Abstract:
The invention provides an aircraft fuel system comprising a fuel storage facility for storing fuel and ullage, a separate ullage storage facility for storing ullage, and a transfer arrangement for transferring ullage between the fuel storage facility and the ullage storage facility, wherein the transfer arrangement is capable of controlling the transfer of ullage based on a pressure input to the transfer arrangement. The invention also provides an aircraft with such a fuel system and a method of operating an aircraft.

Description:
RELATED APPLICATIONS 
     The present application claims priority from Great Britain Application Number 1412311.1, filed Jul. 10, 2014, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an aircraft fuel system comprising a fuel storage facility for storing fuel and ullage. The present invention also relates to an aircraft with such a fuel system and a method of operating an aircraft. 
     Aircraft fuel is stored in one or more aircraft tanks. Air that contains fuel vapour, such as the air above the fuel in the fuel tanks, is known as ullage. When an aircraft is refuelled, the pressure in the fuel tanks is high and, in order to prevent a large difference in pressure between the tank and the atmosphere (to prevent damage to the wing), ullage is expelled into the atmosphere. Similarly, upon aircraft ascent, ullage is expelled to the atmosphere. This causes fuel vapour to be emitted, which may be harmful to the environment. 
     Upon aircraft descent, especially after fuel has been depleted, in order for the pressure in the fuel tanks to be increased to reduce the pressure difference between the fuel tanks and the atmosphere, air is injected back into the fuel tanks. 
     In addition, the air that has been injected back in is often treated to reduce its oxygen content so as to maintain low levels of oxygen in the aircraft tanks and to reduce the flammability risk of the fuel tanks. The inerting system often is required to produce a large amount of inert (low oxygen level) air upon descent and this results in a weight, drag and power consumption penalty for the aircraft. Also, when air is injected back into the tanks, water is often ingested too. This is because the humid outside air is drawn into the tanks and the water then condenses on the fuel and the cold surfaces within the tank. This leads to regular water drain maintenance being required and may result in fuel system equipment failures due to, for example, water ingress into valves and pressure switches, or switches seizing up or seals malfunctioning due to freezing. 
     An example of an inerting system is shown in EP 2439141. This discloses an aircraft fuel system where ullage is taken from the fuel tank, using engine bleed air, processed through a main catalytic unit, to reduce its oxygen content, and returned to the fuel tank. The engine bleed air is used to control the speed of ullage flow through the main catalytic unit, based on the temperature in the unit. In other words, the ullage flow is caused to bypass the main catalytic unit and return to the fuel tank (to reduce the amount of reaction occurring) if the main catalytic unit is getting too hot. The inerting system is known as a GOBIGGS (Green On Board Inert Gas Generation System). 
     The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft fuel system. 
     SUMMARY OF THE INVENTION 
     The present invention provides, according to a first aspect, an aircraft fuel system comprising a fuel storage facility for storing fuel and ullage, a separate ullage storage facility for storing ullage, and a transfer arrangement for transferring ullage between the fuel storage facility and the ullage storage facility, wherein the transfer arrangement is capable of controlling the transfer of ullage based on a pressure input to the transfer arrangement. 
     Such an arrangement allows ullage to be stored on the aircraft and can be resupplied to the fuel storage facility when required. This reduces the burden on an inerting system, as the ullage replaced back into the fuel storage facility from the ullage storage facility has a lower oxygen content than air that would otherwise have had to be taken from the atmosphere. This gives a weight, drag and power consumption benefit. It also reduces the amount of water ingested into the fuel storage facility, as the ullage replaced back into the fuel storage facility from the ullage storage facility has a lower water content than air that would otherwise have had to be taken from the atmosphere.This reduces the water-related maintenance burden. This could be a large cost saving, as water-related maintenance of a fuel system could be as much as half of the total fuel system maintenance cost. This arrangement also reduces the amount of fuel vapour emitted to the atmosphere. 
     The transfer arrangement is capable of controlling the transfer of ullage based on a pressure input to the transfer arrangement. This allows the natural pressure changes caused during flight to be used to transfer the ullage between the fuel storage facility and the ullage storage facility. 
     If an inerting system is used, this could be used on the ullage in the ullage storage facility during flight and before the stored ullage is needed. Hence, the use of the inerting system could be optimised and the weight or size etc. of the inerting system could be reduced. 
