Definition
A condition in which liquid fuel changes to vapor inside fuel lines or fuel system components, interrupting or stopping the flow of fuel to the engine. It is typically caused by excessive heat acting on the fuel, often when operating at high ambient temperatures, after shutdown of a hot engine, or at high altitude where lower atmospheric pressure makes fuel vaporize more easily.
Plain English
Fuel turns into gas bubbles inside the fuel lines instead of staying liquid, which blocks fuel from reaching the engine. The engine then runs rough or quits because it isn't getting the fuel it needs.
Context Anchor
Pilots may encounter vapor lock in discussions of fuel systems, hot weather operations, high-altitude operation, engine roughness, or loss of power.
Derivation
From 'vapor' (gas form of a liquid) and 'lock' (something that blocks movement). The fuel vapor literally locks up the line — liquid fuel can't push past the gas bubbles to reach the engine.
Why Pilots Care
It can cause sudden power loss, rough running, or complete engine stoppage, especially after shutdown on hot days or during prolonged low-fuel-flow operations.
Analogy
It is like trying to drink through a straw that keeps pulling air gaps instead of a steady stream of liquid.
Intuition Check
Vapor lock does not mean a mechanical lock or latch is stuck. It means fuel vapor bubbles are interfering with the flow of liquid fuel.
Example Sentence 1
After shutting down on a hot ramp, the pilot waited a few minutes before restart to reduce the chance of vapor lock in the fuel lines.
Example Sentence 2
High-wing aircraft with gravity-fed systems are less prone to vapor lock than low-wing aircraft that rely on an engine-driven pump.