Abstract:
The invention provides a back up power supply for avionics equipment that can be installed in existing avionics equipment trays without having to rewire the aircraft. According to a preferred embodiment of the invention (described for illustrative purposes in the context of providing a power supply back up for a cockpit voice recorder (CVR)), the power supply/CVR combination is retrofit in an existing CVR rack. The power supply is modular; removable/replaceable; and is “independent” relative to the main aircraft power source and wiring. The modular power supply is designed to fit into a conventional CVR tray; and the modular power supply is designed to accept an existing CVR. According to a preferred embodiment, the power supply is “plug compatible” with the existing electrical connector in the CVR tray and includes circuitry which allows both the power supply and CVR to be tested using the existing CVR test button in the cockpit; and to report faults from either unit using existing aircraft wiring. The power supply includes circuitry which automatically electrically disconnects it from the CVR and exposed pins when the power supply is removed from the CVR tray.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The invention relates generally to avionics equipment, such as flight recorders. More particularly, the invention relates to a back up power supply for avionics equipment that can be installed in existing equipment trays located for example, but not exclusively, in the tail section of an aircraft, without having to rewire the aircraft. 
     2. State of the Art 
     The state-of-the-art is best introduced with the excerpts and paraphrases from the National Transportation Safety Board, Safety Recommendation A-99-16-18, dated Mar. 9, 1999. 
     Although the recommendations set forth hereinafter are directed to back up power supplies for cockpit voice recorders (CVR&#39;s) per se; those skilled in the art will readily appreciate that back up power supplies may be useful to power other types of avionics equipment should power from the aircraft&#39;s power supply be lost. Hence reference hereinafter to CVR&#39;s is intended for illustrative purposes only; and not to limit the scope of the invention described and claimed herein. 
     Many recent aircraft accidents have illustrated the need for an “independent” back-up power supply for the aircraft CVR. As used herein, the word “independent” means independent from the aircraft power supply, not necessarily independent from the CVR unit. 
     For example, on May 11, 1996, the crew of ValuJet flight 592, a DC-9-32, reported smoke and fire shortly after departing Miami, Fla. The flight recorders stopped about 40 to 50 seconds before the airplane crashed on its return to the airport. 
     On Jul. 7, 1996, TWA flight 800, a Boeing 747-100, on an international passenger flight from New York to Paris, exploded about 13 minutes after takeoff as it was climbing through 13,700 feet. Both flight recorders stopped at the time of the explosion, but the airplane did not hit the water off Long Island, N.Y., for another 40 to 50 seconds. 
     On Dec. 19, 1997, SilkAir flight 185, a Boeing 737, entered a rapid descent from 35,000 feet, which ended with a high speed impact in the Sumatran River near Palembang, Indonesia. The Indonesian investigation determined that both flight recorders stopped prior to the airplane entering the rapid descent. 
     On Sep. 2, 1998, Swissair flight 111, an MD11, on a regularly scheduled passenger flight from New York to Geneva, Switzerland, diverted to Halifax after the crew reported smoke in the cockpit; the airplane crashed into the waters near Peggy&#39;s Cove, Nova Scotia. Thus far, the Canadian Transportation Safety Board&#39;s (TSB) investigation has been severely hampered by the lack of data from the cockpit voice recorder (CVR), which stopped nearly 6 minutes before the airplane hit the water. 
     These recent accidents are just the latest in a long history of accident and incident investigations that were hindered by the loss of flight recorder data due to the interruption of aircraft electrical power. 
     Since 1983, the origin date of the NTSB database, there have been 52 accidents and incidents, including the 4 recent accidents mentioned above, in which information from either a CVR or FDR (flight data recorder) or both were lost due to interruption of electrical power following an engine or generator failure or crew action. Until recently, recorder technology did not offer a practical solution to the problem of loss of electrical power to the on-board recorders. However, recent innovations in recorder and power supply technologies have made it possible to provide an independent power source that would provide sufficient power to operate a solid-state flight recorder for a predetermined period of time, for example, 10 minutes. 
     In assessing the feasibility of an independent power source, strong consideration must be given to reliability, complexity, maintainability, and cost. The independent power source must also automatically engage when power to the recorders is lost; and disengage after a predetermined period of time so as not to overwrite recorded data. 
