This invention relates to aircraft fuel tank systems and to methods for inerting aircraft fuel tanks. Regulations require that the fuel tanks of civil aircraft are rendered inert, that is to say that the flammability hazard posed by the fuel tank is reduced, by maintaining the oxygen concentration below a pre-set limit— typically 11.9% oxygen by volume at sea level but can vary from 9% to 12%. It will be appreciated that an inflow of gas into the aircraft fuel tank is required both to make up for the burn rate of the fuel exiting the tank, and also to maintain the pressure differential across the tank within the structural design limits during descent. The mass flow rate required during descent is therefore relatively high as a substantial mass is required to pressurize the tank.

Aircraft are fitted with systems to reduce the flammability of the mixture of gas and fuel vapour that exists in the (ullage) space above the liquid fuel within the tanks. A means of flammability reduction is to replace the air within the ullage space with Nitrogen Enriched Air (NEA), in which the concentration of nitrogen is greater than ambient air and the concentration of oxygen is lower. A preferred type of system is the "continuous flow" as opposed to the store gas system.

In the continuous flow system type the NEA is generated on demand by an air separation device. In stored gas systems nitrogen or NEA is held in storage vessels as compressed gas or by physical adsorption in a medium. The Continuous Flow type of system tends to be lighter and less complex than the stored gas class of system The size of a continuous flow system may be minimised by filling the fuel tanks with NEA having a low concentration of oxygen prior to the period of descent or rapid descent and supplying sufficient NEA to the tanks to ensure that the bulk average of the oxygen concentration at the end of descent is less than the pre-set limit. A problem with this sizing condition is that in the case where the flow of NEA is insufficient to prevent the inflow of air, the air enters the tank at one or more vent valves (typically two per tank). Consequently in the zones local to the vent port or valve the oxygen concentration is greater than the pre-set limit creating an ignition "pocket". Such a pocket tends to be concentrated by the wing ribs or any internal structure or baffles.

There is therefore a need for an aircraft fuel tank inerting system capable of overcoming or mitigating at least some of the disadvantages of existing systems.

According to one aspect of the invention there is provided an aircraft fuel tank system comprising at least one fuel tank comprising a number of interconnected bays and a vent arranged to allow the inward venting of atmospheric air, the system being arranged to direct inwardly vented air along a vent duct, the duct being arranged to convey inwardly vented air to substantially each of the interconnected bays.

Advantageously, according to this aspect of the invention, the inwardly vented air may be effectively distributed across the ullage of the tank, resulting in smaller pockets of increased oxygen levels that mix relatively quickly in the ullage gases; thus reducing risk. This may be contrasted with known systems.

One such system is shown in Figure 1. Figure 1 shows a fuel tank 20, which is divided into inter-connecting bays by wing ribs and/or tank baffles 22. The fuel tank also includes a duct 28 for carrying nitrogen-enriched air. Venting to the external atmosphere occurs via a vent box 34 and a vent distribution duct 36 leading to a vent valve 38 located in the fuel tank. Generally, during inward venting a volume of oxygen rich gas 21 may develop in the region of the vent valve. This may exceed the pre-set oxygen concentration limit, posing a risk to the aircraft, for some time until the oxygen dissipates throughout the ullage.

Another such system is one for mixing of the incoming vent gas with the NEA already in the fuel tank ullage space, through the use of a jet pump or ejector, the secondary flow of which is arranged to pick up the incoming vent gas. A disadvantage of this arrangement is that the zone between the vent float valve inlet and the suction inlet of the jet pump and the zone at the exit of the jet pump ducting are generally flammable and of uncontrolled magnitude.

In one aspect the invention may distribute vent air inflow to an inert aircraft fuel tank using a float valve to direct the inflowing air into the fuel tank structure so that it is directed by "stringers" that form the wing structure between ribs or equivalent structure or pipework throughout the fuel tank and mixed evenly. The efficient mixing of vent air inflow reduces the demand on the fuel tank inerting system.

