Abstract:
A mount yoke assembly comprises a first main hanger bracket and a first anchor bracket. The first main hanger bracket includes a first receiving portion for a first auxiliary power unit (APU) mount with first and second hanger arms. The first hanger arm includes a first hanger flange extending proximate a distal end of the first hanger arm. The first anchor bracket includes a first plurality of anchor holes and a first anchor flange. The first plurality of anchor holes are disposed substantially along a length of the first anchor bracket for securing the mount yoke assembly to a component of an APU assembly. The first anchor flange extends from the first anchor bracket for hingeably securing the first anchor bracket to the first main hanger bracket.

Description:
BACKGROUND 
     This application relates generally to systems and methods for securing aircraft components to the airframe, and more particularly systems and methods for securing the auxiliary power unit (APU) and its related components. 
     An APU assembly with its constituent components is typically secured to the body of an aircraft by a plurality of mount struts. The struts connect at one end to the airframe while the opposing end of each strut is attached to the APU component and/or housing via individual mounts. These mounts were previously fixed directly to the subject component or housing to transfer inertial, thrust, torsional and other loads to the multiple connecting struts between each mount and the airframe. 
     Critical systems and components like the APU must be able to resist failure during a number of emergency situations to optimize flight safety and maintain certification. One set of airworthiness regulations define the minimum duration of time that various aircraft components and systems must be able to withstand a fire. When the mount attaches directly to the assembly, and particularly when it is housed in a lightweight structure, the housing may be weakened or fail during a fire concentrated near one of the mounts. Thus it would be helpful to provide a system whereby the APU assembly can to withstand a fire for an increased time period without structural or operational failure. 
     SUMMARY 
     A mount yoke assembly comprises a first main hanger bracket and a first anchor bracket. The first main hanger bracket includes a first receiving portion for a first auxiliary power unit (APU) mount with first and second hanger arms. The first hanger arm includes a first hanger flange extending proximate a distal end of the first hanger arm. The first anchor bracket includes a first plurality of anchor holes and a first anchor flange. The first plurality of anchor holes are disposed substantially along a length of the first anchor bracket for securing the mount yoke assembly to a component of an APU assembly. The first anchor flange extends from the first anchor bracket for hingeably securing the first anchor bracket to the first main hanger bracket. 
     An auxiliary power unit (APU) mount assembly comprises a first APU mount, a strut for linking the first APU mount to a rigid structure, a first main hanger bracket and a first anchor bracket. The first main hanger bracket includes a first receiving portion removably securing the first APU mount to the first main hanger bracket. The first receiving portion is with first and second hanger arms. The first anchor bracket includes a first plurality of anchor holes and a first anchor flange. The first plurality of anchor holes are disposed substantially along a length of the first anchor bracket affixing the APU mount assembly to a first plurality of yoke connections on a first component of the APU assembly. The first anchor flange hingeably secures the first anchor bracket to the first main hanger bracket. 
     A method for retrofitting an APU mount assembly to an existing APU mount is disclosed. A first existing APU mount is removed from a first component of an APU assembly at a first mounting location. A first anchor bracket is secured to the first component of the APU assembly at a first plurality of yoke connections disposed proximate the first mounting location. The first anchor bracket has a plurality of anchor holes along its length, with a shape of the first anchor bracket and a location of at least one of the plurality of anchor holes corresponding to a position of at least one of the first plurality of yoke connections. The first anchor bracket is secured to a first hanger bracket at a first hinge portion disposed proximate a first distal end of the first hanger arm. The the first existing APU mount is secured to an APU mount receiving portion of the first hanger bracket. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  schematically depicts an APU assembly secured via a plurality of mounts and a mount yoke assembly. 
         FIG. 1B  shows APU assembly with one example mounting location for the mount yoke assembly. 
         FIG. 2  shows the APU mount yoke assembly. 
         FIG. 3A  depicts the main hanger bracket portions of the mount yoke assembly. 
