The invention relates to fuel systems for aircraft, particularly, although not exclusively, fixed-wing aircraft.

Aircraft flying for extended periods of time in low temperatures may be subject to freezing of fuel within fuel storage tanks. In very cold environments this can be a particular problem at lower altitudes as a result of the higher air pressure and air density that occur at lower altitudes, compared to higher altitudes, which leads to more rapid heat loss. In fixed-wing aircraft having fuel storage tanks integrated into the wings, fuel freezing within such tanks during extended flying in low temperatures is very likely. This is a particular problem where the wings have a low aspect ratio, due to the relatively high surface area of the wings in relation to fuel tank volume.

The present invention provides an aircraft fuel system characterised by a fuel storage tank arrangement, a collector fuel tank arrangement, a first fuel line connecting the fuel storage tank arrangement to the collector fuel tank arrangement and a second fuel line connecting the collector fuel tank arrangement to an aircraft engine in use of the system, wherein the system further comprises a first heat-transfer arrangement arranged to transfer heat energy within unspent fuel output from the aircraft engine to fuel within the fuel storage tank arrangement.

In use of a fuel system of the invention, heat energy within unspent fuel output by the aircraft engine is used to heat fuel within the fuel storage tank arrangement, instead of simply being wasted. This reduces the likelihood of fuel freezing within the fuel storage tank arrangement, without the need for dedicated heating and/or insulation apparatus. Preferably the system further comprises a second heat-transfer arrangement arranged to transfer heat energy from aircraft electronic and/or computer equipment to fuel within the fuel storage tank arrangement. This provides additional heating of fuel within the fuel storage tank arrangement using further waste heat from aircraft electronic and/or computer equipment and further reduces the likelihood of fuel freezing within the fuel storage tank arrangement.

A third fuel line may connect the aircraft engine to the collector fuel tank arrangement in use of the system, whereby at least a portion of unspent fuel output by the aircraft engine is returned to the collector fuel tank arrangement, the first heat-transfer arrangement comprising:

(i) a fuel line arrangement arranged to draw fuel from, and return that fuel to, the fuel storage tank arrangement; and

(ii) a heat-exchanger arranged to thermally couple fuel in the third fuel line to fuel in the fuel line arrangement.

This represents one convenient arrangement for transferring heat within unspent fuel to fuel in the fuel storage tank arrangement.

The fuel storage tank arrangement may comprise a fuselage fuel tank and a wing fuel tank connected by a fuel line, the first fuel line connecting the fuselage fuel tank to the collector tank arrangement, and the fuel line arrangement being arranged to draw fuel from, and return that fuel to, the wing fuel tank. In cases where the fuel storage tank arrangement comprises a wing fuel tank and a fuselage fuel tank, it is fuel in the wing fuel tank that is most susceptible to freezing because the limited space within an aircraft wing means that measures such as providing insulation may be difficult or impossible to apply. In other embodiments, the first heat-transfer arrangement may comprise a third fuel line connecting the aircraft engine to the fuel storage tank arrangement in use of the system, whereby at least a portion of unspent fuel output by the aircraft engine is returned directly to the fuel storage tank arrangement. This is a particularly efficient arrangement for transferring heat within unspent fuel to the fuel storage tank arrangement because heat within unspent fuel is transferred to the fuel storage tank arrangement by movement of the unpsent fuel itself. In these embodiments, in cases where the fuel storage tank arrangement comprises a fuselage fuel tank and a wing fuel tank connected by a fuel line, the first fuel line may connect the fuselage fuel tank to the collector fuel tank arrangement, the third fuel line connecting the aircraft engine to the wing fuel tank. Again, this allows heat within unspent fuel to be tranferred to the wing fuel tank, rather to to the fuselage fuel tank, which is typically less likely to suffer from fuel freezing.

The second heat-transfer arrangement conveniently comprises a fuel line arrangement arranged to draw fuel from, and return that fuel to, the fuel storage tank arrangement and a heat-exchanger arranged to couple heat from the aircraft electronic and/or computer equipment to fuel within the fuel line arrangement.

Preferably the heat-exchanger is in thermal contact with a compressor arranged to compress a refrigerant fluid within a cooling circuit for cooling the aircraft electronic and/or computer equipment. This allows waste heat collected by the refrigerant to be efficiently coupled to the heat exchanger in addition to waste heat from the compressor itself.

