is the mineral (petroleum) base fluid; it is dyed red and has an odor like penetrating oil, which is how a mechanic tells it apart from the purple or green synthetics and from the phosphate esters. Because it is a petroleum product, it is compatible with synthetic rubber packings such as neoprene and nitrile (Buna-N), while phosphate ester fluids attack those compounds and require butyl or ethylene-propylene instead. MIL-H-6083 is simply the rust-inhibited version of 5606 and is interchangeable with it.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
2. Which statement about MIL-H-83282 hydraulic fluid is true?
- A.Fire resistant, uses the same seals and hoses as MIL-H-5606, and is generally limited to about -40 °F.✓ Answer
- B.Fireproof, uses the same seals and hoses as MIL-H-5606, and is usable down to about -65 °F.
- C.Fire resistant, requires natural rubber seals and hoses, and is generally limited to about -40 °F.
- D.Fire resistant, requires butyl rubber seals and hoses, and is usable down to about -65 °F.
MIL-H-83282 is a fire-resistant hydrogenated polyalphaolefin that was developed as a direct replacement for MIL-H-5606, so it works with the same neoprene and nitrile seals, gaskets and hoses and may be used in the same system. Its one operational drawback is high viscosity at low temperature, which generally limits it to about -40 °F, whereas 5606 remains usable much colder. No hydraulic fluid in service use is fireproof; 83282 is only far more flame resistant than 5606, and contaminating it with 5606 can destroy that fire resistance.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
3. A transport-category airplane uses a phosphate ester hydraulic fluid. Which statement about that fluid and its packings is true?
- A.The fluid is fire resistant but not fireproof, and its packings are butyl or ethylene-propylene rubber.✓ Answer
- B.The fluid is fire resistant but not fireproof, and its packings are neoprene or nitrile rubber.
- C.The fluid is fireproof, and its packings are neoprene or nitrile rubber.
- D.The fluid is fireproof, and its packings are butyl or ethylene-propylene rubber.
Phosphate ester fluids used on most transport aircraft are fire resistant, not fireproof: they will burn if the ignition source is strong and sustained enough, so they must never be treated as a substitute for fire protection. They are chemically incompatible with the neoprene and nitrile packings used for petroleum-base fluids and require butyl or ethylene-propylene elastomers instead; installing the wrong compound causes the seal to swell and fail. These fluids also dissolve skin oils, so gloves and eye protection are required when handling them.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
4. How does the aviation mechanic determine which hydraulic fluid to use when servicing an aircraft hydraulic system?
- A.From the color and odor of the fluid drawn from the system sampling valve.
- B.From the type certificate data sheet or the aircraft weight and balance record.
- C.From the manufacturer's maintenance manual or the instruction plate on the reservoir.✓ Answer
- D.From the pilot's operating handbook or the placard posted on the instrument panel.
The fluid a system requires is established by the aircraft manufacturer and is published in the maintenance or service manual, and is repeated on the instruction plate attached to the reservoir or to the unit being serviced. The pilot's operating handbook and the type certificate data sheet are not maintenance servicing documents and do not carry that instruction, and judging by color is unreliable because more than one specification can carry a similar dye. Servicing with the wrong fluid requires draining, flushing and reseal of the system.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
5. In a hydraulic component subjected to pressure from one direction only, a single backup ring is installed with the O-ring
- A.in a groove of its own beside the gland, to wipe dirt from the moving piston rod.
- B.on the low-pressure side of the O-ring, to keep it from extruding into the clearance gap.✓ Answer
- C.on both sides of the O-ring, to center it in the gland and keep it from twisting.
- D.on the high-pressure side of the O-ring, to shield it from the fluid pressure surge.