     Older model tape-based recorders require too much electrical power and are not easily adapted to a direct current (d.c.) battery or capacitor. However, the relatively low power requirements of solid-state flight recorders (about 10 to 12 watts from a 28-volt d.c. system) would permit the use of an independent power source. Thus, it is evident that the use of an independent power source would also require the use of solid-state flight recorders. 
     Current regulations call for a CVR with a minimum 30-minute recording duration. This minimum requirement was based on the limitations of 1960s recorder technology, which was constrained by the amount of magnetic tape that could be impact/fire protected. 
     In the years since CVRs became mandatory, the Safety Board has investigated many accidents and incidents for which the 30-minute CVR recording was not sufficient to retain key events. This prompted the Safety Board to recommend in 1996 that all newly manufactured CVRs be SSCVRs (solid state CVRs) with a 2-hour recording duration (Safety Recommendation A-96-171). Accident investigations in which the Safety Board has participated subsequent to the issuance of Safety Recommendation A-96-171 continue to demonstrate that a lack of recorded voice and other aural information can inhibit safety investigators and delay or prevent the identification of safety deficiencies. 
     The CVR installed on Swissair flight 111 used a continuous-loop magnetic tape with a 30-minute duration. The earliest information on the CVR tape was recorded about 15 minutes before the crew noted an unusual odor. Crew conversations and cockpit sounds prior to the beginning of this 30-minute recording might have provided insight into any initiating or precursory events that led to the accident. 
     About 38 minutes prior to the crew noting an unusual odor, Boston Center issued flight 111 a radio frequency change. During the following 13 minutes, Boston Center made repeated attempts to contact flight 111 but did not establish contact. Any cockpit conversations, flight deck noises, or attempted crew transmissions that occurred during this period were subsequently overwritten on the CVR, and thus were not available to the accident investigators. 
     Although 30-minute magnetic tape CVRs are no longer being manufactured, units still exist and could be installed on aircraft today. Given the continued need for longer periods of recorded sound to capture the initiating events of aviation accidents, and the availability of and trend toward 2-hour CVRs, the Safety Board has stated that a retrofit program is warranted. 
     The Safety Board has further stated that the Federal Aviation Administration (FAA) should require the retrofit after Jan. 1, 2005, of all CVRs on all airplanes required to carry both a CVR and a FDR with a CVR that meets TSO C123a and is capable of recording the last 2 hours of audio. 
     A number of options have been identified for realizing an independent power source. In assessing the feasibility of these options, batteries and capacitors appear to be the most effective means of providing aircraft independent power. These and other power sources may be selected by those skilled in the art based on various well known design criteria and objectives. For example, the capacitor method would provide a power source that requires very little maintenance when compared with a rechargeable battery since, unlike a rechargeable battery, the capacitor method would not require any power sensing circuitry for normal operation. Nevertheless, total package weight, center of gravity, packaging constraints (dimensional), etc., will all factor in when selecting among the possible candidates for an independent power supply. 
     To maximize recorder reliability and to minimize any crew intervention, the independent power source should automatically engage whenever the normal electrical power to the recorder ceases, even when the aircraft is powered down normally. To increase the probability of recording accident data, the independent power source should be capable of powering the recorder for 10 minutes after main power to the recorder shuts off; and again, be capable of automatic shut down after a predetermined period of time to avoid overwriting data. 
     In the case of a CVR, the area microphone is powered by the CVR; as long as the CVR receives power, the area microphone will continue to operate and the CVR will continue to record sounds from the area microphone, provided that the connection between the microphone and the recorder is not compromised. 
     In the case of the SilkAir accident, the 2-hour SSCVR stopped recording 5 minutes 58 seconds prior to the DFDR (digital flight data recorder) stopping, which stopped 1 minute 54 seconds prior to impact. Thus, any valuable dialog in the cockpit regarding the airplane status for those 7 minutes 52 seconds was lost. However, providing a 30-minute CVR with 10 minutes of independent power after main power ceases would result in about one-third of the in-flight audio being recorded over. Thus, it would not be acceptable to fit 30-minute CVRs with an independent power supply that automatically engages when aircraft electrical power is terminated for any reason. 
     With maintenance-free independent power sources, it is now feasible to provide an independent power source for new-technology CVRs for a specific period of time, in the event that aircraft power sources to the CVR are interrupted or lost. 
