Definition
A condition in which fuel in the lines, pump, or carburetor has vaporized due to excessive heat, forming gas bubbles that block the normal flow of liquid fuel to the engine and cause power loss or engine stoppage.
Plain English
The fuel got hot enough to turn into vapor inside the fuel system. Because the pump is designed to move liquid, not gas, the flow stops and the engine starves for fuel.
Context Anchor
Encountered in engine operation discussions, especially during hot-weather operations, high-altitude operations, or situations where the engine runs rough or loses power because fuel flow is interrupted.
Derivation
From 'vapor' (gas form of a liquid, from Latin vapor meaning steam) and 'lock' (something that blocks movement). The fuel system is literally locked up by vapor where liquid should be flowing.
Why Pilots Care
It can cause power loss, rough running, or complete engine stoppage until the vapor bubbles are cleared by cooling or priming.
Grounding Statement
On a hot day, fuel in a line can boil into bubbles, and those bubbles can interrupt the flow of fuel to the engine.
Intuition Check
Do not read “locked” as a mechanical lock or a stuck control. In this term, “locked” means the fuel flow is being blocked by vapor bubbles.
Example Sentence 1
After landing on a hot summer afternoon, the pilot waited a few minutes before restarting to reduce the chance of becoming vapor locked.
Example Sentence 2
The pilot cleared the vapor locked condition by opening the throttle and turning the boost pump on for several seconds.