Abstract:
A gas turbine engine assembly is connected to a pylon for mounting the gas turbine engine to an aircraft. The assembly has a frame supporting at least one accessory independently of the gas turbine engine. Frame is attached to the pylon at forward and rearward engine mounting locations. The frame includes at least one hollow tube and at least one hollow tube is fluid tight. The one hollow tube is evacuated or contains pressurised fluid and a pressure sensor is provided to detect a change in pressure in the at least one hollow tube to determine if there is a leak in the at least one hollow tube and hence if the frame is damaged. This ensures that the frame may be repaired or replaced before there is a loss of operation of one or more of the accessories which may result in a failure of the gas turbine engine.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an accessory mounting for a gas turbine engine and in particular to an accessory mounting arrangement for a turbofan gas turbine engine. 
       BACKGROUND TO THE INVENTION 
       [0002]    Our European patent application 12196908.3, published as EP2607658A2, discloses a gas turbine engine assembly connected to a pylon structure for mounting the gas turbine engine to an aircraft. The assembly has a frame attached to the pylon at forward and rearward engine mounting locations and the frame supports accessories independently of the gas turbine engine. Our European patent application 12196908.3, EP2607658A2, discloses that the accessories are arranged in an accessory mounting zone between the core engine casing and an inner nacelle surrounding the core engine. 
         [0003]    The accessory mounting frame is subject to vibrations and/or other loads and these vibrations and/or other loads may result in damage to the accessory mounting frame. If the damage to the accessory mounting frame were to remain undetected and therefore unrepaired, or replaced, it may lead to a failure of one or more portions or the whole of the accessory mounting frame. 
         [0004]    The accessory mounting frame carries all the accessories for the gas turbine engine and a failure of the accessory mounting frame may lead to loss of operation of one or more of the accessories, e.g. may lead to a loss of hydraulic supply, a loss of fuel supply, a loss of lubricant supply or a loss of electrical power which may result in a failure of the gas turbine engine. 
         [0005]    Therefore the present invention seeks to provide a novel arrangement for detecting damage to the accessory mounting frame which reduces or overcomes the above mentioned problem. 
       STATEMENTS OF INVENTION 
       [0006]    Accordingly the present invention provides a gas turbine engine assembly connected to a pylon structure for mounting the gas turbine engine to an aircraft, the assembly having a frame supporting at least one accessory independently of the gas turbine engine, the frame being attached to the pylon at forward and rearward engine mounting locations, the frame comprising at least one hollow tube, the at least one hollow tube being fluid tight, the at least one hollow tube being evacuated or containing pressurised fluid and a pressure sensor being arranged to detect a change in the pressure in the at least one hollow tube to determine if there is a leak in the at least one hollow tube and hence if the frame is damaged. 
         [0007]    The frame may provide a direct load path between the at least one accessory and one or more forward and rearward engine mounting locations of the gas turbine engine. 
         [0008]    Anti-vibration mounts may be provided between the frame and the forward and/or rearward mount engine mounting locations of the gas turbine engine. 
         [0009]    Anti-vibration mounts may be provided between the frame and the at least one accessory. 
         [0010]    The frame may comprise at least one jointed strut for providing axial movement for compensation of thermal expansion. 
         [0011]    The frame may further comprise a ring structure for accessory mounting purposes, the ring structure being arranged coaxially with the axis of the engine. 
         [0012]    The ring structure may be provided by a plurality of curved segments welded or bolted together. 
         [0013]    An integral mounting plate may be provided to interface between the pylon structure and the frame. 
         [0014]    The frame may comprise a front support and a rear support. The front support may have two front struts arranged on opposite sides of the engine and the rear support may have two rear struts arranged on opposite sides of the engine. 
         [0015]    The at least one accessory may be an accessory gearbox, a hydraulic pump, an oil pump, a fuel pump, an electrical generator or an electrical motor. 
         [0016]    The assembly may have an accessory gearbox and the accessory gearbox may be connected to the front mount by the front support and being connected to the rear mount by the rear support. 
         [0017]    An axial link may be provided between a front structure and the accessory gearbox or the frame structure to reduce unwanted axial movement of the accessory gearbox or the frame structure. 
