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:-

1. Inner tanks to feed tanks
2. Mid tanks to feed tanks when the inners are empty
3. Trim tank to feed tanks when the mid tanks are empty
4. 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!

Dave.

The A380 wing is a remarkable piece of engineering. Made in Broughton, North Wales, it is one of the defining features of the A380. The largest wing used on a commercial aircraft before the A380 was designed belonged to the Boeing 747-400. It has a wingspan of 64.9 metres. Consequently, most major airports have been designed with this wingspan in mind. The A380 wing changed the rules! With a maximum takeoff weight of 575 tonnes, some 178 tonnes greater than the 747-400, a much larger wing was required.

This photograph clearly shows the much larger wing of the A380 compared to the 747.

The A380 has a wingspan of 79.676m and an area of 845.8m². Compare that to the wing of the 747-400 which has an area of 541.2m². And we should remember that the 747-400 is a very large aircraft. Trivia fans will note this means there is space to park 144 cars on each side of the the A380 wing!

Each A380 wing has 3 ailerons, 8 spoilers, 8 slats and 3 flaps.

These are terms you may well have heard before. But if not, what does each one mean?

Ailerons – These are used mainly at low speed (below 240 knots) to make the aircraft bank (or roll) from side to side. They move in opposite directions on each wing. Therefore, if we want the aircraft to bank left, the ailerons on the left wing will move up and those on the right will move down. This has the effect of dropping the left wing and raising the right wing. The ailerons on the A380 are also used as part of an active system to reduce the effect of turbulence. More of that later… Each aileron has a maximum deflection of 20° down and 30° up.

Spoilers – These are only on the upper surface of the wing. They have several uses. Firstly, the outer 6 spoilers are used for roll control at higher speeds. In this case, only the spoilers on one wing will move, with a maximum deflection of 45°. Secondly, all 8 spoilers on each wing can be used as speed brakes. You may have seen the spoilers on an aircraft rise during descent. They effectively reduce the lift from the wing and act as air brakes, useful if we want to slow down or go down more rapidly. This will be accompanied by a slight rumbling feeling inside the aircraft. Nothing to worry about. But if ever you have felt it, now you know why!

Their final function is to ‘dump’ the lift from the wing on landing to prevent the aircraft going back into the air. Cleverly, this ground spoiler function has two phases. When one main landing gear is sensed as having touched down the ground spoilers partially extend (spoilers 1 and 2 by 10° and 3 to 8 by 15°. This slightly reduces the lift produced by the wing and aids in a gentle touchdown. Once three main landing gears are sensed on the ground the spoilers extend fully (1 and 2 to 35°, 3 to 8 up to 50°). The ailerons also deflect upwards to 25° acting as additional spoilers and air brakes.  This movement of the spoilers and ailerons will also occur if a rejected takeoff is performed from a speed greater than 72 knots.

Slats and Flaps – Slats and flaps provide lift augmentation. Simply put, aircraft wings are designed to work best at their cruising speed. But we wouldn’t want to try and land the aircraft at that speed! The normal wing shape has a range of operating speeds, but even on superb design such as the A380, this will not be below around 190 knots at landing weight, and more like 230 knots at takeoff weight. In order to allow the aircraft to fly at lower speeds during takeoff and landing the shape of the wing has to be changed. In order to do this there are various stages of slats (front or leading edge of the wing) and flaps (rear or trailing edge) which can be extended.

For those interested in the technical working of these systems, the A380 slats are moved by an electric motor and a hydraulic motor whereas the flaps are driven purely by hydraulic motors. If you have flown on an A380 you will quite probably have heard a quite high pitched ‘whining’ noise when the slats move. It is these motors which generate that noise.

Airbus aircraft use a standard set of slat/flap configurations which depend on flight phase and speed. Convention is that we always ask for a flap setting rather than a slat and flap setting. This is true on Boeing and Airbus aircraft. We ask for the required flap setting knowing that the slats will also move to a corresponding setting. The slats and flaps are controlled using a lever on the centre console between the pilots.

