Abstract:
The present invention describes an outer fuel access tank cover (FTAC) of an aircraft, a wing comprising such outer FTAC of an aircraft and an aircraft. The invention belongs to the field of “designing auxiliary pieces in the wing of aircrafts to control the risk of gases from an explosion occurred in the FTAC entering the fuel tank”.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of the filing date of European Application Serial No. EP12382180.3 filed May 16, 2012 the disclosure of which is hereby incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention describes an outer fuel access tank cover (FTAC) of an aircraft, a wing comprising such outer FTAC of an aircraft and an aircraft. The invention belongs to the field of “designing auxiliary pieces in the wing of aircrafts to control the risk of gases from an explosion occurring in the FTAC entering the fuel tank”. 
       BACKGROUND 
       [0003]    Manholes in aircrafts provide access to the fuel tank. Manholes comprise an inner fuel tank access cover (inner FTAC), an outer FTAC and a void area between the two covers. Fuel Tank Access Covers (FTACs) are mechanically fastened and clamped against the aircraft wing skin to provide fuel tank access sealing. FTAC are designed to meet a wide array of requirements, some of them are: no fuel leaks, fire resistance, resistance to a tire impact, resistance to impacts resulting from an UERF (Uncontained Engine Rotor Failure), EMH/lightning strike, seal friction, and wing bending. Despite the “no fuel leaks” requirement, in the worst case scenario from an operational and certification standpoint, it is not uncommon that seals may leak if the FTAC is installed incorrectly. Therefore, it is impossible to guarantee 100% that small amounts of fuel will never under any circumstance be present in the void zone between covers. This may also happen when FTAC are dismantled on ground and fuel drips and saturates the area until the FTAC is reinstalled. If a lighting strike or an electric static discharge occurs, an explosion can take place inside the void area of the FTAC. The current design approach to address this is to contain the explosion of the fuel/air mixture within the void zone between the inner and outer FTAC. The problem with this approach is that the FTACs require a structure with high stiffness to prevent it from being deformed after an explosion in the void area. Said deformation of the inner FTAC or the outer FTAC or both would compromise the seal integrity between the inner FTAC and the lower wing skin. In essence, currently pressure vessels are created with no relief valves. 
         [0004]    The mentioned prior art approach is seen in the patent U.S. Pat. No. 4,291,816 wherein a fluid tight closure for an aperture, adapted to form a fuel tank access door for an aircraft, and providing fail-safe features and resistance to lightning strikes is described. 
       SUMMARY 
       [0005]    A solution for the stated problems is achieved by an outer FTAC according to claim  1 . The particular embodiments of the invention are defined in the dependent claims. 
         [0006]    The present invention approaches the technical problem described by providing a path of less resistance for the expansion of explosion gases through an outer fuel tank access cover (outer FTAC) of an aircraft adapted for being used to cover the outer opening of a void area of a manhole for accessing the interior of a wing of an aircraft wherein the interior of the wing comprises a fuel tank characterized in that the outer FTAC comprises an explosion gases relief means adapted for allowing the explosion gases to escape from the void area to the atmosphere. 
         [0007]    The invention lets the gases escape from the void area through an exit door or fusible feature which eliminates the need to contain the explosion. In case of an explosion in the void area of the manhole the invention avoids the explosion from entering into contact with the fuel tank. As an advantage, the stiffness of the FTACs&#39; structure is reduced. By reducing the stiffness requirement, a lighter structure is used yielding weight saving opportunities. 
         [0008]    A second aspect of the invention presents a wing of an aircraft comprising at least one fuel tank access comprising an outer FTAC according to the first aspect of the invention. 
         [0009]    A last aspect of the invention presents an aircraft comprising a wing according to the second aspect of the invention. 
         [0010]    All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of such mutually exclusive features and/or steps. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    These and other characteristics and advantages of the invention will become clearly understood in view of the detailed description of the invention which becomes apparent from a preferred embodiment of the invention, given just as an example and not being limited thereto, with reference to the drawings. 
           [0012]      FIG. 1A  This figure represents an embodiment of an aircraft wherein the left wing is shown in black. 
           [0013]      FIG. 1B  This figure shows the locations of the wing of the aircraft shown in  FIG. 1A  where the FTACs are located. 
           [0014]      FIG. 1C  In this figure an embodiment of an outer FTAC is represented. 
           [0015]      FIG. 1D  In this figure a sectional view of the inner area of the wing wherein the fuel tank is located is represented. Besides, the relative position of the FTACs to the fuel tank is shown. The different embodiments of the invention are located on the outer FTAC. 
