Abstract:
An access door structure for a wing skin is received in an access hole opening which incorporates a protective glove having a set of mistake resisting features and engaging the periphery of the access hole opening. An inner door seals an interior of the access hole opening and has multiple fastener attachment elements and a set of mating mistake resisting features. An outer door engaged over an exterior of the access hole opening has fastener holes for mating alignment with the fastener attachment elements and receives a plurality of fasteners for engagement of the inner and outer doors.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is copending with application Ser. No. 12/606,331 filed on Oct. 27, 2009 entitled Composite Access Door having a common assignee with the present application. 
     
    
     BACKGROUND INFORMATION 
       [0002]    1. Field 
         [0003]    Embodiments of the disclosure relate generally to the field of mating systems for structural access doors and more particularly to a system for alignment and providing mistake resistance for fuel tank access doors with features for pressure distribution to avoid tank seal breech. 
         [0004]    2. Background 
         [0005]    Structural access doors in large commercial aircraft are required to provide ingress and egress from various compartments such as wing fuel tanks for maintenance and inspection requirements. The doors are typically designed with fastener systems (with self-retained nuts) and have minimal surface protrusion on the outer surface to provide aerodynamic smoothness for the door surface. In large aircraft a significant number of wing access doors are present, many with similar or identical size and planform. These wing access doors are designed to differing strengths depending on where on the wing the door is placed. For instance doors near the landing gear tires must withstand large pieces of rubber and debris that can collide with the door after tire failure. Doors that are further way from the tires do not need to be as strong and so a lighter and, consequently, weaker door is made. To assure that certain doors are not installed at incorrect locations, some type of physical mistake resistance is required. This insures that the medium or light doors are not used in areas where heavier doors are required. In certain prior art systems radial blade and slot engagement of mistake resistance features has been employed. 
         [0006]    Additionally, aircraft are susceptible to electrical discharges due to static or other natural phenomena, particularly with respect to wing fuel tank access doors, control or diversion of the energy from electrical discharges is required. In prior art aircraft having metallic structure, conductive paths provided by the structure itself typically rendered sufficient protection. However, with greater reliance on composite structural materials which are generally less conductive than metals typically used in commercial aerospace, alternative design methods are required. One embodiment of a prior art door design for use in carbon fiber reinforced plastic (CFRP) structures employed a conductive clamp ring mounted against a conductive door surface to allow currents to flow across the door and wing surface. In advanced aircraft designs, use of CFRP for greater portions of the structural content of the aircraft is desired for further weight reduction. A change from metal to CFRP as the primary access door material and removing the clamp ring from the design is there for desirable. Not having a continuous conductive path between a door retention fastener to the door surface to the wing skin may lead to an electrical discharge. This may result in expanding gas in the volume between the inner/outer doors and the access hole wing skin cutout (“racetrack”). 
         [0007]    It is therefore desirable to provide access door interface designs for (CFRP) structures which provide the desired mistake resistance and avoid impeding any expanding gas in the racetrack thereby preventing redirection of the gas with possible seal breech into the fuel tank. 
       SUMMARY 
       [0008]    Embodiments disclosed herein provide an access door structure for a wing skin having an access hole opening which incorporates a protective glove engaging the periphery of the access hole and having a set of mistake resisting features. An inner door seals an interior of the access hole opening and has multiple fastener attachment elements and a set of mating mistake resisting features. An outer door engaged over an exterior of the access hole opening has fastener holes for mating alignment with the fastener attachment elements and receives a plurality of fasteners for engagement of the inner and outer doors. 
         [0009]    In one example, a composite fuel tank door assembly employs an outer access door received over an access hole in a wing skin. An inner access door has a channel housing fastener attachment elements. Multiple securing fasteners extend through the outer access door to be received in the fastener attachment elements in the inner access door to securely seal the access door assembly at a periphery of the access hole opening. At least one mistake resisting feature extends from a wall of the access hole opening into the channel. At least one mating mistake resisting feature extends from a surface of the inner access door into the channel for operative engagement of the at least one mistake resisting feature in a singular configuration. The mistake resisting feature and mating mistake resisting feature provide clearance in the channel for any gas or shockwave progression. 