         FIG. 3B  shows the anchor bracket portions of the mount yoke assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows APU assembly  10 , APU  12 , gearbox  14 , housing  16 , struts  18 , APU mounts  22 A,  22 B, mount yoke assembly  24 , main hanger brackets  26 , yoke brackets  28 , and anchor holes  30 . APU assembly  10  includes APU  12  and gearbox  14  contained within housing  16 . APU  12  provides electrical and/or hydraulic power for an aircraft, primarily upon startup and during emergencies. In many cases, including this example, APU  12  is a gas turbine engine configured in-line with gearbox  14 . Gearbox  14  transfers rotational power of APU  12  to one or more electrical and/or hydraulic generators. The generators and other ancillary APU equipment are not shown in  FIGS. 1A and 1B  to better illustrate mount yoke assembly  24 . In this example, these generators would be located in-line with APU  12  and gearbox  14  on the side of gearbox  14  opposing APU  12 . In other words, the generator(s) would project outwardly from the page. 
     Because it is both an emergency and an auxiliary component, APU assembly  10  must meet stringent airworthiness standards and be available in the case of emergencies including those due to fire. Airworthiness regulations require that an APU and its housing be able to withstand a fire for a sufficient period of time. For example, current U.S. regulations mandate that the APU and gearbox be able to withstand 15 minutes of a 2000° F. (˜1100° C.) fire. Previous mounting hardware included only struts and localized mounts fixed directly to the housing or other APU structure. This simple arrangement can generally meet certification standards if the fire generally occurs away from the mounts. 
     However, if the fire is concentrated directly around one or more of the localized mounts linking the housing to the airframe, the risk of failure increases. Failure from a directed conflagration is more likely when the housing comprises a lightweight material like aluminum or its alloys. Though aluminum alloys are often used to reduce overall weight, one option to meeting the standard would be to utilize a stronger material for the housing. However, aircraft structural materials are selected with an eye toward balancing cost, weight, and strength to minimize overall aircraft weight and improve efficiency. Components like APUs and their ancillary structures can add substantial mass to the aircraft. Thus, high-strength aluminum alloys are frequently used for housing the APU and gearbox due to cost and weight considerations, despite the lower melting point of such alloys as compared to steel or titanium. 
     When mounts without yokes or other brackets are used to secure an APU assembly, the support forces are often concentrated over a small mount footprint on the housing. As such, an aluminum or other lightweight housing is more likely to fail relatively quickly in the event of a fire occurring proximate one or more of the localized art mount locations. Once the aluminum alloy begins to soften from elevated temperatures, inertial and operational forces (which are often more intense due to emergency maneuvers occurring during an onboard fire), can overcome the ability of that portion of the housing to support the APU assembly. The remaining struts and mounts must then absorb the remaining forces. However, support imbalances become more pronounced, which increases the likelihood that the housing will fail in other locations, particularly if the fire spreads toward the remaining mounts. 
     In contrast to a localized mounting structure,  FIG. 1A  shows mount yoke assembly  24  which engages with APU mounts  22 A,  22 B to support APU assembly  10  substantially around the perimeter of housing  16 . Mount yoke assembly  24  includes at least one main hanger bracket  26 A and at least one yoke anchor bracket  28 A. Main hanger brackets  26 A,  26 B each include a receiving portion (shown in  FIGS. 2 and 3A ), which is shaped to slidably or mountably engage corresponding mounts  22 A,  22 B, while anchor brackets  28 A,  28 B are hingeably connected to respective hanger brackets  26 A,  26 B. Anchor brackets  28 A,  28 B are secured to housing  16  using anchor holes  30 . For example, anchor holes  30  can receive bolts, pins, or other fasteners passing through anchor holes  30  and yoke connections on housing  16 . (examples shown in  FIG. 1B ). 
