Definition
A hydraulic system in which the selector valves are arranged so that, when no actuator is being operated, fluid from the pump flows through the valves and returns to the reservoir at low pressure. When a selector valve is moved to operate an actuator, the return path closes and the pump's output is directed to the actuator under pressure.
Plain English
A hydraulic system where the pump keeps moving fluid in a continuous loop back to the tank when nothing is being used. Only when the pilot calls for something — like gear or flaps — does the fluid get redirected to do work.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially when comparing how different systems route pump flow when no hydraulic component is being moved.
Derivation
‘Open-center’ describes the center (neutral) position of the selector valve: it is open, meaning fluid can flow straight through it back to the reservoir. The opposite is a ‘closed-center’ system, where the valve blocks flow in neutral and pressure is held ready in the lines.
Why Pilots Care
The design is simple, lightweight, and uses little engine power when controls are not moving, making it common on single-engine piston aircraft.
Intuition Check
“Open-center” does not mean the system is open to the air or leaking. It means the valve’s neutral position leaves an open path for fluid to flow back to the reservoir.
Example Sentence 1
Because the trainer uses an open-center hydraulic system, the pump unloads back to the reservoir whenever the pilot is not selecting gear or flaps.
Example Sentence 2
Because the system is open-center, the pilot can hold the gear handle in the up position without creating continuous high pressure in the lines.