A&P Hydraulic and Pneumatic Systems 2 — Questions and Answers
Question 1: What type of pump is most commonly used as the primary hydraulic pump in turbine-powered aircraft?
- Gear-type pump
- Vane-type pump
- Variable-displacement piston pump (Correct answer)
- Centrifugal pump
Correct answer: Variable-displacement piston pump
Variable-displacement axial piston pumps are standard on turbine aircraft because they automatically adjust output to maintain constant system pressure without a separate pressure regulator.
Question 2: How is a hydraulic selector valve different from a check valve?
- A selector valve directs fluid flow to different actuators, while a check valve only allows one-directional flow (Correct answer)
- A selector valve filters fluid, while a check valve controls pressure
- A check valve controls flow direction to multiple actuators, while a selector valve only allows one-way flow
- Both perform the same function in different parts of the system
Correct answer: A selector valve directs fluid flow to different actuators, while a check valve only allows one-directional flow
A selector valve (directional control valve) routes pressurized fluid to one side of an actuator while returning fluid from the other side, whereas a check valve simply prevents reverse flow.
Question 3: What does a hydraulic fuse (thermal relief valve) protect against?
- Excessive fluid temperature in the reservoir
- Fluid loss downstream of a ruptured line by automatically closing when flow exceeds a set rate (Correct answer)
- Contamination entering the actuator from a broken seal
- Overpressure at the pump outlet
Correct answer: Fluid loss downstream of a ruptured line by automatically closing when flow exceeds a set rate
A hydraulic fuse detects abnormally high flow rates (indicating a downstream rupture) and closes to isolate that section, preserving fluid for the rest of the system.
Question 4: When bleeding a hydraulic brake system on an aircraft, fluid is introduced at the:
- Wheel cylinder bleeder fitting and flows toward the master cylinder (Correct answer)
- Master cylinder and flows toward the wheel cylinder bleeder fitting
- Reservoir and is pumped manually through the entire system
- Brake metering valve and flows outward in both directions
Correct answer: Wheel cylinder bleeder fitting and flows toward the master cylinder
When pressure-bleeding aircraft brakes, fluid is forced upward from the wheel cylinder bleeder fitting toward the master cylinder to push air bubbles up and out.
Question 5: What material are most modern aircraft hydraulic lines made from in high-pressure sections?
- Copper tubing
- 5052-O aluminum alloy tubing
- 21-4N stainless steel or titanium tubing (Correct answer)
- Neoprene hose only
Correct answer: 21-4N stainless steel or titanium tubing
High-pressure hydraulic lines in areas subject to vibration and high stress are made from corrosion-resistant steel (CRES) or titanium alloy tubing rated for the system pressure.
Question 6: What is the purpose of the desiccant in an aircraft pneumatic system?
- To lubricate moving parts within the compressor
- To remove moisture from compressed air before it reaches system components (Correct answer)
- To filter carbon particles from bleed air
- To regulate system pressure to a usable level
Correct answer: To remove moisture from compressed air before it reaches system components
Desiccant (moisture-absorbing material) in the pneumatic system removes water vapor from compressed air to prevent corrosion, ice formation, and seal damage in system components.
A&P Hydraulic and Pneumatic Systems 2
What type of pump is most commonly used as the primary hydraulic pump in turbine-powered aircraft?