Definition
A hydraulic system in which fluid under pressure is held ready in the lines at all times, but flow stops at the selector valves until a component is actuated. When no hydraulic device is being operated, the pump pressure builds against closed valves and the pump unloads or cycles off; when a control is selected, the valve opens and pressurized fluid flows to that component.
Plain English
A hydraulic setup where the pump keeps the lines pressurized and ready, but the fluid sits still until you actually move a control. As soon as you select something — like the landing gear or flaps — the valve opens and the waiting pressure drives that part.
Context Anchor
Seen in aircraft hydraulic system descriptions, maintenance manuals, and troubleshooting for hydraulically operated parts such as landing gear, flaps, brakes, or flight controls.
Derivation
‘Closed-center’ refers to the center position of the selector valve. When the valve is in its neutral (center) position, it is closed — meaning no fluid flows through. That is the defining feature: the system rests with its valves closed, holding pressure, rather than letting fluid circulate freely.
Why Pilots Care
Provides instant pressure availability for flight controls while reducing pump wear and heat buildup compared with continuous-flow designs.
Analogy
Think of a garden hose connected to a tap that is fully open, with a spray nozzle at the end. The water is pressurized right up to the nozzle, but nothing comes out until you squeeze the trigger. The moment you do, water sprays immediately.
Intuition Check
Closed-center does not mean the whole hydraulic system is shut down. It means the control valves are closed to flow when they are centered or neutral, while pressure is still kept available.
Example Sentence 1
Because the airliner uses a closed-center hydraulic system, the landing gear begins retracting the moment the pilot moves the gear handle up.
Example Sentence 2
Technicians verified that the closed-center hydraulic system bypassed excess flow correctly during ground checks.