Definition
The fuel remaining in an aircraft's fuel system that cannot be drained out through the normal fuel system drains under standard ground conditions. It is trapped in lines, fittings, low points, and pump cavities, and is not considered part of the usable or drainable fuel supply.
Plain English
Fuel that stays stuck inside the aircraft's fuel system and cannot be removed through the regular drain valves, no matter how long you let it drain.
Context Anchor
Seen in aircraft weight-and-balance information, fuel system discussions, and maintenance procedures.
Derivation
From 'un-' (not) plus 'drainable' (able to be drained off). The term simply names what it is: fuel the system cannot drain out by gravity or normal means.
Why Pilots Care
This fuel is counted as part of the aircraft's basic empty weight and must be known for accurate performance and loading calculations.
Intuition Check
Do not assume “undrainable” means the tank is completely dry except for this fuel. It means this fuel cannot be removed through the normal drains in the specified ground position.
Example Sentence 1
The mechanic noted that the small amount of undrainable fuel left in the lines after sumping was already accounted for in the aircraft's empty weight.
Example Sentence 2
Before weighing the airplane, the mechanic drained the tanks but noted the undrainable fuel still present for the weight and balance form.