the following detailed description is merely exemplary in nature and is not intended to limit the present disclosure or the application and uses of the present disclosure . furthermore , there is no intention to be bound by any theory presented in the preceding background or the following detailed description . fig1 outlines the system 2 according to various embodiments for supplying an aircraft with inert gas , which system 2 comprises a fuel cell 4 and a membrane device 6 . for operation in an aircraft , the fuel cell 4 is generally adapted for using air as an oxidant , which air can be bleed air , fresh air from an air - conditioning system , cabin exhaust air or ram air . there is thus no need for carrying along oxygen in separate high - pressure bottles . accordingly , the fuel cell 4 comprises an air inlet 8 and a fuel inlet 10 which is , for example , supplied with hydrogen ( h 2 ) or a hydrogen - containing gas mixture . during the process taking place in the fuel cell at least part of the oxygen contained in the air is consumed , and consequently oxygen - depleted or inert exhaust air subsequently flows from an exhaust air outlet 12 . the exhaust air of a fuel cell 4 is usually relatively humid because during the fuel cell process the hydrogen contained in the fuel reacts with the oxygen contained in the air , thus forming water . for the oxygen - depleted , slow - reacting inert exhaust air to be able to be used for inerting purposes it is necessary to carry out dehumidification . for this purpose the membrane device 6 with a semi - permeable or water vapor permeable membrane follows on from the exhaust air outlet 12 , with the aforesaid guiding air from an inlet 14 to an outlet 16 , wherein because of the semi - permeability the water vapor contained in the exhaust air , from the exhaust air issues through the membrane to the outside while the exhaust air does not leave the flow cross - section of the membrane device 6 between the inlet 14 and the outlet 16 . depending on the design of the membrane device 6 the oxygen - depleted exhaust air flowing from the outlet 16 is significantly drier than the inflowing air . subsequently , the dried inert exhaust air can be channeled through an inert gas inlet 18 into a region 20 of a fuel tank , which region 20 is not full of fuel , in order to prevent a potentially combustible or explosive fuel - air mixture from forming this is to be considered necessary because in particular during emptying of the fuel tanks and during a landing approach and the resulting higher ambient pressure when compared to cruise flight , this region 20 must be filled with gas or air in order to prevent negative pressure therein . fig2 shows one exemplary embodiment of a membrane device in the form of an exhaust air pipe 24 whose wall 26 at least in part comprises a suitable water - vapor - permeable membrane material . exhaust air 28 , which by way of an inlet 25 flows through the exhaust air pipe 24 to an outlet 27 , gives off dissolved water molecules by way of the membrane material 26 to the surroundings of the exhaust air pipe 24 . consequently , the exhaust air 28 is dried and exits at the outlet 27 in a drier state than it entered the exhaust air pipe 24 at the inlet 25 . the thickness of the membrane material can be selected so that the exhaust air pipe 24 withstands the mechanical loads occurring during operation of the aircraft . for alternative designs , reference is made to the explanations provided below in the context of fig4 a and 4b . the degree of drying provided by this variant strongly depends on the membrane material used , on the temperature gradients resulting between the exhaust air 28 and the surroundings of the exhaust air pipe 24 , and on the corresponding discharge of the water vapor issued through the membrane material . the latter could be improved by arranging the exhaust air pipe 24 in a particularly well - ventilated region so that there is no local concentration of water vapor or local increase in the relative atmospheric humidity on the outside of the exhaust air pipe 24 . by correspondingly routing the exhaust air pipe 24 in regions of known temperature behavior that promotes the passage of water vapor , the temperature gradient can be influenced . in particular , the temperature should be significantly above the freezing point of water . fig3 a shows a modification of the variant shown in fig2 , in which modification an exhaust air pipe 30 , whose wall 32 also is made from or comprises a membrane material , additionally encloses a fuel line 34 . the latter is adapted for channeling fuel 36 from a fuel tank 22 to fuel - consuming devices , which apart from engines , apus or the like also could comprise a reformer ( not shown ) that produces a hydrogen - containing gas from a fuel 36 comprising hydrocarbons . in one example , there is a space between the wall 32 of the exhaust air pipe 30 and the internal fuel line 34 , through which space exhaust air 28 can be conveyed from an inlet 31 to an outlet 33 generally against the direction of flow of the fuel 36 . the fuel lines in a modern aircraft are frequently already of a double - walled design , and consequently this double - walled design according to fig3 a could be modified in order to achieve advantageous passive drying of exhaust air 28 with little or even without any additional weight . at the same time the flow of oxygen - depleted air 28 could be suitable to flow around any leaks in the fuel line 34 so that any leaking fuel 36 can largely be prevented from spreading as a result of the flow of oxygen - depleted exhaust air 28 . with an adequate volume flow and pressure of the oxygen - depleted exhaust air 28 , furthermore , flowing of the fuel 36 from any leakages to the fuel tank 22 can take place and it can be ensured that the dripping - out fuel does not flow in the wrong direction . because of the exposed position of fuel tanks in an aircraft , for example