A method for extending a fuel capacity of a tanker aircraft, the method comprising: mounting a plurality of fuel containers (26A-26F) on a plurality of pallets shaped to roll on and off of a cargo area of an aircraft to form a plurality of modular fuel container units; attaching an outlet conduit and an inlet conduit to each of the fuel containers (26A-26F), wherein each of the outlet conduits is adapted to connect alternatively to the inlet conduit of another one of the plurality of modular fuel container units and to a fuel port in the cargo area of the aircraft connected to aircraft fuel tanks, and wherein each of the inlet conduits is adapted to connect alternatively to the outlet conduit of another one of the plurality of modular fuel container units and to a source of fuel external to the aircraft; and providing each of the outlet conduits with an adjustable outlet valve, and each of the inlet conduits with an adjustable inlet valve; the method further comprising; rolling the plurality of modular fuel container units into the cargo area of an aircraft; filling the plurality of fuel containers (26A-26F) with fuel; connecting the outlet conduit of one of the plurality of fuel containers (26A-26F) to the fuel port; and connecting the fuel containers (26A-26F) of a remainder of the plurality of modular fuel container units to form a chain of the plurality of fuel container units that is connected to the one of the plurality of fuel containers (26A-26F) so that the fuel stored in each of the fuel containers (26A-26F) flows through the chain to the one of the plurality of fuel containers (26A-26F) having the outlet conduit connected to the fuel port; the method further comprising selectively actuating the outlet valves and the inlet valves to regulate fuel flow between the fuel containers (26A-26F) of the fuel container units to maintain a desired fuel level in the plurality of fuel containers (26A-26F) as fuel flows to the aircraft fuel tanks and thereby maintain a desired center of gravity of the plurality of modular fuel container units within the cargo area of the aircraft. mounting a plurality of fuel containers (26A-26F) on a plurality of pallets shaped to roll on and off of a cargo area of an aircraft to form a plurality of modular fuel container units; attaching an outlet conduit and an inlet conduit to each of the fuel containers (26A-26F), wherein each of the outlet conduits is adapted to connect alternatively to the inlet conduit of another one of the plurality of modular fuel container units and to a fuel port in the cargo area of the aircraft connected to aircraft fuel tanks, and wherein each of the inlet conduits is adapted to connect alternatively to the outlet conduit of another one of the plurality of modular fuel container units and to a source of fuel external to the aircraft; and providing each of the outlet conduits with an adjustable outlet valve, and each of the inlet conduits with an adjustable inlet valve; the method further comprising; rolling the plurality of modular fuel container units into the cargo area of an aircraft; filling the plurality of fuel containers (26A-26F) with fuel; connecting the outlet conduit of one of the plurality of fuel containers (26A-26F) to the fuel port; and connecting the fuel containers (26A-26F) of a remainder of the plurality of modular fuel container units to form a chain of the plurality of fuel container units that is connected to the one of the plurality of fuel containers (26A-26F) so that the fuel stored in each of the fuel containers (26A-26F) flows through the chain to the one of the plurality of fuel containers (26A-26F) having the outlet conduit connected to the fuel port; the method further comprising selectively actuating the outlet valves and the inlet valves to regulate fuel flow between the fuel containers (26A-26F) of the fuel container units to maintain a desired fuel level in the plurality of fuel containers (26A-26F) as fuel flows to the aircraft fuel tanks and thereby maintain a desired center of gravity of the plurality of modular fuel container units within the cargo area of the aircraft.