     Accordingly, the Safety Board recommended that for the CVR retrofit after Jan. 1, 2005, the FAA should also require the CVR to be provided with an independent power source that is located with the CVR and that automatically engages and provides 10 minutes of operation whenever aircraft power to the recorder ceases, either by normal shutdown or by a loss of power to the bus. 
     The Canadian Transportation Safety Board (TSB) shares the view that a CVR retrofit is warranted and has developed a similar recommendation, which was issued to the Canadian and European regulators in early March 1999. The Safety Board and the TSB have urged that the actions recommended by the two investigative authorities be adopted by civil aviation regulators worldwide. 
     Accordingly, the National Transportation Safety Board has recommended that the Federal Aviation Administration: Require retrofit after Jan. 1, 2005, of all cockpit voice recorders (CVRS) on all airplanes required to carry both a CVR and a flight data recorder (FDR) with a CVR that (a) meets Technical Standard Order (TSO) C123a, (b) is capable of recording the last 2 hours of audio, and (c) is fitted with an independent power source that is located with the digital CVR and that automatically engages and provides 10 minutes of operation whenever aircraft power to the recorder ceases, either by normal shutdown or by a loss of power to the bus. 
     Although the aforementioned recommendations are directed to CVR&#39;s specifically, back up power supplies which can be installed in existing equipment storage racks, do not require rewiring the aircraft, are easy to maintain and test, etc., would be useful for providing a back up power source for other types of avionics equipment including, for example, FDR&#39;s, etc. 
     Accordingly, although the invention will be described hereinafter with reference to an illustrative embodiment thereof (a modular back up power supply for a CVR), the scope of the invention is not intended to be limited thereby, i.e., the invention covers back up power supplies for avionics equipment in general; modular or not. 
     Furthermore, although the recommendations of the NTSB are quite specific; and although the DOT and ARINC have issued detailed specifications for the new CVRs, many questions still remain as to how to provide the recommended back up power supply, how the back up power supplies will be installed in the confines of an aircraft where space and other constraints limit where and how to locate and install such units; etc. 
     For example, will it be necessary to rewire at least some portion of the aircraft to install a back up power supply? Will it be further necessary to rewire the aircraft so that the CVR and it&#39;s back up power supply may be tested from the cockpit? Will it be necessary to require and/or reconfigure the equipment storage racks in order to accommodate the back up power supply and a CVR? 
     It should be noted that industry has recommended that any separate power supply be located outside the existing equipment racks. This recommendation is problematic in that additional space in the aircraft is required; safety and aircraft rewiring become issues if external wiring is required to couple rack mounted equipment to a back up power supply unit located outside a rack; where to locate mounts for the power supplies becomes an issue; the weight of the added independent power supply units needs to be considered, etc. 
     Accordingly, and notably in addition to the aforementioned recommendations, it would be desirable to be able to provide a back up power supply unit for avionics equipment which, together with the avionics equipment, can use existing racks. 
     It would be further desirable to be able to provide a back up power supply that does not require an independent mechanical mount in the confines of the aircraft. 
     Further still, it would be desirable to provide a back up power supply that does not require any aircraft rewiring. 
     It would also be desirable to provide a back up power supply/avionics equipment combination that has a combined weight that is less than or equal to the weight of a piece of conventional (not backed up) avionics equipment alone. 
     Still further, it would be desirable to provide a modular back up power supply for use with avionics equipment to allow for easy installation and testing of the back up unit independent (where the modular design is used) of the avionics equipment being powered. This would avoid having to re-test an integrated back up power supply/avionics equipment combination to meet all the specs of the avionics equipment (if opened), as would be the case where an integrated unit (which also comes within the scope of the invention) is used. 
     Further yet, it would be desirable to provide a module fault reporting capability for both a back up power supply module and avionics equipment module, using the existing wiring for reporting equipment module faults; and to provide a “push to test” feature for testing either (or both) of the back up power supply module and equipment backed up, using the existing cockpit button and electrical connection to a rack presently used to test the equipment (not backed up) alone. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary object of the invention to provide a back up power supply for avionics equipment in general. 
     It is a more specific object of the invention to provide a tray adapted back up power supply for avionics equipment in general, i.e. a back up power supply unit for avionics equipment which, together with the avionics equipment, can use existing racks. 
     It is a further object of the invention to provide a back up power supply for avionics equipment that does not require additional wires or cables to couple the power supply to the equipment. 
