In aviation, 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.[1]
Aviation, or air transport, refers to 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 balloons and airships.
Aviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications, such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperature, among other properties.
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".[2]
The figure usable fuel is used when calculating or defining other key figures of an aircraft such as MTOW, zero-fuel weight etc.[citation needed]
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.
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.[citation needed]
In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.
Although the term is mainly used in aviation it is sometimes also used for other craft with engines.
Sources and References
↑ Websters dictionary on Unusable fuel, visited 19 March, 2012
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