FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
A technician is comparing two engine-driven hydraulic pump designs. One pump delivers a constant volume of fluid per revolution regardless of system demand, requiring a separate downstream device to control system pressure. Which pump type is being described?
- APower transfer unit pump
- BFixed-displacement gear (gerotor) pump
- CHand-operated wobble pump
- DVariable-displacement piston pump
Show answer & explanationAnswer & explanation
Correct answer: B. Fixed-displacement gear (gerotor) pump
A fixed-displacement pump, such as a gear or gerotor pump, always delivers the same volume of fluid per revolution regardless of system pressure or demand. Because output is constant, a separate unloading or pressure-control valve is required to regulate system pressure, unlike a variable-displacement pump that internally adjusts its own output.
Why the other options are wrong
- A. A PTU transfers power between two systems, it is not a primary displacement pump type.
- C. A wobble pump is manually operated, not engine-driven.
- D. Variable-displacement pumps self-regulate output based on system demand.
Fixed-Displacement (Gerotor) Pump
A hydraulic pump that delivers a constant volume of fluid per shaft revolution regardless of system pressure, requiring external pressure regulation.
- Constant output per revolution
- Requires unloading/relief valve for pressure control
- Simpler design than variable-displacement pumps
Memory trick: Fixed pump = fixed scoop size every spin, no matter the thirst.