If you’re a true aviation geek, maybe you already knew airplanes carry their fuel in the wings. But do you know why, exactly where, and how it all works? Here’s a quick primer:
Why in the Wings?
First of all, for support. In order to prevent stress in the union of the wing with the fuselage, or needing reinforcement with very heavy materials, the weight of the fuel counteracts the forces that would bend the materials upwards. In this way, we avoid excessive flexing and fatigue of the material.
For space considerations: the cabin and cargo hold housed within the fuselage are used for passengers, suitcases, merchandise, mail, and so forth, so that placing a large tank there would be unfeasible.
For balance and centering. To carry so many tons of fuel on the airplane, they must go as close to the center of gravity as possible.
How Does Fuel Get to the Engines?
Gravity-driven. This is generally used in combustion engines in light aircraft. As the name implies, the force of gravity transfers fuel to the engine. The tanks, located one on each wing, drop the fuel to the filter and take it to the carburetor, where it is finally mixed with the air that will go to the cylinders and produce the combustion that generates the engine’s operation. This system is only found on aircraft whose wings are located above the carburetor.
Pump-driven. Normally used in jet engines in commercial aviation. In this case, gravity is not possible (for example, because the tanks are not above the carburetor), so the way to send the fuel in the wings to the engine is through a system of pumps (a main pump and an electrical backup in case the other fails). It is the most common system on commercial aircraft.
Wings, Fuel, and Balance
Inside the airplane there;’s is a fuel selector valve. The most common layout (because there are also planes with extra tanks, for example, in the tail) is: right (receives fuel from the right wing), left, both or closed. The “both” position is the one that allows fuel to be pumped from both tanks, and can be selected left or right to balance the amount left on each wing. At this point, it’s quite clear that the amount of fuel on both sides must be the same to avoid lateral imbalance of the aircraft.
There are a number of gauges that allow you to monitor the fuel system; among them, the amount in each tank, the total on board, its temperature, the consumption of each engine or total consumption. This allows us to know in detail the status of each tank, as well as the amount stored in each.
How Do the Tanks Get Filled?
As we’ve already seen, the main fuel tanks from which the engines are fed directly are on the wings. In general, they have a filling port either at the top or at the bottom, as in Iberia‘s Airbus, and a cap that seals them. In addition, there is a ventilation system to the outside to maintain atmospheric pressure inside the tank, and a drainage mechanism, independent or linked to the ventilation, to compensate for expansion of the fuel when the temperature rises, something relatively common if the tanks are filled on a hot day.
How Is the Fuel Drawn from the Wings?
Fuel consumption begins with the central tank, which is sometimes of great capacity, as in the case of the Airbus A350. Then it’s taken from the inner tanks and, finally, from the outer ones (inner and outer ones are in the wings), to counteract the lift force and prevent the wings from bending.
Fuel loading is done the other way around: first, the tip of the airplane (that is, the outer tanks located on both sides of the wings), inner tanks and, finally, the central tank. In principle, the aircraft’s engines receive fuel directly from the inner wing tanks. Of course, airplanes are designed so that any engine receives fuel from either of the two tanks. This is achieved through cross-feed valves.
Thus the sequence is as follows: the engines take fuel from the internal tanks until their quantity falls below the limit established for that specific model.