         [0010]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present disclosure or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a pictorial view of an aircraft showing an example under-wing access door location; 
           [0012]      FIG. 2  is a bottom view of an access hole opening in the wing skin; 
           [0013]      FIG. 3  is a bottom view of an outer access door as installed; 
           [0014]      FIG. 4  is a bottom view of an inner access door with the outer access door omitted; 
           [0015]      FIG. 5  is an upper pictorial view of an access hole opening with the protective glove installed on the periphery of the hole and outer access door in place with the inner access door omitted; 
           [0016]      FIG. 6  is an upper pictorial view of the access door with the inner access door installed; 
           [0017]      FIG. 7  is a section view of both an inner and outer access installed in an access hole opening. 
           [0018]      FIG. 8  is a bottom view of an access hole opening with the outer access door removed and the inner access door in place; 
           [0019]      FIG. 9  is a detailed view of a portion of the access door periphery in bubble  9 - 9  of  FIG. 8 ; 
           [0020]      FIG. 10  is a detailed view of a portion of the access door periphery in bubble  10 - 10  of  FIG. 8 ; 
           [0021]      FIG. 11  is a detailed pictorial view of the portion of the access door periphery designated by line  11 - 11  in  FIG. 10 ; 
           [0022]      FIG. 12  is a detailed section view along line  12 - 12  of  FIG. 10 ; 
           [0023]      FIG. 13  is a table showing mistake resisting glove and pin location for multiple door types; 
           [0024]      FIG. 14  is the detailed view of  FIG. 9  showing exemplary flow within the racetrack; 
           [0025]      FIG. 15  is the detailed view of  FIG. 10  showing exemplary flow within the racetrack; and, 
           [0026]      FIG. 16  is a flow chart of a method for providing mistake resisting features for access door assemblies. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    Embodiments disclosed herein provide a structural access door configuration in which mistake-resisting features are provided to prevent an access door from being installed in non-designated locations. Shape and installation profiles of the mistake-resisting features within a channel forming the racetrack prevents expanding gases from being directed towards the seal between the fuel access door and wing fuel tank. 
         [0028]    Referring to the drawings,  FIG. 1  shows an aircraft  10  having a structural access door assembly  12  located in a bottom skin  14  of a wing  16 . For the embodiment shown the aircraft structure and wing skin as well as the structural access door assembly are primarily fabricated from CFRP or other composite materials. The location shown may be one of multiple substantially identically shaped access doors in the wings of the aircraft. As shown in  FIG. 2 , the wing skin  14  incorporates an access hole  18  in which a protective glove  24  is installed. For the exemplary embodiment shown, the access door assembly  12  includes an outer door  20  shown installed in the access hole  18  in  FIG. 3  and an inner door  22  shown installed in the access hole  18  with the outer door removed in  FIG. 4 . For tooling consistency in manufacturing it is desirable for multiple doors to have substantially identical elliptical dimensions with minimum hole size to accommodate the average airline mechanic. However to help prevent miss-installation of similarly sized doors in the wrong locations mistake resistance is required. 
         [0029]    A protective glove  24  is installed around the periphery of access hole  18  as seen in  FIG. 5 . The glove  24  includes, as mistake resisting features, bumps  26   a  and  26   b,  the function of which will be described in greater detail subsequently. A spacer  32  (to be described in greater detail subsequently) surrounds the access hole  18 . Outer door  20  incorporates multiple fastener holes  28  and in certain embodiments may employ a copper foil or other conductive sheathing. Inner door  22 , shown in  FIG. 6  includes a sealing flange  30  which is received over the spacer  32 . As shown in the section view of  FIG. 7 , outer door  20  is received in a chamfer  34  in the wing skin  14 . Fasteners  36  received through the fastener holes  28  in the outer door extend into mating contact with threaded inserts  38  installed in domes  40  formed in the inner door. Sealing flange  30 , extending over the spacer  32  and wing skin  14  surrounding access hole  18 , incorporates a fuel seal  42  for sealing the wing tank volume  44  with the assembled inner door  22  and outer door  20 . The sealing flange  30  terminates in a wall  46  that creates a channel  48  with the periphery of access hole  18  having a width  58  to provide space for the fasteners  36 . 
         [0030]      FIG. 8  demonstrates an example of the mistake resistance employed in the present embodiments for installation of the door assembly  12  into the access hole  18 . Installation fit is provided by a combination of bumps ( 26   a,    26   b  for the glove configuration shown) on the protective glove  24  and posts ( 50   a,    50   b  for the door assembly shown) extending from an exterior surface  52  of the inner door  22 . Inner door  22  is symmetrical about the longitudinal axis with two sets of posts  50   a  and  50   b  located on the door adjacent the curvature of protective glove  24  on the periphery of access hole  18  allowing proper placement of the door at either of two 180° orientations. 
