Abstract: This invention relates to a collapsible fuel storage tank for storing fuel on an aircraft. According to this invention, there is provided an aircraft including a fuselage portion, a wing portion connected thereto and an expandable fuel storage tank disposed adjacent the intersection of the wing portion and the fuselage portion. The fuel storage tank has an external surface which includes spanwise spaced inner and outer edge regions extending in a generally chordwise direction. The inner edge region of the tank extending alongside the fuselage portion and the outer edge region being attached to the wing portion to allow movement of the external surface between an empty position in which it lies generally flush with the surrounding surface of the wing portion and a fuel storing position in which it lies at an acute angle to said surrounding surface forming a fillet region between the wing portion and the fuselage portion.