Patent Publication Number: US-2022227474-A1

Title: Keel beam assembly for an aircraft

Description:
CROSS-REFERENCES 
     This application claims the benefit under 35 U.S.C. § 119(e) of the priority of U.S. Provisional Patent Application Ser. No. 63/139,722, filed Jan. 20, 2021, the entirety of which is hereby incorporated by reference for all purposes. 
    
    
     BACKGROUND 
     Commercial aircraft for cargo and passenger transport utilize a keel beam extending along the bottom of the fuselage. The keel beam braces the fuselage centrally along its length and helps to couple sections of the fuselage to one another. Commercial aircraft, such as a midsize commercial aircraft, may have a keel beam including a box beam portion (a keel box) located in an aft region of the keel beam where loads can be higher. However, integrating this type of keel beam into the aircraft can slow the rate of aircraft production significantly. Moreover, this integration presents substantial design challenges to ensure that the keel box is robust but relatively lightweight, while still creating an efficient load path without excessive bending moments under high load. A novel keel box to enable more efficient manufacture of aircraft would be desirable. 
     SUMMARY 
     The present disclosure provides a keel beam assembly, an aircraft including the keel beam assembly, and a method of aircraft manufacture using the keel beam assembly. An illustrative keel beam assembly comprises a pair of keel chords extending outside an aircraft fuselage and forming at least part of a lower keel box portion of a keel box. An upper keel box portion of the keel box is coupled to the pair of keel chords and has an aft end engaged with and secured to an aft wheel well bulkhead (AWWB). 
     In some examples, an aircraft comprises a fuselage, an aft wheel well bulkhead (AWWB), and a keel beam assembly. The keel beam assembly includes a pair of keel chords extending outside the fuselage and forming at least part of a lower keel box portion of a keel box. The keel beam assembly also includes an upper keel box portion of the keel box coupled to the pair of keel chords and having an aft end mounted onto a forward side of the AWWB. 
     In some examples, a method of aircraft manufacture uses an aircraft section including a fuselage portion coupled to a pair of keel chords. The keel chords extend outside the fuselage portion, form at least part of a lower keel box portion of a keel box, and are coupled to an upper keel box portion of the keel box. In the method, an aft end of the upper keel box portion is mounted onto a forward side of an aft wheel well bulkhead (AWWB). 
     Features, functions, and advantages may be achieved independently in various examples of the present disclosure, or may be combined in yet other examples, further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic side view of an illustrative keel beam assembly coupled to an illustrative aircraft fuselage. 
         FIG. 2  is a magnified fragmentary schematic side view of the keel beam assembly and aircraft fuselage of  FIG. 1 , taken generally around the region indicated at “ 2 ” in  FIG. 1  where an upper portion of a keel box of the keel beam assembly is mounted onto an aft wheel well bulkhead (AWWB) inside the aircraft fuselage, and a lower portion of the keel box is spliced to aft chord extensions outside the aircraft fuselage. 
         FIG. 3  is a further magnified fragmentary schematic side view of the keel beam assembly and aircraft fuselage of  FIG. 1 , taken generally around the region indicated at “ 3 ” in  FIG. 2  and depicting illustrative fasteners securing the upper portion of the keel box to a web of the AWWB and to a backup structure located aft of the AWWB web inside the aircraft fuselage. 
         FIG. 4  is fragmentary schematic top view of the keel beam assembly and aircraft fuselage of  FIG. 1 , taken generally along line  4 - 4  of  FIG. 3  and depicting fasteners securing left- and right-side portions of the upper portion of the keel box to the AWWB web and to respective left and right portions of a backup structure located aft of the AWWB web inside the aircraft fuselage. 
         FIG. 5  is a fragmentary oblique side view of an illustrative, somewhat simplified keel beam assembly coupled to an aircraft fuselage, taken around an aft portion of a keel box of the keel beam assembly and a lower forward portion of an aft aircraft section coupled to an AWWB. 
         FIG. 6  is a sectional view of the keel box of  FIG. 5 , taken generally along line  6 - 6  of  FIG. 5  through the keel box. 
         FIG. 7  is another fragmentary oblique view of the keel beam assembly, AWWB, and aircraft fuselage of  FIG. 5 . 
         FIG. 8  is yet another fragmentary oblique view of the keel beam assembly, aircraft fuselage, and AWWB of  FIG. 5 , taken from a position in the aft aircraft section toward the AWWB, with the fuselage&#39;s structural framework and skin removed. 
         FIG. 9  is a fragmentary schematic sectional view of the keel beam assembly of  FIG. 5 , taken generally along line  9 - 9  of  FIG. 7  around a right-side portion of the keel box. 
         FIG. 10  is a schematic fragmentary side view of the keel box, AWWB, and aft aircraft section of  FIG. 5 . 
         FIG. 11  is an oblique sectional view of another exemplary keel beam assembly, taken in isolation around an aft portion of a keel box of the keel beam assembly. 
         FIG. 12  is a fragmentary schematic sectional view of only a right-side portion of the keel box, taken generally along line  12 - 12  of  FIG. 11 . 
         FIG. 13  is a fragmentary side view of the keel beam assembly of  FIG. 11  mounted to a web of an AWWB and to an illustrative backup structure rearwardly adjacent the AWWB web. 
         FIG. 14  is a fragmentary side view of the keel beam assembly of  FIG. 11  mounted to a web of an AWWB and to another illustrative backup structure rearwardly adjacent the AWWB web. 
         FIG. 15  is a fragmentary side view of the keel beam assembly of  FIG. 11  mounted to a web of an AWWB and to yet another illustrative backup structure rearwardly adjacent the AWWB web. 
         FIG. 16  is a fragmentary side view of the keel beam assembly of  FIG. 11  mounted to a web of an AWWB and to still another illustrative backup structure rearwardly adjacent the AWWB web. 
         FIG. 17  is a fragmentary schematic view of another illustrative keel beam assembly, AWWB, and backup structure for an aircraft. 
         FIG. 18  is a fragmentary schematic sectional view of the keel beam assembly and AWWB of  FIG. 17 , taken generally along line  18 - 18  of  FIG. 17 . 
         FIG. 19  is another fragmentary schematic view of the keel beam assembly and AWWB of  FIG. 17 , taken with another illustrative backup structure rearwardly adjacent a web of the AWWB. 
         FIG. 20  is a flowchart depicting steps of an exemplary method of aircraft manufacture. 
         FIG. 21  is a schematic diagram of an illustrative aircraft. 
         FIG. 22  is a flowchart depicting steps of an illustrative aircraft manufacturing and service method. 
