The A380 Fuel System.
This is the one part of the A380 course which I worked my way through, closed my eyes, took a deep breath, then went back to the start of the topic and started again! In the words of Amy Pond…..
The ultimate aim of any fuel system is to deliver the right amount of fuel at the right pressure to the engines at all times. But as an ultra-longhaul aircraft of such size, the percentage of the total weight of the aircraft at maximum weight which is fuel can be very high. For our British Airways aircraft the maximum takeoff weight is 569000kg. Of this, up to 254000kg could be fuel. Around 44.5% of the total weight. As the flight progresses this will obviously decrease. But storing the fuel the wings, as shown by this diagram, brings with it some challenges and complications.
The design of the A380 wing is what is known as a ‘swept wing’. This means it doesn’t go out from the side of the fuselage at 90°, but instead is swept back at an angle of 33.5°. Therefore, as the fuel is used during flight, the centre of gravity, ie. the balance point of the aircraft, moves quite significantly. All aircraft designs have an optimum point for the centre of gravity (C of G). In order to keep the A380 C of G at the optimum for as long as possible the aircraft has a large fuel tank in the horizontal stabiliser at the rear. During flight fuel is transferred out of this into the other tanks, so maintaining the optimal balance point.
Moving fuel around the aircraft between the various tanks is what makes the A380 fuel system so complex. There are 11 main tanks used to store fuel. Each wing has 5 main tanks. An outer tank, a mid tank, an inner tank and two feed tanks. The final storage tank is the one in the horizontal stabiliser at the rear, known as the trim tank. In addition to these tanks there are various surge tanks and vent tanks. Surge tanks are there to collect any overflow from the main tanks which may occur when they are full. This can happen if the fuel expands or if it ‘sloshes’ out of the tanks during tight turns during taxi. The vent tanks connect the main tanks to the outside atmosphere. Using a vent tank limits the differential pressure between the main tanks and the atmosphere, keeping it within structural limits.
Fuel is supplied to the engines via the feed tanks. The outer, mid, inner and trim tanks can be considered as storage tanks which are used to keep the feed tanks full. Each engine has it’s own feed tank. Feed tanks 1 and 4 have a capacity of 27632 litres (21691kg) with feed tanks 2 and 3 being slightly larger at 29349 litres (23039kg). If there is a problem with a feed tank an engine can be supplied with fuel from other feed tanks using a crossfeed system.
Before describing any more of the fuel system it may be useful to explain the difference between the litres and kg figures given above. Aviation fuel has a typical specific gravity of around 0.785 kg/l. This means that each litre of fuel weighs 0.785kg. For anyone not used to dealing with specific gravities this can be a slightly strange concept. We are all used to dealing with water, which has a specific gravity of approximately 1kg/l. (It does vary with temperature, but let’s ignore that for now!). Simply put, if we pour 1 litre of water into a jug which is placed on a set of scales, we will find it weighs 1kg. Aviation fuel is less dense than water, so if we were to do the same again we would find out 1 litre of aviation fuel would only weigh 0.785kg. For us it is the weight of fuel which is important rather than the volume. Our aircraft systems are calibrated in kgs (or pounds in some cases) rather than litres. Consequently you will hear pilots talk about how many kg or tonnes of fuel they have ordered for the flight, rather than how many litres.
This diagram shows one of the fuel system pages we can display in the flight deck. Let’s work our way down from top to bottom to explain what we are seeing here.
FU TOTAL is the total fuel used on our flight so far, 11000kg. The four sets of 2750 show how much fuel each engine has used. Each engine has a line with an arrow pointing toward the top. This shows fuel flowing into the engine. The circles just below the arrows depict fuel valves. In this case you can see the green line is going through the circle, showing the valve is open and fuel is flowing. Slightly below and to the side of each of these open valves you will see a corresponding valve which is still coloured green, but is depicted as being closed. This means the valve is in the correct position which has been selected by the fuel system, but at the moment there is no fuel flowing through it. This is the general way Airbus have set up their information systems for us. If a valve or other component is coloured green it means it is in the correct setting as instructed. If it is amber it means something is wrong or it is in the process of moving.
Below these valves we come to a series of small boxes. Some of these are coloured green and some white. These are the engine fuel pumps. Here, a green pump shows it is working properly and pumping fuel. A white pump means it is turned off. The main body of this display is a representation of the wing. Below each of the engine pumps is a box representing a feed tank. The numbers indicate the remaining fuel in kg. You will see that each feed tank is actually split into two chambers. The smaller one, in this case containing 1000kg of fuel, is called a collector cell. The engine fuel pumps are actually contained in the collector cell, where the fuel is used as a cooling agent.
In the lower half of the feed tanks you will see a number, -10. This is a fuel temperature gauge. In most areas of the world we use either Jet A1 or Jet A fuel. Jet A1 is the standard in the UK and has a freeze point of -47°C. Jet A is more common in the USA and has a freeze point of around -40°C. With outside air temperatures typically -55°C, but sometimes as low as -70°C and below, it is important that we monitor the temperature of the fuel to make sure it is still a liquid! If it is getting too cold, we would either have to fly faster (increased friction increases the temperature of the air over the wings) or descend into warmer air.
The numerous white pointed arrows you can see in the diagram depict valves which are not in use at the moment, but show how we can move fuel from one tank to another. The next row of tanks down in this diagram show the inner, mid and outer tank in each wing, and their respective fuel quantities. Finally, we have a line to another tank at the bottom of the display. This shows the trim tank contained at the rear of the aircraft. Finally we have an indication of the total fuel flow to the engines at present, shown as ALL ENGines Fuel Flow.