In one aspect an unmodified, conventional float valve that can be purchased from the Applicant is used in combination with an adapter plate which allows it to be combined with and interact with the structure or pipework of embodiments of the invention.

The invention allows for an inexpensive method of augmenting the inerting capabilities of an aircraft fuel system. It may be conveniently retrofitted and may reduce the load placed on a NEA system allowing for a reduced capacity NEA system to be deployed, reducing costs and weight.

The invention may be performed in various ways, and embodiments thereof will now be described by way of example only, with reference being made to the accompanying drawings, in which:

Figure 1 is a schematic view of a prior art aircraft fuel tank inerting system;

Figure 2 is a schematic view of an aircraft fuel tank inerting system in accordance with a first embodiment of the invention;

Figure 3 is a schematic view of an aircraft fuel tank inerting system in accordance with a second embodiment of the invention; Figure 4 schematically illustrates a third embodiment of the invention;

Figure 5 illustrates a vent valve and vent duct in accordance with the first and second embodiments of the invention;

Figure 6 illustrates an alternative configuration of vent valve, adapter plate and vent duct assembly for use with the first and second embodiments of the invention; Figures 7a and 7b illustrate schematically cross sectional views of stringer assemblies for use in the first or second embodiments.

Figure 8 illustrates the loss in energy of inwardly vented air directed directly at the wing skin;

Figures 9 and 10 illustrate deflectors that may be deployed in embodiments of the invention in order to direct the inwardly vented air along the length of a vent duct.

Referring to Figure 2 an embodiment of the present invention is illustrated. In this embodiment, a fuel tank 20 is schematically illustrated. The fuel tank is divided into inter-connecting bays (or volumes), separated by wing ribs and/or tank baffles 22. The fuel tank has an upper surface 24 and a lower surface 26. Upper surface 24 and lower surface 26 may be the upper surface and the lower surface respectively of the aircraft wing. The fuel tank also includes an NEA duct 28. The source of NEA may be any suitable form of air-separation device, typically implementing Hollow Fibre Membrane technology. The aircraft fuel tank 10 is connected to a source of NEA via a control valve (not shown). The term "nitrogen-enriched air" or NEA is used in this specification to mean air which has passed through an air-separation device including separation means intended to increase the nitrogen content in the air with a commensurate reduction in the oxygen content. NEA passes along the duct 28 whilst the inerting system is operational and passes via orifices 30 in the duct to the main volume of the fuel tank.

The tank is vented to the external atmosphere. Inwardly vented air 32 passes from the external atmosphere via a conventional vent box 34 and a vent distribution duct 36 to a vent valve 38 located in the fuel tank. It will be appreciated that the same vent arrangement may also be used to allow outward venting. In this embodiment the vent valve is a conventional float valve, which is shown in more detail in Figures 5 and 6. Other suitable valves may instead be used. In the position illustrated in the figure, the float valve is in the open position allowing fluid communication between the vent distribution duct 36 and a vent duct 42. As the level of fuel approaches the float of the float valve, the buoyancy of the float causes the valve to close the vent distribution duct 36 at point 40, sealing the tank from the external atmosphere. In some embodiments however, a controlled leakage at the interface between the vent valve and vent distribution duct 36 is acceptable.

The inward venting air is evenly distributed throughout the fuel tank by the vent duct 42 that is fed by the vent valve. In this implementation the vent duct is formed by a stringer or stringers of the wing, to form a substantially closed conduit, or pipe, examples of which are shown in Figures 7a and 7b. Figure 7a shows in cross section two conventional "I" section stringers, 62 and 64. The stringers 62 and 64 are arranged adjacent to and connected to the aircraft wing skin 56 and a closure surface 68 to form an enclosed volume 70 there between. Figure 6b shows a box shaped stringer in cross section having four sides 72, again enclosing a volume 70.