         [0018]    The assembly may having an accessory gearbox, the frame comprising a ring structure arranged coaxially with the axis of the engine, the ring structure being connected to the rear mount by the rear support, the accessory gearbox being connected to the front mount by the front support and the accessory gearbox being connected to the ring structure by at least one additional strut. 
         [0019]    The at least one additional strut may be connected to the front support. 
         [0020]    A nacelle may surround the core engine wherein the at least one accessory is positioned within the nacelle. 
         [0021]    The nacelle may have an opening in a forward position of the nacelle to supply cooling air over the accessories. 
         [0022]    The nacelle may have an opening in a rearward position of the nacelle to discharge cooling air out of nacelle. 
         [0023]    The nacelle may be an inner nacelle separating the core engine from a bypass duct. 
         [0024]    There may be a plurality of accessories. 
         [0025]    The frame may define a plurality of fluid tight chambers, each chamber being evacuated or containing pressurised fluid and each chamber having a pressure sensor arranged to detect a change in the pressure in the associated chamber to determine if there is a leak in the associated chamber and hence if the frame is damaged. 
         [0026]    The frame may comprise a plurality of hollow tubes, each hollow tube being fluid tight and defining a respective chamber, each hollow tube being evacuated or containing pressurised fluid and each hollow tube having a pressure sensor arranged to detect a change in the pressure in the associated hollow tube to determine if there is a leak in the associated hollow tube and hence if the frame is damaged. 
         [0027]    The pressurised fluid may be an inert gas. The inert gas may be nitrogen. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present invention will be more fully described by way of example with reference to the accompanying drawings, in which: 
           [0029]      FIG. 1  is schematic view of a turbofan gas turbine engine having an accessory mounting frame according to the present invention. 
           [0030]      FIG. 2  is a perspective view of an alternative turbofan gas turbine engine having an accessory mounting frame according to the present invention. 
           [0031]      FIG. 3  is a top view of turbofan gas turbine engine shown in  FIG. 2 . 
           [0032]      FIG. 4  is a perspective view of a further turbofan gas turbine engine having an accessory mounting frame according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    A turbofan gas turbine engine  10 , as shown in  FIG. 1 , comprises, in axial flow series, an air intake  1 , a propulsive fan  2 , an intermediate pressure compressor  3 , a high pressure compressor  4 , combustion equipment  5 , a high pressure turbine  6 , an intermediate pressure turbine  7 , a low pressure turbine  8  and an exhaust nozzle  9 . Air entering the air intake  1  is accelerated by the fan  2  to produce two air flows. A first air flow is supplied into the intermediate pressure compressor  3  and a second air flow  59  that passes over an inner nacelle  40  and though a bypass duct which provides propulsive thrust. The intermediate pressure compressor  3  compresses the air flow directed into it before delivering the air to the high pressure compressor  4  where further compression takes place. 
         [0034]    Compressed air exhausted from the high pressure compressor  4  is directed into the combustion equipment  5 , where it is mixed with fuel that is injected from fuel injectors  61  and the mixture is combusted. The resultant hot combustion products expand through and thereby drive the high, intermediate and low pressure turbines  6 ,  7  and  8  respectively before this flow is exhausted as a core stream  60  through the nozzle  9  to provide additional propulsive thrust. The high, intermediate and low pressure turbines  6 ,  7  and  8  respectively drive the high and intermediate pressure compressors  4  and  3  and the fan  2  by suitable interconnecting shafts: high pressure shaft  68 , intermediate pressure shaft  67  and low pressure shaft  66 . 
         [0035]    A flow of cooling air  58  for the accessories enters the accessory mounting zone through an opening at the front, upstream end, of the inner nacelle  40  and is exhausted through an opening into the bypass air stream  59  at the rear, downstream end, of the inner nacelle  40  or is exhausted into the core flow  60  at the rear, downstream end, of the turbofan gas turbine engine  10  upstream of the exhaust nozzle  9 . 
         [0036]    The turbofan gas turbine engine  10  is mounted to the aircraft structure via a pylon  14  which locates on the turbofan gas turbine engine  10  at a front mount  16  and a rear mount  18 . The mounts  16  and  28  carry the weight of the turbofan gas turbine engine  10  and transfer thrust loads from the turbofan gas turbine engine  10  through the pylon  14  to the aircraft structure. The front of the pylon  14  is attached to the front mount  16  through a front attachment bracket  20  which is integral with or bolted to the pylon  14 . The rear of the pylon  14  is mounted to the rear engine mount  18  through a rear attachment bracket  22  and an engine link  24  which permits axial movement of the turbofan gas turbine engine  10  relative to the rear attachment bracket  22 . The front mount  16  is provided on the front frame  54  in this example, but alternatively may be provided on the fan casing. Thus, the front mount  16  and the rear mount  18  may both be provided on the core engine casing or the front mount  16  is provided on the fan casing and the rear mount  18  is provided on the core engine casing. 