     Preferably, a pressure-controlled valve arrangement of the transfer arrangement is capable of controlling the transfer of ullage based on a pressure input to the valve arrangement. Preferably, the pressure-controlled valve arrangement comprises a number of valves that are opened and closed based on the pressure input. 
     More preferably, the pressure-controlled valve arrangement is designed to control the transfer of ullage based on a continually changing pressure input. 
     Preferably, the pressure input is an indication of the pressure in the fuel storage facility. The pressure input may also include an indication of atmospheric pressure. 
     More preferably, the pressure input is a pressure measured in the fuel storage facility. 
     Alternatively or additionally, the pressure input is based on an ambient pressure and an amount of fuel usage. 
     Preferably, the pressure-controlled valve arrangement is designed to transfer ullage from the fuel storage facility to the ullage storage facility when the pressure input is relatively high and to transfer ullage from the ullage storage facility to the fuel storage facility when the pressure input is relatively low. 
     Preferably, the transfer arrangement is capable of only allowing the transfer of ullage to the ullage storage facility and not to allowing the transfer of fuel to the ullage storage facility. This prevents fuel from transferring from the fuel storage facility to the ullage storage facility. This means the ullage storage facility does not have to be designed to contain fuel. 
     More preferably, the transfer arrangement comprises a valve, such as a float valve, for allowing transfer of ullage, but not transfer of fuel, from the fuel storage facility to the ullage storage facility. 
     Preferably, the aircraft fuel system further comprises a pump for effecting transfer of the ullage between the fuel storage facility and the ullage storage facility. 
     Preferably, the ullage storage facility comprises a variable volume ullage container. For example, the variable volume ullage container may be an expandable bladder. 
     Preferably, the aircraft fuel system further comprises a compressor for compressing the ullage and wherein the ullage storage facility comprises a pressurisable container for storing the compressed ullage. 
     More preferably, the ullage storage facility further comprises an intermediate container connected between the pressurisable container and the pressure-controlled valve arrangement. This allows easier ullage transfer from the pressurisable container to the fuel storage facility. 
     Preferably, the fuel storage facility comprises a surge container for accommodating excess fuel. 
     Preferably, the aircraft fuel system further comprises a climb-dive valve for ensuring a limited pressure difference between a pressure in the fuel storage facility and an atmospheric pressure. 
     Preferably, the aircraft fuel system further comprises an inerting system for reducing the oxygen content of the ullage. 
     More preferably, the inerting system is connected to the ullage storage facility so as to act upon the ullage in the ullage storage facility to reduce its oxygen content. 
     According to a second aspect of the invention there is also provided an aircraft comprising the aircraft fuel system as described above. The aircraft comprises engines to power the aircraft and wherein the engines are arranged to be fuelled by the aircraft fuel system as described above. 
     According to a third aspect of the invention, there is provided a method of operating an aircraft, the method comprising the following steps supplying fuel to a fuel storage facility of an aircraft fuel system of the aircraft, transferring at least some ullage from the fuel storage facility, through a transfer arrangement, to a separate ullage storage facility, based on a pressure input to the transfer arrangement, storing at least some of the transferred ullage in the ullage storage facility, and transferring at least some of the stored ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility, based on the pressure input to the transfer arrangement. 
     Preferably, the transfer of ullage between the fuel storage facility and the ullage storage facility is controlled by a pressure-controlled valve arrangement, based on a pressure input to the pressure-controlled valve arrangement. 
     Preferably, the pressure input is an indication of the pressure in the fuel storage facility. 
     Preferably, the method comprises the step of flying the aircraft, thereby using up some of the fuel in the fuel storage facility and thereby reducing the pressure in the fuel storage facility, and then transferring at least some of the stored ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility, based on the pressure input to the transfer arrangement. This allows the pressure in the fuel storage facility to be maintained at an acceptable level, despite the reduction in pressure due to the fuel usage. 
     Preferably, the method comprises the step of climbing the aircraft to an increased altitude level, thereby increasing the pressure in the fuel storage facility relative to atmospheric pressure, and, at the increased altitude level, transferring at least some ullage from the fuel storage facility, through the transfer arrangement, to the ullage storage facility, based on a pressure input to the transfer arrangement. The transferring of the ullage is preferably done as a result of the increase in relative pressure of the fuel storage facility. In other words, the pressure input reflects the increase in relative pressure of the fuel storage facility. 