In cases where the fuel storage tank arrangement comprises a fuselage fuel tank and a wing fuel tank connected by a fuel line, preferably the the first fuel line connects the fuselage fuel tank to the collector tank arrangement, the fuel line arrangement being arranged to draw fuel from, and return that fuel to, the wing fuel tank. Again, this provides for waste heat to be directed to the wing fuel tank which is more susceptible to fuel freezing.

In any embodiment, the collector fuel tank arrangement may be either a simplex collector tank or a duplex collector tank.

It should be understood that the phrase "fuel line" in this specification may indicate not only a single physical fuel line or fuel pipe, but possibly also a fuel route comprising two or more individual fuel lines or fuel pipes, including any appropriate valves, arranged in combination to transport fuel within the fuel system. Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings in which Figures 1 and 2 each schematically show a respective example aircraft fuel system of the invention. Referring to Figure 1 , a fuel system 100 of the invention comprises a fuel storage tank arrangement comprising a fuselage fuel tank 102 and two wing fuel tanks 104, 106 each of which are connected to the fuselage fuel tank by a respective fuel line 107, 109. The fuselage fuel tank 102 is connected to a collector fuel tank 108 by a first fuel line 101. Fuel is provided to aircraft engines in use of the system 100 from the collector fuel tank 108 via second fuel lines 103A, 103B. Unspent fuel output from the aircraft engines is returned to the collector fuel tank 108 during operation of the system 100, via third fuel lines 105A, 105B which pass through a heat exchanger 110. A fuel line arrangement consisting of individual fuel lines 111A, 111 B, 111 C is arranged to draw fuel from the wing fuel tanks 104, 106 and to return that fuel to the wing tanks 104, 106. The fuel line 111 B passes through the heat exchanger 110 which is arranged to couple heat from unspent fuel output by the aircraft engines within fuel lines 105A, 105B to fuel within the fuel line 111 B. In other words fuel drawn from the wing fuel tanks 104, 106 is heated using heat within unspent fuel output by the aircraft engines, and then returned to the wing fuel tanks 104, 106, thus reducing the likelihood of fuel freezing within the wing fuel tanks 104, 106.

The system 100 further comprises a secondary heater exchanger 112 arranged to couple waste heat from aircraft electronic and/or computer equipment 118 to fuel within the fuel line 111 B. Waste heat from the electronic and/or computer equipment 118 is conveyed to the secondary heat exchanger 112 via refrigerant within a cooling circuit 113. Heat within the refrigerant is coupled to the heat exchanger 112 upon compression within a compressor 116 which is in thermal contact with the heat exchanger 212. An air condenser 114 within the cooling circuit 113 provides a cold source to the electronic and/or computer equipment 118 in the event that fuel drawn from the wing tanks 104, 106 is very hot and/or there is little fuel in the wing tanks 104, 106 such that fuel within fuel line 111 B cannot receive more heat.

In operation of the system 100, fuel drawn from the wing tanks 104, 106 is heated within fuel line 111 B using waste heat within unspent fuel output from the aircraft engines, and also using waste heat from the aircraft electronic and/or computer equipment 118, and then returned to the wing fuel tanks 104, 106, thus reducing the likelihood of fuel freezing within these fuel tanks during extended flying in cold environments.

Figure 2 shows a second example fuel system of the invention indicated generally by 200. In Figure 2, parts corresponding to parts in the system 100 of Figure 1 are given reference numerals corresponding to those labelling the equivalent parts in Figure 1. A fuel line arrangement consisting of individual fuel lines 211 A, 211B, 211C is arranged to draw fuel from wing fuel tanks 204, 206 and to return it to these tanks. Fuel line 211 B passes through a heat exchanger 212 which couples waste heat from aircraft electronic and/or computer equipment 218 to fuel in fuel line 211 B by means of a cooling circuit 213 comprising refrigerant fluid which is compressed by a compressor 216, the compressor 216 being in thermal contact with the heat exchanger 212. The system 200 of Figure 2 operates similarly to the system 100 of Figure 1 , except that unspent fuel output by the aircraft engines is returned directly to wing fuel tanks 204, 206, via fuel lines 215A, 215B, 211C, rather than being returned to the collector fuel tank 208. A heat exchanger for thermally coupling fuel within fuel lines 215A, 215B to fuel in fuel line 211 B is not required, since waste heat within unspent fuel from the aircraft engines is transferred to the wing fuel tanks 204, 206 by directly returning the unspent fuel to these fuel tanks.