Please find below a short briefing about the A340 fuel system, distribution, and sensors.
The fuel system stores fuel in 6 fuel tanks (LH Inner, LH Outer, RH Inner, RH Outer, Center, and Trim).
Note: The photo is taken from AIRBUS VACB CD / Its here for studying purposes only. It is not allowed to copy it for trading purposes without prior notice from AIRBUS.
The fuel is used to feed the engines and APU, controls the position of CG and recirculates fuel for IDG oil cooling.
The fuel system and its related functions are controlled by the FCMS – Fuel Control and Monitoring System-.
The inner tanks are the main feed tanks. They receive fuel from other tanks (Outer, Center, and Trim) by the means of transfer.
The fuel system is controlled by the FCMS which has two identical FCMC – Fuel Control Monitoring Computer -.
The FCMS functions are: 1) Fuel level and temperature sensing. 2) Fuel gauging. 3) Fuel system fault detection and reporting. 4) Control the CG to its optimum position. 5) Fuel monitor and test. 6) Fuel management.
The total useable fuel is 111.08 Tones. The unusable fuel is less than .23% of the fuel tank capacity.
The inner tanks are divided into FWD Inner and AFT Inner. Each tank of them has a collector tank that uses Jet Pumps to keep it always full of fuel. These collector tanks are always full of fuel because the Main fuel pumps and the standby fuel pump are located their. The standby fuel pump operates when the main fuel pump switched OFF or when the main boost pump fails.
The inner aft has main fuel pump and the inner fwd has another main fuel pump.
The inner tank capacity is 33,580 KGS.
The center tank is located in the wings center section. The center tank has two expansion tanks above it to act as the surge tank for the wing tanks.
The center tank capacity is 33,300 KGS.
The outer tanks are useful to provide wing bending relief. The fuel in this tanks transfer to the inner tank when specific amount in the inner tank reached.
The capacity of the outer tank is 2865 KGS.
The trim fuel tank provides extra capacity, CG position is optimum so the drag reduced and the fuel economy is improved.
The trim fuel tank capacity is 4890 KGS.
There is one surge tank at the end of each wing and at the right Hand Side of the trim tank. The surge tank provides tank venting and collect any spilled fuel from the main tanks.
Note: when the refuel operation is completed to the maximum, the fuel can expand 2 % without spillage in the surge tank. The expansion tanks above the center tank allow the center tank to be filled to the maximum.
Access to the wing tanks is done through the man hole access panels and access to the center tank is done through the rear spar. Access to the trim tank is through the THS front spar and through hand holes on the lower skin.
The indicating systems are temperature sensing, fuel quantity, tank level sensing, and MML –Manual Magnetic Level – indicator.
Fuel quantity probes are installed in each tank. Each probe has capacitance value that changes according to the level of the fuel at that tank.
Three compensator probes are installed at the aft of each inner tank and one at the aft of center tank. The compensator probe is of a capacitance type probes. The compensator part has a capacitance which in proportion with the dielectric constant of the fuel changes according to change in temperature.
Two densitometers are installed. One in the lowest part of each inner tank. It measures the density of the fuel and sends the dielectric constant to the FCMC.
Two High level sensors are located in each inner, outer, center, and trim tank. When the High level sensor immersed fully with fuel it will signal the FCMC to close the inlet valve of the related full tank.
Low level sensors (2 in each inner, one in the center and one in the trim) control fuel operation and trigger warnings.
One overflow sensor located in each surge tank. When immersed with fuel the signal sent to the FCMC that in turns closes all the tank inlet valves and the refuel isolation valves.
The fuel temperature is measured through sensor in each collector tank, one in the trim tank and one in the left outer tank.
The recirculated fuel which cooled the IDG oil return to the inner tanks.
2 MML in each outer, 4 MML in each inner and 1 MML in the center tank used to measure the fuel quantity on the ground using the specific gravity and special tables. No electrical power is required.
The FCMC receives data from the FQI probes and the densitometer to give fuel quantity indication.
We have only low and high level sensors in the center tank.
The fuel distribution consists of:
1- Engines, APU feed system.
2- Refuel Defuel system.
3- Main transfer system.
4- Trim transfer system.
The two sections of the inner tank (FWD and AFT) are connected together by normally open EMERGENCY VALVE.
A booster pump from each collector is feeding an engine. Transfer valves are used to isolate each feed system. These transfer valves are opened when cross feeding. LP valves are located one in the pylon of each engine used to isolate the engine feed from the fuel system. This LP valve is controlled through the engine MASTER SWITCH and the FIRE P/B.
There are two APU pumps. One Uses fuel from ENG 2 collector tank and this is called FWD Fuel Pump. Another is called AFT fuel pump that uses fuel in the trim tank through the transfer pipe. The FWD Pumped fuel pass through APU isolation valve and then through the APU LP VALVE while when AFT fuel pumped the fuel passes only through the APU LP VALVE.
The APU FWD fuel pump is used when aircraft on ground or below FL250 (25000FT) with APU isolation valve opened.
The APU AFT fuel pump is used when aircraft above FL250 (25000 FT), or on ground during trim tank refueling defueling, or when fuel low pressure detected from APU FWD FUEL PUMP outlet. It will not operate if the trim tank is empty.
.The refuel/Defuel system controls the fuel flow during refueling defueling. At each wing there is refuel coupling (two adapters) with isolation valve.
Each tank has an inlet valve that allows the fuel to enter the tank from the refuel coupling.
The main transfer system controls the fuel transfer from the outer and center tanks to the inner tanks automatically controlled by the FCMC. Also they can be transferred manually controlled through the overhead panel.
From outer to inner there is a transfer valve when opens the fuel flow by gravity. From center to inner there is transfer pump.
The trim transfer system is to transfer fuel from the center tank to trim tank (AFT TRANSFER) or from the trim to the center tank (FWD TRANSFER).
For the FWD transfer there are trim tank transfer pump, trim tank isolation valve, and auxiliary FWD transfer valve.
When an AFT transfer is required if center tank isn’t empty. The center tank transfer pumps sent fuel via the trim pipe isolation valve and the trim tank inlet valve. If the center tank is empty then the fuel will be transferred from collector cell 2 (LH AFT INNER) and collector cell 3 (RH AFT INNER) via cross feed valves 2, 3 and the aft transfer valves.
The trim transfer is used for optimizing the aircraft performance by putting the CG in the optimum position. The Crew can control the FWD TRANSFER only. The FMGEC provides the FCMC with the CG position and provides excess AFT CG protection.
B.Regards
Ayman Shak'ah
Licensed Aircraft Maint. Engineer
