ACS Hydraulic & Pneumatic Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a hydraulic accumulator in an aircraft system?
- Store hydraulic fluid under pressure to supplement the pump during peak demands (Correct answer)
- Filter contaminants from the hydraulic fluid
- Regulate fluid temperature during high-load operations
- Convert hydraulic pressure to pneumatic pressure
Correct answer: Store hydraulic fluid under pressure to supplement the pump during peak demands
A hydraulic accumulator stores pressurized fluid to assist the pump during peak demand periods, such as when multiple systems operate simultaneously.
Question 2: Which type of hydraulic seal is most commonly used between two parts that have relative motion (dynamic application)?
- O-ring used as a static seal
- Metal crush washer
- O-ring or lip seal in a dynamic groove (Correct answer)
- Anaerobic sealant
Correct answer: O-ring or lip seal in a dynamic groove
Dynamic seals such as O-rings installed in dynamic grooves or lip seals are designed to prevent leakage between moving components.
Question 3: When bleeding an aircraft hydraulic system, why is it important to work from the component farthest from the reservoir first?
- To ensure maximum fluid velocity through the lines
- To purge all air systematically toward the reservoir and prevent air pockets (Correct answer)
- To allow gravity to pull fluid through the system naturally
- To protect the reservoir from contamination during bleeding
Correct answer: To purge all air systematically toward the reservoir and prevent air pockets
Bleeding from the farthest point ensures air bubbles are progressively pushed back toward the reservoir rather than trapped in the system.
Question 4: What does a hydraulic system pressure relief valve protect against?
- Fluid contamination entering the actuator
- Thermal expansion causing pressure spikes that exceed system design limits (Correct answer)
- Pump cavitation during low-fluid conditions
- Filter bypass due to clogging
Correct answer: Thermal expansion causing pressure spikes that exceed system design limits
The pressure relief valve opens to bypass fluid back to the reservoir when system pressure exceeds the maximum design limit, protecting components from overpressure damage.
Question 5: What material are most aircraft hydraulic lines made of when routing through areas subject to vibration?
- Rigid aluminum tubing with compression fittings
- Flexible rubber hose reinforced with braided wire (Correct answer)
- Copper tubing with soldered joints
- PVC plastic tubing with barb fittings
Correct answer: Flexible rubber hose reinforced with braided wire
Flexible braided hose is used in areas subject to vibration or relative movement because it can absorb movement without cracking or failing.
Question 6: What is the function of a hydraulic power transfer unit (PTU)?
- Transfer fluid between two separate hydraulic systems without cross-contaminating the fluids (Correct answer)
- Convert hydraulic energy to electrical energy for emergency power
- Maintain equal pressure in both main and backup hydraulic systems
- Pump fluid when the engine-driven pump fails
Correct answer: Transfer fluid between two separate hydraulic systems without cross-contaminating the fluids
A PTU allows one hydraulic system to power another using a hydraulic motor/pump combination, transferring power but keeping the fluids isolated.
Question 7: If an aircraft hydraulic system uses MIL-PRF-5606 fluid and is inadvertently serviced with MIL-PRF-83282 fluid, what action is required?
- No action; the fluids are completely interchangeable
- Flush the system and replace all seals, as the fluids have different seal compatibility (Correct answer)
- Simply add the correct fluid to dilute the contamination
- Replace only the hydraulic pump since it is most susceptible to damage
Correct answer: Flush the system and replace all seals, as the fluids have different seal compatibility
MIL-PRF-5606 and MIL-PRF-83282 have different base oils and different seal material requirements, requiring a full system flush and seal replacement if mixed.
ACS Hydraulic & Pneumatic Systems 2
What is the primary purpose of a hydraulic accumulator in an aircraft system?