           [0016]      FIG. 1E  In this figure a zoomed view of  FIG. 1D  is represented where the wing skin ( 16 ) is shown to be in contact with the void area ( 2 ) and the inner FTAC ( 3 ) is shown to be sealed to the wing skin ( 16 ) with fuel seals ( 17 ). 
           [0017]      FIG. 2  This figure represents a view of a rupture disk. 
           [0018]      FIG. 3A  Represents a view of an embodiment of a frangible line. 
           [0019]      FIG. 3B  A zoomed view of the frangible line is represented. 
           [0020]      FIG. 4  This figure represents a sectional view of a pop-off door. 
           [0021]      FIG. 5  This figure represents an embodiment of the open vent means in a view from below the wing. 
           [0022]      FIG. 6  This figure represents a sectional view of a solution for the open vent means. 
           [0023]      FIG. 7  This figure represents a perspective view of the inner part of an outer FTAC where an embodiment of the open vent concept is shown. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Once the object of the invention has been outlined, specific non-limitative embodiments are described hereinafter. 
         [0025]    All the embodiments of the invention are located in the outer FTAC ( 4 ). An embodiment of an aircraft is represented in  FIG. 1A  and its left wing is shown in  FIG. 1B  where the locations where the FTACs ( FIG. 1C ) are located are shown. In  FIG. 1D  a sectional view of the inner area of the wing wherein the fuel tank ( 1 ) is located is represented. Besides, in  FIGS. 1D and 1E  the relative position of the FTACs to the fuel tank ( 1 ) is shown. The inner FTAC ( 3 ) is in the fuel tank ( 1 ) and it is sealed with fuel seals ( 17 ) to the lower wing skin ( 16 ). The outer FTAC ( 4 ) is fixed to the lower wing skin ( 16 ) by means of a plurality of mounting holes peripherally distributed. The fixing means, in an embodiment of the invention, are bolts. The different embodiments of the invention are located in the outer FTAC ( 4 ). 
         [0026]    In a first embodiment of the invention the outer FTAC ( 4 ) comprises relief means which is a disk ( 5 ) which is ruptured once a predetermined level of pressure is reached. In  FIG. 2  the disk ( 5 ) is a rupture disk. The rupture disk ( 5 ) is designed to provide a replaceable leak-tight seal within a vessel until the internal pressure rises to a predetermined level. The rupture disk ( 5 ) in the embodiment shown in  FIG. 2  is riveted to the outer FTAC ( 4 ). In an embodiment it comprises a circular region ( 6 ) wherein the thickness is smaller the rest of the disk ( 5 ). The thickness of the centre ( 6 ) of the rupture disk ( 5 ) is such that it is the point with the highest stress when the rupture disk ( 5 ) is pressurized. This high stress point ruptures beyond a predetermined pressure. In the case of rupture, only the rupture disk ( 5 ) has to be replaced when it is damaged and the rest of the outer FTAC ( 4 ) remains undamaged. 
         [0027]    In one embodiment the disk ( 5 ) is manufactured using a high strain material, such as a ceramic material. 
         [0028]    In one embodiment the region ( 5 ) is sized according to an average detonation pressure. This pressure varies depending on the FTAC size and typical values are within the range of 1-4 Atm. 
         [0029]    The outer FTAC ( 4 ) comprises relief means which is a frangible line ( 8 ) represented in  FIG. 3A .  FIG. 3B  shows a sectional zoomed view of the frangible line ( 8 ). 
         [0030]    This pressure relief concept is considered in the form of a fusible feature/device to evacuate the pressure caused by the detonation of fuel/air gases. The frangible line ( 8 ) is calibrated to rupture once a predetermined pressure level due to explosion gases is reached. The outer FTAC ( 4 ) itself is a fusible component, so once damaged, it must be replaced. Rip-stop features are also required to limit the crack propagation to the mounting holes of the outer FTAC ( 4 ) to avoid FTAC detachment from the lower wing skin and to ensure that the sealing requirements are still fulfilled. 
         [0031]    In an embodiment the outer FTAC ( 4 ) comprises relief means which are a door ( 9 ) which pops-off once a predetermined pressure level due to explosion gases is reached. The pop-off door ( 9 ) comprises clipping means ( 10 ), as it can be seen in  FIG. 4 , for fixing the pop-off door ( 9 ) to the outer FTAC ( 4 ) wherein such clipping means ( 10 ) are adapted to be broken or deflected once a predetermined level of pressure due to explosion gases is reached. The clipping means ( 10 ) are designed to deflect/fracture at the minimum detonation pressure along with a conservative reserve factor. 
