Definition
A hydraulic selector valve that, when in the neutral (off) position, blocks the pressure and return ports so fluid is held in the lines and cannot flow back to the reservoir. It is used in hydraulic systems where the pump maintains pressure continuously, with each subsystem isolated until the valve is moved to direct fluid to an actuator.
Plain English
A control valve that shuts off the flow when it is not being used, trapping the fluid in place. It only lets fluid through when the pilot or system selects a specific action, like extending the gear or flaps.
Context Anchor
Seen in aircraft hydraulic system descriptions, especially for landing gear, flap, brake, or other hydraulically operated equipment.
Derivation
Closed-center refers to the valve's middle (neutral) position being closed to flow. The center is the resting position of the valve spool; in this design that center blocks both ports rather than venting them. This contrasts with an open-center valve, where the center position lets fluid circulate freely back to the reservoir.
Why Pilots Care
Maintains hydraulic pressure without constant pump operation or fluid loss when controls are not actuated.
Intuition Check
“Closed-center” does not mean the valve is physically located in the center of the aircraft. It means the valve closes off fluid flow when its control is in the center or neutral position.
Example Sentence 1
Because the aircraft uses a closed-center selector valve for the landing gear, hydraulic pressure remains available the moment the gear handle is moved.
Example Sentence 2
During preflight, the mechanic checked the closed-center selector valve to ensure it properly isolated the hydraulic actuators in neutral.