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Ballast Fuel (Nano)
From SKYbrary Wiki
Ballast fuel is sometimes carried to maintain the aircraft centre of gravity within limits. In certain aeroplanes, a zero fuel weight above a defined threshold requires that a minimum amount of fuel be carried in the wings through all phases of flight to prevent excessive wing bending. In both cases, this fuel is considered ballast and, under anything other than emergency circumstances, is not to be burned during the flight.
Learn more about Fuel - Flight Planning Definitions