FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsHard
An electrically driven auxiliary hydraulic pump is controlled by a pressure switch that starts the pump motor when system pressure drops to 2,700 psi (cut-in) and stops the motor when pressure rises to 3,000 psi (cut-out). What is the pressure differential of this switch, and what is its primary purpose?
- A2,700 psi differential; it limits pump flow rate during startup
- B300 psi differential; it precharges the accumulator with nitrogen
- C300 psi differential; it prevents rapid pump motor cycling on and off
- D5,700 psi differential; it sets the maximum system relief pressure
Show answer & explanationAnswer & explanation
Correct answer: C. 300 psi differential; it prevents rapid pump motor cycling on and off
The differential is the cut-out pressure minus the cut-in pressure: 3,000 psi − 2,700 psi = 300 psi. This differential prevents the pump motor from rapidly cycling on and off (chattering) as system pressure fluctuates near a single setpoint, extending motor and switch life.
Why the other options are wrong
- A. 2,700 psi is the cut-in pressure itself, not a differential, and the switch does not limit flow rate.
- B. The math (300 psi) happens to match, but a pressure switch differential has nothing to do with accumulator nitrogen precharge.
- D. Adding the two pressures together is not how a differential is calculated, and this is not a relief valve setting.
Pressure Switch Differential (Auxiliary Pump Control)
The difference between cut-out and cut-in pressures on a pressure switch controlling an electric auxiliary hydraulic pump, preventing rapid cycling.
- Differential = cut-out pressure minus cut-in pressure
- Example: 3,000 psi cut-out − 2,700 psi cut-in = 300 psi differential
- Prevents motor chatter/rapid cycling near a single pressure point
Memory trick: Differential = a buffer zone so the motor doesn't flicker on/off like a bad light switch