Definition
A one-way valve that uses a spring-loaded ball seated against a port to allow fluid or gas to flow in one direction only. When pressure builds on the inlet side, the ball is pushed off its seat and flow passes through. When pressure drops or reverses, the spring (and back-pressure) pushes the ball firmly against the seat, sealing the port and preventing reverse flow.
Plain English
A small valve with a ball inside that lets fluid flow one way but blocks it from coming back the other way.
Context Anchor
Seen in aircraft fuel, oil, hydraulic, and pneumatic system descriptions, especially in maintenance manuals and component diagrams.
Derivation
Named for its two working parts: a ball (the moving sealing element) and a check (an old mechanical term meaning 'to stop' or 'to restrain'). A check valve is simply a valve that checks — stops — flow in the unwanted direction.
Why Pilots Care
Maintains correct flow direction in fuel delivery and hydraulic systems, preventing loss of pressure or contamination that could lead to system failure.
Analogy
Think of a lightweight ball resting over a drain hole. Flow from one side can lift the ball and pass through, but flow from the other side presses the ball tighter against the hole and blocks it.
Intuition Check
“Check” does not mean “inspect” here. In a ball check valve, “check” means “stop reverse flow.”
Example Sentence 1
The hydraulic pump output line uses a ball check valve to prevent fluid from flowing back into the pump when it's not running.
Example Sentence 2
During hydraulic servicing, the technician verified that the ball check valve allowed fluid to reach the actuators but blocked return flow.