Concorde’s Fuel Systems

On Concorde, the fuel system has three functions; to supply fuel to the engines, to control the position of the aircraft’s Centre of Gravity (CG) and to act as a heat sink to absorb kinetic heating from the structure and to dissipate heat generated by the air-conditioning and hydraulic systems.

For further information on the fuel system, click on the links below!

FUEL TRANSFER & CG SYSTEM

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Trim Transfer, Main Transfer, Fuel Quantity Indicators (FQI), CG Position Indication, CG & Mach Limits Indication, CG & Mach Warnings.

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mConcorde Refuelling Operations

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Safety & Cleanliness Precautions during Refueling. Water Check Operations, Magnetic Fuel Indicator

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Concorde Fuels

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Approved Fuels and Additives for Concorde

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GENERAL DISCRIPTION OF THE FUEL SYSTEM

Development, Fuel Tank Structure, De-Aeration, Fuel Jettison, Engine Feed, Venting & Pressurisation, Load Limit Control CG & Mach Limit Warnings, CG & Mach Limit Indications, CG Position Indication, Trim & Main Transfer.


The Development

While the Concorde fuel system had been designed as far as possible along the conventional lines, employing well tried principles and practices throughout, a number of new problems came to light during the development which had not been previously encountered with other civil transport aircraft at the time. These problems stemmed mostly from the following novel features of Concorde:—

(1) The environment was more severe, extending to high temperatures and very low ambient pressures.

(2) The high, performance of the aircraft lead to a proportionately high performance requirement for the fuel system.

(3) Besides the conventional function of supplying fuel to the engines, Concorde fuel system had to perform the additional functions of controlling and of absorbing surplus heat rejected by other aircraft and engine systems. A number of the particular problems, the research and the development that have gone into the system are described briefly below.

At high temperatures and low ambient pressures, such as experienced by Concorde when cruising at Mach 2.0 at 60,000ft, conventional kerosene may be both highly supersaturated with dissolved air and near its boiling point. It was therefore necessary to take measures to prevent loss of fuel due to boiling-off, to prevent high transient pressures in the tank and loss of fuel due to a rapid-de-aeration of the fuel, and to ensure that the tank pumps would give the required performance at these conditions. The fuel system usage sequence was arranged such as to minimise the intake of heat through the wing skin and thus keep temperatures to a minimum, but a considerable amount of research work was necessary to provide data on fuel vapourisation and aeration characteristics. More particularly, a large effort was necessary to design and develop the fuel tank pumps- which had to both start and give the required performance with a low inlet pressure and with fuel which is both near its boiling point and containing a high percentage of dissolved air.

Although the boundary layer and structure temperatures of Concorde are not such as to constitute an ignition risk, there was, as for subsonic aircraft, the risk of ignition by lightning strike of fuel vapours emitted from the vent outlets. A programme to demonstrate the probability and distribution of lightning strikes was carried out by the Lightning and Transients Institute in the USA and the evidence so derived was used in choosing the positions of the vent outlets. As an additional safeguard, an explosion suppression system was fitted in the main vent gallery adjacent to the outlet orifices.

Although subsonic aircraft have to be capable of sustaining their engines during short periods of negative g, Concorde had a rather special problem. In the case of subsonic aircraft, where both the fuel temperatures and the altitudes are relatively low, when a negative g condition is encountered the tank pumps become uncovered but the engines continue to run for a period on the fuel remaining in the supply lines. An amount of air enters the lines, but this becomes mixed with the fuel before reaching the engine and the effect upon the engine, after the negative g period is over, is small.

For Concorde the situation was quite different in that, at high altitude and high temperature, if the pumps become uncovered the pressure at the inlet end of the fuel line dropped to a mere llb/sq in, which is below the vapour pressure of the fuel at the engine inlet where the temperature is above 100°C. The fuel in the line would therefore boil, and the engine would go out immediately. To overcome this situation it was necessary to design a constantly pressurised fuel accumulator to maintain the fuel flow and pressure during any period when the tank pumps may become uncovered.

