What is meant by undrainable fuel?

• A. The fuel that is left in the tank, lines and components when the aircraft is placed in level flight position and the fuel drained at the main fuel strainer. This is also called residual fuel.
• B. The fuel that is drained from the main fuel strainer during flight.
• C. The fuel that is left in the fuel tanks after fueling.
• D. The fuel that is used for takeoff.

Answer: (D)

Undrainable fuel is the portion of fuel that must remain in the aircraft's fuel system and cannot be drained away before takeoff because it is needed to power the engines during the takeoff and initial climb. In other words, it’s the fuel reserved for the takeoff moment and the early phase of flight, not something you drain out during maintenance or preflight checks. The other ideas describe fuel that can be drained (residual fuel after draining, fuel left after fueling, or fuel drained from a strainer during flight), whereas undrainable fuel specifically refers to the quantity reserved for takeoff.