FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
A technician is inspecting a hydraulic power pack that includes an engine-driven pump. The pump maintains constant output pressure by internally varying the stroke of its pistons based on system demand, rather than relying on an external relief valve to bypass excess flow. What type of pump is this?
- AGerotor pump
- BHand-operated auxiliary pump
- CVariable displacement pump
- DFixed (constant) displacement pump
Show answer & explanationAnswer & explanation
Correct answer: C. Variable displacement pump
A variable displacement pump automatically adjusts the stroke (and thus output volume) of its pistons to maintain a constant system pressure as demand changes, without needing to bypass excess flow through a relief valve. A fixed displacement pump always outputs a set flow rate and requires a separate relief/regulator valve to control pressure.
Why the other options are wrong
- A. Gerotor pumps are a type of fixed displacement pump, often used for low-pressure applications.
- B. Hand pumps are manually operated backup pumps, not engine-driven demand-based pumps.
- D. Fixed displacement pumps output constant flow regardless of demand and need external pressure regulation.
Variable Displacement Pump
An engine-driven hydraulic pump that automatically varies piston stroke to match system demand while maintaining constant pressure.
- Self-regulates output without external relief valve
- Contrasts with fixed displacement pumps needing a separate pressure regulator
- Common on large transport aircraft hydraulic systems
Memory trick: Variable pump breathes in and out with demand.