Definition
Fuel system valves that have only two positions — fully open (on) or fully closed (off) — used to either allow or completely stop the flow of fuel from a tank or fuel line to the engine.
Plain English
A simple two-position fuel valve. It is either letting fuel through, or it is shut. There is no in-between setting and no choosing between tanks with this type of valve.
Context Anchor
Seen in cockpit controls, checklists, and system descriptions where the pilot must confirm whether a flow path is open or closed.
Why Pilots Care
They provide positive, reliable shutoff capability for fuel or other systems during emergencies or maintenance.
Intuition Check
Do not treat on/off valves like dimmer switches or adjustable knobs. In this context, they are meant to be fully ON or fully OFF.
Example Sentence 1
The Cessna 152 uses a simple on/off valve for fuel, since both wing tanks feed the engine together.
Example Sentence 2
During preflight, the on/off valves were verified in the open position to ensure proper fuel flow to the engine.