     Preferably, the method comprises the step of descending the aircraft to a reduced altitude level, thereby decreasing the pressure in the fuel storage facility relative to atmospheric pressure, and, at the reduced altitude level, transferring at least some ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility, based on a pressure input to the transfer arrangement. The transferring of the ullage is preferably done as a result of the decrease in relative pressure of the fuel storage facility. In other words, the pressure input reflects the decrease in relative pressure of the fuel storage facility. 
     More preferably, the method comprises the step of in-taking atmospheric air from outside the aircraft into the fuel storage facility at a first altitude and then allowing transfer of the at least some of the stored ullage from the ullage storage facility, through the transfer arrangement, to the fuel storage facility at a second, lower altitude. 
     It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: 
         FIG. 1 a    shows a schematic rear view of part of an aircraft according to a first embodiment of the invention; 
         FIG. 1 b    shows a schematic plan view of part of an aircraft according to the first embodiment of the invention; 
         FIG. 2 a    shows a schematic rear view of part of an aircraft according to a second embodiment of the invention, whilst on the ground prior to refuel; 
         FIG. 2 b    shows a schematic rear view of part of an aircraft according to the second embodiment of the invention, whilst on the ground after refuel; 
         FIG. 2 c    shows a schematic rear view of part of an aircraft according to the second embodiment of the invention, whilst at the start of cruise; 
         FIG. 2 d    shows a schematic rear view of part of an aircraft according to the second embodiment of the invention, whilst at the start of descent; 
         FIG. 2 e    shows a schematic rear view of part of an aircraft according to the second embodiment of the invention, whilst at the end of decent; and 
         FIG. 3  shows a schematic plan view of an aircraft according to either the first or second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1 a    shows a schematic rear view of part of an aircraft  10  according to a first embodiment of the invention. The aircraft  10  comprises a port wing  20 , a centre wing box  30 , and a starboard wing  40 . The tank arrangement of the starboard wing  40  will now be described in more detail. 
     The starboard wing  40  comprises a fuel tank  51 , taking up most of the space in the wing, and a surge/vent tank  52 , at an outboard tip of the wing  40 . The fuel tank  51  holds fuel  53  and also ullage  54  (the gas above the fuel in the tank). The ullage  54  contains fuel vapour. 
     A climb-dive valve  55  is located adjacent the surge tank. The Figure shows two possible locations for the valve; a first location at the very tip of the wing, outboard of the surge/vent tank  55   a  at a NACA duct outlet (not shown) and a second location inboard of the surge/vent tank  55   b  in the vent system ducting (not shown). 
     In either position, the climb dive valve  55  is a two-way valve that partially pressurises the fuel tank  51 . The climb-dive valve  55  only allows a specified maximum pressure differential across it so that the fuel tank  51  does not experience a pressure very different to atmospheric pressure. This could otherwise cause damage to the wing  40 . 
     In other words, at a specified positive pressure difference (typically +2.9 psi) and higher of the fuel tank  51 , the climb-dive valve  55  opens to allow air out of the tank  51 . At a specified negative pressure difference (typically −1.9 psi) and lower of the fuel tank  51 , the climb-dive valve  55  opens to allow air into the fuel tank  51 . 
     The fuel tank  51  is provided with ducting  56 . This ducting has a first inlet/outlet  56   a  corresponding to the location of ullage in climb  54   a  and a second inlet/outlet corresponding to the location of ullage in early cruise  54   b  (see  FIG. 1 b   ). Another inlet/outlet of the ducting  56  is connected to a first side of a pressure control valve assembly  57 , located in the wall  64  between the starboard wing  40  and the centre box  30 . 
     Inside the centre box  30  a pipe  61  is connected to the second side of the pressure control valve assembly  57 . An opposite end of the pipe  61  is connected to an expandable bladder  62  located within the centre box  30 . 
     The pressure control valve assembly  57  is an electronically controlled set of valves. The valves include a float valve (not shown) so that ullage  54  only (and not fuel  53 ) can pass from the fuel tank  51  to the expandable bladder  62 . Alternatively, the float valves may be situated in the ducting  56 , for example at the first and second inlet/outlets  56   a ,  56   b.    
