AMT Hydraulics and Pneumatics 3 — Questions and Answers
Question 1: What effect does elevated temperature have on hydraulic fluid viscosity?
- Viscosity increases, causing sluggish actuator response
- Viscosity decreases, potentially causing increased leakage and wear (Correct answer)
- Viscosity remains unchanged regardless of temperature
- Viscosity doubles for every 10°C increase
Correct answer: Viscosity decreases, potentially causing increased leakage and wear
Higher temperatures reduce hydraulic fluid viscosity, which can increase internal leakage past seals and accelerate component wear.
Question 2: What is cavitation in a hydraulic pump?
- Formation and collapse of vapor bubbles due to localized low pressure, causing erosion and noise (Correct answer)
- Blockage of the pump inlet by contaminant particles
- Reverse flow through the pump due to excessive system pressure
- Overheating of the pump caused by insufficient lubrication
Correct answer: Formation and collapse of vapor bubbles due to localized low pressure, causing erosion and noise
Cavitation occurs when fluid pressure drops below vapor pressure, forming bubbles that violently collapse and erode metal surfaces inside the pump.
Question 3: In aircraft pneumatic systems, what is the most common source of high-pressure air?
- Dedicated electric air compressors
- Engine bleed air tapped from the compressor stages (Correct answer)
- Ground support pneumatic carts only
- Ram air scoops on the fuselage
Correct answer: Engine bleed air tapped from the compressor stages
Most aircraft pneumatic systems are supplied by bleed air extracted from intermediate or high-pressure compressor stages of the turbine engines.
Question 4: What does an aircraft hydraulic power transfer unit (PTU) do?
- Transfers hydraulic fluid between system reservoirs to equalize fluid levels
- Uses pressure from one hydraulic system to drive a pump that pressurizes another system without fluid transfer (Correct answer)
- Converts electrical power into hydraulic pressure in an emergency
- Measures differential pressure across hydraulic filters
Correct answer: Uses pressure from one hydraulic system to drive a pump that pressurizes another system without fluid transfer
A PTU uses one system's hydraulic pressure to drive a hydraulic motor/pump combination that pressurizes another system, keeping the two fluid circuits separate.
Question 5: What is the purpose of a hydraulic fuse?
- To protect electrical circuits powering hydraulic pumps
- To automatically shut off fluid flow when flow rate exceeds a set value, limiting fluid loss from a ruptured line (Correct answer)
- To provide a thermal relief path when fluid overheats
- To sense low fluid level and alert the crew
Correct answer: To automatically shut off fluid flow when flow rate exceeds a set value, limiting fluid loss from a ruptured line
A hydraulic fuse senses abnormally high flow (indicating a burst line) and closes automatically to conserve remaining fluid and maintain pressure to other actuators.
Question 6: Which type of seal material is NOT compatible with phosphate ester (Skydrol) hydraulic fluid?
- EPDM (ethylene propylene)
- Butyl rubber
- Buna-N (nitrile rubber) (Correct answer)
- Neoprene rated for phosphate esters
Correct answer: Buna-N (nitrile rubber)
Nitrile (Buna-N) seals are compatible with petroleum-based fluids but swell and degrade rapidly when exposed to Skydrol phosphate ester fluid.
Question 7: What is the purpose of a priority valve in an aircraft hydraulic system?
- To ensure essential services (such as flight controls) receive hydraulic pressure before less critical systems during low-pressure conditions (Correct answer)
- To maintain equal pressure distribution to all actuators simultaneously
- To purge air from the hydraulic lines automatically
- To restrict maximum actuator travel speed
Correct answer: To ensure essential services (such as flight controls) receive hydraulic pressure before less critical systems during low-pressure conditions
A priority valve opens only when system pressure exceeds a set threshold, ensuring critical systems like flight controls are satisfied before secondary systems receive flow.
AMT Hydraulics and Pneumatics 3
What effect does elevated temperature have on hydraulic fluid viscosity?