         [0037]    Also attached to the pylon  14  is an accessory mounting frame  26  which is secured to the front attachment bracket  20  and the rear attachment bracket  22  by mounts  28 , which may be anti-vibration mounts  49 . The accessory mounting frame  26  of  FIG. 1  has two front struts  30  that extend from the front attachment bracket  20  and join to respective rear struts  32  that extend from the rear attachment bracket  22 . It will be appreciated that one arrangement of front and rear struts  30  and  32  passes to one side of the core engine and that a corresponding arrangement of front and rear struts  30  and  32  is provided on the opposite side of the core engine. Although the front and rear struts  30 ,  32  are shown as being straight it may be necessary to use curved struts that bow around the core engine. 
         [0038]    Each rear strut  32  may be attached to the rear attachment bracket  22  through a swing link  34  which permits axial movement of the rear strut  32  as required by, for example, thermal growth. 
         [0039]    The front and rear struts  30  and  32  together form a “V” arrangement and join at an accessory  35 , in this case the accessory gearbox. The front and rear struts  30  and  32  are mounted to the accessory gearbox by one or more hinge mounts  36 . The accessory gearbox  35  provides power to other engine accessories such as an electrical generator, hydraulic pumps, oil pumps, fuel pumps etc. The electrical generator may be a starter/generator such that the electrical generator may be used as an electrical motor to start the turbofan gas turbine engine  10 . 
         [0040]    The accessory gearbox  35  can be driven by a radial and an axial drive train, e.g. from the low speed drive train  63  which is driven by the low pressure shaft  66 . The accessory gearbox can be further driven by a radial drive train e.g. by the high speed drive train  64  which is driven by the high pressure shaft  68 . These drive trains have preferably one or more interconnections or couplings (e.g. spline connections)  65  to withstand movement of the accessory gearbox  35 . 
         [0041]    In the example of  FIG. 1  the rear strut  32  has additional frame section  38  mounted to it. The additional frame section  38  advantageously provides additional locations for mounting of further accessories  50 . The shape of the additional frame section  38  may be selected so that the further accessories  50  can be optimally located in order to reduce the length of any mounting brackets, drive trains or circuitry to the further accessories  50  or to ensure all the accessories  50  can fit within the inner nacelle line  40 , which can be a congested volume. Exemplary accessories  50  which may be mounted are generators e.g. starter generators, pumps, gearboxes, brackets, cables, sensors, and wiring. The location of the components may be selected to provide good access for maintenance. The accessories  35 ,  50  are arranged within an accessory mounting zone defined between the inner nacelle  40  and a core engine casing  12 . 
         [0042]    The mounts  28 ,  36  are preferably anti vibration mounts which may beneficially decouple the accessories  35 ,  50  from the engine core vibrations. The mounts  28 ,  36  may be selected to further reduce the vibration response of the accessory mounting frame  26  and finally the vibration input to the accessories  35 ,  50 . The corresponding vibration loads could, if not damped, lead to foot-point excitation of an accessory bracket and early failure of the bracket or accessory. 
         [0043]    Engine thrust links  52  are provided, one on each side of the engine (preferably one arranged on the left of the engine and one arranged on the right of the engine) and are located between the rear attachment bracket  22  and the front frame  54 . No accessories are mounted to these links. 
         [0044]    The accessory mounting frame  26  comprises at least one hollow tube and the front struts  30 , the rear struts  32  and the additional frame section  38  comprise hollow tubes. The hollow tubes of the accessory mounting frame  26  are joined together so that the hollow tubes of the accessory mounting frame define at least one fluid tight, leak free, chamber when in the manufactured condition. The hollow tubes of the accessory mounting frame  26  are joined together by a suitable manufacturing process, e.g. welding, brazing etc. Pressurised fluid, e.g. air, is supplied into the chamber within the accessory mounting frame  26  or fluid, air, is evacuated from the chamber within the accessory mounting frame  26  when the accessory mounting frame is manufactured or during maintenance of the gas turbine engine. The pressurised fluid, air, may be supplied from a gas bottle or the pressurised fluid, air, may be supplied from another suitable source of pressurised air, e.g. a compressor. The pressurised fluid may be supplied into the chamber within the accessory mounting frame through a respective non-return valve  74 . The fluid may be evacuated from the chamber in the accessory mounting frame  26  by a vacuum pump. 