     
    
    
     DETAILED DESCRIPTION 
     Various aspects and examples of a keel beam assembly, keel box, aft wheel well bulkhead (AWWB), chord extension, backup structure, aircraft section, aircraft, and associated methods of the present disclosure in accordance with the present teachings, and/or its various components may, but are not required to, contain at least one of the structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein. Furthermore, unless specifically excluded, the process steps, structures, components, functionalities, and/or variations described, illustrated, and/or incorporated herein in connection with the present teachings may be included in other similar devices and methods, including being interchangeable between disclosed examples. The following description of various examples is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. Additionally, the advantages provided by the examples described below are illustrative in nature and not all examples provide the same advantages or the same degree of advantages. 
     This Detailed Description includes the following sections, which follow immediately below: (1) Overview; (2) Examples, Components, and Alternatives; (3) Illustrative Combinations and Additional Examples; (4) Advantages, Features, and Benefits; and (5) Conclusion. The Examples, Components, and Alternatives section is further divided into subsections A through F, each of which is labeled accordingly. 
     Overview 
     The present disclosure provides a keel beam assembly, an aircraft including the keel beam assembly, and a method of aircraft manufacture involving the keel beam assembly. An illustrative keel beam assembly comprises a pair of keel chords extending outside an aircraft fuselage and forming at least part of a lower keel box portion of a keel box. An upper keel box portion of the keel box is coupled to the pair of keel chords and has an aft end engaged with and secured to an aft wheel well bulkhead (AWWB). 
     In some examples, an aircraft comprises a fuselage, an aft wheel well bulkhead (AWWB), and a keel beam assembly. The keel beam assembly includes a pair of keel chords extending outside the fuselage and forming at least part of a lower keel box portion of a keel box. The keel beam assembly also includes an upper keel box portion of the keel box coupled to the pair of keel chords and having an aft end mounted onto a forward side of the AWWB. 
     In some examples, a method of aircraft manufacture uses an aircraft section including a fuselage portion coupled to a pair of keel chords. The keel chords extend outside the fuselage portion, form at least part of a lower keel box portion of a keel box, and are coupled to an upper keel box portion of the keel box. In the method, an aft end of the upper keel box portion is mounted onto a forward side of an aft wheel well bulkhead (AWWB). 
     The relative positional terms “forward,” “fore,” and “front,” as used herein, have the same meaning as one another, and the relative positional terms “rear,” “back,” “behind,” and “aft,” as used herein, have the same meaning as one another, unless specified otherwise. 
     Examples, Components, and Alternatives 
     The following subsections describe selected aspects of illustrative keel beam assemblies, keel boxes, aft wheel well bulkheads (AWWBs), backup structures, splice configurations, as well as related systems and/or methods. The examples in these subsections are intended for illustration and should not be interpreted as limiting the entire scope of the present disclosure. Each subsection may include one or more distinct examples, and/or contextual or related information, function, and/or structure. 
     A. Illustrative Aircraft and Keel Beam Assembly 
     This subsection describes an illustrative aircraft  100  including a keel beam assembly  102  having a keel box  104  mounted onto a forward side of an aft wheel well bulkhead (AWWB)  106  (see  FIGS. 1-4 ). A schematic overview is provided of illustrative aircraft, keel beam assemblies, keel boxes, keel chord extensions, AWWBs, aft backup structures for keel boxes, and mounting/joint configurations, among others. Any of the aircraft, and/or components and features thereof, described in the subsection may be combined with, modified to include, and/or used for, any suitable components, features, or processes described below in subsections B-F. For instance, within examples, the illustrative aircraft  100  can include the keel beam assembly of aircraft  200 ,  300 ,  400 ,  500 ,  600 ,  700 , or  800 . 
       FIG. 1  shows a schematic side view of keel beam assembly  102  coupled to an aircraft fuselage  108  (interchangeably called a fuselage) of aircraft  100 . Fuselage  108  is formed by multiple aircraft sections arranged along a longitudinal axis  110  of aircraft  100  and joined to one another end to end. In the simplified example depicted in  FIG. 1 , fuselage  108  is formed by respective fuselage portions  112   a,    112   b,  and  112   c  of only three aircraft sections, namely a forward section  114 , a midsection  116 , and a rear section  118 . However, the fuselage may be formed by any suitable number of forward sections and rear sections, each located either forward or rearward of midsection  116 , which itself may include more than one aircraft section. Each aircraft section may have an at least generally cylindrical shape along at least a portion of the section&#39;s central axis. Any of the aircraft sections may be sub-assembled separately and then joined to one another, as described further in subsection E. 
     Midsection  116  has a lower portion under a deck  119  of aircraft and including a central wing box  120  and a wheel well  122  (also see  FIG. 2 ). The central wing box provides anchoring sites for wings of aircraft  100 . Wheel well  122  houses landing gear such as wheels  124 . 
     Keel beam assembly  102  extends along fuselage  108  from forward section  114  to rear section  118  and connects the forward and rear sections (and midsection  116 ) to one another, to strengthen and stabilize the fuselage (see  FIG. 2 ). The keel beam assembly is attached to central wing box  120  and, optionally, to at least one wall member  126  defining at least part of wheel well  122 . 
     Keel beam assembly  102  has a central keel beam portion  128  and a pair of end keel beam portions  130 ,  132  spliced to opposite ends of the central keel beam portion (see  FIG. 2 ). Central keel beam portion  128  extends along the entire length of midsection  116  and is coupled to fuselage portion  112   b  of the midsection. End keel beam portions  130 ,  132  are coupled to respective fuselage portions  112   a,    112   c  of forward section  114  and rear section  118 . Each end keel beam portion  130 ,  132  extends along an aft region of forward section  114  (end keel beam portion  130 ) or along a forward region of rear section  118  (end keel beam portion  132 ). 
     Central keel beam portion  128  includes a pair of keel chords  134   a,    134   b  extending outside fuselage  108  (and/or fuselage portion  112   b ) along longitudinal axis  110  between end keel beam portions  130 ,  132  (see  FIGS. 2 and 3 ). Keel chords  134   a,    134   b  may be described as primary, major, and/or lower keel chords of keel beam assembly  102 . The keel chords form at least part of a lower keel box portion  136  (also called a lower portion) situated under an upper keel box portion  138  (also called an upper portion) of keel box  104  (see  FIG. 3 ). Each keel chord  134   a,    134   b  is located at least predominantly outside fuselage  108  (and/or fuselage portion  112   b ), which means that more than one-half of the keel chord&#39;s mass is situated outside the fuselage or fuselage portion, such as outside an outer mold line defined by the skin/shell of the fuselage or fuselage portion. 
