FAA Commercial Pilot Airplane (CAX)Aircraft SystemsEasy
An airplane's retractable landing gear is actuated by an electrically driven hydraulic power pack consisting of a pump, reservoir, and relief valve. What is the primary function of this unit?
- ATo supply hydraulic pressure on demand to extend and retract the landing gear (and often flaps)
- BTo sense weight on the wheels and prevent inadvertent gear retraction
- CTo cool the hydraulic fluid before it enters the gear actuators
- DTo provide backup electrical power to the gear position indicator lights
Show answer & explanationAnswer & explanation
Correct answer: A. To supply hydraulic pressure on demand to extend and retract the landing gear (and often flaps)
The hydraulic power pack is essentially a self-contained mini hydraulic system: an electric motor drives a pump that pressurizes fluid from the reservoir, which is then directed by selector valves to move the gear (and sometimes flaps) actuators. The relief valve simply protects the system from excessive pressure; it does not cool fluid, provide electrical power, or sense weight.
Why the other options are wrong
- B. Weight sensing is performed by the squat switch, a separate component.
- C. Cooling is not a designed function of this pump/reservoir unit.
- D. Electrical power backup is unrelated to a hydraulic power pack's function.
Hydraulic Power Pack
A self-contained electric pump/reservoir/relief valve assembly that generates hydraulic pressure on demand to actuate landing gear (and often flaps).
- Electric motor drives the pump only when gear/flaps are selected
- Relief valve prevents system over-pressurization
- Common in light retractable-gear singles and twins
Memory trick: Pump on demand, gear responds.