Fuel Feeding Sequence in A320
As we all know there are three fuel tanks in an Airbus A320 and two of them are wing tanks while the other one is located the center of the fuselage belly. Wing tanks mainly divided into two partitions called the inner tank and the outer tank. Apart from those two tanks, there is a tank located towards the tip of the wing and it’s called the surge tank. It collects excess fuel and fuel spillage while venting the fuel tank to the atmosphere.
A318, A319 and A320 aircraft are fitted with booster pumps to supply fuel out from the fuel tanks with a positive pressure. Center tank, inner tank, and the outer tank have two of each booster pumps. When it comes to A321 it has two jet pumps in the center tank instead of ordinary booster pumps. A jet pump is lower in weight and consumes less power when compared to a booster pump. All the newer Airbus aircraft including A320neo will implement this jet pump in the center fuel tank.Jet pump operation is based on the Venturi principle and fuel in the center tanks is sucked into the related wing tank with the aid of fuel flow in a wing tank.
Operating Principle of a Jet Pump
Total fuel capacity of the aircraft is 23,858 L in volume and 18,728 Kgs in weight. So how this fuel is fed to the engines? What is the sequence of feeding?
Fuel in the center tank is fed to the engines initially and when the center tanks if emptied, the fuel from the wing inner tank is taken. Fuel from the wing inner tank is used until fuel level in the inner tank drops down to 750 Kgs. When this limit is reached, two transfer valves open and let the fuel in outer tanks to be filed into the inner tanks. So wing outer tank fuel is used at last. There is a special reason to implement this sequence. The wing is subjected to upward bending force throughout the journey due to the huge lift force created. Heavy upward forces try to bend the wing upwards and weight of the fuel provides the counter-attack for this bending moment. As arm increase when moving towards the tip, even a less amount of fuel will create more counter force to keep the wings straight.
What happens if all the booster pumps failed or inoperative due to a total electrical failure?
Fuel can be fed to the engines using the gravitational force and this method is called gravity feeding. In order to be gravity fed, related tanks should have suction valves. They are held in closed position during normal operations and comes in to act if booster pumps failed. Only wing tanks are fitted with suction valves, hence during a pump failure or electrical failure center tanks, fuel is unusable.