         [0045]    Once the chamber in the accessory mounting frame  26  is pressurised with fluid, or the fluid has been evacuated from the chamber, the accessory mounting frame  26  is sealed and a pressure sensor  76  is provided to monitor the pressure in the chamber in the accessory mounting frame  26 . If the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is below a predetermined level that was provided in the accessory mounting frame  26  this provides an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of fluid out of the hollow tubes of the accessory mounting frame  26 . 
         [0046]    Alternatively if the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is above a predetermined level that was provided in the accessory mounting frame  26  this is an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of air into the hollow tubes of the accessory mounting frame  26 . 
         [0047]    The pressure sensor  76  may be a digital pressure sensor or an analogue pressure sensor. The use of the pressure sensor enables an early warning that there is a fault with the accessory mounting frame  26  during routine maintenance inspections of the gas turbine engine. It is important to detect impending failure of the accessory mounting frame  26  because the accessory mounting frame  26  carries all the accessories  35 ,  50  for the gas turbine engine and a failure of the accessory mounting frame  26  may lead to loss of operation of one or more of the accessories  35 ,  50 , e.g. could lead to a loss of hydraulic supply, a loss of fuel supply, a loss of lubricant supply or a loss of electrical power which may result in a failure of the gas turbine engine. Thus, an advantage of the present invention is that the use of the pressure sensor  76  enables an early warning that there is fault, a crack, at the joint, weld or braze, between the hollow tubes of the mounting frame  26  or indeed actually in the hollow tubes of the mounting frame  26 . 
         [0048]    An alternative embodiment is shown in  FIG. 2 . Where possible the same numerals have been used to identify similar parts with the embodiment of  FIG. 1 . It will be appreciated that although these embodiments are separate it may be possible to combine features of one embodiment with features of the other embodiments. 
         [0049]      FIG. 2  depicts a turbofan gas turbine engine with the inner nacelle removed. The turbofan gas turbine engine is mounted to the airframe via a pylon  14  that is mounted to the main aircraft structure through lugs  15  at the midpoint and aft end of the pylon  14 . The pylon  14  has a front mounting bracket  20  which is attached to the front frame structure  54  at a front mount  16  and a rear mounting bracket  22  attached to a rear mount  18  on the turbine casing or engine rear structure  55  via links  53  at the rear of the turbofan gas turbine engine. The pylon  14  is positioned at the top of the turbofan gas turbine engine and struts  30 ,  32  extend down from the pylon  14  to form a frame  26  for mounting accessories. Two front struts  30  are provided, one for each side of the core engine. These front struts  30  are substantially vertical and diverge from the pylon  14  and are mounted to respective lugs  42  on the accessory gearbox  35 , one lug on each side of the core engine. Two rear struts  32  are provided, one for each side of the core engine. These rear struts  32  are angled forwards and diverge from the pylon  14  and are mounted to respective rear lugs  44  on the accessory gearbox  35 , one lug on each side of the core engine. 
         [0050]    In this arrangement of struts  30  and  32  the accessory gearbox  35  is securely mounted to the pylon  14  but an optional axial link  46  may be provided between the front frame structure  54  and the accessory gearbox  35  to reduce unwanted axial movement of the accessory gearbox  35 . Further accessories and dressings  50  may be mounted off the accessory gearbox  35  or the accessory mounting frame  26  directly. 
         [0051]    A further support strut  48  may be provided between the front strut  30  and the rear strut  32  onto which additional accessories  50  may be mounted. 
         [0052]    In the embodiment of  FIG. 2  an engine thrust link  52  is located between the rear attachment bracket  22  and the front frame structure  54 . No accessories are mounted to this thrust link  52 . The engine thrust link  52  and optional axial link  46  at one side of the engine are secured to the front frame structure  54  by brackets at substantially the same point on the front frame structure  54 . 