     Furthermore, it is an object of the invention to provide a back up power supply that does not require any aircraft rewiring. 
     Further still, it is an object of the invention to be able to provide a back up power supply that does not require an independent mechanical mount in the confines of the aircraft. 
     It is also an object of the invention to provide a back up power supply/avionics equipment combination that has a combined weight that is less than or equal to the weight of a piece of conventional (not backed up) avionics equipment alone. 
     Still further, it is an object of the invention to provide a modular back up power supply for use with avionics equipment to allow for easy installation and testing of the back up unit independent (where the modular design is used) of the avionics equipment being powered. 
     Further yet, it is an object of the invention to provide a module fault reporting capability for both a back up power supply module and avionics equipment module, using the existing wiring for reporting equipment module faults; and to provide a “push to test” feature for testing either (or both) of the back up power supply module and equipment backed up, using the existing cockpit button and electrical connection to a rack presently used to test the equipment (not backed up) alone. 
     It is also an object of the invention to provide a cockpit voice recorder (CVR) with an independent power supply. 
     Still another object of the invention is to provide a CVR having an independent power supply which meets the requirements and recommendations of the NTSB, DOT, and ARINC. 
     It is another object of the invention to provide a CVR having an independent power supply which does not require any rewiring of an aircraft in order that the backed up avionics equipment (e.g., a CVR) and independent power supply be tested from the cockpit. 
     It is also an object of the invention to provide an independent power supply which does not occupy an undue amount of space. 
     It is still another object of the invention to provide a CVR and independent power supply which is retro-fittable into the existing equipment rack which now receives a prior art CVR. 
     It is also an object of the invention to provide an independent power supply which does not require an independent mechanical mount in the confines of the aircraft. 
     It is still another object of the invention to provide a CVR and independent power supply which has a similar weight and center of gravity as the existing CVRs. 
     It is also an object of the invention to provide a modular back up power supply which is easy to install and test. 
     It is still another object of the invention to provide a CVR and independent power supply which utilize the existing “push to test” circuits in the cockpit. 
     Further still, it is an object of the invention to provide methods for providing back up power for avionics equipment in accord with the aforestated objects. 
     In accord with these and other objects of the invention, which will be discussed in detail below, the backup power supply for avionics equipment will be described with reference to an illustrative embodiment, namely a cockpit voice recorder and independent power supply. 
     The cockpit voice recorder and independent power supply of a preferred embodiment of the present invention includes separate, modular CVR and power supply units. The power supply is designed to fit into a conventional CVR tray and the new CVR is designed to fit into the power supply. The combination is designed to fit within approximately the footprint and space occupied by a “conventional” CVR as defined hereinafter (including specific dimensions being set forth), with reference to the description of the illustrative prior art tray and CVR shown in FIGS. 1-6. 
     According to the preferred embodiment of the invention, the power supply is “plug compatible” with the existing electrical connector in the existing CVR tray and includes circuitry which allows both the power supply and the CVR to be tested using the existing CVR test button in the cockpit. Moreover, for safety purposes, the power supply includes circuitry which automatically electrically disconnects it from the CVR and exposed pins when the power supply is removed from the existing CVR tray. 
     Furthermore, according to a preferred embodiment of the invention, the power supply utilizes the same “push to test” circuitry as the existing CVRs; and the same wiring used to output fault indications from the recorder alone are used to output fault indications from both the power supply and recorder (whether modular units or integrated as one device). 
     The invention also contemplates methods for providing a back up power supply for avionics equipment, comprising the steps of: (a) utilizing a back up power supply that includes a self contained power source; (b) combining said back up power supply with the avionics equipment such that the combination of equipment fits into a conventional equipment tray; and (c) retrofitting the combination of said power supply and said avionics equipment into a conventional equipment tray without rewiring the aircraft. 
     Further methods contemplated by the invention include the steps of: (a) fabricating a back up power supply module, including a self contained power source, that is plug compatible with a conventional tray avionics equipment connector; and (b) fabricating said power supply module to be plug compatible with said avionics equipment, the combination of said power supply and said avionics equipment being able to fit substantially within said tray. 
     The CVR and power supply of the present invention are preferably (though not necessarily) modular, easy to install, require no cables, occupy approximately the same space previously occupied by the existing CVR, and have substantially the same center of gravity as the existing CVR. 
