The (un)usable fuel figure is calculated for an aircraft in level flight (not true - "under the most adverse fuel feed condition" - see 14 CFR 23.959): thus with the wings horizontal and the nose slightly upwards. The figure doesn't change when the aircraft is ascending/descending or making a turn although it will affect the ability to get fuel from its tanks.
Usable fuel is the total amount of fuel in an aircraft minus the fuel that cannot be fed into the engine(s): fuel under the pump-intake, fuel behind ribs of a tank, fuel in lines between the tanks and the engines etc. 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.
Although the term is mainly used in aviation it is sometimes also used for other craft with engines.
Sources and References
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