For takeoff, the A380 has three possible slat/flap positions (known as configurations). These are 1, 2 and 3. We action a takeoff performance calculation using one of the apps installed on the aircraft in order to determine the best flap setting for each individual takeoff. There are several factors to consider here. Flap 1 gives the highest takeoff speeds and greatest after takeoff climb gradient, but due to the higher speed needed before takeoff uses more runway. Flap 3 gives the lowest speeds for takeoff but also a reduced initial climb gradient because in addition to producing more lift at lower speeds, flap 3 also produces more drag. For landing we can use Flap Full or Flap 3. In most cases flap 3 is used.

The angle of slat and flap extended for each configuration is shown in the table below.

What is aileron droop? In certain configurations the A380 extends the ailerons down on both wings by 5° in order to provide additional low speed lift, effectively turning them into small flaps.

You will notice that there are two possible configurations when the flap lever is in position 1. When we select flap 1 on the ground as a takeoff flap setting, we get 20° of slat extension and 8° of flaps. This is called configuration 1+F (one plus F). We always have some flap extension for takeoff. However, when we are in flight and reducing speed for our initial approach, there may be a time when we only want the slats to be extended in order to allow a reduction in speed below our ‘minimum clean’ speed, the minimum speed at which we can fly with the wing in the normal shape. Having slats only extended in this case allows us to fly slightly slower than our minimum clean speed, but not get the additional drag which would be produced by the flaps. This would typically happen if we were relatively heavy and air traffic control asked us to maintain a speed of 210 knots. Our minimum clean speed could be around 220 knots, but extending flap 1 (which actually wouldn’t give us any flap at all, just slats to 20°) would allow us to fly at 210 knots. When we subsequently reduce speed below 205 knots, the flaps auto extend to 8°. This is the AES (Automatic Extension System)

You will see from the table that each configuration has a maximum speed. We need to be mindful of this when making flap selections. In addition to the AES we also have the wonderfully named ARS system! (Always raises a smile when we discuss it during takeoff briefings!) ARS = Automatic Retraction System. For higher weight takeoffs the speed at which we would normally retract the flaps setting from 1+F to zero can be close to the limiting speed of 222 knots. In this case, in order to protect the flaps from overspeed, the ARS automatically retracts them to zero leaving only the slats extended when the aircraft accelerates past 212 knots. Very clever!

The wings have one further vital function. They act as massive fuel tanks.

Each wing contains five fuel main fuel tanks, a surge tank, and a vent tank. The surge tanks temporarily collect fuel which may overflow from any tank when they are close to being full. For example, fuel may overflow a full tank during a tight turn while taxiing. The vent tanks connect the fuel tanks to exterior atmospheric pressure in order to limit the differential pressure between the tanks and the atmosphere.

The fuel tanks are huge! Each wing has a capacity of 149924 litres! That’s around 120 tonnes. The trim tank at the rear can hold 23698 litres, which is almost the same as the total fuel capacity of an A320! If you want to ‘fill-her-up’ you will need to put 323546 litres in the tanks.

One complication of making these amazing wings in North Wales is they have to be transported to the Airbus factory in Toulouse where the aircraft is assembled. This is a major logistical operation! A Multi-Purpose Vehicle is used to transport the wings on the 1.6km journey from the Broughton factory to the River Dee. This MPV is 22 metres long, has 96 wheels, and with the wing plus the carrying jig, carries around 140 tonnes. The MPV drives on board the Dee River Craft to place the wing in position for its 24km journey along the river to the Port of Mostyn. From here, another MPV collects the wings where they are put on the roll-on-roll-off ship Cuidad de Cadiz where they sail to Pauillac, the nearest port to Toulouse. On arrival in Pauillac they are transferred on to barges which transport them 95km up the Garonne river to Langon. From there the final 240km of the journey is by road.

The Langon to Toulouse journey passes through 21 towns and villages. Much of this journey is done at night to avoid disruption. I was lucky enough to be invited to the Airbus factory in Toulouse in April 2017. This included a visit to the nearby town of Levignac, through which the convoy passes. Although the particular convoy I saw didn’t have a wing shipment included, it did have some A380 fuselage sections. As you can see on the video below (click the link), the clearance between the fuselage and buildings in the town is rather small! I was informed the wing sections are slightly wider than this, resulting in even less room for manoeuvre!

A380 Convoy through Levignac

Finally, a few interesting facts about the A380 wing..

Each set of wings has 20 panels and 314 stringers. They contain 750000 rivets or bolts! And lots of wire – 23 miles of it!