Definition
A fuel that exists in the gaseous state at normal atmospheric temperature and pressure, such as natural gas, propane, or hydrogen, and is burned in an engine in vapor form rather than as an atomized liquid.
Plain English
A fuel that is already a gas at normal outside temperatures, instead of a liquid that has to be sprayed and turned into vapor before it can burn.
Context Anchor
Seen in discussions of aircraft fuel types, experimental or alternative-fuel aircraft, engine fuel systems, and fuel storage safety.
Derivation
From the Latin chaos by way of Dutch gas, meaning a state of matter that fills its container. Used here in the simple physics sense — the fuel is already in gas form before it reaches the engine, unlike avgas or jet fuel which are liquids.
Why Pilots Care
Gaseous fuels behave differently from liquid fuels in storage, metering, and fire risk. They must be kept under pressure or refrigerated, and any leak disperses quickly into the surrounding air rather than pooling, which changes how leaks are detected and handled.
Grounding Statement
Picture fuel that cannot sit in an open bucket; it would escape into the air unless it is held in a sealed system.
Intuition Check
Do not confuse gaseous fuel with liquid fuel vapor near a fuel tank opening. Gaseous fuel means the fuel is intended to be used as a gas, not just that a liquid fuel is giving off fumes.
Example Sentence 1
Some ground support tugs run on a gaseous fuel like propane, which is stored in a pressurized tank on the vehicle.
Example Sentence 2
Before flight the crew verified that the gaseous fuel pressure regulators were functioning correctly.