System pressure pushes the O-ring toward the clearance gap on the far side of the gland, and above roughly 1,500 psi the elastomer will begin to extrude into that gap and shear. The backup ring is a harder Teflon washer that fills the gap, so it must sit downstream of the packing, on the side away from the pressure. Where pressure alternates and can come from either direction, two backup rings are installed, one on each side of the O-ring.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
6. Which statement about the color coding of hydraulic O-ring packings is true?
- A.Color codes are applied only to packings intended for phosphate ester fluid systems.
- B.Color codes reliably identify both the fluid compatibility and the size of the packing.
- C.Color codes are not a reliable means of identification and never indicate packing size.✓ Answer
- D.Color codes identify the size of the packing but not the fluid it is compatible with.
Color coding is only a supplementary aid: the dots and stripes can be omitted by the manufacturer, can be obscured by the packing's own dye, and in no case do they convey the size of the O-ring. The mechanic identifies a packing from its part number on the sealed package, not from its color. When codes are present, blue always appears in codes compatible with MIL-H-5606, a green stripe marks packings for phosphate ester fluid, and hydrocarbon-fuel codes always contain red and never blue.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
7. The hydraulic unit that automatically causes one actuator to begin moving only after a second actuator has completed its travel is the
- A.priority valve, which passes fluid to the second actuator when system pressure falls low.
- B.relief valve, which passes fluid to the second actuator when the first one has stalled.
- C.shuttle valve, which passes fluid to the second actuator from the emergency system.
- D.sequence valve, which passes fluid to the second actuator at a preset pressure setting.✓ Answer
A sequence valve, also called a timing valve, resembles a relief valve except that when the preset pressure is reached it diverts fluid to a second actuator instead of dumping it to return. Pressure in the line only builds to that setting after the first actuator reaches the end of its stroke and stops accepting flow, so the two units always operate in the intended order. A priority valve, by contrast, cuts off non-critical subsystems when pressure falls, and a shuttle valve isolates the normal system from the emergency system.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
8. In a hydraulic system using a constant-delivery pump and a pressure regulator, the term "kick-out" refers to the pressure at which the regulator
- A.opens the bypass and returns pump output to the reservoir, unloading the pump.✓ Answer
- B.opens to relieve pressure trapped by thermal expansion in an isolated section of line.
- C.closes the bypass and directs pump output back into the system pressure manifold.
- D.reduces pressure to a subsystem that must operate below main system pressure.
A constant-delivery pump moves the same volume every revolution, so something must relieve it once the system is up to pressure. The regulator, through which all pump output must pass, opens its bypass at the kick-out pressure so the pump turns against almost no resistance while a check valve holds system pressure. Kick-in is the lower pressure, reached after the accumulator has discharged, at which the regulator again routes pump output into the system; too narrow a spread between the two makes the system cycle rapidly.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
9. A hydraulic fuse installed in a brake or leading edge flap line protects the system by acting so that it
- A.seals off the normal system line whenever the emergency system is pressurized.
- B.restricts flow in one direction through the line while allowing free flow in the other.
- C.shuts off flow to the line when it senses a sudden excessive rate of flow.✓ Answer
- D.shuts off flow to the line when system pressure exceeds the relief valve setting.
A hydraulic fuse is a flow-sensing device, not a pressure-sensing one: a ruptured line or a failed seal downstream lets fluid escape at a rate far above normal, and the fuse closes at that abnormal flow rate to preserve the remaining fluid for the rest of the system. It is fitted where a single failure could drain the reservoir, such as brake, slat, nose gear and thrust reverser lines. Pressure limiting is the job of the relief valve, one-way restriction is the orifice check valve, and isolating the emergency system is the shuttle valve.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
10. A constant-pressure hydraulic system cycles its pump on and off much more often than normal, and no external leakage can be found. The most likely cause is
- A.an air preload lower than the value specified for the accumulator.✓ Answer
- B.a clogged micron element in the system return line filter.
- C.an air preload higher than normal system operating pressure.
- D.a pressure relief valve set below the regulator kick-out pressure.