         [0031]    Details of the mistake resisting interface are shown in  FIGS. 9 through 12 . Bumps  26   a  and  26   b  extending from the protective glove  24  will only allow positioning of an inner door  22  having posts spaced and circumferentially located to be received around one or more of the bumps; in the instance shown, bump  26   b  received between posts  50   a  and  50   b.  As shown in  FIG. 13 , a matrix of bump and post positions  60  along a specifically defined length of the circumference of the glove and door creates assured interference between the posts and bumps except for the specific mating pairing. For the example shown, five positions  60  are defined with respect to door type  62 . Position  1  is shown in  FIG. 8  as element  101 , position  2  as  102 , position  3  as  103 , position  4  as  104  and position  5  as  105 . For a first door type, impact heavy, position  1  is occupied by a post on the inner door. Positions  2 - 4  are occupied by bumps on the glove. Position  5  is occupied by a post on the inner door. A second door type, impact, has position  1  occupied by a bump on the glove, position  2  by a post extending from the inner door, positions  3  and  4  occupied by a bump on the glove and position  5  by a post extending from the inner door. As seen from these first two door types, an “impact” door could not be inserted in a “impact heavy” access hole since the pin in position  2  of the impact door would interfere with the bumps on the glove extending over position two of the impact heavy access hole preventing installation. Similarly for each door type, at least one post (or in limited cases an installation fastener) from an alternate door type would impact a bump location inhibiting installation. 
         [0032]    As shown in the matrix of  FIG. 13 , the five position definition with two posts extending from each inner door allows ten door types with assured mistake resistance. In a general case, the matrix incorporates positions for n door types with p positions and z posts and p-z bumps. Bump locations at adjacent positions may result in a “merged bump” extending over the multiple locations as opposed to individual bumps at each location. The door assembly embodiment shown in  FIGS. 9-12  corresponds to a “Mid-Extra Heavy” door type with posts at positions  1  and  3 , a single bump at position  2  intermediate the posts and a merged bump at positions  4  and  5 . 
         [0033]    As seen in  FIGS. 9-12 , posts  50   a  and  50   b  are elliptical in planform having a major axis perpendicular to the cross sectional area of the channel  48 . For each post, a post body  54  expands into a filleted base  56  providing stress and aerodynamic smoothness within the channel. For the embodiment shown, the geometric shape and positioning of the mistake resisting features are provided to specifically accommodate gas expansion in the channel due to electrical discharge or other environmental effects. The rounded geometries avoid deflection of the expanding gases towards the seal between the inner door and fuel tank skin. Additionally, sizing of the posts is less than the diameter of the fasteners  36  for the embodiments shown. Bumps  26   a  and  26   b  extend into the channel  48  to occlude less than 25% of a width  58  of the channel avoiding any impedance to gas expansion within the channel  48 . 
         [0034]    As shown in  FIGS. 14 and 15 , the minimal intrusion of the bumps into the channel and reduced profile of the posts prevents blockage in the channel which might divert expanding gases, shown notionally as arrows  68 ,  70  and  72 , toward the seal  42  (seen in  FIGS. 7 and 12 ). Additionally, placement of the mistake resisting positions on the larger radius portions  66  of the elliptical shape substantially centered on the minor axis (as shown in  FIG. 8 ), reduces any flow turning resistance induced by the shape of the channel which might be exacerbated by the mistake resisting features. 
         [0035]    The embodiments disclosed provide a method for mistake resistant installation of composite access door structures with minimal impedance of gas expansion in the racetrack as shown in  FIG. 16  by providing an inner door received over an elliptical access hole opening, step  1602 . An outer door is received on the access hole opening for attachment to the inner door with fasteners extending through a channel formed by a sealing flange on the inner door, step  1604 . A periphery of the access hole opening is provided with a protective glove having mistake resisting features extending from the periphery of the hole, step  1606 . Mating mistake resisting features extending from an outer surface of an inner door that engage the mistake resisting features on the protective glove, step  1608 . The mistake resisting features and mating mistake resisting features are sized to occlude less than 25% of a cross sectional area of the channel, step  1610  and may be located in a portion of the periphery of the access hole opening having larger radius associated with a minor axis, step  1612 . 
         [0036]    Having now described various embodiments of the disclosure in detail as required by the patent statutes, those skilled in the art will recognize modifications and substitutions to the specific embodiments disclosed herein. Such modifications are within the scope and intent of the present disclosure as defined in the following claims.