Definition
The fuel aboard an aircraft that is available to the engine in flight. It is the total fuel in the tanks minus the unusable fuel — the small amount that cannot be drawn into the fuel system under normal flight conditions and attitudes.
Plain English
The fuel you can actually burn. Not every drop in the tank can reach the engine, so usable fuel is the part the engine can really use during flight.
Context Anchor
Seen in aircraft weight-and-balance data, fuel planning, performance charts, and aircraft operating handbooks.
Derivation
Usable comes from use, meaning to put something to practical service. Fuel comes from an old word meaning material used to feed a fire. In aviation, the phrase points to fuel that is not just present, but practically available to feed the engine.
Why Pilots Care
Using total fuel instead of usable fuel in planning can produce an over-optimistic range estimate and increase the risk of fuel exhaustion.
Analogy
It is like water in a bottle with a straw that does not reach the very bottom. The water below the straw may still be in the bottle, but you cannot drink it through the straw.
Intuition Check
Do not assume usable fuel means all fuel physically in the tanks. It means the portion the aircraft can reliably deliver to the engine for operation.
Example Sentence 1
The Cessna 172's tanks hold 56 gallons total, but only 53 gallons are usable fuel for flight planning.
Example Sentence 2
After the preflight, the instructor reminded the student to subtract unusable fuel when determining how much fuel remained available.