     The valves in the pressure control valve assembly  57  are controlled to allow ullage  54  to flow from the tank  51  to the bladder  62  in some circumstances and to allow ullage  54  to flow from the bladder  62  to the tank  51  in other circumstances. Typically, the valve assembly  57  is controlled to allow/urge ullage  54  to flow into the bladder  62  during refuel, be closed during climb, and allow/urge ullage  54  to flow into the tank  51  during cruise and/or descent. This will be described in more detail in relation to  FIGS. 2 a    to  2   e.    
     The assembly  57  also comprises a pump or compressor (not shown) for urging ullage  54  between the tank  51  and bladder  62  in certain circumstances. The assembly  57  is connected to a pressure sensor (not shown) in the tank  51  and is controlled based on the pressure sensed by that sensor. Alternatively, it could be controlled according to atmospheric pressure and a rate of usage of fuel  53 . 
     The aircraft  10  also has an “OBIGGS” (On Board Inert Gas Generation System) (not shown), which is used to produce oxygen depleted air (ODA). This ODA is injected into the bladder  62 , when necessary. 
       FIG. 1 b    shows a schematic plan view of part of an aircraft  10  according to the first embodiment of the invention. This Figure simply shows where the ullage  54  is located at different times during the aircraft flight cycle. Typically, in climb, the ullage  54   a  is located at the front inboard root of the wing: Typically, in early cruise, the ullage  54   b  is located at the outboard tip of the wing. The location of inlet/outlets  56   a ,  56   b  of the ducting  56  correspond to these locations  54   a ,  54   b.    
       FIGS. 2 a  to 2 e    show schematic rear views of part of an aircraft  110  according to a second embodiment of the invention. This second embodiment is very similar to the first embodiment. However, the ducting  56  and the surge/vent tank  52  are not shown, for clarity. All other elements are similar to the elements of the first embodiment, and are labelled with the same reference numerals, with the addition of a preceding “1”. In this embodiment, the climb-dove valve  155   a  is located at the first location ( 55   a  in the first embodiment) at the very tip of the wing  140 , outboard of the surge/vent tank at a NACA duct outlet (not shown). 
       FIG. 2 a    shows the aircraft  110  whilst on the ground prior to refuel. Here, the pressure in the fuel tank  151  is slightly lower than atmospheric pressure as the aircraft has recently descended (into higher pressure air) and the climb-dive valve  155   a  allows a certain negative pressure difference. The tank  151  has little fuel  153  in it (as a lot has recently been used up from the previous flight) and there is a large amount of ullage  154  in the tank  151 . There is no stored ullage in the bladder  162 . 
       FIG. 2 b    shows the aircraft  110  whilst on the ground after refuel. Here, the fuel tank  51  has been refueled with fuel  153  and now contains very little ullage  154 . During refuel, the ullage  154  that was in the tank  151  was allowed through the pressure control valve assembly  157  into the bladder  162 . The pressure valve assembly  157  was opened to allow this ullage flow  154  to the bladder  162  in response to an increasing pressure in the tank  151  (sensed by a pressure sensor in the tank  151 , connected to the valve assembly  157 ). Hence, the bladder  162  here is fully expanded to hold the ullage  154 . The pressure of the ullage  154  in the tank  151  and also in the bladder  162  is higher than atmospheric pressure because of the refuel process and because the climb-dive valve  155   a  allows a certain positive pressure difference. 
     Storing the ullage  154  in the bladder  162  prevents the ullage being ejected out of the aircraft (through the climb-dive valve  155   a ) into the atmosphere. 
       FIG. 2 c    shows the aircraft  110  whilst at the start of cruise, after the aircraft has climbed to or near cruising altitude. The atmospheric pressure here is low and the fuel  153  level is still high (as not much has been consumed yet). The climb-dive valve  155   a  prevents the ullage  154  in the fuel tank  151  from being too high in comparison to the atmospheric pressure so once this maximum positive pressure difference is reached (i.e. once the aircraft has climbed to a low enough atmospheric pressure), some ullage  154  is expelled out of the fuel tank  151  through the climb-dive valve  155   a . This ullage  154  must be expelled so that the wing  140  does not experience a large pressure differential with the atmosphere. The ullage  154  cannot be stored in the bladder  162 , as the bladder  162  is full. 
     During flight, the fuel is gradually used up and some of the ullage  154  stored in the bladder  162  is allowed through the pressure control valve assembly  157  into the tank  151  during flight to replace this volume. The pressure valve assembly  157  was opened to allow this ullage flow  154  to the fuel tank  151  in response to a decreasing pressure in the tank  151  (sensed by a pressure sensor in the tank  151 , connected to the valve assembly  157 ). 