         [0053]    Anti-vibration mounts  49  may be used between the front struts  30  and the front attachment bracket  20  and between the rear struts  32  and the rear attachment bracket  22 . The vibration mounts  49  permit the engine loads or eigenfrequency behaviours of the turbofan gas turbine engine and engine casing structure to be decoupled from the accessory units  35 ,  50 . Beneficially, the effects of whole engine vibration need not be considered to a high extent for the environment of the accessories. 
         [0054]    Applied vibration effects of the accessory mounting frame  26  may be adjusted by the shape of the accessory mounting frame  26  and the attachment points to the pylon  14 . The anti-vibration mounts  49 , where used, have a different damping characteristic to the rest of the accessory mounting frame  26  and may be made from softer materials such as high temperature elastomeric or composite metal assemblies with implemented spring effects. 
         [0055]    The attachment points at the rear of the frame can include swing links which mitigate the effects of thermal expansion of the engine casing or other components. 
         [0056]    The accessory gearbox  35  is driven by the low speed drive train  63  and/or is driven by a high speed drive train  64 . 
         [0057]    Both drive trains should be covered by covers  62  for protection purpose and to avoid oil and heat contamination between the drive trains and adjacent hardware (e.g. accessories  50 ). 
         [0058]      FIG. 3  is a simplified top view of the embodiment of  FIG. 2 . Some details shown in  FIG. 2 , such as the thrust struts  52 , are omitted for clarity. 
         [0059]    The accessory gearbox  35  is joined to the pylon  14 , in between the core engine casings  12 , by symmetrically arranged front struts  30  and symmetrically arranged rear struts  32 . The front and rear struts  30  and  32  respectively diverge from the pylon  14  with the spacing at the distal ends being determined by the size of the accessory  35 . It will be appreciated that this spacing may be provided by a spacing strut to which the accessory may be mounted. In this arrangement the front and rear struts  30  and  32  may not be directly attached to the accessory  35 . Main accessory support structures  48  are attached to these struts  30  and  32  to further stiffen up the whole structure and provide preferred attachment place for further accessories  50 . 
         [0060]    The accessory mounting frame  26  shown in  FIGS. 2 and 3  also comprises at least one hollow tube and the front struts  30 , the rear struts  32  and the further support struts  48  comprise hollow tubes. The hollow tubes of the accessory mounting frame  26  are joined together so that the hollow tubes of the accessory mounting frame define at least one fluid tight, leak free, chamber when in the manufactured condition. The hollow tubes of the accessory mounting frame  26  are joined together by a suitable manufacturing process, e.g. welding, brazing etc. Pressurised fluid, e.g. air, is supplied into the chamber within the accessory mounting frame  26  or fluid, air, is evacuated from the chamber within the accessory mounting frame  26 . The pressurised fluid, air, may be supplied from a gas bottle or the pressurised fluid, air, may be supplied from another suitable source of pressurised air, e.g. a compressor. The pressurised fluid may be supplied into the chamber within the accessory mounting frame through a respective non-return valve  74 . The fluid may be evacuated from the chamber in the accessory mounting frame  26  by a vacuum pump. 
         [0061]    Once the chamber in the accessory mounting frame  26  is pressurised with fluid, or the fluid has been evacuated from the chamber, the accessory mounting frame  26  is sealed and a pressure sensor  76  is provided to monitor the pressure in the chamber in the accessory mounting frame  26 . If the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is below a predetermined level that was provided in the accessory mounting frame  26  this provides an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of fluid out of the hollow tubes of the accessory mounting frame  26 . Alternatively if the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is above a predetermined level that was provided in the accessory mounting frame  26  this is an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of air into the hollow tubes of the accessory mounting frame  26 . 
         [0062]    The pressure sensor  76  may be a digital pressure sensor or an analogue pressure sensor. The use of the pressure sensor enables an early warning that there is a fault with the accessory mounting frame  26  during routine maintenance inspections of the gas turbine engine. It is important to detect impending failure of the accessory mounting frame  26  because the accessory mounting frame  26  carries all the accessories  35 ,  50  for the gas turbine engine and a failure of the accessory mounting frame  26  may lead to loss of operation of one or more of the accessories  35 ,  50 , e.g. could lead to a loss of hydraulic supply, a loss of fuel supply, a loss of lubricant supply or a loss of electrical power which may result in a failure of the gas turbine engine. Thus, an advantage of the present invention is that the use of the pressure sensor  76  enables an early warning that there is fault, a crack, at the joint, weld or braze, between the hollow tubes of the mounting frame  26  or indeed actually in the hollow tubes of the mounting frame  26 . 
