FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsEasy
A technician observes that on a particular transport-category aircraft, the hydraulic pump runs continuously whenever the engine is operating, and fluid can be felt flowing warm through the return line even when no flight control or landing gear selector is being actuated. This is characteristic of which type of hydraulic system design?
- AA power-transfer system
- BA closed-center system
- CAn open-center system
- DA demand-pump system
Show answer & explanationAnswer & explanation
Correct answer: C. An open-center system
In an open-center system, fluid circulates continuously from the pump through the selector valves and back to the reservoir when no unit is being actuated. When a selector valve is moved, the flow path is redirected to the actuator instead of back to the reservoir.
Why the other options are wrong
- A. A power-transfer unit moves fluid between two independent systems, unrelated to this continuous-flow description.
- B. A closed-center system uses a pressure-demand pump that only strokes to maintain a set pressure, not continuous flow.
- D. Not a standard system classification used in airframe hydraulics.
Open-Center Hydraulic System
A hydraulic system in which fluid flows continuously from the pump through all selector valves (in series) and back to the reservoir until a valve is actuated to direct flow to a subsystem.
- Selector valves are wired in series
- Pump delivers constant flow, not constant pressure
- Common on older and lighter aircraft with fixed-displacement pumps
Memory trick: Open-center: 'Open road' — fluid keeps cruising until a valve redirects it.