A modern system

Citation

Ford, T. (2002), "A modern system", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 5. https://doi.org/10.1108/aeat.2002.12774eaf.001

A modern system

Keywords: Airbus, Aircraft, Fuel systems

Both the Airbus A340-600 and A340-500 provide significant increases in capacity and/or range compared with the earlier A340-300. The A340-600 is 10.6m (34ft 10in) longer than the A340-300 and can carry up to 380 passengers for 7,500n miles while the very long range A340-500 accommodates 313 passengers over 8,500n miles.

Because of the increases in size and range and the consequent changes in the design, the wing area is increased by 20 per cent and the wing sweep is also changed. The latter is the principal reason for the fuel systems of the –600 and –500 being different from the earlier A340s. models. The effect of this is to change the trajectory of any debris from an uncontained engine rotor failure, and thus prevent the use of tank boundaries similar to those on earlier A340s, For certification reasons the boundaries for the engine feed tanks and associated systems architecture had to be changed. Also, additional changes have been made as the result of the requirement for increased refuelling flow rates of 400,000l/h.

The functions of the –600 and –500 fuel systems remain similar to the previous aircraft in this series, but the differences can be summarised as follows: an increase in the number of fuel tanks; a revised KCAM (Electronic Centralized Aircraft Monitor) fuel system page; secondary wing refuel and transfer galleries; dedicated fuel jettison/ transfer and engine feed pumps; separate APV and trim transfer tanks; segregation of computing and fuel probe interface functions into separate boxes; (–500 only) a fuel tank positioned between the forward end of the rear cargo hold and the centre landing gear bay; and the fuel control monitoring system is provided by Parker.

On the A340-500 fuel is stored in nine fuel tanks. Three tanks are in each wing, comprising one outer wing transfer tank and two engine feed tanks called "inners" (Within each inner tank is a dedicated engine feed collector cell). One transfer tank is in the centre wing box, called the "centre" tank. One transfer tank is in the horizontal stabiliser, called the "trim tank, and one transfer tank is positioned at the forward end of the rear cargo hold, called the "rear centre tank" or RCT. The A340-600 has only eight tanks, the RCT not being fitted.

Refuel and defuel

The refuel system is designed to refuel the aircraft from empty to full in 33min on the – 500 and 30min on the –600, when a fuel pressure of 3.45 bar is applied at all four of the refuel couplings. A lower fuel pressure is also available. The four couplings are fitted as two pairs, one pair to each wing. The automatic refuel distribution is defined in two steps: in the first stage by specific masses for each tank, followed by a second "top up" phase to the volumetric high level for all the tanks.

The total volume of fuel for the A340-500 is 56,746 US galls weighing 171,846kg (378,854lb), and for the A340-600 is 51,841 US galls weighing 155,902kg (943,709lb). Control of the refuel function is by the two fuel control and monitoring computers (FCMC) and two fuel data concentrators (FOC). The operator interface is either through the standard refuel panel fitted in the lower surface of the fuselage just aft of the undercarriage bay, or through the optional refuel panel and multipurpose control and display unit (AICDV) in the cockpit. Normal refuel is fully automatic. However, in the event of system failures, a manual refuel facility is available through the standard refuel/defuel control panel

The outer, trim and RCT tanks each have a simple tank inlet valve. To minimise any surge pressure in the refuel gallery the centre and inner tanks have two valves which are closed in sequence. The centre tank has a tank inlet valve in series with a restrictor valve and the inner tanks each have two tank inlet valves in parallel. To enter the aircraft, fuel must pass through one of the two refuel isolation valves which form part of the refuel couplings fitted to each wing. To prevent spillage of fuel, the detection of fuel within either of the wing tip surge tanks or a jettison valve detected open will automatically stop the refuelling of the complete aircraft by closing the refuel isolation valves. In addition, detection of the fuel in the horizontal stabiliser surge tank will automatically stop automatic refuelling and manual refueling of the trim tank.