         [0063]    In the accessory mounting frame  26  of  FIGS. 2 and 3  each of the front struts  30  comprises a hollow tube, each of the rear struts  32  comprises a hollow tube and each of the further support struts  48  comprises a hollow tube. Each of the tubes defines a respective chamber in the accessory mounting frame and each chamber is evacuated, or pressurised, and the pressure in each chamber of the accessory mounting frame  26  is monitored using a respective pressure sensor  76 . If a pressure sensor  76  indicates that the pressure in the associated chamber within the accessory mounting frame  26  is below, or above, the predetermined level that was provided in that chamber within the accessory mounting frame  26  this provides an indication that the portion of the accessory mounting frame  26  defining that chamber is damaged or there is a fault, e.g. there is a crack, due to leakage of fluid out of, or into, that particular chamber within hollow tubes of the accessory mounting frame  26 . 
         [0064]      FIG. 4  depicts an alternative embodiment, where the same reference numerals are used for the same components of the previous embodiments. The accessory mounting frame  26  of the turbofan gas turbine engine assembly is provided with a hooped accessory support structure  48  which is coaxially arranged with the engine axis. The hooped accessory support structure  48  may be provided as separate elements which are hinged, bolted, welded or otherwise secured together so that it can be assembled around the engine. 
         [0065]    The hooped accessory support structure  48  has the primary function of carrying a number of accessories  50  other than the main accessory gearbox  35 . Beneficially by mounting accessories  50  in this way it is possible to isolate the accessories  50  attached to the hooped accessory support structure  48  from vibrations and loads from both the accessory gearbox  35  and the engine. 
         [0066]    The accessory mounting frame  26  again comprises front struts  30  and rear struts  30 . Each front strut  30  extends from the front mount  16  to the accessory gearbox  35  and each rear strut  32  extends from the rear mount  18  to the hooped accessory support structure  48  on the respective sides of the turbofan gas turbine engine. Additional struts  47  extend from the front frame structure  54  to the hooped accessory support structure  48  on respective sides of the turbofan gas turbine engine. Each additional strut  47  intersects a respective one of the front struts  30 . 
         [0067]    Engine thrust links  52  are located between the rear attachment bracket  22  and the front frame structure  54 . Optional axial links  46  are provided between the front frame structure  54  and the accessory gearbox  35  to reduce unwanted axial movement of the accessory gearbox  35 . The engine thrust link  52 , the optional axial link  46  and the additional strut  47  at one side of the engines are secured to the front frame structure  54  by brackets at substantially the same point on the front frame structure  54 . 
         [0068]    The accessory mounting frame  26  shown in  FIG. 4  also comprises at least one hollow tube and the front struts  30 , the rear struts  32 , the additional struts  47  and the hooped accessory support structure  48  comprise hollow tubes. The hollow tubes of the accessory mounting frame  26  are joined together so that the hollow tubes of the accessory mounting frame define at least one fluid tight, leak free, chamber when in the manufactured condition, e.g. each front strut  30  is joined to the associated rear strut  32  via associated additional strut  47  and hooped accessory support structure  48 . The hollow tubes of the accessory mounting frame  26  are joined together by a suitable manufacturing process, e.g. welding, brazing etc. Pressurised fluid, e.g. air, is supplied into the chamber within the accessory mounting frame  26  or fluid, air, is evacuated from the chamber within the accessory mounting frame  26 . The pressurised fluid, air, may be supplied from a gas bottle or the pressurised fluid, air, may be supplied from another suitable source of pressurised air, e.g. a compressor. The pressurised fluid may be supplied into the chamber within the accessory mounting frame through a respective non-return valve  74 . The fluid may be evacuated from the chamber in the accessory mounting frame  26  by a vacuum pump. 
         [0069]    Once the chamber in the accessory mounting frame  26  is pressurised with fluid, or the fluid has been evacuated from the chamber, the accessory mounting frame  26  is sealed and a pressure sensor  76  is provided to monitor the pressure in the chamber in the accessory mounting frame  26 . If the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is below a predetermined level that was provided in the accessory mounting frame  26  this provides an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of fluid out of the hollow tubes of the accessory mounting frame  26 . Alternatively if the pressure sensor  76  indicates that the pressure in the chamber in the accessory mounting frame  26  is above a predetermined level that was provided in the accessory mounting frame  26  this is an indication that the accessory mounting frame  26  is damaged or there is a fault, e.g. there is a crack, due to leakage of air into the hollow tubes of the accessory mounting frame  26 . 
