Definition
A condition in a fuel system in which liquid fuel turns to vapor inside the fuel lines, pump, or carburetor, blocking or interrupting the flow of liquid fuel to the engine. It is typically caused by excessive heat, low fuel pressure, high altitude, or volatile fuel, and can result in rough running or complete engine stoppage.
Plain English
Fuel that should be flowing as a liquid has boiled into a gas inside the fuel line. The bubble of vapor blocks the line, so fuel can't get through to the engine and the engine starves.
Context Anchor
Seen in aircraft fuel system discussions, especially during hot-weather operation, high-altitude operation, and maintenance troubleshooting for poor fuel flow.
Derivation
From 'vapor' (a substance in gas form, from Latin vapor meaning steam or exhalation) and 'lock' (something that stops movement). The vapor literally locks the fuel from moving down the line.
Why Pilots Care
It can produce rough running, power loss, or engine stoppage, especially after shutdown on hot days.
Analogy
It is like trying to drink through a straw that has air gaps in it. Instead of getting a steady pull of liquid, the flow breaks up and becomes unreliable.
Grounding Statement
When fuel gets too hot or the pressure around it drops, bubbles can form in the fuel line and interrupt the flow to the engine.
Intuition Check
Do not picture a physical lock or latch. In this term, “lock” means vapor has blocked or disrupted the normal fuel flow.
Example Sentence 1
After shutting down on a hot ramp, the pilot waited a few minutes before restarting to reduce the chance of vapor lock in the fuel lines.
Example Sentence 2
The pilot cleared the vapor lock by priming the engine and waiting several minutes for the lines to cool.