One purpose of the accumulator is to supply the small volume of fluid needed to make up normal internal leakage, so the pressure switch or regulator is not triggered by every minor pressure decay. If the gas preload is low, the accumulator stores very little fluid at system pressure and empties almost at once, so pressure falls to the kick-in point repeatedly and the pump cycles. Checking and restoring the preload with air or nitrogen to the value specified by the manufacturer is the corrective action.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
11. Before a hydraulic accumulator is removed from the aircraft or disassembled, the aviation mechanic must
- A.open the reservoir bleed valve and leave the gas preload to expel the residual fluid.
- B.release the trapped fluid pressure and discharge the gas preload from the accumulator.✓ Answer
- C.release the trapped fluid pressure and charge the gas side to its normal preload.
- D.close the system shutoff valve and leave the gas preload in place to seat the piston.
An accumulator stores energy in a compressed gas charge, and that energy remains stored after the aircraft is shut down and the hydraulic system is depressurized. Loosening a fitting or an end cap while the preload is still trapped can launch the piston, end cap or fitting with enough force to cause serious injury, so both the fluid pressure and the entire gas preload must be released first. The mechanic must also identify the type of high-pressure air valve fitted before attempting to vent it.
Source: AC 43.13-1 Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, Ch. 9Report a problem with this question
12. Why is a spring-loaded bypass relief valve built into the head of a hydraulic system filter?
- A.So the element can be back-flushed in place, since a paper element is cleaned rather than replaced.
- B.So the element is protected from surges, since a pressure surge can collapse the convolutions.
- C.So contaminated fluid returns to the reservoir, since the reservoir filter will trap the particles.
- D.So fluid keeps flowing when the element loads up, since unfiltered flow beats no flow at all.✓ Answer
As a filter element loads with contaminant, the pressure differential across it rises; at a differential of roughly 50 psi the bypass valve lifts and lets fluid pass around the element. The design accepts unfiltered fluid because losing flow entirely would disable the subsystem the filter serves, which is the greater hazard. Micron-rated paper elements are disposable and are replaced rather than cleaned, and a bypassing filter is a defect to be corrected at the next servicing, not a normal operating condition.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
13. Before removing the filler cap to service a pressurized hydraulic reservoir, the aviation mechanic should
- A.drain the reservoir down to the top of the standpipe before opening the cap.
- B.increase the reservoir air pressure so that fluid cannot siphon out of the cap.
- C.operate every subsystem so that the reservoir is at its highest fluid level.
- D.bleed off the reservoir air pressure at the bleed valve before opening the cap.✓ Answer
A pressurized reservoir is held at roughly 30 to 50 psi to keep the pump supplied and prevent cavitation and foaming at altitude, and that air pressure is still present after shutdown. Opening the filler cap against it can blow the cap off and spray fluid with enough force to injure the eyes or skin, so the air must be released at the reservoir bleed valve first, with a rag wrapped around the valve to catch the escaping fluid. Servicing through a ground cart connection at the service port avoids opening the reservoir at all and reduces the chance of introducing contamination.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
14. A hydraulically actuated subsystem operates in a spongy and erratic manner, although reservoir quantity and system pressure are normal. The mechanic should suspect
- A.air entrained in the fluid, corrected by cycling or bleeding the subsystem.✓ Answer
- B.a loaded return filter element, corrected by resetting the indicator button.
- C.an accumulator preload above system pressure, corrected by venting gas.
- D.a fluid level below the standpipe, corrected by refilling the reservoir.
Liquids are practically incompressible, so a properly filled hydraulic system moves an actuator firmly and predictably; a spongy, erratic response means a compressible gas is present in the lines or the actuator. Air is purged by operating the subsystem through several complete cycles so the entrained air is carried back to the reservoir and released, or by pumping fluid through a bleed port until air-free fluid flows. A clogged filter or worn pump would slow the actuator rather than make it feel spongy.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
15. Which statement about hydraulic power pumps is true?
- A.A constant-displacement pump requires a pressure regulator to control pressure and unload the pump.✓ Answer
- B.A variable-displacement pump requires a pressure regulator to control pressure and unload the pump.