         [0070]    The pressure sensor  76  may be a digital pressure sensor or an analogue pressure sensor. The use of the pressure sensor enables an early warning that there is a fault with the accessory mounting frame  26  during routine maintenance inspections of the gas turbine engine. It is important to detect impending failure of the accessory mounting frame  26  because the accessory mounting frame  26  carries all the accessories  35 ,  50  for the gas turbine engine and a failure of the accessory mounting frame  26  may lead to loss of operation of one or more of the accessories  35 ,  50 , e.g. 
         [0071]    could lead to a loss of hydraulic supply, a loss of fuel supply, a loss of lubricant supply or a loss of electrical power which may result in a failure of the gas turbine engine. Thus, an advantage of the present invention is that the use of the pressure sensor  76  enables an early warning that there is fault, a crack, at the joint, weld or braze, between the hollow tubes of the mounting frame  26  or indeed actually in the hollow tubes of the mounting frame  26 . 
         [0072]    In the accessory mounting frame of  FIG. 4  the hollow tubes of the accessory mounting frame  26  are joined together to form a number of chambers in the accessory mounting frame  26  and each chamber is evacuated, or pressurised, and the pressure in each chamber of the accessory mounting frame  26  is monitored using a respective pressure sensor  76 . If a pressure sensor  76  indicates that the pressure in the associated chamber within the accessory mounting frame  26  is below, or above, the predetermined level that was provided in that chamber within the accessory mounting frame  26  this provides an indication that the portion of the accessory mounting frame  26  defining that chamber is damaged or there is a fault, e.g. there is a crack, due to leakage of fluid out of, or into, that particular chamber within hollow tubes of the accessory mounting frame  26 . In  FIG. 4  each hollow tube, each of the front struts  30 , each of the rear struts  32 , each of the additional struts  47  and the hooped accessory support structure  48  define its own respective chamber and has a respective pressure sensor  76 . Alternatively, the hollow tubes of the front struts  30 , the rear struts  32 , the additional struts  47  and the hooped accessory support structure  48  may be arranged to define a single chamber with a single pressure sensor. 
         [0073]    One of the many advantages of using an accessory mounting frame is that it can be removed from the engine assembly without having to remove the engine from the pylon structure. Accessories may be pre-mounted on the accessory mounting frame prior to the accessory mounting frame being mounted to the front and/or rear mounts. This is advantageous as it can speed up repair and replacement of accessories. The provision of the pressure sensors to detect changes in the pressure within the chamber, or chambers, defined by the hollow tube, or hollow tubes, of the accessory mounting frame enables early detection of damage to the accessory mounting frame and enabling the accessory mounting frame to be repaired or replaced. 
         [0074]    The turbofan gas turbine engine is mounted to the pylon  14  by engine rear mount attachment struts  53 . A fail safe swing link  51  further connects the turbofan gas turbine engine  10  via the rear attachment bracket  22  to the pylon  14  to prevent the turbofan gas turbine engine becoming detached from the pylon  14  should the engine rear mount attachment struts fail for any reason. 
         [0075]    Optionally the frame maybe used to stiffen the engine main structure. However the vibration isolation effect described before may be affected, the rotor tip clearance will be improved and the material thickness of the core engine casings can be reduced in thickness. 
         [0076]    Although the present invention has been described with reference to the use of air to pressurise the hollow tubes of the accessory mounting frame any suitable fluid may be used. However, it may be particularly advantageous to use an inert gas so that the inert gas inhibits, or prevents, corrosion of the hollow tubes. An inert gas is a gas which will not react with the hollow tubes, for example nitrogen, helium, argon, neon etc. These gases may also be supplied into the hollow tubes of the accessory mounting frame from suitable gas bottles. 
         [0077]    Although the present invention has been described with reference to a turbofan gas turbine engine it may be possible to provide a frame according to the present invention on a turbo-shaft gas turbine engine, a turbo-propeller gas turbine engine or a turbojet gas turbine engine. Although the present invention has been described with reference to a three shaft engine gas turbine engine it is equally applicable to a two shaft gas turbine engine and to a single shaft gas turbine engine. 
         [0000]    
       