     One or more struts  18  then join each mount  22 A,  22 B to the airframe or other rigid structure (not shown). As such, struts  18 , mounts  22 A,  22 B, and mount yoke assembly  24  cooperate to support APU assembly  10  and transfer operational and inertial forces to the body of the aircraft. The combination of mounts  22 A,  22 B and mount yokes  24  takes forces ordinarily concentrated only around the relatively small footprint of individual localized mounts and distributes them over a much larger footprint on housing  16 . Mount yoke assembly  24  supports APU assembly  10  over a substantial portion of housing  16 , so thus it would take a much larger and more widespread fire over the larger footprint in order to cause failure of housing  16  or other APU components. 
     Mounts  22 A,  22 B can be new mounts fabricated specifically for this purpose. They can alternatively be existing mounts which are modified if necessary to improve engagement with yoke assembly  24 . In some cases, mounts  22 A,  22 B are modified or replaced to more easily accommodate corresponding hanger brackets  26 A,  26 B. For example, portions of existing mounts  22 A,  22 B may be extended, shortened, or reshaped to improve the transfer of forces between APU assembly  10  and struts  18 . And while the design may not be optimal as compared to a new installation, retrofitting mount yoke assembly  24  to APU assembly  10  can inexpensively and simply allow existing APU assemblies to meet more stringent airworthiness requirements. 
     Many existing mounts  22 A,  22 B are based primarily on austenitic steel, such as but not limited to ASTM 304L. However, new or existing mounts  22 A,  22 B can be any material of sufficient strength and thickness to meet certification requirements. Mounts  22 A,  22 B can also include other features such as but not limited to various joints, knuckles, etc. to help withstand bending and twisting motion of APU assembly  10  relative to struts  18 . Regardless of its exact features, mounts  22 A,  22 B include one or more structures to connect with mount yoke assembly  24 , an example of which will be seen in more detail in  FIGS. 2 and 3A . 
       FIG. 1B  includes APU assembly  10  from  FIG. 1A  with APU  12 , gearbox  14 , housing  16 , gears  32 , and yoke connections  34 . Struts  18 , mounts  22 A,  22 B, mount yoke assembly  24 , main hanger brackets  26 A,  26 B, and yoke anchor brackets  28 A,  28 B, also from  FIG. 1A , are shown in phantom to better illustrate the underlying features of gearbox  14  and housing  16 . 
       FIG. 1B  shows a portion of housing  16  cut away from gearbox  14  to better illustrate gears  32 . Gears  32  optimize APU shaft power for auxiliary accessories such as electrical and/or hydraulic generators (not shown). It can be seen that anchor brackets  28  are sized and shaped to avoid interfering with the operation and interaction of gears  32  and other internal components of gearbox  14 . This can be done, for example by providing yoke connections  34  substantially around the perimeter of housing  16 , which correspond to anchor holes  30  on brackets  28 A,  28 B. Anchor brackets  28 A,  28 B also can be shaped to avoid interference with generators or other APU-driven components (not shown) to be mounted proximate the visible face of gearbox  14 . In this example, securing yoke assembly  24  to housing  16  does not interfere with the operation of gearbox  14  even when the securing means such as bolts or pins extend into the interior of housing  16 . 
     Housing  16  also includes yoke connections  34  to engage fasteners like bolts or anchors. In the example shown in  FIGS. 1A and 1B , brackets  28 A,  28 B are shaped to fit around parts of gearbox  14 , and are aligned to correspond with anchor holes  30  (shown in  FIG. 1A ) of anchor brackets  28 A,  28 B. Yoke connections  34  can be existing pre-drilled connections for generators or other ancillary APU components, or they can be added specifically for the purpose of attaching mount yoke assembly  24 . Yoke connections  34  can include simple threaded or unthreaded bolt holes or flanges, and can additionally or alternatively include one or more bosses. 
