In
aviation
Aviation includes the activities surrounding mechanical flight and the aircraft industry. ''Aircraft'' includes fixed-wing and rotary-wing types, morphable wings, wing-less lifting bodies, as well as lighter-than-air craft such as hot a ...
, usable fuel is the
fuel on board an aircraft that can actually be used by its engines. The opposite of usable fuel is unusable fuel.
[Websters dictionary o]
Unusable fuel
visited 19 March, 2012
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".
[Cornell Law online library https://www.law.cornell.edu/cfr/text/14/23.959, accessed 24 November 2017]
The figure usable fuel is used when calculating or defining other key figures of an aircraft such as
MTOW
The maximum takeoff weight (MTOW) or maximum gross takeoff weight (MGTOW) or maximum takeoff mass (MTOM) of an aircraft is the maximum weight at which the pilot is allowed to attempt to take off, due to structural or other limits. The analogous ...
,
zero-fuel weight The zero-fuel weight (ZFW) of an aircraft is the total weight of the airplane and all its contents, minus the total weight of the usable fuel on board. Unusable fuel is included in ZFW.
For many types of airplane, the airworthiness limitations incl ...
etc.
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
Aircraft operations
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