         [0032]    In an embodiment the outer FTAC ( 4 ) comprises clipping means ( 10 ) designed for a detonation pressure of 1-4 Atm. 
         [0033]    In an embodiment the pop off door ( 9 ) is made of metal. 
         [0034]    In an embodiment the pop off door ( 9 ) is made of molded plastic. This embodiment has the advantage of being lighter and more economical to manufacture than the metal one. 
         [0035]    In an embodiment of the invention, the outer FTAC ( 4 ) comprises relief means which are open vent means such that the void area ( 2 ) of the manhole is in communication with the atmosphere or the outside environment of the outer FTAC ( 4 ). In this concept, the outer FTAC ( 4 ) is open to atmosphere, so that air-fuel gases, either before the explosion or after it, are let outside. This is called the open vent concept. 
         [0036]    In an embodiment of the invention, the open vent means comprise at least one orifice ( 12 ) on it. 
         [0037]    In an embodiment of the invention, the solution of the pop-off door ( 9 ) is used in with a fire trap ( 11 ) to provide a barrier if the door is exposed to fire and it melts, in case of having a pop-off door made with a material that can melt. It is also used to prevent direct flame from touching the inner FTAC ( 3 ). 
         [0038]    In an embodiment of the invention, a number of orifices ( 12 ) are located in such a way that they follow a direction according to the streamlines of the air flow ( 18 ) when the aircraft is flying under cruise conditions in order to minimize the aerodynamic drag. These orifices ( 12 ) provide an open door venting path between void area and the atmosphere. The size and shape of the orifices will depend on the volume and shape of the void area between the inner FTAC ( 3 ) and outer FTAC ( 4 ). 
         [0039]    In an embodiment, the outer FTAC ( 4 ) is elongated showing two ends wherein it comprises two pluralities of orifices ( 12 ), preferably six orifices ( 12 ). Each plurality of orifices ( 12 ) is a cluster distributed as a line according to the stream lines the air flow close to one end, providing a two hole pattern, one pattern at each end. This solution is represented in  FIG. 5 . 
         [0040]    In an embodiment the size of the orifices ( 12 ) is in the range from 0.3 cm to 0.8 cm. These sizes depend on the gas velocity inside the void area. 
         [0041]    In an embodiment of the invention, the outer FTAC ( 4 ) comprises an inner cover ( 15 ) located in the inner side of the outer FTAC ( 4 ) following a diagonal direction ( 18 ) according to the streamlines of the air flow when the aircraft is flying under cruise conditions. Between the inner cover ( 15 ) and the outer FTAC ( 4 ) a chamber ( 19 ) is enclosed such that:
       the orifices ( 12 ) are in communication with the chamber ( 19 ); and,   the chamber ( 19 ) is also in communication with the void area ( 2 ) by means of an opening ( 13 ).       
 
         [0044]    The inner cover ( 15 ) prevents any possible flame, in case of an external fire when the aircraft is on earth, from travelling towards the fuel tank ( 1 ). 
         [0045]    In an embodiment the inner cover ( 15 ) or the outer FTAC ( 4 ) comprises a pressure baffle ( 14 ) to reduce shock waves due to the explosion, as it is represented in  FIG. 6 , such that the chamber is divided at least in two sub-chambers:
       a first sub-chamber ( 19 . 1 ) in communication with the void area ( 2 ) through the opening ( 13 ); and,   a second sub-chamber ( 19 . 2 ) in communication with the first sub-chamber ( 19 . 1 ) and also in communication with the open vent means.       
 
         [0048]    In the embodiment represented in  FIG. 6 , the pressure baffle ( 14 ) has a predetermined length and it is shaped in such a way that one end is parallel and in contact with the inner cover ( 15 ) and the other end forms a predetermined angle with the inner cover ( 15 ) towards the direction of the opening ( 13 ). 
         [0049]    Pressure baffles ( 14 ) are used to reduce the shockwave velocity to sub-sonic speeds so that the flow is not choked at the exit ports. Therefore, super-sonic conditions are avoided. 
         [0050]      FIG. 7  represents a perspective view of the inner part of an outer FTAC ( 4 ) where an embodiment of the open vent concept is shown. The different elements for this embodiment can be differentiated:
       outer FTAC ( 4 ),   orifices ( 12 ),   inner cover ( 15 ) riveted to the internal surface of the outer FTAC ( 4 ),   pressure baffle ( 14 ),   inlet ( 13 ).       
 
         [0056]    The elements located inside of the inner cover ( 15 ) are represented using a slashed line.