October 22, 2021

Why does aircraft fuel stay on the wings ?

All planes, regardless of size, carry fuel on their wings. The simplest and most obvious explanation for this would be to use space, but that is just one reason. There are also many other technical issues involved.

Fuel represents a significant portion of the plane’s weight. An Airbus A380, for example, 4040 can carry 254 thousand kilos of fuel, the equivalent of more than 320 thousand liters of kerosene. The A320 can carry 23 thousand kilos, or more than 29 thousand liters.

In airplanes, weight distribution is an essential item to ensure flight stability. After takeoff, only one item can change the total weight of the plane, which is precisely the fuel. The more the plane flies, the more kerosene is consumed and the lighter the plane gets.

For stability, airplanes are designed to have the center of gravity (equilibrium point) at the height of the wing. If the fuel was in the fuselage region, as the fuel was consumed, there could be an imbalance between the weight on the front and rear of the plane. This would change the position of the aircraft’s center of gravity, creating instability in the control of the aircraft.

With the fuel stored in the wings, that doesn’t happen. The system works so that the engines are fed with fuel from both wings. If the flow of the tank from one wing is greater than the other, it is possible to rebalance the weight by opening a cross feed valve.

Some larger planes still have central tanks, located on the fuselage in the region under the wings or even on the horizontal tail stabilizer. The fuel stored in these tanks is the first to be consumed in flight. This is because the weight of the fuel in the wings is also important for structural reasons.

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The wings are responsible for providing the necessary support for the plane to take off and stay in flight. The lift force causes the wings to curve upwards. If the fuselage were much heavier than them, the wings might not support and even break. With the weight of the fuel, these forces are more balanced.

And there is also the question of space. If it were not for the fuel, the internal part of the wing would be completely empty, while the fuel would occupy the space that is used to transport cargo.

Why is the fuel calculation by weight?

The amount of fuel used by an airplane is calculated according to its weight, which can be in pounds or pounds. This is because the volume changes according to the temperature. A plane can take off from Rio de Janeiro in the summer at 40 ºC and, in a cruise flight, face a negative temperature of -40 ºC.

The hotter, the lower the density. The lower the density, the greater the volume. This means that the plane would have much more fuel on the ground than in flight, and not just by burning the fuel. On the other hand, the weight is always the same, regardless of the temperature.

Another curiosity is that airplanes rarely fly with a full tank. This is for the sake of performance and economy.

The heavier, the greater the fuel consumption. Filling the tank means the plane will have to spend fuel unnecessarily. This is only done on routes close to the plane’s autonomy limit.

In addition, filling the tank can make it impossible to operate at certain airports. Heavier, the plane needs more speed to have lift, which requires a longer runway length for takeoff. Upon landing, the plane also needs more space to land and brake.