Methods and systems are provided to comply with structural load requirements applicable to aircraft additional fuel tank systems. A plurality of aircraft fuel tanks may be positioned adjacent to one another, preferably within the fuselage (e.g., a cargo compartment) of the aircraft so as to be disposed generally along a longitudinal axis of the aircraft. The tank body defining an interior space for holding aircraft fuel, an intercommunication conduit assembly between the fuel tank modules configured to refuel and transfer fuel from the tank modules by a cascade mode and an intentional air-filled ullage space are operatively associated with the tank body to prevent an overpressure condition within the interior space of the fuel tank body. The intentional air-filled ullage is obtained through the predetermined positioning of the terminal open end of the intercommunication tube inside the respective fuel tank module. The intentional air-filled ullage can be configured in all or in only some of the fuel tank modules according to the design of the auxiliary fuel tanks or aircraft structural loads requirements.