- C.A variable-displacement pump uses a centrifugal impeller to control pressure and limit its output.
- D.A constant-displacement pump uses an internal compensator to control pressure and destroke itself.
A constant-displacement pump delivers the same volume for every revolution regardless of demand, so an external pressure regulator, or unloading valve, is required both to hold system pressure within its range and to let the pump turn against little resistance once the system is up. A variable-displacement pump needs no such regulator, because its internal compensator senses system pressure and reduces the piston stroke to near-zero delivery as the compensator setting is approached. The two arrangements are functionally equivalent, and a system relief valve is still installed in each as a backup.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
16. In an aircraft pneumatic power system, the moisture separator is installed
- A.downstream of the storage bottle, where it removes water from the delivery line.
- B.upstream of the relief valve, where it removes oil vapor from the compressed air.
- C.upstream of the compressor, where it removes water from the incoming air.
- D.downstream of the compressor, where it removes water from the compressed air.✓ Answer
Compressing air raises its temperature and then, as it cools, drives water vapor out as liquid, so the separator must be placed after the compressor where that condensate actually forms. The unit contains a reservoir, a pressure switch, a dump valve and a check valve; when the pressure switch de-energizes the dump valve, the collected water is purged overboard along with the line back to the compressor, and the check valve prevents pressure loss and reverse flow during the dump. Removing water matters because moisture left in the lines can freeze and block a valve or an orifice at altitude.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
17. During inspection of a pneumatic system, the mechanic finds that the chemical drier cartridge is a color other than blue. This indicates that
- A.the cartridge is rated for low-pressure service and must be relocated.
- B.the cartridge is new and fully serviceable and may be left in service.
- C.the cartridge is contaminated with oil and must be flushed and reinstalled.
- D.the cartridge has absorbed moisture and must be replaced with a new one.✓ Answer
The desiccant in a chemical drier is a color-indicating material that is blue when dry and serviceable and changes color as it takes up moisture, giving the mechanic a direct visual check of its remaining capacity. Once it has changed color the cartridge is saturated and can no longer protect the downstream lines and valves from water, so it is replaced rather than dried or cleaned. Leaving a saturated cartridge installed allows moisture to pass into the system, where it can freeze and jam a valve or orifice.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
18. The identification stripe, or lay line, running the length of a flexible hydraulic hose is used to show
- A.whether the hose has been twisted during installation.✓ Answer
- B.the temperature range within which the hose may be used.
- C.the maximum operating pressure of the hose assembly.
- D.the direction in which fluid must flow through the hose.
The lay line is a straight reference mark printed along the hose, and if it spirals around the hose after installation the assembly has been twisted, which puts a shear load on the reinforcement braid and can burst the hose in service. Pressure rating, temperature limits and specification are given by the printed hose markings, not by the direction of the stripe, and hose is not flow-directional. Correct installation also allows about 5 to 8 percent slack, because a hose changes length roughly +2 to -4 percent when it is pressurized.
Source: FAA-H-8083-31 Aviation Maintenance Technician Handbook (Airframe), Ch. 12 (Hydraulic and Pneumatic Power Systems)Report a problem with this question
Practice questions based on the Aviation Mechanic Airman Certification Standards (FAA-S-ACS-1), 14 CFR parts 43, 65, and 91, and the FAA Aviation Maintenance Technician Handbook—Airframe (FAA-H-8083-31). This site is not affiliated with or endorsed by the Federal Aviation Administration. This bank covers the AIRFRAME written test only; the General and Powerplant written tests and the oral and practical tests are separate. Torque values, cable tensions, inflation pressures, servicing quantities, wear limits, and repair criteria always come from the manufacturer's current maintenance data, the structural repair manual, and the applicable airworthiness directives — never from a practice test. Confirm current eligibility and testing requirements with the FAA before you test. About the A&P mechanic certificate →