The enclosed stringers include holes 48 (illustrated in Figures 2 and 3) in the surface 68, 74 opposite the wing skin. The holes 48 meter the incoming air into each zone formed by the wing ribs or baffles such that the bulk average oxygen concentration within each zone is less than the pre-set limit. The distribution of the incoming air 50, which may occur at vent locations on descent, gives rise to multiple relatively small and localised volumes of increased oxygen content. This enables an even distribution and rapid mixing of the inwardly vented oxygen with the existing NEA in the ullage, limiting any risk of ignition to a short period during vent inflow. Moreover, this helps ensure that in the event of any local ignition of an oxygen pocket, by an electrical fault or impact damage or other means, the ignition does not propagate throughout the fuel tank and limits the resulting overpressure to less than the structural capability of the fuel tank. In the present embodiment, the vent duct 42 is closed at both ends 44 and 46 The number and/or size of the orifices 48 machined in it is arranged such that the volume of inwardly vented air entering the main volume of the fuel tank between each pair of wing ribs and/or tank baffles 22 is proportional to the volume between the respective pair of wing ribs and/or tank baffles 22. More generally, the flow characteristics of the orifices 48 may be chosen such that the localised volumes of increased oxygen content 50 mix with the existing NEA in the ullage and return to a level less than the pre-set limit at approximately the same rate; thereby further limiting the risk of combustion.

Whist in this embodiment the vent duct 42 is formed from a stringer, in other embodiments longerons or other stiffening or loadbearing structures may alternatively be used. Alternatively the vent duct 42 may be formed by a pipe or tube in the fuel tank that serves no other function.

Whist in this embodiment the vent duct 42 is arranged such that the inwardly vented air is delivered via the orifices 48 to each of the interconnected bays between each pair of wing ribs and/or tank baffles 22 this may be varied in other embodiments. In other embodiments, inwardly vented air may be delivered to the majority or more of the bays, or other proportion of the total.

Referring to Figure 3 a second embodiment of the present invention is illustrated. The second embodiment is similar to the first embodiment. In this and subsequent embodiments, like features are not further described and the same reference numbers are used. In this embodiment the NEA is mixed with the incoming air 32 in the vent box 34 prior to being delivered to the fuel tank when inward venting of atmospheric air occurs. When no inward venting of atmospheric air occurs the NEA may be routed directly to the tank, under the control of a control system (not shown). The premixing of the inwardly vented air with the NEA gives rise to an advantage over the system of the first embodiment in that the inwardly vented air 32 is of lower oxygen content on arrival in the fuel tank. Consequently it takes less time fall below the pre-set oxygen limit figure, through the process of mixing with ullage gases, than would otherwise be the case. Referring to Figure 4, a third embodiment of the present invention is illustrated. As can be seen from the figure, two "I" section stringers 54 are shown connected to the wing skin 56. Unlike the first and second embodiments, the space between the members 54a of the stringer is left open, illustrated in the figure by the line 57. Thus, as the inwardly vented air 60 travels along the length of the stringer, mixing will occur along the plane or surface 57 between the inwardly vented air 60 and the other gas in the ullage. The characteristics of the system (such as cross sectional area between the stringers, the length of the stringers and the velocity and flow characteristics of the inwardly vented air) will determine the maximum length of stringer assembly that may be traversed by the inwardly vented air. This in turn affects the degree to which the inwardly vented air is distributed across and mixed with the ullage. Where the maximum distance is insufficient to meet requirements, the mixing surface 57 may be narrowed to reduce the degree of mixing per unit length of the vent duct, thus increasing the distance which the inwardly vented air travels along the vent duct. This may be achieved by fixing a strip of material to, and along the length of one of the members 54a. The advantage of leaving an open slot in the vent duct assembly is that the machining operations to create the orifices 48 of Figure 2 may be avoided. Referring now to Figure 5, the vent valve 38 (a conventional float valve) is shown in more detail. As is conventional with such valves, it has a float portion 62 and a sealing portion 64, arranged to pivot as a fixed assembly about pivot 66. In the position shown the sealing unit closes the valve. When the valve is open it is arranged to connect the vent distribution duct 36 and the vent duct 42 via an air flow passage in its cast aluminium bo dy 68.

