Where do planes carry their fuel?

The wing of a modern aircraft is a hollow box: two spars, the ribs, and the upper and lower skin form a structural box resistant to bending and torsion. By sealing the joints with an elastic compound, that same box becomes a tank with no walls of its own, what engineers call an integral tank. There is no added rubber or metal tank: the fuel sits directly against the inside of the wing. The solution saves weight and makes use of a volume that would otherwise be empty, while leaving the cabin and cargo holds free for passengers and cargo. On an Airbus A320 the two wing tanks and the center tank together hold about 24,000 L; on a Boeing 787-9, 126,000 L, almost all of it in the wing.

There is a significant structural reason. In flight the wing holds up the aircraft and tends to flex upward; the weight of the fuel spread along the span pulls downward and reduces the bending moment at the root, so the structure can be lighter. That is why the manuals require the center tank to be consumed first, leaving the wing fuel for last, and why maximum zero fuel weight (MZFW) is a certified limitation: an aircraft with a full cabin and empty wings experiences greater stresses than one with full wings. Placing the fuel near the center of gravity also keeps the aircraft's balance from shifting much as it is consumed.

The center tank occupies the torsion box that joins both wings under the cabin floor, between the landing gear wells. On long-haul aircraft such as the A330, A340, A380 or 747-400, a trim tank is added in the horizontal stabilizer: during cruise fuel is transferred aft to shift the center of gravity rearward and reduce trim drag, and it is moved forward again before descent. Some models, such as the long-range A321 or executive 737s, fit auxiliary tanks in the cargo hold. Fighters and military aircraft also carry disposable external tanks under the wings and fuselage, and receive fuel in flight through probes or refueling booms.

Each tank has submerged electric pumps that feed the engines, cross-feed valves so that any engine can receive fuel from any tank, capacitive probes that measure the quantity, and venting systems that prevent overpressure and vacuum conditions. Since the TWA 800 accident in 1996, the center tanks of new aircraft also incorporate an inerting system that fills the empty space with oxygen-poor air to prevent the ignition of vapors.