An aircraft fuel tank assembly comprises a first fuel tank, a second fuel tank and a tank-to-tank overpressure protector therebetween. The overpressure protector has a closed state and an open state. The fuel tank assembly further comprises an indicator for indicating whether the overpressure protector is in the open or closed state, the indication of the indicator being able to be sensed at a location outside of the fuel tanks. The fuel tank assembly may comprise a test mechanism arranged to effect movement of the overpressure valve from the closed state to the open state, the test mechanism being operable from a location outside a fuel tank. The fuel tank assembly may comprise an overpressure valve re-set mechanism, the re-set mechanism being operable from a location outside the fuel tanks.