Definition
A mechanical device that controls the flow of a fluid (liquid or gas) through a passage by opening, closing, or partially obstructing the path. In aircraft systems, valves regulate fuel, oil, hydraulic fluid, air, and exhaust gases, and may operate manually, mechanically, electrically, or by pressure differential.
Plain English
A part that opens and closes to let a liquid or gas flow through, stop, or be controlled. It is the on/off or volume control for anything moving through a tube or passage.
Context Anchor
Seen in engine, fuel, oil, brake, air, and hydraulic system descriptions, and in checklist items such as opening or closing a fuel valve.
Derivation
From the Latin valva, meaning the leaf or panel of a folding door. The image is exact: a valve is a small door that opens to let something through and closes to hold it back.
Why Pilots Care
Proper valve operation is essential for engine performance, fuel management, and system integrity; a stuck or failed valve can cause loss of power or system failure.
Analogy
A household faucet is a simple valve: turn it one way and water flows; turn it the other way and the flow stops.
Intuition Check
Do not think of a valve only as an engine part or a tire stem. In aircraft use, a valve can be any control point that starts, stops, directs, or limits the flow of liquid or gas.
Example Sentence 1
Before starting the engine, the pilot moved the fuel selector valve to the BOTH position to draw from both tanks.
Example Sentence 2
The pilot checked the fuel selector valve before starting the engine to ensure proper tank selection.