Definition
A hydraulic selector valve that, when in its neutral (off) position, allows fluid from the pump to flow freely through the valve and back to the reservoir rather than being directed to an actuator. When the pilot or technician moves the valve to a working position, the return path is blocked and pump pressure is routed to one side of the actuating cylinder while the opposite side is connected to return.
Plain English
A control valve in a hydraulic system that, when not being used, lets the pump's fluid circulate back to the tank without building pressure. When you select a position to operate something, like landing gear or flaps, the valve redirects the fluid to do the work.
Context Anchor
Seen in aircraft hydraulic system diagrams and maintenance descriptions, especially where a pump supplies fluid to parts such as landing gear, flaps, or other hydraulically moved equipment.
Derivation
Open-center refers to the valve's center (neutral) position being open to return flow. Selector means it selects which actuator port receives pressure. Together: a valve whose neutral position keeps the system's center path open.
Why Pilots Care
Keeps hydraulic fluid moving at low pressure until a control input is made, preventing heat buildup and ensuring immediate response when the valve is shifted.
Analogy
Think of a water hose with a bypass. In the neutral position, water keeps flowing through the bypass back to where it started. When you move the control, the water is sent to do work instead.
Intuition Check
Open-center does not mean the valve is physically open to the outside air. It means the inside flow path is open in the centered position so fluid can return instead of being blocked.
Example Sentence 1
With the open-center selector valve in neutral, hydraulic fluid circulates back to the reservoir and the system sits at low pressure.
Example Sentence 2
The technician cycled the open-center selector valve to confirm it directed fluid to the actuator only when commanded.