Another fuel characteristic which had demanded a good deal of research was its thermal stability. When subjected to high temperatures, certain elements in the fuel became unstable and a gum deposit was formed in pipes, heat exchangers and filters. A considerable amount of laboratory research was undertaken, mostly by the oil companies, to provide data upon which to establish what could be the maximum temperatures for the fuel in the tanks and throughout the aircraft and engine systems.

In addition to the laboratory tests, over 3,000hr testing had been carried out on a representative rig at the Shell Thornton Laboratories to confirm that the decisions taken were valid. Because of the additional functions, the Concorde fuel system had to perform, and because of the need to use the fuel in a certain manner to minimise temperature problems, Concorde fuel system is somewhat more complex, than the subsonic aircraft. It has a number of sub-systems for the transfer of fuel between tanks and there is a good deal of interdependence and interaction of the sub-systems. ; It was therefore necessary to prove the satisfactory functioning of the total system by means of a full-scale test rig which embodied all the tanks and the sub-systems and was capable of achieving all the attitudes, pressures (altitudes), temperatures and rates of temperature change which Concorde’s fuel system would experience in flight.

The Concorde Fuel System

Concorde, like most airliners, has multiple fuel tanks which have been detailed in the picture below. The only difference concerning Concorde is that during the flight, fuel is transferred from tank to tank to maintain trim and balance of the aircraft as Concorde does not have a full tail plane which other aircraft use on a subsonic flights to perform this task. Any swept wing designed to fly close to Mach 1; experiences changes in the pressure pattern over it, these always have the effect of moving the centre of lift rearwards as speed increases. As a result there is a tendency to pitch down, and on subsonic aircraft this is trimmed out by an upwards deflection of the elevators, or, more usually, by moving the tail-plane itself, as this cause less drag.

An aircraft such as Concorde flying at Mach 2 and above needs to do rather more, as the rearwards movement of the centre of lift is much greater. Some of the effects of the aerodynamic changes are countered by the gentle cambers and twists of Concorde’s wing, but there still remains a shift of about six feet to be accounted for. That may not sound much, but the forces involved (the lift) is opposing, and therefore equal to, the weight – which may be 170 tons at the time.

Moving the elevons up to compensate for this would obviously produce an appalling increase in drag, and would leave precious little further upward deflection available for control purposes. Instead, fuel is moved to change the internal weight distribution. In fact most of Concorde’s 95 tons of fuel is kept in tanks in the wings, but the forward two, and another in the tail cone, are used for trim as well as storage. Together they hold about 33 tons of fuel.

During supersonic flight, Centre of Gravity movement is a critical task, and therefore fuel is required to be moved around the aircraft to shift the Centre of Gravity for different speeds.

The fuel system and centre of gravity (CG) are inseparable, they are has one; CG and flight controls are also inseparable, they are as one. They are highly integrated in function but not automated in control.

The fuel is also used as a heat sink for cooling purposes. Surplus heat from the air conditioning and hydraulic systems from the constant speed drive and generator and also from the engine lubricating oil is rejected through heat exchangers to the fuel.

Fuel Tank Structure

One of the BA engineers fitting liners in the fuel tanks

Concorde’s fuel is stored in thirteen sealed tanks which are integral with the wing and fuselage structures. The fuel tanks are formed as sealed cells integral with the wing, centre fuselage and rear fuselage structure.To prevent fuel vapour entering the cabin, a vapour-seal membrane forms a double skin over the fuselage fuel tank cells. This area is pressurized and vented overboard.

Intermediate ribs and spars within the tanks reduce fuel surging and sloshing. As a result of modifications carried out between the years 2000 & 2001, tank numbers 5, 6, 7 and 8 are fitted with liners on the wing lower surface, which limit fuel leakage to a minimum in case of foreign object damage. The tanks also have structural expansion joints, located on the lower surface between the wing and fuselage, to allow for expansion and contraction of the aircraft structure caused by the thermal cycle induced by the supersonic/subsonic flight profile. The expansion joints are formed from two top hat sections which ramp down to a flat surface at either end where they attach to the spar cap flanges. The inner expansion joint forms part of the aircraft fuel tanks.