integrated in wings , at times the temperature of the fuel can be very low so that there is a danger of condensed water or even ice accumulating on the outside of the fuel line 34 through which cold fuel flows . in order to avoid this , in fig3 b a modification of the exhaust air pipe 30 is shown in which the interior fuel line 34 is enclosed by an insulation layer 38 , followed at a certain distance by the wall 32 . the insulation layer can be made from any material suitable for thermal insulation . in one example , in order to maintain a constant external diameter the use of a foamed plastic material of a certain rigidity suggests itself , which plastic material should nevertheless be as flexible as possible in order to allow easy installation of this double - walled line construction . in order to prevent fuel 36 from being absorbed by the insulation layer 38 in the case of a leakage in the fuel line 34 the insulation layer 38 should in one example , be made as closed - cell foamed plastic . as a result of the uniform diameter the conveyance of fuel is not impeded by the inert gas 28 . the membrane material to be used can be relatively thin in order to be able to provide particularly effective giving - off of water vapor . however , in the context of installing an exhaust air pipe comprising a membrane material it should be noted that the strength of this membrane material is not sufficient as the sole wall material of an exhaust air pipe . purely as examples fig4 a and 4b show that an exhaust air pipe thus need not exclusively comprise a thin membrane material , but instead for reinforcement could comprise a suitable frame structure that can absorb the mechanical loads arising while nevertheless allowing a degree of shaping . as an example fig4 a shows an exhaust air pipe 40 that comprises a frame structure with stiffening elements 42 extending in the axial direction and with radial stiffening rings 44 spaced apart from each other in the axial direction , over which radial stiffening rings 44 a membrane material 46 , which for example is relatively thin , has been stretched , which membrane material 46 defines an inlet 41 and an outlet 43 . in a manner that differs slightly from the above , fig4 b shows an exhaust air pipe 48 with an inlet 49 and an outlet 51 , wherein the exhaust air pipe 48 comprises a lattice structure 50 over which also a membrane material 46 has been stretched . the exhaust air pipes 40 and 48 can of course also be used with double - walled designs as shown in fig3 a and 3b . fig5 shows a further variant of a membrane device in the form of a gas - gas dehumidifier 52 . said dehumidifier 52 comprises an exhaust air inlet 54 , an exhaust air outlet 56 , a gas inlet 58 and a gas outlet 60 . between the exhaust air inlet 54 and the exhaust air outlet 56 again an exhaust air pipe 62 extends whose wall 64 is made of or comprises a membrane material . water vapor dissolved in the through - flowing exhaust air 28 passes through the wall 64 into a housing 66 that encloses the exhaust air pipe 62 . the gas flowing through the gas inlet 58 into the housing 66 takes up the water vapor and in a humidified state exits to the outside through the gas outlet 60 . in this arrangement it is not necessary to accommodate an exhaust air pipe 62 in a ventilated region ; instead , a gas that flows in the aircraft anyway , for example from an air - conditioning system , could be used to take up the water vapor . with the above it is possible , for example , to supply slightly humidified air to a space to be air conditioned . of course , the exhaust air pipes from fig2 to 4b that are used in such a gas - gas dehumidifier 52 can be situated upstream or downstream of the aforesaid in order to carry out dehumidification that is as ideal as possible . the arrangement of exhaust air pipes in an aircraft is particularly located in heavily ventilated regions , in the case of a modern commercial aircraft in particular in a triangular region 68 , in other words between an outer skin 70 and a lateral delimitation 78 of a cargo compartment 72 underneath a cabin floor 74 . at that location air is sucked , by way of one or several recirculation fans , from a cabin that follows on at the top , from where at least some of the air enters the ventilation circuit again . in one example , a drainage line 76 can be situated in the triangular region 68 so that the issuing humidity is admixed to the cabin exhaust air in order to be partially recirculated . lastly , fig7 shows an aircraft 80 which , for example , in an aft region comprises a fuel cell 4 that is connected to an exhaust air pipe 82 , wherein this exhaust air pipe 82 , for example , extends forward towards the triangular region 68 . fuel tanks 86 are located in wings 84 , wherein the regions of said fuel tanks 86 , which regions are not filled with fuel , are to be inerted . the exhaust air pipe branches in a nodal point 88 and conveys the inert gas to the individual tanks 86 . the particular exhaust air pipe 82 is , for example , designed according to the characteristics shown in fig2 to 4b or , in a manner that differs from the above , can comprise a conventional line material , wherein in this case a gas - gas dehumidifier 52 follows . as an alternative to this it would also be possible to use a combination of an exhaust air pipe from fig2 to 4b and of a gas - gas dehumidifier 52 . while at least one exemplary embodiment has been presented in the foregoing detailed description , it should be appreciated that a vast number of variations exist . it should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples , and are not intended to limit the scope , applicability , or configuration of the present disclosure in any way . rather , the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment , it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the present disclosure as set forth in the appended claims and their legal equivalents .