       FIG. 2 d    shows the aircraft  110  whilst at the start of (or just before) descent. Here, a lot of fuel  153  has been used up so there is not much fuel  153  left in the tank  151 . It has been replaced by some ullage  154  from the bladder  162  (allowed through the pressure control valve assembly  157 , as described above) in order to keep the pressure of the tank  151  at an acceptable level. The ullage  154  in the tank  151  is at low pressure because of the low atmospheric pressure and the climb-dive valve  155   a  maintaining a similar pressure in the tank  151 . 
     During descent, the ullage  154  in the bladder  162  is moved to the tank  151  (allowed through the pressure control valve assembly  157 ) to increase the pressure of the ullage  154  in the tank  151  in order to keep it within a certain negative pressure difference of the increasing atmospheric pressure. The pressure valve assembly  157  is open to allow this ullage flow  154  to the fuel tank  151  in response to the increasingly negative pressure difference of the fuel tank  151  compared to the atmospheric pressure (sensed by a pressure sensor in the tank  151 , connected to the valve assembly  157 ). 
       FIG. 2 e    shows the aircraft  110  whilst at the end of decent, after landing on the ground. Here, it can be seen that all the ullage in the bladder  162  has all been moved to the tank  151 . In addition, air from the OBIGGS system (oxygen depleted air—ODA) has also been passed from the bladder  162  to the tank  151  (allowed through the pressure control valve assembly  157 ) to ensure the tank  151  remains inerted and to increase the pressure in the tank  151  sufficiently. The fuel tank  151  is now back to the same state as shown in  FIG. 2   a.    
     The tank arrangement as described in relation to the first and/or second embodiments may be used with an aircraft, such as that shown in  FIG. 3 . 
     Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described. 
     A similar arrangement may also, or alternatively, be provided in the port wing  20 , 120 . 
     The position of the ullage storage facility, the inerting system (OBIGGS), the pressure controllable valve assembly  57 ,  157  or the climb-dive valve  55 ,  155 , as well as the wing geometry shown is only indicative. 
     For example, the ullage  54 ,  154  could be stored outside of the centre wing tank area  30 ,  130 , for example, in a belly fairing area of an aircraft, or just forward or aft of the centre tanks (for example, in cargo space) or in the tail cone of the aircraft. Storing ullage in the tail cone would work especially well with tailplane fuel tanks. 
     The ullage  54 ,  154  may be urged (not just allowed) to flow through the pressure valve assembly  57 ,  157  by a pump or compressor. 
     Also, the ullage  54 ,  154  stored in the bladder  62 ,  162  could be dried (for example, by cycling the ullage through a drier to remove water) in preparation for returning it to the fuel tank  51 ,  151 . 
     Upon descent, or just before descent ( FIG. 2 d   ), air from the atmosphere (generally dry air at high altitude, compared to low altitude) could be taken in to the tank  51 ,  151  or the bladder  62 ,  162  to help achieve the required pressure in the tank  51 ,  151 . This drier air minimises the problems of water ingestion. However, importantly, the amount ingested would be less than would have been needed if the bladder  62 ,  162  was not present. Hence, the problems of water ingestion are less than in the prior art. 
     Also, the OBIGGS could be operated after landing for a short amount of time (e.g. a few minutes) to allow the pressure in the tank  51 ,  151  to be restored with ODA. 
     The OBIGGS could be operated to produce ODA and store it in the bladder  62 ,  162 , in advance. (This is opposed to producing the ODA at the point when or shortly before it is needed.) This allows the OBIGGS to be operated more effectively and hence, the OBIGGS may be able to be smaller in size and weight. The OBIGGS could be used to treat the ullage in the bladder  62 ,  162  and return it (with less oxygen present) to the bladder  62 ,  162 . 
     Also, the ullage  54 ,  154  may be stored, instead of or as well as in an expandable bladder  62 ,  162 , in a compressed air tank (using a pump or compressor to pressurise the gas). This would allow more ullage to be stored and would be more space efficient (although it would incur a weight penalty). There may also be an accumulator bladder for controlling ullage transfer between the compressed air tank and the fuel tank  51 ,  151 . The ullage may also be stored in a number of different containers, rather than one single container. 
     Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.