As a result of these sealed cells expanding and contracting, as part of normal flight, cracks can develop, which results in fuel seepage/leakage from the tanks. Fuel leaks are continually assessed by engineering staff and monitored in accordance with the Aircraft Maintenance Manual (AMM). They are categorised as ’seepage’ or a ‘running leak’. Seepage is assessed for an area six inches square such that once the area is wiped clean, fuel should not flow or fall in droplets for a period of 15 minutes.

For a ‘running leak’, fuel reappears immediately after the surface is wiped clean and falls in drops; the leak rate is assessed as the number of drops per minute. Allowable fuel leaks and seepage are classified by specific aircraft regions, according to risk, and are detailed in the AMM. For seepage or a running leak of less than 15 drops per minute, no immediate action is required for some areas, but frequent checks must be conducted to ensure that a leak is not worsening and repair work must carried out at the next scheduled maintenance check. For other, more critical areas with the same condition, repairs are required before the next flight.

For a ‘running leak’, fuel reappears immediately after the surface is wiped clean and falls in drops; the leak rate is assessed as the number of drops per minute. Allowable fuel leaks and seepage are classified by specific aircraft regions, according to risk, and are detailed in the AMM. For seepage or a running leak of less than 15 drops per minute, no immediate action is required for some areas, but frequent checks must be conducted to ensure that a leak is not worsening and repair work must carried out at the next scheduled maintenance check. For other, more critical areas with the same condition, repairs are required before the next flight.

The tanks are arranged in three principal groups:

- Engine Feed

- Main Transfer

- Trim Transfer

This arrangement ensures that the fuel is delivered to the engines at suitable flow rates, temperatures and pressures to satisfy all engine operating conditions. The use of a number of separate tanks, together with their internal bracing, reduces the amount of surging of the stored fuel. Because of the high climb rate of Concorde, the tanks that store fuel during the climb require to be de-aerated, to ensure that air in solution in the fuel does not become hazard.

RED TANKS – 1, 2, 3 & 4.

Only Red coloured tanks 1, 2, 3, and 4 can feed the four Olympus engines. The rest of the fuel is transferred into them, hence their alternative name “collector tanks”. If you can notice in the picture that tanks 1 and 4 are ahead of the CG with 2 and 3 behind, so no change of CG as their contents vary during flight.

BLUE TANKS 5, 6, 7 & 8

Blue coloured tanks 5, 6, 7, & 8 are the main transfer tanks. Their job is to keep the collector tanks topped up. Tank 5 and 7 are an operating pair; 5 supplies 1 and 2, while 7 looks after 3 and 4. Once again they are disposed symmetrically about the CG so that no CG changes occur during their operation – note that 5 and 7 are also accepting fuel from the tanks colored Green this fuel is part of trim Transfer. When 5 and 7 are empty, 6 and 8 take their place and function similarly. The remaining blue colored tanks in the picture, tanks 5a and 7a, are transferred into tanks 5 and 7 upon reaching Mach 2

GREEN TANKS 9, 10 & 11

Green coloured tanks are trim transfer tanks. It is their job to shift the CG aft by some 5ft during transonic acceleration, keeping it nicely matched to Centre of Pressure (CP). First, Tank 9 contents will be pumped aft to Tank 11. When that is full, the remainder of 9 will be shared between 5 and 7, where there will be room as they have been keeping the red tanks topped up since before take-off. Tank 10 will empty into 5 and 7, whereupon the CG should be just about right for Mach 2.

If you look at diagrams 1 and 2 above and below, you will see how the Fuel Quantity Indicators (FQIs) and their pumps and valves are placed within the aircraft, and then relate them to the tank location. It must be stressed that none of the fuel on Concorde is for trim purposes only; it is all usable and multitasking, as it serves to cool engine oil, generator drive oil, hydraulic oil and aircon supply.

