Where do passenger planes carry their fuel?

In the vast majority of commercial aircraft, fuel does not travel in separate tanks as in a car, but in "integral tanks": the wing's own interior structure, between spars and ribs, is sealed and turned into a large tank. It is an efficient solution in terms of weight and space, because it makes use of a structure that has to exist anyway to hold the aircraft up in flight, and it places the fuel's weight near the center of lift, where the wing flexes less.

Many wide-body aircraft and some large single-aisle jets add a center tank, located in the lower part of the fuselage, right where the two wing halves meet the central structure known as the torsion box. It is used for long flights that cannot be covered by the wings alone. Some very long-haul aircraft, such as the Airbus A330, the A340 or the A380, also incorporate a trim tank in the horizontal tail stabilizer: besides adding capacity, it allows fuel to be shifted aft during cruise to move the center of gravity, reduce tail drag and save fuel.

Storing fuel under the passenger cabin, far from the wings, would complicate the structure and weight distribution, and would increase the risk in the event of an impact. That is why engineers concentrate fuel in the wings and in the wing box, the structurally most reinforced area of the aircraft and one protected against external damage by the very design of its ribs and spars, and only resort to tail tanks in designs optimized for very long transoceanic routes.