Bahnhof Strasse
Let's av ya fuckin' dancin'!
Cap't Dave...
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.
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.