     Mounts  22 A,  22 B and mount yoke assembly  24  can either be designed for incorporation into a new APU assembly  10 , or for retrofit onto an existing design. In the case of a new design, the shape of anchor brackets  28 A,  28 B, along with the arrangement of anchor holes  30 , and yoke connections  34  are selected as part of the design process to achieve an overall optimal balance between strength, size, cost, and weight. In a retrofit application, locations on housing  16  are selected or added for yoke connections  34  with these factors in mind as well. However, the locations may deviate from the optimal support arrangement in order to utilize existing features of housing  16 , while still being sufficient to meet or exceed fire safety and airworthiness regulations. 
     In the case of a retrofit, there are often unused potential mounting locations on the components of APU assembly  10 . These mounting locations are identified and reinforced if necessary to act as yoke connections  34 . Anchor brackets  28  are shaped to align with these yoke connections  34  and provided with corresponding anchor holes  30 . Additional yoke connections  34  can be added if needed to further increase the footprint of mount yoke assembly  24 . Hanger brackets  26  are then provided with a receiving portion for a mount  22 A,  22 B and with flanges or other hinge-like structures to engage with complementary flanges on anchor brackets  28 A,  28 B. As such, anchor holes  30  and corresponding yoke connections  34  each provide a plurality of individual connections over a substantial portion of housing  16  that replace the single connection originally provided by each single APU mount location. 
     Regardless of whether yoke assembly  24  is for a new design or a retrofit, it operates to substantially increase the footprint linking APU assembly  10  (including housing  16 ) to struts  18 . While yoke assembly  24  often causes a marginal weight increase as compared to mounting APU assembly  10  directly to mounts  22 A,  22 B, the increased margin of safety in the event of a fire is well worth any de minimis efficiency loss. Further, yokes  24  nevertheless have a substantial weight advantage over replacing the entire housing  16  with a stronger and more temperature resistant material like titanium or steel which may otherwise be required to meet certification standards. 
     In this illustrative example, mount yokes  24  have been shown relative to a gearbox  14  mounted in-line with APU  12 . However, mount yokes  24  are applicable to any configuration of an APU and gearbox to improve fire resistance of one or more connections between the mounts and the APU assembly. In addition, the example in  FIGS. 1A and 1B  depict mount yoke assembly  24  as being removably secured to the exterior of housing  16 . However, it will be appreciated that yoke assembly  24  can be similarly secured to the interior of APU  12 , such as to an inner surface of housing  16 . 
       FIG. 2  depicts mounts  22 A,  22 B yoke assembly  24 , main hanger brackets  26 A,  26 B, anchor brackets  28 A,  28 B, anchor holes  30 A,  30 B, hinges  36 A,  36 B,  38 A,  38 B, mount receiving portions  40 A,  40 B, and mount bases  42 A,  42 B. 
     As described above, mount yoke assembly  24  includes at least one of two primary components, main hanger brackets  26 A,  26 B and anchor brackets  28 A,  28 B. In this example, hanger bracket  26 A is secured to anchor bracket  28 A at hinges  36 A and  38 A via pins or bolts. Optionally, the pins or bolts can also extend into a boss or other receiving structure on the APU assembly. In addition, hinges  36 A,  38 A can alternatively be other similar connecting structures such as flanges  39 A,  39 B, clevises, or shackles, to name a few. 
     Hanger brackets  26 A,  26 B are respectively secured to mounts  22 A,  22 B at receiving portions  40 A,  40 B. In this illustrative example, mounts  22 A,  22 B are slidably connected to respective hanger brackets  26 A,  26 B between receiving portions  40 A,  40 B and mount bases  42 A,  42 B. Alternatively, mounts  22 A,  22 B and/or hanger brackets  26 A,  26 B can include other suitable complementary engaging structures. Hanger brackets  26 A,  26 B can also be permanently or semi-permanently integrated with mounts  22 A,  22 B. 