     Each end keel beam portion  130 ,  132  includes a pair of chord extensions extending outside fuselage  108  (see  FIGS. 2 and 3 ). End keel beam portion  130  has a pair of forward chord extensions  140   a,    140   b,  and end keel beam portion  132  has a pair of aft chord extensions  142   a,    142   b.  Each chord extension is spliced end-to-end with one of keel chords  134   a,    134   b  using at least one splicing device  144  secured with fasteners  146 . For example, a pair of fastener-secured splicing devices, such as splice plates, can join each chord extension to an end of one of the keel chords (see subsections B-D). Each chord extension is located at least predominantly outside fuselage  108  (and/or fuselage portion  112   a  or  112   c ), which means that more than one-half of the mass of the chord extension is situated outside the fuselage or fuselage portion, such as outside an outer mold line defined by the skin/shell of the fuselage or fuselage portion. 
     Upper keel box portion  138  of keel box  104  is located over aft portions of keel chords  134   a,    134   b  (see  FIGS. 2-4 ). Upper keel box portion  138  is engaged with, secured to, and/or mounted onto aft wheel well bulkhead (AWWB)  106  of rear section  118 . More specifically, an aft end  148  of upper keel box portion  138  may be engaged with, secured to, and/or mounted onto a forward side  150  of AWWB  106 . 
     The upper keel box portion has several example purposes. It strengthens and stabilizes the keel beam assembly and helps to share loads with, and transfer and distribute loads between, sections of the fuselage. By mounting the upper keel box portion onto the AWWB, loads are transferred both inside and outside the fuselage at the junction between the midsection and rear section of the aircraft. Furthermore, the upper keel box portion raises the center of mass of the keel box to a position closer to the outer mold line of the fuselage. This configuration advantageously reduces the eccentricity in the keel box-to-fuselage splice, resulting in reduced bending moments in the keel beam runout and connecting fuselage structure, thereby allowing a lighter-weight design. 
       FIGS. 2-4  schematically illustrate how upper keel box portion  138  may be mounted to AWWB  106  and coupled to a backup structure  152  located in rear section  118 , aft of at least part of the AWWB. AWWB  106 , interchangeably called an AWWB assembly, includes a web  154  and a frame member  156 , each located under deck  119  inside fuselage  108 . Frame member  156  extends in a circumferential direction along the edge of web  154  and couples the web to a peripheral portion  158  of rear section  118  including a skin  160  (interchangeably called a shell) over a structural framework  162  (also called a peripheral framework). The structural framework may include an assembly of longitudinal stringers and circumferential frame members located inwardly adjacent the skin. 
     AWWB  106  also may include additional components. For example, one or more stiffeners, such as horizontal stiffeners, may be attached to web  154 , such as mounted on a forward side or an aft side of the web. Alternatively, or in addition, one or more beams, such as vertical beams, may be attached to web  154 , such as mounted on an aft side or a forward side of the web. The vertical beams may include a left vertical beam  163   a  and a right vertical beam  163   b,  which are aligned respectively with left- and right-side portions of keel box  104 . The left and right vertical beams perform a dual function, namely, forming part of AWWB  106  and backup structure  152 . Accordingly, these two vertical beams can be described as being included in AWWB  106  or backup structure  152 . Subsections B-D provides further examples of an AWWB including horizontal stiffeners and vertical beams. The term “vertical beam,” as used in this disclosure, means a beam defining a long axis that is within 20 degrees of exactly vertical. Any vertical beam of the present disclosure may be described as a “transverse beam,” which is a beam having its long axis oriented transverse (e.g., orthogonal) to longitudinal axis  110  of the aircraft. 
     Backup structure  152  may include any suitable number of structural members, such as beams or struts, each arranged vertically, horizontally, or obliquely. The backup structure may include left and right backup portions that are engaged with, secured to, and/or mounted onto an aft side  164  of web  154  and/or peripheral portion  158  of rear section  118 . 
     Fasteners  166  secure upper keel box portion  138  to AWWB  106  (see  FIGS. 3 and 4 ). Each fastener  166  extends into and/or through a set of aligned apertures  168 , which may be coaxial with one another. The apertures of the set may, for example, be defined by a flange  170  of upper keel box portion  138 , AWWB  106  (e.g., by web  154  and/or frame member  156 ), and a flange  172  of backup structure  152 . The fastener may be a single piece, such as a bolt  174  in threaded engagement with an internal thread of the aft-most or forward-most aperture of the set of apertures  168 . Alternatively, the fastener may be a fastener assembly of two or more pieces, such as bolt  174  in threaded engagement with a nut  176 . In other examples, the fastener may include a pin, rivet, anchor, or the like. In yet other examples, the upper keel box portion may be secured to the AWWB without fasteners, such as by welding. In some examples, the upper keel box portion has a pair of side portions spaced laterally from one another, and each side portion is separately bolted through a web of the AWWB to a backup structure engaged with an aft side of the web and secured to a peripheral portion of the fuselage. In some examples, each side portion is bolted through the web to a respective vertical beam of the backup structure. 
     Upper keel box portion  138  has a body  178  fastened to a pair of brackets  180  (see  FIG. 4 ). Flanges  170  may be part of brackets  180  and engaged with forward side  150  of AWWB  106 . The upper keel box portion includes a pair of side portions  182   a,    182   b  (i.e., left- and right-side portions) that are spaced laterally from one another, and an upper portion  184  that connects the side portions to one another. Brackets  180  may be used to receive fasteners  166  that separately fasten, such as bolt, each side portion  182   a,    182   b  to AWWB  106 . Upper portion  184  also may be fastened to AWWB  106  separately from side portions  182   a,    182   b  (see subsections B-D). 
     B. Illustrative Keel Beam Assembly, Mounting Configuration, and Backup Structure 
     This subsection describes an illustrative keel beam assembly  202 , keel box  204 , AWWB  206 , mounting configuration, and backup structure  252  of an aircraft  200 ; see  FIGS. 5-10 . 
       FIG. 5  shows an aft portion of keel box  204  of keel beam assembly  202  coupled to a rear section  218  of aircraft  200 . A lower keel box portion  236  and an upper keel box portion  238  of keel box  204  are generally indicated by a pair of dashed boxes. An aft end  248  of upper keel box portion  238  is mounted onto a forward side  250  of aft wheel well bulkhead (AWWB)  206  and engaged with a web  254  of the AWWB. 
       FIG. 6  shows a sectional view of keel box  204 , which is hollow. The vertical extents of lower keel box portion  236  and upper keel box portion  238  are indicated. Lower keel box portion  236  is formed at least in part by lower keel chords  234   a,    234   b  and a lower web  286 . Upper keel box portion  238  includes upper keel chords  288   a,    288   b,  side webs  290   a,    290   b,  and upper web  292 . In at least an aft portion of keel box  204 , upper keel chords  288   a,    288   b  are significantly thinner and lighter than lower keel chords  234   a,    234   b , and under normal flight conditions may bear only a minor fraction of the load on the keel box. For example, in the aft portion of keel box  204 , the upper keel chords may have a mass per unit length and/or a thickness that is less than 50% that of the lower keel chords. Within examples, the upper keel chords may have a mass per unit length and/or a thickness that is less than 25%, 20%, 15%, or 10% that of the lower keel chords. The aft portion of the keel box is located adjacent the AWWB and has a length that is less than 50% of the full length of the keel box. Within examples, the aft portion of the keel box has a length that is less than 33% or 25% of the full length of the keel box. 
