Airframe Mechanic Certification Hydraulic and Pneumatic Power 3 — Questions and Answers
Question 1: What happens to hydraulic fluid viscosity as temperature increases?
- Viscosity increases
- Viscosity decreases (Correct answer)
- Viscosity remains constant
- Viscosity first increases then decreases
Correct answer: Viscosity decreases
Hydraulic fluid becomes thinner (less viscous) as temperature rises, which can affect sealing effectiveness and pump efficiency.
Question 2: A hydraulic power pack is BEST described as:
- A reservoir used to store extra fluid
- A self-contained unit with pump, reservoir, and controls for a specific subsystem (Correct answer)
- A type of high-pressure filter
- An emergency hand pump assembly
Correct answer: A self-contained unit with pump, reservoir, and controls for a specific subsystem
A hydraulic power pack is a self-contained unit integrating a pump, reservoir, filter, and associated controls to power a specific aircraft subsystem.
Question 3: When bleeding a hydraulic system, the purpose is to:
- Remove contaminated fluid from the system
- Expel trapped air to ensure solid, responsive actuator operation (Correct answer)
- Lower system pressure to a safe level for maintenance
- Test the relief valve setting
Correct answer: Expel trapped air to ensure solid, responsive actuator operation
Bleeding removes air pockets that cause spongy or sluggish actuator response because air is compressible unlike hydraulic fluid.
Question 4: Skydrol hydraulic fluid is classified as:
- Petroleum-based with corrosion inhibitors
- A phosphate ester-based fire-resistant fluid (Correct answer)
- A water-glycol mixture
- A silicone-based fluid
Correct answer: A phosphate ester-based fire-resistant fluid
Skydrol is a phosphate ester-based, fire-resistant hydraulic fluid widely used in commercial transport aircraft.
Question 5: What is the function of a hydraulic fuse?
- To prevent reverse flow in suction lines
- To automatically shut off flow if a downstream line ruptures and flow exceeds a preset rate (Correct answer)
- To regulate pressure to actuators
- To connect hydraulic lines of different diameters
Correct answer: To automatically shut off flow if a downstream line ruptures and flow exceeds a preset rate
A hydraulic fuse senses excessive flow caused by a downstream rupture and closes to isolate the failed section, preserving fluid for the rest of the system.
Question 6: A pneumatic high-pressure storage bottle must be hydrostatically tested every:
- 12 months
- 2 years
- 3 years
- 5 years (Correct answer)
Correct answer: 5 years
High-pressure pneumatic cylinders used in aircraft systems are required to undergo hydrostatic testing every 5 years per DOT/FAA regulations.
Question 7: In a hydraulic system, cavitation in a pump is caused by:
- Excessive system pressure
- Insufficient fluid supply to the pump inlet, causing vapor bubbles to form (Correct answer)
- Contaminated hydraulic fluid
- A stuck pressure regulator
Correct answer: Insufficient fluid supply to the pump inlet, causing vapor bubbles to form
Cavitation occurs when the pump inlet is starved of fluid, creating low-pressure voids that collapse violently and damage pump components.
Airframe Mechanic Certification Hydraulic and Pneumatic Power 3
What happens to hydraulic fluid viscosity as temperature increases?