Engine and APU feed

The A340-500 and A340-600 are each powered by four Rolls-Royce Trent turbofans, each certificated at 60,000lb take-off thrust and developing 56,000lb for the A340-600 and 53,000lb on the A340-500. Under normal operation each engine is fed by an independent fuel feed system. This consists of main and standby engine feed booster pumps located within a collector cell, which in turn is located within an engine feed tank (inner). The main pump operates continuously, the standby pump only operating if (the main pump becomes elective or is set to OFF).

The collector cells are maintained fully until the inner wing tanks are nearly empty to help ensure a supply of fuel to the engine under negative "G" maneuvers. The cells are maintained fully by the use of jet pumps driven by fuel flow taken from the main engine feed booster pumps. All engine feed systems can be joined to the crossfeed gallery by the independent crossfeed valve. The crossfeed system is used under abnormal operating conditions such as loss of all electrical power requiring gravity feeding or to connect all engines to a single engine feed boost pump when only the emergency electrical supply is available, or to allow the crew to correct an imbalance between symmetrical wing tanks,

In common with all other Airbus Industrie programmes, all fuel pump and valve electrical wiring is routed outside of the fuel tanks to eliminate the potential for introducing ignition sources into the fuel tanks. The APV is fed via a tapping off the number one engine fuel feed line. The number one engine booster pumps normally supply the fuel pressure. However, if these pumps are not selected then a dedicated APV pump is installed in the line to supply the fuel pressure.

A jettison system is provided to avoid the necessity for heavy maintenance tasks, by providing a means to minimise the potential for an overweight landing, although the system is not required for certification reasons relating to the performance of the aircraft. The system is activated by means of two dedicated pushbuttons, or automatically if all jettison/transfer pumps are running dry (low pressure) or when the fuel quantity drops below a predetermined target input into the MCDV by the flight crew.

The system jettisons fuel through two jettison valves positioned in the number three flap fairing between the two engines on each wing. Up to ten jettison/transfer pumps are used to provide the fuel flow, one situated in each of the four inner tank and two in the centre tank, two in the trim tank and two in the RCT. The refuel gallery is used to connect the jettison valves to the pumps. The centre, trim and RCT pumps only function if the associated tank contains fuel. In addition, if the outer tanks contain fuel a transfer of the fuel to the inner tank is automatically initiated,

Fuel transfer and useage

On the A340-600 under normal operation all fuel transfers, except those for centre of gravity control, are to the four inner tanks, prior to transfer to the collector cells, and are controlled automatically. Automatic transfers are controlled to balance the fuel quantities in symmetrical wing tanks to prevent an imbalance which could adversely affect the aircraft handling. On the A340-500 transfers from the RCT are to the centre tank. On the cockpit overhead panel four pushbuttons are provided to allow manual transfer control of the outers to inners, centre to inners, trim to centre, and RCT to centre.

On the A340-500 transfers from the RCT to the centre tank are by means of two pumps located in the RCT and two valves, one of the latter situated at each end of the RCT refuel/ transfer line. Transfers from the centre to the inner tanks are normally by means of two pumps located in the centre tank which are connected to the forward refuel/transfer gallery are independently controlled tank inlet valves, one for each inner tank. In the event of certain failures, transfers are by means of two pumps located to the centre tank which are connected to the aft refuel/transfer gallery and independently controlled tank inlet valves, one for each inner tank.

Transfers from the trim to inners is by means of two pumps located in the trim tank, two valves situated at opposite ends of the trim tank transfer line add independently controlled tank inlet valves, one for each inner tank, connected to the aft refuel/transfer gallery. If the centre tank contains useable fuel, then trim transfers will be to this tank,

Transfers from the outer tanks to the inner tank are by gravity. For transfers to inner tanks one and four the refuel inlet valves, for the concerned tank and refuel gallery, are used. For transfers to inner tanks two and three dedicated transfer valves and associated pipework is used.