     The position of a fragmentary portion of an envelope  293  (also called a trace or skin line) defined by fuselage  208  of aircraft  200  is indicated with a dashed line. Envelope  293  is defined by the exterior surface of skin  260  of the fuselage. The envelope corresponds to the exterior surface of the skin, and theoretical extension or a projected profile of the exterior surface across open regions adjacent the exterior surface where the skin does not physically exist (e.g., where small or large openings are formed in the skin). Keel box structure above the dashed line is located in the fuselage (i.e., inside envelope  293 ), while keel box structure below the dashed line is located outside the fuselage (i.e., outside envelope  293 ). Accordingly, lower keel box portion  236  is located outside the fuselage, and upper keel box portion  238  is located inside the fuselage. A center of mass  294  of an aft portion of keel box  204 , also called the aft portion&#39;s centroid or center of gravity, is situated close to the fuselage exterior (formed by the fuselage shell/skin). Within examples, the center of mass  294  is located less than 10 centimeters from envelope  293 . Within examples, the center of mass  294  is located less than 4, 3, or 2 centimeters from envelope  293 . Positioning the center of mass of the aft portion of the keel box near the fuselage exterior can be advantageous to reduce eccentricity of the keel box-to-fuselage splice, thereby reducing bending moments of the keel beam assembly at high loads. 
     A pair of brackets  280   a,    280   b  enable mounting opposite side portions of the upper keel box portion to AWWB  206  (see  FIGS. 7 and 9 ). Each bracket  280   a,    280   b  has a flange  270  engaged with web  254  of AWWB  206 . The flange defines apertures  268  to receive fasteners  266  that extend through the flange and the AWWB web. In  FIG. 9 , the positions and orientations of apertures  268 , and fasteners  266  received in the apertures, are indicated schematically with crosses. In this schematic convention, each cross indicates the presence of an aperture, or two or more coaxial apertures, defining an aperture axis parallel to the viewing direction of the drawing. The cross also indicates a fastener received in the aperture(s) along the aperture axis. The same schematic convention is used for drawings described in subsections C and D. 
     In the depicted example, each bracket  280   a,    280   b  defines two columns of four apertures  268  each, but more or fewer apertures and fasteners may be used in other examples. Each bracket  280   a,    280   b  may be aligned vertically with a corresponding upper keel chord  288   a,    288   b  and a corresponding lower keel chord  234   a,    234   b,  as shown for bracket  280   a,  upper keel chord  288   a,  and lower keel chord  234   a  in  FIG. 9  (also see  FIG. 6 ). 
     Upper keel box portion  238  also may be secured to AWWB  206  along an upper region of the upper keel box portion at positions intermediate to brackets  280   a,    280   b  (see  FIG. 7 ). The upper region forms a central flange  298  engaged with the AWWB. The central flange may define apertures to receive fasteners that extend into and/or through AWWB  206 . 
     Lower keel chords  234   a,    234   b  are spliced to aft chord extensions  242   a,    242   b  under upper keel box portion  238  and rear section  218  (see  FIGS. 5-9 ). A pair of splice plates  244  are mounted on opposite sides of each lower keel chord and corresponding chord extension using fasteners  246 , to join the lower keel chord and the chord extension to one another axially. 
       FIGS. 8 and 10  show an illustrative backup structure  252  located in rear section  218 . In  FIG. 8 , the skin and structural framework of the rear section have been removed. In  FIG. 10 , crosses indicate positions of apertures/fasteners having axes parallel to the viewing direction, as described above, while sets of parallel dashed lines indicate positions of aperture/fasteners having axes orthogonal to the viewing direction. 
     Backup structure  252  has a backup flange(s)  272  defining apertures  268  that are coaxial with corresponding apertures  268  defined by keel flange  270  of each bracket  280   a,    280   b,  and by web  254  and frame member  256  of AWWB  206 . Fasteners  266  received in sets of coaxially aligned apertures  268  secure each of the brackets (and thus opposite side portions of the upper keel box portion) to the backup structure through AWWB  206 . 
     The backup structure depicted has a left portion aligned with bracket  280   a,  and a right portion aligned with bracket  280   b  (also see  FIG. 7 ). Each of the left and right portions includes a vertical beam  263 , a horizontal beam  203 , and a strut  207  coupled to one another. The vertical beam is mounted onto an aft side of AWWB web  254  and has a lateral position, defined by its distance from a plane of symmetry of the aircraft, matching that of the corresponding bracket, one of the upper keel chords, one of the lower keel chords, and one of the aft chord extensions. The horizontal beam also has a lateral position matching that the same structures and is elongated parallel to the longitudinal axis of the aircraft, along a peripheral portion  258  of rear section  218  that includes a skin  260  and a structural framework  262  (also called a peripheral framework) located inwardly adjacent the skin. A transverse beam  205  is secured to, and extends horizontally between the left and right portions of backup structure  252 , to stabilize the backup structure (see  FIG. 8 ). The backup structure spans two frame bays (see  FIG. 10 ). In other instances, the backup structure spans at least one frame bay, only one frame bay, or less than one frame bay. 
     C. Other Illustrative Keel Beam Assemblies, AWWBs, and Backup Structures 
     This subsection describes other illustrative keel beam assemblies, AWWBs, and backup structures; see  FIGS. 11-16 . Similar to the illustrative keel beam assemblies of subsections A and B, within examples of the illustrative keel beam assemblies of subsection C, an aft portion of the keel box has a center of mass located close to an envelope defined by an exterior surface of a skin of the aircraft fuselage. 
     With the splicing configuration of subsection B, a thick flange of the aft chord extensions, and the thick skin over the aft chord extensions drive high loads into a lower portion of the splice structure. The aft splicing configuration presented in this subsection differs from that described in subsection B, by splicing only the upper and lower keel chords at the AWWB interface, which can provide better access and/or simplify/facilitate assembly of the splice structure, by removing overlapping structural elements. 
       FIG. 11  shows an aft portion of a keel box  304  of a keel beam assembly  302 . Keel box  304  is structured generally as described above in subsection B for keel box  204 . Keel box  304  has a pair of lower keel chords  334   a,    334   b  forming at least part of lower keel box portion  336  of keel box  304 . An upper keel box portion  338  of keel box  304  has a pair of brackets  380   a,    380   b  each defining apertures  368  for receiving fasteners  366  to mount the bracket, and thus the upper keel box portion, to an aft wheel well bulkhead (AWWB) (also see  FIG. 12 ). However, each bracket  380   a,    380   b  is engaged with a smaller vertical extent of the AWWB than the brackets of keel box  204  of subsection B. 
