AMT Hydraulics and Pneumatics 2 — Questions and Answers
Question 1: What is the purpose of a hydraulic system relief valve?
- To filter contaminants from the fluid
- To limit maximum system pressure and prevent overpressure damage (Correct answer)
- To regulate fluid temperature during operation
- To maintain constant flow rate to actuators
Correct answer: To limit maximum system pressure and prevent overpressure damage
A relief valve limits maximum system pressure by bypassing fluid back to the reservoir when pressure exceeds the set value.
Question 2: Which hydraulic fluid is used in most modern commercial transport aircraft?
- MIL-H-5606 (red mineral oil)
- MIL-H-83282 (red fire-resistant)
- Skydrol (purple phosphate ester) (Correct answer)
- Ethylene glycol-based fluid
Correct answer: Skydrol (purple phosphate ester)
Skydrol (phosphate ester-based) is used on most commercial transports because of its fire-resistant properties.
Question 3: In a hydraulic system, what does a check valve do?
- Allows fluid flow in one direction only (Correct answer)
- Regulates pressure to a fixed value
- Measures volumetric flow rate
- Controls the speed of actuator movement
Correct answer: Allows fluid flow in one direction only
A check valve permits fluid flow in only one direction, preventing backflow that could cause loss of pressure or actuator drift.
Question 4: What type of pump is most commonly used as the main hydraulic pump on large transport aircraft?
- Gear pump
- Vane pump
- Variable-displacement axial piston pump (Correct answer)
- Centrifugal pump
Correct answer: Variable-displacement axial piston pump
Variable-displacement axial piston pumps are preferred because they maintain constant pressure by adjusting displacement rather than bypassing excess flow.
Question 5: What is the function of a hydraulic accumulator?
- To store a small volume of pressurized fluid for emergency use and dampen pressure surges (Correct answer)
- To convert hydraulic pressure into electrical power
- To separate water contamination from hydraulic fluid
- To control the direction of actuator movement
Correct answer: To store a small volume of pressurized fluid for emergency use and dampen pressure surges
An accumulator stores pressurized fluid to supplement pump output during peak demand, provide emergency power, and absorb pressure surges.
Question 6: When bleeding a hydraulic system, what is the primary goal?
- Remove excess hydraulic fluid from the reservoir
- Purge all air from the lines to prevent spongy actuator response (Correct answer)
- Replace the hydraulic fluid with fresh fluid
- Increase system operating pressure to specification
Correct answer: Purge all air from the lines to prevent spongy actuator response
Bleeding removes entrapped air from hydraulic lines; air is compressible and causes spongy, unpredictable actuator movement.
Question 7: A selector valve in a hydraulic system is used to:
- Limit maximum system pressure
- Direct fluid flow to the appropriate actuator port to control direction of movement (Correct answer)
- Filter particulate contamination from the fluid
- Convert mechanical energy into hydraulic pressure
Correct answer: Direct fluid flow to the appropriate actuator port to control direction of movement
A selector (directional control) valve routes pressurized fluid to one side of an actuator while connecting the opposite side to return, reversing actuator direction.
