FAA Flight Instructor Airplane (FIA)Aircraft Systems and InstrumentsMedium
During cruise flight in a retractable-gear airplane equipped with a hydraulic power pack, a pilot notices the gear pump motor cycling on briefly every few minutes, even though the landing gear has been up and locked for over an hour. What does this behavior most likely indicate?
- AThis is a normal, expected characteristic requiring no attention
- BThe landing gear is slowly extending on its own
- CA minor internal hydraulic leak is allowing system pressure to bleed down
- DThe pump has failed and gear extension will be impossible
Show answer & explanationAnswer & explanation
Correct answer: C. A minor internal hydraulic leak is allowing system pressure to bleed down
A hydraulic power pack maintains system pressure using a pressure switch that activates the pump when pressure drops below a set threshold. Periodic pump cycling while the gear is up and locked usually indicates a minor internal leak allowing pressure to slowly bleed off, prompting the pump to restore it. While not usually an immediate emergency, it should be noted and inspected on the ground.
Why the other options are wrong
- A. Frequent, unexplained cycling is not considered fully normal and warrants maintenance attention.
- B. Uplock mechanisms mechanically hold the gear up; a leak alone would not typically extend the gear.
- D. Pump cycling shows the pump is still functioning, not failed.
Hydraulic Power Pack Cycling
A landing gear hydraulic power pack uses a pressure switch to activate the electric pump whenever system pressure drops below a set value, often due to a minor internal leak.
- Power pack includes reservoir, pump, selector valve, actuators, and relief valve
- Pressure switch cycles pump automatically to maintain pressure
- Frequent cycling with gear up may indicate an internal leak needing maintenance
Memory trick: Pump cycles = system 'topping off' pressure lost to a slow leak.