All the airplanes in service can store fuel in the belly of the aircraft, in the wings, in the rear tail-plane and/or in the cargo sections (for extending the range). Now, out of all the fuel storages available, many airplanes carry around 70–80% of the fuel on board in its two wings.
While refuelling, the fuel is first filled in the wings and then, only if required, the centre tank is filled last.
What is the main reason behind this fuel arrangement in most airplanes?
The main and only reason behind it is providing Wing Bending Relief. In order to understand this we must know what happens when the wing generates lift. When the wings are producing lift, the immense force under the wings causes them to bend upwards, specially towards the tips of the wings.
That doesn’t mean that the wings will break open during flight. The wings are actually designed to be flexible enough to bear the structural loads in flight (specially G-forces due to gusts, turbulence, and turns), without breaking or discomforting the passengers inside. This is known as Wing Flex.
But as always, there are limits to such flexes. They also put stress on the wing roots while they bear those G-forces.
That is why, fuel is stored in the wings. When the wings are laden with tons of loads of fuel it provides some sort of relief to the wings, when it comes to wing bending moments. The reason for not filling so much of fuel in the body tank is to keep the fuselage of the airplane as light as possible. Since the wings are now heavier and the fuselage is lighter, it will reduce the stress on the wing roots.
Secondly, it also utilises the available space in the wings, converting them into fuel wing tanks, which would otherwise have been wasted.
There are airplanes like the Airbus A380, which hold fuel only in the wings and in the tailplane. It has no tanks in the main body of the aircraft. It holds 120 tons (150,000 litres) of fuel in each of the wings, and 18 tons (22,500 litres) in the back of the airplane. Total fuel capacity is 258 tons (322,500 litres).
That’s why, it is always advised that while filling the aircraft with fuel, fill the wings tanks first and body tanks last; and while consuming fuel, use the fuel in the body tanks first and wing tanks last to provide maximum wing bending relief.