         
               
             
               
               
             
               
               
             
           
               
                   
               
               
                 Drawing References 
               
             
          
           
               
                 Ref. No. 
                 Description 
               
               
                   
               
             
          
           
               
                 1 
                 Air Intake 
               
               
                 2 
                 Propulsive fan 
               
               
                 3 
                 Intermediate pressure compressor 
               
               
                 4 
                 High pressure compressor 
               
               
                 5 
                 Combustion Equipment 
               
               
                 6 
                 High Pressure turbine 
               
               
                 7 
                 Intermediate pressure turbine 
               
               
                 8 
                 Low pressure turbine 
               
               
                 9 
                 Exhaust nozzle 
               
               
                 10 
                 Gas turbine engine 
               
               
                 12 
                 Core engine casing 
               
               
                 14 
                 Pylon structure 
               
               
                 15 
                 Pylon lugs 
               
               
                 16 
                 Front mount 
               
               
                 18 
                 Fail safe rear mount 
               
               
                 20 
                 Front attachment bracket 
               
               
                 22 
                 Rear attachment bracket 
               
               
                 24 
                 Engine link 
               
               
                 26 
                 Accessory mounting frame 
               
               
                 28 
                 Accessory mounting frame mounts 
               
               
                 30 
                 Front strut 
               
               
                 32 
                 Rear strut 
               
               
                 34 
                 Swing link 
               
               
                 35 
                 Accessory or Accessory Gearbox 
               
               
                 36 
                 Hinge mount 
               
               
                 38 
                 Additional frame section 
               
               
                 40 
                 Inner nacelle 
               
               
                 41 
                 Opening in Inner Nacelle 
               
               
                 42 
                 Forward on accessory gearbox 
               
               
                 44 
                 Rearward lug on accessory gearbox 
               
               
                 46 
                 Optional axial link 
               
               
                 47 
                 Additional Struts 
               
               
                 48 
                 Main accessory support structure 
               
               
                 49 
                 Anti vibration mount 
               
               
                 50 
                 Accessory or Unit 
               
               
                 51 
                 Fail save swing link 
               
               
                 52 
                 Thrust strut 
               
               
                 53 
                 Engine rear mount attachment strut 
               
               
                 54 
                 Engine front frame structure 
               
               
                 55 
                 Engine rear structure 
               
               
                 58 
                 Fan cooling flow 
               
               
                 59 
                 Fan bypass stream 
               
               
                 60 
                 Core exhaust stream 
               
               
                 61 
                 Fuel Injector 
               
               
                 62 
                 Cover for drive train 
               
               
                 63 
                 Low speed axial drive train 
               
               
                 64 
                 High speed radial drive train 
               
               
                 65 
                 Accessory gearbox drive shaft coupling 
               
               
                 66 
                 Low pressure shaft 
               
               
                 67 
                 Intermediate pressure shaft 
               
               
                 68 
                 High pressure shaft 
               
               
                 74 
                 Non-return valve 
               
               
                 76 
                 Pressure gauge