Usable fuel

In aviation, usable fuel is the fuel on board an aircraft that can actually be used by its engines. It is defined as the total quantity of fuel in all fuel tanks, minus the fuel quantity necessary to comply with the unusable fuel supply determination requirements.^[1] It is the only value that should be used when calculating the amount of fuel required.^[2] It is also used when calculating or defining other key parameters of an aircraft such as MTOW, zero-fuel weight etc.^{[citation needed]}

As this figure is calculated/defined for a plane in level flight it is possible that the engines of an aircraft run dry (out of fuel) even when the amount of usable fuel is still above zero, such as if the wings are not level and/or the angle of attack is higher or lower than when cruising. The inverse is also possible; in some conditions, fuel can continue to be fed to the engines when the usable fuel is below zero.^{[citation needed]}

The opposite of usable fuel is unusable fuel.^[3] This is the fuel quantity stored in the aircraft fuel tank that cannot be supplied to the engine, due to the design and positioning of fuel pumps, tank ribs, such as fuel under the pump-intake, fuel behind ribs of a tank, fuel in lines between the tanks and the engines. However the value is often used to help calibrate the aircrafts fuel gauge.^[4]

The unusable fuel figure is calculated for an aircraft fuel tank in "the most adverse fuel feed condition occurring under each intended operation and flight maneuver involving that tank".^[5]

Although the term is mainly used in aviation it is sometimes also used for other craft with engines.

See also

• Index of aviation articles