     A pair of splice plates  344  are mounted to opposite sides of each lower keel chords  334   a,    334   b.  Each splice plate has a stepped shape, with the thickness of the plate increasing towards its longitudinal center where a clip  309  is attached. Each clip  309  is configured to be mounted onto an AWWB at a position under a bracket  380   a  or  380   b  (see  FIGS. 11 and 12 ) and stabilizes lower keel chords  334   a,    334   b.  The clip has a clip flange  311  to engage the forward side of the AWWB and defines apertures  313  to receive fasteners  315  that secure the clip to the AWWB and a backup structure behind the AWWB. The clip may be formed separately from, or integrally with, the corresponding splice plate. Clips  309  stabilize the lower keel chords, and in conjunction with brackets  380   a,    380   b,  are advantageous because they provide better load distribution, which reduces the undesirably high loads that can be driven into the lower portion of the keel box/AWWB splice structure. 
       FIG. 13  shows a fragmentary view of an aircraft  300  including keel beam assembly  302  coupled to a rear section  318  of the aircraft that includes a backup structure  352 . Two regions that are empty space are indicated with the word “open”. Bracket  380   a  and clip  309  are engaged with, and mounted onto, a web  354  of an AWWB  306  of rear section  318 . Splice plate  344  under clip  309  is secured to lower keel chords  334   a  and to an aft chord extension  342   a  using fasteners, to splice the lower keel chord to the aft chord extension. 
     Each bracket  380   a,    380   b  is secured to backup structure  352  through AWWB  306  using fasteners (also see  FIGS. 11 and 12 ). Each of the fasteners extends through a keel flange  370  provide by one of the brackets, web  354 , frame member  356 , and a backup flange  372  provided by an integration fitting of backup structure  352 . Further aft, the backup structure may be similar to that described above in subsection B. 
     Backup structure  352  has left and right portions each including a vertical beam  363  and a horizontal beam  303  coupled to one another, as described above in subsection B for backup structure  252 . Horizontal beam  303  is arranged longitudinally in the aircraft and is mounted to a peripheral portion  358  of the fuselage. Each of the portions of the backup structure, and each bracket  380   a,    380   b,  has an oblique wall  329  or  331  extending obliquely away from AWWB  306  to form an open region under the oblique wall. 
       FIG. 14  shows a fragmentary view of an aircraft  400  including a keel beam assembly  402  coupled to a rear section of the aircraft that includes a backup structure  452 . Keel beam assembly  402  has a keel box  404  that is identical to keel box  304  (see  FIG. 13 ), but backup structure  452  is different from backup structure  352 . More specifically, like backup structure  352 , backup structure  452  has left and right portions aligned with corresponding brackets of the keel box. However, each portion has a vertical beam  463  equipped with a foot  417  (also called a boot) that projects aft along the skin and structural framework of the rear section. The horizontal beam present in backup structure  352  has been eliminated in backup structure  452 . Also, vertical beam  463  fits closely over frame member  456  such that the oblique wall of the backup structure and the opening under this oblique wall are eliminated (compare with  FIG. 13 ). 
       FIG. 15  shows a fragmentary view of an aircraft  500  including a keel beam assembly  502  coupled to a rear section of the aircraft that includes a backup structure  552 . Keel beam assembly  502  has a keel box  504  that is identical to keel box  304  (see  FIG. 13 ), but backup structure  552  is different from backup structure  352 . More specifically, like backup structure  352 , backup structure  552  has left and right portions aligned with corresponding brackets of the keel box. Each of the left and right portions has a vertical beam  563  coupled to a horizontal beam  503 . However, unlike backup structure  352 , backup structure  552  and a frame member  556  collectively define a rectangular opening, instead of a triangular opening. 
       FIG. 16  shows a fragmentary view of an aircraft  600  including a keel beam assembly  602  coupled to a rear section of the aircraft that includes a backup structure  652 . Keel beam assembly  602  has a keel box  604  that is identical to keel box  304  (see  FIG. 13 ), but backup structure  652  is different from backup structure  352 . More specifically, like backup structure  352 , backup structure  652  has left and right portions aligned with corresponding brackets of the keel box. Each of the left and right portions has a vertical beam  663 , which fits closely over a frame member  656  along a vertical portion of the frame member but may be spaced from a horizontal portion of the frame member, for instance, to form a gap. In other examples, no gap is formed. Moreover, unlike backup structure  352 , there is no horizontal beam. Instead, an oblique strut  607  couples vertical beam  663  to a peripheral portion of the rear section. 
     D. Further Illustrative Keel Beam Assemblies, AWWBs, and Backup Structures 
     This subsection describes further illustrative keel beam assemblies, AWWBs, and backup structures; see  FIGS. 17-19 . Similar to the illustrative keel beam assemblies of subsections A, B, and C, within examples of the illustrative keel beam assemblies of subsection D, an aft portion of the keel box has a center of mass located close to an envelope defined by an exterior surface of a skin of the aircraft fuselage. 
       FIGS. 17 and 18  show fragmentary views of an aircraft  700  taken around an aft portion of a keel box  704  of a keel beam assembly  702  and a forward portion of a rear section  718  of the aircraft. Keel box  704  has a lower keel box portion  736 , formed at least in part by lower keel chords  734   a,    734   b,  and an upper keel box portion  738 . Lower keel chords  734   a,    734   b  are spliced to aft chord extensions using splice plates  744 , as depicted in  FIG. 17  for lower keel chord  734   a  and aft chord extension  742   a,  and as described in more detail above in subsections A-C. 
     Upper keel box portion  738  is mounted onto an AWWB  706  of rear section  718  along opposite side portions and along an upper portion. A pair of brackets  780   a,    780   b  are fastened to opposite side portions of a body  778  of the upper keel box portion using fasteners  721 . More specifically, each bracket is fastened at an upper keel chord  788   a  or  788   b,  a side web  790   a  or  790   b,  and/or a chord flange  723  of a lower keel chord  734   a  or  734   b.  Each bracket  780   a,    780   b  defines apertures  768  for fasteners  766  that extend through a web  754  and/or a frame member  756  of AWWB  706  to a backup structure  752 , such that the bracket is secured to the backup structure through the AWWB. An upper portion of body  778  of upper keel box portion  738  is secured to a horizontal stiffener  725  of AWWB  706 . More specifically, fasteners  727  secure a horizontal ledge  729  of horizontal stiffener  725  to an upper web  792  and upper keel chords  788   a,    788   b  of body  778 . Other fasteners, such as a fastener  731 , secure a stiffener flange  733  to web  754  of the AWWB. 
     Backup structure  752  has left and right portions each including a vertical beam  763  coupled to an oblique strut  707 . An aft end of the strut is coupled to a peripheral portion  758  of the fuselage. 
