Definition
The change of liquid fuel into vapor (gas) within the fuel system before it reaches the carburetor or fuel injection unit. When this happens inside fuel lines or pumps, the resulting vapor bubbles can interrupt the steady flow of liquid fuel to the engine, causing rough running, power loss, or engine stoppage.
Plain English
Fuel turning from liquid into gas inside the fuel lines before it gets to the engine. Those gas bubbles can block the flow of fuel and starve the engine.
Context Anchor
Encountered in engine-failure and fuel-system discussions, especially during hot weather, high-altitude operation, climbs, or operation after the engine compartment has become heat-soaked.
Derivation
Vapor comes from the Latin vapor, meaning steam or exhalation. It refers to a substance that has shifted from liquid into a gas-like state. In aviation fuel systems, the concern is that liquid fuel can quietly turn to vapor inside the lines and disrupt the engine's fuel supply.
Why Pilots Care
Uncontrolled vapor formation can produce vapor lock, resulting in partial or total loss of engine power without warning.
Grounding Statement
Picture liquid fuel in a warm line near the engine turning into bubbles before it can reach the engine smoothly.
Intuition Check
Do not assume this means the airplane has run out of fuel. Fuel vapor formation means the fuel is present, but some of it has changed into gas bubbles that can interfere with normal fuel flow.
Example Sentence 1
On a hot summer afternoon at a high-elevation airport, the pilot turned on the auxiliary fuel pump before start to reduce the chance of fuel vapor formation in the lines.
Example Sentence 2
After restarting following a ground shutdown, the pilot waited for the engine to stabilize before takeoff to avoid fuel vapor formation.