Definition
In a hydromechanical flight control system, control valves are mechanical devices that direct the flow of pressurized hydraulic fluid to and from the actuators that move the flight control surfaces. The valve opens, closes, or proportions fluid flow in response to pilot input, allowing fluid pressure to extend or retract the actuator and move the surface accordingly.
Plain English
A control valve is a device that decides where the hydraulic fluid goes. When the pilot moves the controls, the valve opens the right path so fluid can push the part that moves the control surface.
Context Anchor
Seen in flight control system diagrams, especially hydromechanical systems where pilot input is carried through mechanical parts and hydraulic pressure.
Derivation
From 'control' (to direct or regulate) and 'valve' (from Latin valva, meaning the leaf of a folding door). A valve is something that opens and closes a passage — here, the passage carries hydraulic fluid, and the pilot's input controls when and how it opens.
Why Pilots Care
In a hydromechanical system, the control valve is the link between what the pilot does with the stick or yoke and what the airplane actually does. If the valve fails or sticks, the corresponding flight control surface will not respond correctly even if everything else in the system is working.
Analogy
Think of control valves like faucets inside the system. A faucet does not create the water pressure, but it controls where the water can go and how much can pass through.
Intuition Check
Do not read control valves as the cockpit controls themselves. In this context, they are internal valves that control hydraulic fluid flow after a pilot or system command.
Example Sentence 1
When the pilot pushes the yoke forward, the control valve opens to let hydraulic fluid flow into the actuator, which pushes the elevator down.
Example Sentence 2
During preflight the mechanic checks that the control valves respond smoothly to full control travel.