       FIG. 19  shows a fragmentary view of an aircraft  800  including a keel box  804  coupled to a rear section of the aircraft that includes a backup structure  852 . Keel box  804  is identical to keel box  704  (see  FIG. 17 ), but backup structure  852  is different from backup structure  752 . More specifically, backup structure  852  has a vertical beam  863  equipped with a foot  817  that projects aft, as described above for backup structure  452  (see subsection C and  FIG. 14 ). 
     E. Illustrative Method of Aircraft Manufacture 
     This subsection describes an illustrative method  900  of aircraft manufacture involving a keel beam assembly; see  FIG. 20 . The method steps described here may be performed in any suitable order, including simultaneously, in any suitable combination, including with one or more of the steps omitted, and may be performed using any keel assembly, keel box, AWWB, backup structure, and/or aircraft section(s) of the present disclosure. 
     At step  902 , a keel box and an AWWB are selected. The keel box may be part of a keel beam assembly including a pair of keel chords forming at least part of a lower keel box portion of the keel box. The AWWB may be a single component or an assembly of coupled components including any combination of a web, one or more horizontal stiffeners mounted on a forward side (or on an aft side) of the web, a frame member extending along and secured to an edge of the web (optionally on an aft side of the web and/or along an arcuate edge of the web), and/or one or more vertical beams mounted on an aft side (or a forward side) of the web. 
     In some examples, an aircraft section including a fuselage portion may be selected. The fuselage portion, which may be described as a fuselage shell portion, may be coupled to the keel beam assembly, and the keel chords may extend outside an exterior surface of the fuselage portion and/or fuselage shell portion (i.e., outside an envelope defined an exterior surface of the skin of the fuselage portion). In some examples, the keel chords may form part of a keel box having an aft portion, and in the aft portion, the keel chords may be located at least predominantly outside the fuselage portion and/or fuselage shell portion (i.e., at least predominantly outside the envelope (greater than 50% outside the envelope by mass or volume)). 
     In some examples, first and second aircraft sections may be selected. When selected, the aircraft sections may have no mechanical connection to one another. The first aircraft section may be a midsection for an aircraft. The first aircraft section may include a first fuselage shell portion. The first fuselage shell portion may already be coupled to the pair of keel chords. Each of the keel chords may extend outside the first fuselage shell portion (i.e., outside an envelope defined by an exterior surface of a skin of the shell portion) and may be located at least predominantly outside the fuselage shell portion (i.e., at least predominantly outside the envelope), particularly in an aft portion of a keel box formed in part by the keel chords. The keel chords may form at least part of the lower portion of the keel box and may be coupled already to the upper portion of the keel box when the aircraft sections are selected. The second aircraft section may include a second fuselage shell portion, which may or may not already coupled to an aft wheel well bulkhead (AWWB) and/or to at least one or a pair of aft chord extensions, when the second aircraft section is selected. 
     At step  904 , the upper keel box portion is mounted onto the AWWB, on a forward side of the AWWB. Mounting may include positioning the entire upper keel box portion forward of a web of the AWWB, and/or such that the upper keel box portion does not extend through the web of the AWWB. More specifically, mounting may be performed by extending fasteners through the AWWB, such as through a web, a peripheral frame member, a horizontal stiffener, and/or a vertical beam thereof. Mounting may create an initial mechanical connection between the aircraft sections or may be performed after the aircraft sections have already been connected to one another mechanically. 
     At step  906 , the keel chords are spliced to aft chord extensions. More specifically, an aft end of at least one keel chord may be joined to a forward end of at least one aft chord extension. Splicing may be performed with a splicing device, such as a splice plate, secured with fasteners to one of the keel chords and to an aft chord extension. 
     F. Illustrative Aircraft and Associated Method 
     Examples disclosed herein may be described in the context of an illustrative aircraft  1000  (see  FIG. 21 ) and an illustrative aircraft manufacturing and service method  1100  (see  FIG. 22 ). Method  1100  includes a plurality of processes, stages, or phases. During pre-production, method  1100  may include a specification and design phase  1104  of aircraft  1000  and a material procurement phase  1106 . During production, a component and subassembly manufacturing phase  1108  and a system integration phase  1110  of aircraft  1000  may take place. Thereafter, aircraft  1000  may go through a certification and delivery phase  1112  to be placed into in-service phase  1114 . While in service (e.g., by an operator), aircraft  1000  may be scheduled for routine maintenance and service phase  1116  (which may also include modification, reconfiguration, refurbishment, and so on of one or more systems of aircraft  1000 ). While the examples described herein relate generally to the production and operational use of aircraft  1000 , they may be practiced at other stages of method  1100 . 
     Each of the processes of method  1100  may be performed or carried out by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include, without limitation, any number of aircraft manufacturers and major-system subcontractors; a third party may include, without limitation, any number of vendors, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on. 
     As shown in  FIG. 21 , aircraft  1000  produced by illustrative method  1100  may include a frame  1002  with a plurality of systems  1004  and an interior  1006 . Examples of plurality of systems  1004  include one or more of a propulsion system  1008 , an electrical system  1010 , a hydraulic system  1012 , an environmental system  1014 , and a flight control system  1016 . Each system may comprise various subsystems, such as controllers, processors, actuators, effectors, motors, generators, etc., depending on the functionality involved. Any number of other systems may be included. Although an aerospace example is shown, the principles disclosed herein may be applied to other industries, such as the automotive industry, rail transport industry, and nautical engineering industry. Accordingly, in addition to aircraft  1000 , the principles disclosed herein may apply to other vehicles, e.g., land vehicles, marine vehicles, etc. 
     Apparatuses and methods shown or described herein may be employed during any one or more of the stages of the aircraft manufacturing and service method  1100 . For example, components or subassemblies corresponding to component and subassembly manufacturing phase  1108  may be fabricated or manufactured in a manner similar to components or subassemblies produced while aircraft  1000  is operating during in-service phase  1114 . Also, one or more examples of the apparatuses, methods, or combinations thereof may be utilized during manufacturing phase  1108  and system integration phase  1110 , for example, by substantially expediting assembly of or reducing the cost of aircraft  1000 . Similarly, one or more examples of the apparatus or method realizations, or a combination thereof, may be utilized, for example and without limitation, while aircraft  1000  is in in-service phase  1114  and/or during maintenance and service phase  1116 . 
     Illustrative Combinations and Additional Examples 
     This section describes additional aspects and features of the keel beam assemblies, keel boxes, chord extensions, AWWBs, backup structures, aircraft sections, aircraft, and associated methods of the present disclosure, presented without limitation as a series of paragraphs, some or all of which may be alphanumerically designated for clarity and efficiency. Each of these paragraphs can be combined with one or more other paragraphs, and/or with disclosure from elsewhere in this application, in any suitable manner. Some of the paragraphs below expressly refer to and further limit other paragraphs, providing without limitation examples of some of the suitable combinations. 