In order to minimise the aircraft's aerodynamic drag in cruise the fuel system is used to optimise the aircraft's angle of attack by controlling the aft centre of gravity. Depending on the aircraft's zero fuel weight (ZFW), CG and the actual fuel on board the fuel system will control the aircraft's CG to a target of 2 per cent mean aerodynamic chord (MAC) forward of the certified aft limit.

Control is achieved by means of fuel transfers to and from the trim tank and in addition on the A340-500 transfers from the RCT. Aft transfers to the trim tank arc performed if the tank is not full and the aircraft's CG is forward of the target. Forward transfers from the trim tank are performed if the aircraft's CG drifts aft of the target, due to fuel burn. On the A340-500 forward transfers from the KCT are delayed until the aircraft's CG drifts aft of the target due to fuel burn, or the centre tank has less than 11 tonnes of fuel. (At the end of cruise all trim tank fuel is transferred forward). For integrity of the system, an independent CG monitor is performed by the flight management system.

Measurement and controls

Unlike other Airbus aircraft, the fuel quantity measurement/indication and level sensing functions are combined and use similar capacitive type probes to perform the two functions. The probe excitation and return signal processing is performed within two independent FDCs. The conditioned data from the FDCs is sent to both FCMCs, which process the data for fuel quantity indication (FQI), fuel level indication, refuel control, CG calculations and control and general fuel transfer control. Automatic compensation is applied for changes in fuel density, fuel permitivity and aircraft effective attitude. The probes are configured into two separate groups per tank and the interfaces through the FDCs and FCMCs are designed so that any single failure (e.g. probe, harness. FDC or FCMC) will not cause a loss of indication.

Within the normal ground standing attitude range of the aircraft the accuracy of the FQI system will be in the order of 0.4 per cent of the lull capacity near empty to 1 per cent at full. To further enhance the security of the fuel systems design on Airbus aircraft the potential for an ignition source to be present in the fuel tank has been mitigated by the following measures: the harnesses to the capacitive probes are segregated from all other aircraft wiring; and, the length of the harnesses is minimised (the FDCs being fitted within the centre fuselage section). These measures limit the potential for a short circuit to power cables,

In the case of a complete failure of the fuel control and monitoring system (FCMS), a secondary manual fuel lever indication system is installed in the six wing tanks and the centre tank for ground use. Height data from the probes is used in conjunction with aircraft attitude information, fuel density measurement and a set of look up tables to calculate the fuel mass in the tank being measured. The level sense function is used to detect high and low fuel states in all fuel tanks and a fuel overflow in the three surge tanks. The information is used to control refuel, inner tank transfers, transfer pump shut- down, overflow protection, and indication of low fuel state.

Dual element in-tank temperature sensors are fitted to the trim, outer and engine feed tanks. These sensors enable the flight crew to monitor the evolution of the tank fuel temperatures to ensure that they are within the operational limitations for the specific fuel types being used (e.g. JET A or JET Al).

Due to the differences in the fuel system architecture, the fuel system control panel and CCAM system page differ from the A340-300. Despite these differences, there is still a pushbutton for each engine feed and transfer pump for each crossfeed valve and each transfer function, as well as a dedicated toggle switch associated with the trim tank isolation. The latter can be seen on the main fuel system overhead panel. Under normal operation after initialisation at the start of a mission, no crew action is required on the panel. Manual transfer control is by selection of the dedicated transfer pushbuttons or the deselection of the transfer pump pushbuttons. Adjacent to the main panel are the pushbuttons for Jettison and the inner 2 and 3 wing tank isolation valves.

On the ECAM fuel page fuel system data is displayed including, fuel quantity, fuel temperature, fuel transfers, pump status, fuel used, and fuel flow. Warnings and the total fuel on board are displayed on the engine page.

Terry Ford