Definition
A simple one-way check valve consisting of a flexible disc, flap, or hinged plate that opens to allow fluid or air to pass in one direction and closes against a seat to block flow in the opposite direction.
Plain English
A small flap that lets air or fluid move one way and stops it from going back. When pressure pushes from the right side, the flap lifts and opens. When pressure tries to push the other way, the flap is pushed shut against its seat.
Context Anchor
Seen in aircraft system descriptions and maintenance work involving fuel, oil, air, drains, vents, or other passages where flow must be controlled.
Derivation
From 'flapper,' meaning something that flaps or moves loosely on a hinge. The name describes exactly how it works -- a small flap that opens and closes with the flow.
Why Pilots Care
Prevents backflow that could starve the engine of fuel or cause loss during maneuvers.
Analogy
Like a cat flap on a door that only swings one way -- push from the correct side and it opens; push from the other side and it stays shut.
Intuition Check
Do not assume “flapper” means an informal or unimportant part. Here it means the actual moving flap that controls flow inside the valve.
Example Sentence 1
The mechanic replaced a worn flapper valve in the vacuum system after the attitude indicator began responding sluggishly.
Example Sentence 2
During inspection the mechanic checked that the flapper valve in the vent line moved freely.