Definition
A flow-control valve consisting of a flat, disc-shaped plate mounted on a shaft that runs across the inside of a pipe or duct. Rotating the shaft turns the disc from edge-on (open, allowing flow) to face-on (closed, blocking flow), with intermediate angles producing partial flow. In aircraft powerplants, butterfly valves are commonly used as the throttle valve in carburetors and fuel injection systems, where they regulate the volume of air entering the engine.
Plain English
A round flat disc on a shaft inside a pipe. Turn the shaft and the disc swings open or closed to control how much air or fluid passes through.
Context Anchor
Seen in aircraft engine systems such as carburetors, throttle bodies, and other air or fluid passages where flow must be controlled.
Derivation
Named for the way the disc, when partly open, looks like a butterfly's wings hinged in the middle of the pipe. The picture is the whole point of the name — it tells you immediately how the valve is shaped and how it moves.
Why Pilots Care
The valve directly governs engine power and mixture response; sticking, wear, or misalignment can cause rough running, power loss, or failure to respond to throttle inputs.
Analogy
Think of a small round door inside a tube. Turn the door edge-on and flow passes easily; turn it flat across the tube and flow is mostly blocked.
Intuition Check
“Butterfly” does not refer to an insect in the system. It describes the shape and turning action of the valve plate.
Example Sentence 1
Pushing the throttle forward opens the butterfly valve in the carburetor, allowing more air into the engine.
Example Sentence 2
During the 100-hour inspection the mechanic checked the butterfly valve for carbon deposits that could restrict its movement.