AM AM Hydraulic & Pneumatic Systems 1 — Questions and Answers
Question 1: What is the typical operating pressure range for a large transport aircraft hydraulic system?
- 500–1,000 psi
- 1,500–2,000 psi
- 3,000–5,000 psi (Correct answer)
- 8,000–10,000 psi
Correct answer: 3,000–5,000 psi
Large transport aircraft hydraulic systems typically operate between 3,000 and 5,000 psi, with many modern aircraft using 5,000 psi systems for reduced weight.
Question 2: What fluid is specified for most Boeing commercial aircraft hydraulic systems?
- MIL-PRF-5606 (red mineral oil)
- Skydrol (phosphate ester) (Correct answer)
- MIL-PRF-83282 (synthetic hydrocarbon)
- Water-glycol
Correct answer: Skydrol (phosphate ester)
Boeing commercial aircraft use Skydrol, a fire-resistant phosphate ester hydraulic fluid, to reduce fire risk in the event of a hydraulic leak.
Question 3: What is the function of a hydraulic accumulator?
- Filter contaminants from fluid
- Store pressurized fluid and dampen pressure surges (Correct answer)
- Cool hydraulic fluid
- Measure system pressure
Correct answer: Store pressurized fluid and dampen pressure surges
A hydraulic accumulator stores pressurized fluid under a pre-charged gas pressure to supplement pump output during peak demands and dampen pressure spikes.
Question 4: Which seal material is NOT compatible with Skydrol hydraulic fluid?
- Butyl rubber
- Ethylene propylene (EPDM)
- Buna-N (nitrile) rubber (Correct answer)
- Teflon (PTFE)
Correct answer: Buna-N (nitrile) rubber
Nitrile (Buna-N) seals swell and deteriorate rapidly when exposed to Skydrol phosphate ester fluid and must not be used in these systems.
Question 5: What is the purpose of a hydraulic system relief valve?
- Reduce flow to actuators
- Limit maximum system pressure to protect components (Correct answer)
- Control actuator speed
- Separate fluid from air
Correct answer: Limit maximum system pressure to protect components
A relief valve opens to return fluid to the reservoir when system pressure exceeds the set maximum, protecting pumps, actuators, and lines from over-pressurization.
Question 6: How is hydraulic fluid contamination level typically classified in aircraft maintenance?
- By color and viscosity only
- By NAS 1638 or ISO 4406 cleanliness codes (Correct answer)
- By FAR Part 43 fluid specs
- By MIL-SPEC gravity rating
Correct answer: By NAS 1638 or ISO 4406 cleanliness codes
NAS 1638 and ISO 4406 cleanliness codes classify hydraulic fluid contamination by counting particle sizes per unit volume using automated particle counters.
AM AM Hydraulic & Pneumatic Systems 1
What is the typical operating pressure range for a large transport aircraft hydraulic system?