Definition
The fuel remaining in an aircraft's fuel system that cannot be safely delivered to the engine in flight. It is the portion of total fuel onboard that is not available for use because of tank shape, fuel pickup location, or aircraft attitude limitations. Unusable fuel is determined by the manufacturer during certification and is included in the aircraft's empty weight.
Plain English
The small amount of fuel that stays trapped in the tanks and lines and can never reach the engine, no matter how full you fill up. It still has weight, so it counts as part of the aircraft itself, not as part of your usable fuel load.
Context Anchor
You will see this term in aircraft specifications, weight-and-balance data, fuel planning, and pilot operating handbooks.
Why Pilots Care
Pilots must subtract unusable fuel from total fuel load when calculating range, endurance, and legal reserves.
Analogy
It is like liquid left below the tap in a container. The liquid is still in the container, but you cannot count on getting it out through the tap when you need it.
Intuition Check
Do not assume “unusable” means the fuel is physically gone or impossible to burn. In aviation, it means the fuel is not approved to count as available for safe flight planning.
Example Sentence 1
When calculating endurance, the pilot subtracted the unusable fuel from the total tank capacity to find how much fuel was actually available for the flight.
Example Sentence 2
Even with full tanks, the unusable fuel amount reduced the total usable quantity available for the cross-country flight.