So that is the basic design and layout of the A380 fuel system. The next thing the designers had to consider is how to move all this fuel around while maintaining the optimum centre of gravity for the aircraft during flight. To to this, they use a network of pipes and valves known as galleries. There are two of these, termed the forward and aft galleries.
Complicated, but bear with me! I earlier described the inner, mid and outer tanks as storage tanks. Now we know there is a transfer system it is easier to consider these as transfer tanks.
Each transfer tank has a pump connected to the forward gallery. Each feed and transfer tank can receive fuel from the forward gallery via an inlet valve. The inner and mid tanks have a pump connected to the aft gallery, and again, each feed and transfer tank can receive fuel from the aft gallery via an inlet valve. The trim tank is connected to both the forward and aft galleries.
The forward gallery is used to transfer fuel between all the wing tanks. The aft gallery is used to transfer fuel from the trim tank to the wing tanks. The trim tank can accept fuel during refuel operations before the flight and while on the ground in order to change the centre of gravity. However, in flight, fuel can only from from the trim tank to the wing tanks, not the other way.
The design of the gallery system means if there is a failure in one of the galleries, the other can take over and complete the fuel transfer.
Refuelling is carried out using the galleries. There are two refuelling points installed under the wings, each of which can accept two fuel hoses from the refuelling vehicle. When both hoses are in use it takes around 45 minutes to upload 200 tonnes of fuel.
This photograph show how enormous the A380 wings and engine are as they dwarf the refuelling truck!
Rather than refuel via tankers, most major airports have an underground network of fuel pipes supplying fuel to each parking stand. The refuelling truck connects to this underground network and uses a pump used to load the fuel into the aircraft.
The refuel control panel is located underneath the aircraft. In normal operation it is completely automatic. The refueler selects the amount of fuel required using the PRESELECT control. The fuelling system then uses the gallery system to direct fuel to each tank as required to give the optimum centre of gravity for takeoff, which is 39.5%
On the flight deck there is a dedicated fuel control panel above the pilots. One of our setup actions is to turn on 20 fuel pumps!
In flight the transfer of fuel between tanks is completely automatic (so long as the system is working properly!). Shortly after takeoff what is known as a Load Alleviation Transfer takes place. Here, fuel is transferred from the inner or mid tanks to the outer tanks in order to reduce the upward bending of the wing. Anyone who has watched an A380 take off from a window seat may have noticed the wing tips lifting by up to 4 metres during takeoff due to the airflow. Transferring fuel to the outer tanks reduces this. You may ask why these tanks are not, therefore, filled before takeoff. That is because the weight of the engines makes the wings bend down and any extra fuel in the outer tanks would only increase this. Filling the outer tanks also has the effect of moving the C of G rearward to around 41% This is the approximate targeted C of G for the cruise.
As the flight progresses the fuel transfer system keeps the fuel level in the feed tanks at the same level, to within 1000kg. The sequence of fuel transfer is as follows:-
- Inner tanks to feed tanks
- Mid tanks to feed tanks when the inners are empty
- Trim tank to feed tanks when the mid tanks are empty
- Outer tanks to feed tanks when the trim tank is empty
The fuel transfer rate from inner or mid tanks to the feed tanks is around 10000 kg per hour per feed tank. Once trim tank transfers start they are performed in a way which maintains the optimum C of G for as long as possible, until eventually the trim tank is empty. From this point on the C of G will continue to move forward as fuel is used and transferred from the outer tanks.
I mentioned the freeze point of aviation fuel earlier. Usually in the range -40 to -47°C. The temperature of the fuel in the outer tanks tends to decrease more rapidly than in the other tanks during flight. In order to avoid this fuel freezing the system automatically transfers it from the outer to the feed tanks if the fuel temperature drops below -35°C. If this results in the feed tanks being filled any extra fuel is transferred to the inner tanks.
Towards the end of the flight there are two additional fuel transfers. Any fuel remaining in the trim tank when the time remaining to destination drops below 80 minutes is pumped forward. Similarly, when the time remaining to destination drops below 30 minutes any fuel left in the outer tanks is moved.
By now you have probably come to the conclusion that the A380 fuel system is reasonably complex. I’d agree! Now consider that everything I have described above is what happens when it is all working correctly! With so many pumps, valves and sensors we also have to consider how we handle things when part of the system stops working. It is these considerations which make the A380 fuel system quite so challenging for us as pilots. I don’t intend to go into detail here about what we would do in each failure case. That would change this from a blog to a book! You only have to imagine how much extra work it would be if all the automatic transfers I mentioned above didn’t work as designed. Indeed, the system on the A380 is somewhat similar to that used on Concorde. However, on the supersonic airliner a Flight Engineer had to do all the fuel transferring by manual switch and pump selection. In the event of certain fuel system problems we have to do the same on the A380, as the centre of gravity of an aircraft is critical in flight.
I hope the above has given you some insight into the extremely important role fuel plays in the way we operate the A380. For most aircraft fuel is just loaded into the tanks to the required level, used in flight, and that is it. The nature and sheer size of the A380 means we have a much more complex fuel system. But by imaginative use of the fuel in order to maintain the optimum centre of gravity for as long as possible in flight, the designers have found a way to not only power the aircraft but also make use of it to improve efficiency.
I’ll just keep my fingers crossed that the automatic fuel control systems keep working, otherwise the flight will get just a little busier for us up in the flight deck!
Thanks for reading. Hope you found it interesting. Well done if you made it this far!
One final and interesting point is the fuel capacity of the A380 trim tank is almost identical to the total fuel capacity of an Airbus A320!
I think we will move to something a little less complicated for my next blog….How to land an A380!