The vent valve 38 may require an adapter plate to allow it to be connected to the vent duct 42. Such an adapter 70 is illustrated in Figure 6 and may be manufactured from formed aluminium. An adapter plate enables the use of a commercial off the shelf vent float valve. The adapter plate may optionally provide a deflector, or shroud 70a which aerodynamically efficiently directs the flow of incoming vent gas along the duct with low pressure loss. It will be understood therefore that in embodiments of the invention a float valve is employed which has its outlet into the tank interfaced, either with an adapter or without an adapter, with a duct arranged to carry and/or distribute inwardly vented air into the fuel tank. This may be necessary in such a system where preventing the loss of fuel from the tank is required.

Referring now to Figure 8, a longitudinal section of a stringer such as that of Figure 4 is shown. It will be understood that in the system of the third embodiment, where the inwardly vented air is directed directly at the wing skin (as is illustrated by the arrow in the figure), a significant portion of the kinetic energy of the air is lost. This results in the air 60 travelling a reduced distance along the length of the stringer assembly. This in turn may result in the inwardly vented air not being adequately distributed across and mixed with the ullage gas.

Figures 9 and 10 each schematically illustrates a deflector, or shroud 76 which may be attached to the vent valve 38 (Figure 9) or to the vent duct 42 (Figure 10) and serves to re-direct the inwardly vented air along the length of the vent duct 42 as illustrated by the arrows 78. Preferably the deflector 76 is so shaped as to aerodynamically efficiently redirect the inwardly vented air. In this manner the distance along the vent duct 42 that the inwardly vented air may travel is increased, giving rise to an improved distribution across and mixing with the ullage gas.

Whilst the present invention has been described with reference to particular embodiments, the skilled reader will understand that the scope of the present invention extends beyond these described embodiments. For example:

The first or second embodiments may be modified to give rise to an alternative arrangement employing smaller vent valves at each vent outlet, or orifice 48, along the vent distribution duct instead of a single vent valve 38. This solution may provide improved redundancy. However it may increase the probability of a dormant failure of one or more of the multiplicity of small vent valves. Any of the preceding embodiments may be modified by eliminating the vent valve 38. Aircraft fuel systems exist in which vent ports have no float valve (or other equivalent valve) fitted. It will be understood that the present invention may equally be applied to such systems, particularly those employing a deflector, or shroud, as described above, for efficiently directing the flow of incoming vent gas along the duct with low pressure loss.

Any of the preceding embodiments may be modified by routing the NEA or other inerting gas, such as nitrogen, via the vent duct. This allows for the elimination of the tubes or pipes used in distributing the NEA, such as the NEA duct 28 shown in Figure 2, with associated weight gains. In such a modified system the NEA or other inerting gas might enter the vent duct either or upstream or downstream of the vent valve (to the extent one is employed). As an alternative to this arrangement a separate NEA plenum formed by a stringer duct could be used. In this manner the NEA, or other inerting gas such as nitrogen, could be routed by a further duct formed by one or more stringers or other structure as described in the embodiments above. In this arrangement, two or more stringer type ducts could be used in a fuel tank, one to convey NEA to the fuel tank and another to convey inwardly vented air. In such an arrangement the NEA flow would be separate to the vent flow which would avoid premature mixing of the two gas flows. As a further alternative to the above arrangement NEA may be routed along a duct that is housed inside the vent stringer duct carrying vent air (42). In this arrangement, and the NEA duct may comprise holes placed to ensure the correct mixing of the NEA and the inwardly vented air. Such holes may be aimed upwards, or towards the wing skin forming part of the fuel tank enclosure.

- In yet a further embodiment of the invention a single stringer duct could be employed in a fuel tank or an aircraft, the stringer duct being arranged to carry NEA but not inwardly vented air.