     A1. A keel beam assembly, comprising: (i) a pair of keel chords extending outside an aircraft fuselage (e.g., outside a shell/skin of the aircraft fuselage) and forming at least part of a lower keel box portion of a keel box; and (ii) an upper keel box portion of the keel box coupled to the pair of keel chords and having an aft end engaged with and secured to an aft wheel well bulkhead (AWWB). 
     A2. The keel beam assembly of paragraph A1, wherein an aft portion of the keel box has a center of mass located close to an envelope defined by an exterior surface of a skin of the aircraft fuselage. 
     A2a. The keel beam assembly of paragraph A2, wherein the center of mass is located less than 10 centimeters from the envelope. 
     A2b. The keel beam assembly of paragraph A2, wherein the center of mass is located less than 4, 3, or 2 centimeters from the envelope. 
     A3. The keel beam assembly of any of paragraphs A1 to A2b, wherein the upper keel box portion is bolted to the AWWB. 
     A4. The keel beam assembly of any of paragraphs A1 to A3, wherein the AWWB includes a web, wherein the upper keel box portion is secured to a backup structure through the web, and wherein the backup structure is engaged with an aft side of the web and secured to a peripheral portion of the aircraft fuselage. 
     A5. The keel beam assembly of paragraph A4, wherein the keel box includes a body and one or more brackets fastened to the body, and wherein each bracket has one or more flanges engaged with the web of the AWWB and defining apertures through which the bracket is secured to the backup structure through the web using fasteners. 
     A6. The keel beam assembly of any of paragraphs A1 to A5, further comprising a pair of aft chord extensions spliced to the pair of keel chords at aft ends of the keel chords and extending outside the aircraft fuselage. 
     A7. The keel beam assembly of any of paragraphs A1 to A6, wherein the AWWB includes a web and a horizontal stiffener located on a forward side of the web, and wherein the upper keel box portion is engaged with and fastened to the horizontal stiffener. 
     A8. The keel beam assembly of any of paragraphs A1 to A7, further comprising a clip having a first portion mounted to the keel box at one of the keel chords and a second portion engaged with and secured to a forward side of the AWWB. 
     A9. The keel beam assembly of any of paragraphs A1 to A8, wherein the keel box is hollow. 
     A10. The keel beam assembly of any of paragraphs A1 to A9, wherein the keel box has an upper web, a lower web, and a pair of side webs (e.g., left- and right-side webs) spaced laterally from one another. 
     A11. The keel beam assembly of any of paragraphs A1 to A10, wherein the pair of keel chords is a pair of lower keel chords, wherein the keel box includes a pair of upper keel chords, an upper web extending between the upper keel chords and a lower web extending between the lower keel chords, and a pair of side webs (e.g., left- and right-side webs) laterally spaced from one another and each extending between an upper keel chord and a lower keel chord. 
     A12. The keel beam assembly of any of paragraphs A1 to A11, wherein the pair of keel chords are a left lower keel chord and a right lower keel chord, wherein the upper keel box portion includes a left upper keel chord and a right upper keel chord. 
     A12a. The keel beam assembly of paragraph A12, wherein the lower keel chords are primary/major keel chords and the upper keel chords are minor/secondary keel chords, wherein the left lower keel chord and the left upper keel chords are aligned with one another in a left plane (optionally a vertical left plane), and wherein the right lower keel chord and the right upper keel chord are aligned with one another in a right plane (optionally a vertical right plane). 
     A13. The keel beam assembly of paragraph A12 or A12a, wherein the upper keel box portion includes a left bracket and a right bracket each engaged with the AWWB, wherein the left bracket is aligned with the left lower keel chord and the left upper keel chord in the left plane, and wherein the right bracket is aligned with the right lower keel chord and the right upper keel chord in the right plane. 
     A14. The keel beam assembly of paragraph A12, A12a, or A13, wherein the AWWB includes a web, and wherein the upper keel box portion is coupled through the web to a left vertical beam and a right vertical beam located aft of the web, wherein the left vertical beam is aligned with the left lower keel chord and the left upper keel chord in the left plane, and wherein the right vertical beam is aligned with the right lower keel chord and the right upper keel chord in the right plane. 
     A15. The keel beam assembly of any of paragraphs A1 to A14, wherein an aft portion of the keel chords of the pair of keel chords extends along an aft portion of the keel box and is located at least predominantly outside the aircraft fuselage. 
     A15a. The keel beam assembly of paragraph A15, wherein at least 50%, 60%, 70%, or 80% of the mass of the aft portion of the keel chords is located outside an envelope defined by an exterior surface of a skin/shell of the aircraft fuselage. 
     A16. The keel beam assembly of any of paragraphs A1 to A15a, wherein the keel beam assembly is mounted to a backup structure that is rearwardly adjacent a web of the AWWB. 
     A17. The keel beam assembly of paragraph A16, wherein the backup structure includes a strut having a longitudinal axis arranged obliquely to a longitudinal axis of the aircraft fuselage. 
     A18. The keel beam assembly of paragraph A17, wherein the strut has a forward end coupled to a vertical beam of the backup structure and an aft end coupled to a structural framework of the aircraft fuselage located inwardly adjacent a skin of the aircraft fuselage. 
     A19. The keel beam assembly of any of paragraphs A16 to A18, wherein the backup structure includes a vertical beam mounted onto an aft side of the web of the AWWB. 
     A20. The keel beam assembly of paragraph A19, wherein the vertical beam extends to a deck located over the backup structure. 
     A21. The keel beam assembly of paragraph A20, wherein the vertical beam includes a body defining a longitudinal axis, optionally oriented parallel to a plane defined by the AWBB, and also includes a foot projecting rearwardly from the body and secured to a structural framework located inwardly adjacent a skin of the aircraft fuselage. 
     A22. The keel beam assembly of paragraph A21, wherein the body and the foot are formed integrally with one another. 
     A23. The keel beam assembly of paragraph A21, wherein the body and the foot are formed separately from one another. 
     A24. The keel beam assembly of any of paragraphs A16 to A23, wherein the backup structure includes a horizontal beam elongated parallel to a longitudinal axis of the aircraft, and wherein the horizontal beam is located on and/or secured to a structural framework and/or a skin of the aircraft fuselage. 
     A25. The keel beam assembly of any of paragraphs A16 to A24, wherein the backup structure spans at least one or two frame bays of the aircraft fuselage. 
     A26. The keel beam assembly of any of paragraphs A16 to A24, wherein the backup structure spans less than one frame bay of the aircraft fuselage. 
     A27. The keel beam assembly of any of paragraphs A1 to A26, wherein the AWWB includes a web having a peripheral edge and a frame member extending along the peripheral edge and secured to web and the aircraft fuselage, and wherein the upper portion of the keel box is mounted onto a forward side of the web using fasteners that extend through the web and the frame member. 