The fuel also needs extra care too. This is to prevent the release of entrapped air during high rates of climb in thin air, the fuel in tanks not in use must be constantly agitated to provide gradual release – known as de-air process. Climbing through 42,000ft, vents are closed-off and tanks lightly pressurised to minimise evaporation losses in low pressure atmospheres.

DE-AERATION

De-aeration is provided in tank 10 by a special pump, and in tanks 11, 6, 8, 5A and 7A by normal pumps.

De-aeration is required in fuel tanks where the fuel remains static for relatively long periods during the climb. Under these conditions it is possible that as the fuel air pressure decreases, air in solution will expand causing fuel pump cavitations or transient increases in tank pressure and subsequent fuel transfer via the vent gallery.

ENGINE FEED

Each engine has its own feed system from a collector tank; however, a cross-feed system allows any engine or group of engines to be supplied from a collector tank.

Accumulator is used to provide a limited amount of fuel when a low pressure is sensed in the engine feed.

Between the Low Pressure (LP) valve and engine driven pump each feed system contains air conditioning and hydraulic heat exchangers. A fuel LP protection system, when armed, causes the fuel to the engine to bypass the air conditioning and hydraulic heat exchangers in the event of a low fuel pressure. Disarming the bypass valve circuit ensures a constant fuel flow through the heat exchangers.

IN-FLIGHT FUEL JETTISON

The fuel jettison system utilizes parts of the main trim transfer system to move fuel from the trim tanks and collector tanks to a jettison outlet at the rear of the aircraft. Two engine feed pumps in each collector tank supplies fuel to the jettison system.

The system ensures that sufficient fuel is retained for operation of the engines.

VENTING AND PRESSURISATION

The tanks vent into a ring main gallery and thence into a scavenge tank which connects to atmosphere through vents in the rear fuselage. A scavenge pump automatically removes any fuel that has entered the scavenge tank and returns it to tank 3.

At high altitudes the fuel tanks are pressurized, thus facilitating fuel pumping and preventing fuel boiling, to a maximum of between 1.2 and 1.5 psig. This increasing differential pressure is necessary to maintain a minimum absolute tank pressure with increasing altitude.

FUEL QUANTITY INDICATION (FQI)

The FQI system measures the fuel contents of the tanks by means of capacitance type gauging channels, and provides individual indication of each tank content at the Flight Engineers fuel management panel or alternatively, at the refuel control panel for refueling.

The fuel gauging information is also used to provide the following:

Total fuel indication at the centre dash panel, the fuel management panel and the refuel control panel.

Tank load limit control during trim transfer and refueling operations.

CG position indication at the pilots dash panels and fuel management panel.

CG and mach limit warnings at two levels within the defined flight envelope.

LOAD LIMIT CONTROL

During the normal mode of fuel trim transfer, fuel is pumped either from tanks 9 and 10 into tanks 11, 5, and 7 to obtain a rearward CG shift, or from tank 11 into tanks 9, 5 and 7 to obtain a forward CG shift. The trim tank contents are pre-selected on two load limit selectors, one for tank 9 and 10 and the other for tank 11. Any fuel in excess of the trim tank requirements is transferred into tanks 5 and 7. The load limit control channels are duplicated and each one automatically continues controlling should the other channel fail.

MAIN TRANSFER

The collector tanks are replenished from the main transfer tanks 5, 6, 7, & 8 in a sequence that minimizes the movement of the aircraft centre of gravity

The main transfer sequence is manually initiated using the pumps in tanks 5 and 7. Each collector tank is equipped with main and standby 115V AC electric motor driven pumps. The electrical loads of the fuel system are supplied from the aircraft’s electrical power system via circuit breakers on the distribution busbars. The main AC and DC circuit breakers are located above the equipment racks on both sides of the flight compartment; the essential AC and DC circuit breakers are located forward of the racks on the left side.

The main transfer sequence is:-

Tank 5 – Replenishment tank 1 via the left-hand pump, and tank 2 via the right-hand pump.

