A&P A&P Aircraft Hydraulic & Pneumatic Systems 1 — Questions and Answers
Question 1: What type of fluid is most commonly used in aircraft hydraulic systems certified under modern FAA standards?
- Mineral-based oil (MIL-H-5606)
- Phosphate ester-based fluid (Skydrol) (Correct answer)
- Water-glycol mixture
- Castor oil
Correct answer: Phosphate ester-based fluid (Skydrol)
Skydrol (phosphate ester-based fluid) is the most common hydraulic fluid in large commercial aircraft due to its fire-resistant properties, while MIL-H-5606 (mineral oil) is used in many general aviation and military aircraft.
Question 2: In a hydraulic system, what is the primary function of the hydraulic accumulator?
- Filter contaminants from the fluid
- Store pressurized fluid to supplement the pump during peak demand and absorb pressure surges (Correct answer)
- Control the direction of fluid flow
- Convert hydraulic pressure to electrical power
Correct answer: Store pressurized fluid to supplement the pump during peak demand and absorb pressure surges
The accumulator stores a quantity of pressurized hydraulic fluid (using a gas charge) to supplement the pump during high-demand situations and to dampen pressure surges.
Question 3: What component in a hydraulic system is responsible for maintaining a constant system pressure by allowing fluid to bypass back to the reservoir when pressure reaches a set limit?
- Check valve
- Selector valve
- Pressure relief valve (Correct answer)
- Filter bypass valve
Correct answer: Pressure relief valve
The pressure relief valve protects the hydraulic system from overpressure by opening and directing fluid back to the reservoir once system pressure exceeds the preset limit.
Question 4: How is hydraulic fluid contamination most commonly introduced into an aircraft hydraulic system?
- During normal operation
- During maintenance when seals, lines, or components are replaced improperly (Correct answer)
- Through the hydraulic pump
- Through the accumulator charge valve
Correct answer: During maintenance when seals, lines, or components are replaced improperly
Contamination most often enters hydraulic systems during maintenance activities such as opening lines, replacing seals, or handling components with dirty tools or hands.
Question 5: What does a hydraulic system's power transfer unit (PTU) do?
- Converts hydraulic power to electrical power
- Transfers hydraulic pressure from one system to another without fluid transfer (Correct answer)
- Stores hydraulic fluid
- Actuates the landing gear doors
Correct answer: Transfers hydraulic pressure from one system to another without fluid transfer
A PTU allows hydraulic pressure from one system to drive a pump that pressurizes a second system, transferring power but not fluid between the two circuits.
Question 6: When bleeding a hydraulic system, what is the primary goal?
- Replace old fluid with new fluid
- Remove all trapped air from the system to ensure positive pressure and proper actuator operation (Correct answer)
- Increase system pressure
- Clean the hydraulic filter
Correct answer: Remove all trapped air from the system to ensure positive pressure and proper actuator operation
Air in a hydraulic system is compressible and causes spongy or erratic actuator response; bleeding removes trapped air to restore firm, predictable system operation.
A&P A&P Aircraft Hydraulic & Pneumatic Systems 1
What type of fluid is most commonly used in aircraft hydraulic systems certified under modern FAA standards?