     B1. An aircraft, comprising: (i) a fuselage; (ii) an aft wheel well bulkhead (AWWB); and (iii) a keel beam assembly including a pair of keel chords extending outside the fuselage and forming at least part of a lower keel box portion of a keel box, the keel beam assembly also including an upper keel box portion of the keel box coupled to the pair of keel chords and having an aft end mounted onto a forward side of the AWWB. 
     B2. The aircraft of paragraph B1, further comprising a pair of aft chord extensions spliced to the pair of keel chords at aft ends of the keel chords and extending outside the fuselage. 
     B3. The aircraft of paragraph B2, wherein an aft portion of the keel box has a center of mass located close to an envelope defined by an exterior surface of a skin of the fuselage. 
     B3a. The aircraft of paragraph B3, wherein the center of mass is located less than 10 centimeters from the envelope. 
     B3b. The aircraft of paragraph B3, wherein the center of mass is located less than 4, 3, or 2 centimeters from the envelope. 
     B4. The aircraft of any of paragraphs B1 to B3b, wherein the AWWB includes a web, and wherein the upper keel box portion is mounted onto a forward side of the web, further comprising a backup structure engaged with an aft side of the web and fastened to the upper keel box portion through the web. 
     B5. The aircraft of paragraph B4, wherein the upper keel box portion has a pair of side portions (e.g., left- and right-side portions) spaced laterally from one another, and wherein each side portion is separately bolted through the web to a backup structure engaged with an aft side of the web and secured to a peripheral portion of the fuselage. 
     B6. The aircraft of paragraph B5, wherein each side portion is bolted through the web to a respective vertical beam of the backup structure. 
     B7. The aircraft of any of paragraphs B1 to B6, wherein the keel box includes a body and one or more brackets fastened to the body, and wherein each bracket has one or more flanges engaged with the AWWB and defining apertures through which the bracket is mounted onto the AWWB. 
     B8. The aircraft of any of paragraphs B1 to B7, further comprising any limitation(s) of any of paragraphs A1 to A27 of this section. 
     C1. A method of aircraft manufacture using an aircraft section including a fuselage portion coupled to a pair of keel chords, wherein the keel chords extend outside the fuselage portion, form at least part of a lower keel box portion of a keel box, and are coupled to an upper keel box portion of the keel box, the method comprising: mounting an aft end of the upper keel box portion onto a forward side of an aft wheel well bulkhead (AWWB). 
     C2. The method of paragraph C1, wherein the AWWB includes a web, and wherein mounting includes extending a plurality of fasteners through the web of the AWWB. 
     C3. The method of paragraph C2, wherein mounting includes bolting the upper keel box portion through the web of the AWWB to a vertical beam located on an aft side of the web. 
     C4. The method of any of paragraphs C1 to C3, wherein the aircraft section is a first aircraft section, the method further comprising coupling the AWWB to a fuselage portion of a second aircraft section prior to mounting. 
     C5. The method of any of paragraphs C1 to C4, wherein the AWWB includes a web, and wherein mounting comprises positioning the keel box such that no part of the keel box extends through the web. 
     C6. The method of any of paragraphs C1 to C5, further comprising any limitation(s) of any of paragraphs A1 to A27 or B1 to B7 of this section. 
     Advantages, Features, and Benefits 
     The different examples of a keel box, a keel beam assembly, an AWWB, a backup structure, aircraft sections, and corresponding aircraft and associated manufacturing methods described herein, provide several advantages over known solutions. For example, illustrative examples described herein of a keel box and AWWB provide any combination of improved load transfer, separate fabrication and construction, simplified joining, lighter weight, lower cost, fewer parts, and faster more economical aircraft production. 
     Additionally, and among other benefits, illustrative examples described herein of a keel box place primary chords of the keel box outside the fuselage shell/skin of an aircraft or aircraft section, and an upper portion of the keel box inside the fuselage shell/skin. This configuration advantageously positions the center of gravity of an aft portion of the keel box close to the shell/skin exterior surface (e.g., less than 10, 4, 3, or 2 centimeters from an envelope defined by the exterior surface of the skin/shell). As a result, the overall induced moment fora large load transfer is minimized, which provides better aerodynamic control and less stress on the aircraft. Moreover, this configuration optimally lines up the keel box with the fuselage skin at the aft end of the keel box, where the loads are highest. 
     Additionally, and among other benefits, illustrative examples herein of a keel box fully splice an upper portion of the keel box at the AWWB, without any pass-through or continuous members of the keel box extending through and/or interrupting the web of the AWWB. This configuration enables faster, more efficient aircraft production, as it allows the keel box and AWWB to be fabricated and sub-assembled independently, and then joined to one another at a later stage of aircraft manufacture using fasteners such as bolts, with a relatively small amount of time and effort. 
     Additionally, and among other benefits, illustrative examples described herein simplify the fabrication and construction of the AWWB for an aircraft. 
     Additionally, and among other benefits, illustrative examples described herein of a keel box save weight by having the centroid of the keel box align closely with the fuselage skin. This configuration provides a structurally efficient load path that reduces overall eccentricity-induced bending, which allows for a lighter-weight design of the keel box. 
     Additionally, and among other benefits, illustrative examples described herein simplify the fabrication and assembly of the AWWB, by reducing the usage of titanium and the time needed to drill/slot the titanium. This reduced usage and machining of titanium reduces cost and weight. 
     Additionally, and among other benefits, illustrative examples described herein enable production of midsize aircraft. By fully splicing a keel box at the AWWB, this approach allows the keel box, the AWWB, and an aft fuselage body to be independently fabricated and then assembled without requiring complicated slotting or threading of parts of the keel box through the web of the AWWB. This approach allows for faster and simpler end item joining. 
     Additionally, and among other benefits, illustrative examples described herein use one or more parts of the AWWB for splicing to the keel box, which reduces the number of parts needed for assembly. 
     The disclosed keel box, keel beam assembly, AWWB, aircraft, and aircraft production system can provide these example benefits over known keel boxes, keel beam assemblies, AWWB, aircraft, and aircraft production systems. However, not all examples described herein provide the same advantages or the same degree of advantage. 
     CONCLUSION 
     The disclosure set forth above may encompass multiple distinct examples with independent utility. Although each of these has been disclosed in one or more illustrative forms, the specific examples thereof as disclosed and illustrated herein are not to be considered in a limiting sense, because numerous variations are possible. To the extent that section headings are used within this disclosure, such headings are for organizational purposes only. The subject matter of the disclosure includes all novel and nonobvious combinations and subcombinations of the various elements, features, functions, and/or properties disclosed herein. The following claims particularly point out certain combinations and subcombinations regarded as novel and nonobvious. Other combinations and subcombinations of features, functions, elements, and/or properties may be claimed in applications claiming priority from this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.