Definition
Fuel that remains in an aircraft's fuel system but cannot be delivered to the engine under normal flight conditions. It is held in low spots, lines, filters, sumps, and other parts of the system where the fuel pumps and feed lines cannot draw it out. Trapped fuel is counted as part of the aircraft's empty weight, not as usable fuel.
Plain English
Fuel that is stuck inside the fuel system and cannot be used by the engine. It still adds weight to the aircraft, but you cannot fly on it.
Context Anchor
Seen in aircraft weight-and-balance, maintenance, and fuel-system discussions, especially when deciding what fuel weight is included in the aircraft’s empty weight.
Derivation
“Trapped” comes from the idea of something being caught or held in place. In aviation, the word helps because this fuel is held in parts of the fuel system where normal draining does not remove it.
Why Pilots Care
It reduces the amount of usable fuel, directly affecting range and requiring accurate preflight calculations for safety.
Analogy
It is like the small amount of water left in a hose after you drain it. The hose is mostly empty, but some water remains in low spots or bends.
Intuition Check
“Trapped” does not mean the fuel is deliberately stored for backup use. Here it means fuel that remains in the system after normal draining and is not normally available for flight.
Example Sentence 1
The aircraft's empty weight includes the trapped fuel that stays in the lines and sumps after the tanks are drained.
Example Sentence 2
During a steep turn, additional fuel became trapped in the outer wing tanks.