     Also note in this illustrative example, hanger bracket  26 B is secured at opposing ends to both anchor brackets  28 A,  28 B. Hinge  36 B links hanger bracket  26 B to anchor bracket  28 B, while hinge  36 A joins anchor bracket  28 A to hanger bracket  26 B. Connecting hanger bracket  26 B to multiple anchor brackets  28 A,  28 B and/or connecting one anchor bracket  28 A to multiple hanger brackets  26 A,  26 B, can be done such as is illustrated here in yoke assembly  24 . One reason for doing this, for example, is when anchor bracket  28 A is required to support a greater load that is better distributed among multiple mounts  22 A,  22 B. 
       FIG. 3A  depicts hanger brackets  26 A,  26 B with hinges  36 A,  36 B,  38 A,  38 B mount receiving portions  40 A,  40 B, and hanger arms  46 A,  46 B. 
     As described with respect to  FIG. 2 , hanger brackets  26 A,  26 B provide the respective connections between mounts  22 A,  22 B and anchor brackets  28 A,  28 B (shown in detail in  FIG. 3B ). Each hanger bracket  26 A,  26 B is connected to at least one anchor bracket  28 , which can be seen in the case of hanger bracket  26 A, which includes arms  46 A and hinges  36 A,  38 A. In some alternative cases, hanger arms  46 A are symmetrical depending on the old and new position of mounts  22 A,  22 B and the loads to be supported by yoke assembly  24 . In the case of hanger bracket  26 B, it is connected to both anchor brackets  28 A,  28 B via hinges  36 B and  38 B at respective distal ends of hanger arms  46 B. 
     Selection of the mounting arrangement of hanger brackets  26  and anchor brackets  28  is primarily governed by the orientation of brackets  28 A,  28 B and the positions of mounts  22 A,  22 B relative to APU assembly  10 . In this illustrative case of retrofitting yoke assembly  24  to APU assembly  10 , the configuration of anchor brackets  28 A,  28 B is determined in part by the presence of existing mounting locations or bosses around housing  16  that can also serve as yoke connections  34  (shown in  FIG. 1B ). Other configurations will necessarily require a slightly different structure for hanger brackets  26 A,  26 B and/or anchor brackets  28 A,  28 B. 
       FIG. 3B  shows anchor brackets  28 A,  28 B with anchor holes  30 A,  30 B, flanges  48 A,  48 B and hanger holes  50 A,  50 B. 
     Brackets  28 A,  28 B include flanges  48 A,  48 B and hanger holes  50 A,  50 B for connection to hanger brackets  26 A,  26 B. In addition, brackets  28 A,  28 B include a plurality of anchor holes  30 A,  30 B for securing brackets  28 A,  28 B to APU assembly  10  generally and (in this example) housing  16  as shown in  FIGS. 1A and 1B . As previously described, in this example, brackets  28  are shaped to correspond to the underlying elements of gearbox  14  (such as gears  32 ) or other component of APU assembly  10 . Brackets  28 A,  28 B then can be fixed via bolts or other securing means through anchor holes  30 A,  30 B to housing  16  or other component of APU assembly  10 . 
     Yoke assembly  24  increases the available support footprint such that the area around each yoke connection  34  (shown in  FIG. 1B ) is not failure critical. Thus, the surrounding housing experiences a much more manageable load between each individual pair of anchor hole  30  and yoke connection  34 . The risk of failure of APU assembly  10  due to a fire around the mount locations is thus significantly decreased due to the extremely unlikely event of a fire extending along the entire area of one or more brackets  28 A,  28 B. 
     In this example, both hanger brackets  26 A,  26 B and anchor brackets  28 A,  28 B are fire resistant metal alloys, which can include certain high-temperature grades of austenitic steel such as, but not limited to ASTM 304L. Other example grades include ASTM 321 and 347. Many titanium alloys can also withstand fires for the required time period. Suitable examples include but are not limited to ASTM Grade 5 and 6 titanium alloys. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.