Tank 7 – Replenishment tank 3 via the left-hand pump, and tank 4 via the right-hand pump.

When tanks 5 and 7 are empty, the pumps in tanks 6 and 8 continue the transfer by:-

Tank 6 – Replenishment tank 1 via the left-hand pump, and tank 2 via the right-hand pump

Tank 8 – Replenishment tank 3 via the left-hand pump, and tank 4 via the right-hand pump.

Transfer of fuel from the auxiliary tanks 5A and 7A is into their respective main tanks 5 and 7.

TRIM TRANSFER

The trim transfer system is used to redistribute the fuel in the trim tanks and main transfer tanks so that the aircraft centre of gravity can be moved to optimum positions for take-off, subsonic and supersonic flight.

The trim transfer is normally automatically sequenced and controlled from the Flight Engineer’s Panel; however there is a forward transfer override control available to the Pilots for use in abnormal circumstances requiring a rapid forward transfer of fuel.

The aft trim tank (Tank number 11) has four pumps two of which PUMP GREEN and PUMP BLUE are powered by their respective hydraulic system. Thus forward transfer capability is available using electric or hydraulic power.

The trim transfer system is augmented in the aft trim condition by a reduced level operation in collector tanks 1 and 4. As tanks 1 and 4 are located well forward this moves the aircraft centre of gravity further rearward. The resultant rearward centre of gravity is the optimum for minimum trim drag in supersonic cruise

CG POSITION INDICATION

Each CG indicator is derived from any one of three channels. The main channel is the one normally used with Standby 1 and Standby 2 channels availably in the event of a main channel failure. Each channel has its own CG pack in which the CG position is computed using fuel quantity information and other fixed and manually introduced inputs.

The main channel uses fuel quantity indication from all tanks in computing the CG.

Standby 1 channel uses fuel quantity indication from only those tanks on the left-hand side of the aircraft and channel A of tanks 9, 10 and 11. For computation purposes the sum of the left-hand tank moments is doubled.

Standby 2 channel computes the CG in a similar manner to Standby 1 but uses the tanks in the right-hand side of the aircraft and channel B of tanks 9, 10 and 11.

CG & MACH LIMITS INDICATION

Bugs on the CG indicators show the forward (fwd) and rear (aft) boundaries of the CG corridor relative to Mach number. Bugs on the Machmeters show the maximum and minimum mach limits relative to the aircraft CG position. Both the CG and Mach number band widths move relative to Mach number and CG respectively.

The limits display is provided by two identical but separate channels. One is contained in the Standby 1 CG pack and serves the Captain’s CG Indicators and Machmeters.

The other channel is contained in the Standby 2 CG pack and serves the First Officers Machmeter and the Flight Engineers CG Indicators.

CG & MACH LIMIT WARNINGS

CG and Mach limits warnings are provided at two levels of CG/Mach number values. The first warning activates at a normal boundary level and the second warning activates when the normal boundary limits have been exceeded by a further margin.

The second warning level is defined as an extreme boundary. Its purpose is to indicate when corrective actions, taken at the normal boundary warning, are not producing a rapid enough correction of CG/Mach valves.

The warnings are initiated through two separate channels; one in STANDBY 1 CG pack and the other STANDBY 2 CG pack. Both channels function the master warning system, but STANDBY 1 channel activates the pilots Machmeter/CG indicator warning and STANDBY 2 channel activates the identical warnings at the First Officers Machmeter and the Flight Engineer’s CG indicator.

For further information on the fuel system, click on the links below!

FUEL TRANSFER & CG SYSTEM

Including – Trim Transfer, Main Transfer, Fuel Quantity Indicators (FQI), CG Position Indication, CG & Mach Limits Indication, CG & Mach Warnings.

Concorde Refuelling Operation

Including – Safety & Cleanliness Precautions during Refueling. Water Check Operations, Magnetic Fuel Indicator, Types of Fuel and requirements

CONCORDE FUELS

Approved Fuels and Additives for Concorde

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