Definition
A measure of how readily a liquid fuel evaporates and changes from a liquid into a vapor at a given temperature and pressure. In aviation gasoline, volatility is carefully controlled so the fuel vaporizes easily enough to start and run the engine, but not so easily that it forms vapor in the fuel lines and disrupts fuel flow.
Plain English
How easily a liquid turns into vapor. A highly volatile fuel evaporates quickly; a low-volatility fuel evaporates slowly.
Context Anchor
Seen in aircraft fuel, engine starting, carburetor, and fuel-system discussions.
Derivation
From the Latin volatilis, meaning 'flying' or 'fleeting.' The idea is of something that 'flies away' into the air -- which is exactly what a volatile liquid does as it evaporates.
Why Pilots Care
Fuel volatility directly affects engine starting, smooth running, and the risk of vapor lock (where fuel vaporizes in the lines and starves the engine). Using the wrong fuel, or fuel contaminated with a more volatile substance, can cause real performance and safety problems.
Analogy
Rubbing alcohol has high volatility because it evaporates quickly from your skin. Cooking oil has low volatility because it stays liquid and does not give off vapor easily at normal temperatures.
Grounding Statement
Leave a small dish of water and a small dish of gasoline outside on a warm day. The gasoline disappears much faster -- that is volatility in action.
Intuition Check
Volatility does not mean the fuel is emotionally or financially “unstable.” Here it means how easily the fuel changes from liquid into vapor.
Example Sentence 1
Aviation gasoline is blended to have the right volatility for reliable starting in cold weather without causing vapor lock in hot weather.
Example Sentence 2
Mechanics watched for vapor lock when the summer heat raised fuel volatility.