What is meant by usable fuel in the context of flight planning?

Usable fuel refers to the quantity of fuel that is available for the engine during flight operations. It is the amount of fuel that can be accessed by the aircraft’s fuel system and utilized by the engine to ensure safe operation. This includes considerations of the fuel's position in the tanks, as factors such as the angle of the aircraft and the fuel tank design can affect how much fuel is genuinely accessible during flight.

In flight planning, understanding usable fuel is critical since it directly impacts the aircraft's weight, balance, and range calculations. Pilots must be aware of how much usable fuel will remain at the start of the flight to ensure that they have enough for takeoff, flight, reserves, and landing. Simply put, usable fuel is what you can rely on to complete the planned flight safely, taking into account any contingencies or factors that might reduce fuel availability.

The other definitions do not accurately capture the concept of usable fuel. Fuel that can be stored but not used does not contribute to operational capabilities, and fuel that is only usable during taxi does not relate to the fuel needed for flight. Lastly, fuel mixed with oil is typically relevant for two-stroke engines but does not define usable fuel in general aviation context. Thus, the correct understanding of