     Again, although the presently preferred embodiment includes separate CVR and power supply modules, the invention contemplates and encompasses a single integrated unit which fits into an existing equipment rack. 
     Further advantages and features of the invention include providing a back up power supply unit for avionics equipment which, together with the avionics equipment, can use existing racks; a back up power supply that does not require an independent mechanical mount in the confines of the aircraft; a back up power supply that does not require any aircraft rewiring; a back up power supply/avionics equipment combination that has a combined weight that is less than or equal to the weight of a piece of conventional (not backed up) avionics equipment alone; a modular back up power supply for use with avionics equipment to allow for easy installation and testing of the back up unit independent (where the modular design is used) of the avionics equipment being powered; a module fault reporting capability for both a back up power supply module and avionics equipment module, using the existing wiring for reporting equipment module faults; and to provide a “push to test” feature for testing either (or both) of the back up power supply module and equipment backed up, using the existing cockpit button and electrical connection to a rack presently used to test the equipment (not backed up) alone. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a prior art CVR tray. 
     FIG. 2 is a side elevation view of the prior art CVR tray of FIG.  1 . 
     FIG. 3 is a front end view of the prior art CVR tray of FIGS. 1 and 2. 
     FIG. 4 is a top view of a prior art CVR tray with a prior art CVR installed therein. 
     FIG. 5 is a side elevation view of the prior art CVR tray and CVR of FIG.  4 . 
     FIG. 6 is a front end view of the prior art CVR tray and CVR of FIGS. 4 and 5. 
     FIG. 7 is a top view of an independent power supply according to a preferred embodiment of the invention. 
     FIG. 8 is a side elevation view of the independent power supply of power supply depicted in FIG.  7 . 
     FIG. 9 is a rear end view of the independent power supply power supply depicted in FIG.  7 . 
     FIG. 10 is a front end view of the independent power supply power supply depicted in FIG.  7 . 
     FIG. 11 is a top view of the independent power supply power supply depicted in FIG. 7, installed in an existing CVR tray. 
     FIG. 12 is a side elevation view of the independent power supply depicted in FIG. 7, installed in an existing CVR tray. 
     FIG. 13 is a front end view of the independent power supply depicted in FIG. 7, installed in an existing CVR tray. 
     FIG. 14 is a top view of a CVR according to a preferred embodiment of the invention, installed in an independent power supply of a type further contemplated by a preferred embodiment of the invention, with the power supply being installed in an existing CVR tray. 
     FIG. 15 is a side elevation view of a CVR according to a preferred embodiment of the invention, installed in an independent power supply of a type further contemplated by a preferred embodiment of the invention, with the power supply being installed in an existing CVR tray. 
     FIG. 16 is a front end view of a CVR according to a preferred embodiment of the invention, installed in an independent power supply of a type further contemplated by a preferred embodiment of the invention, with the power supply being installed in an existing CVR tray. 
     FIG. 17 is a simplified schematic diagram of a circuit to electrically decouple the independent power supply from the CVR when the independent power supply is removed from the existing CVR tray. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 through 3, a prior art CVR tray  10  is shown. The tray  10  is supported by four non-conductive vibration isolators  12 ,  14 ,  16 ,  18  and is electrically grounded by a corresponding four grounding straps  20 ,  22 ,  24 ,  26 . 
     As seen best in FIG. 3, the tray  10  has a pair of spaced apart parallel rails  28 ,  30  upon which a CVR is to be received and slid toward the back of the tray  10  where a male electrical connector  32  electrically couples with a female electrical connector in the CVR. 
     In order to assure proper alignment between the connectors, a pair of tapered alignment rods  34 ,  36  as well as a pair of upstanding side supports  38 ,  40  are provided adjacent to the end of the rails  28 ,  30 . A pair of locking nuts  42 ,  44  are provided at the end of the rails  28 ,  30  opposite the electrical connector  32 . These will secure the CVR in place as shown in prior art FIGS. 4-6. 
     Each of the locking nuts, e.g.  42 , has a threaded member  42   a , coupled to the tray by a pivot pin  42   b  and having an end stop  42   c . On the threaded member  42   a  are a sliding collar  42   d  with an undercut (not shown) and a wing nut  42   e.    
     The standard (“conventional”) avionics rack shown in FIGS. 1-3 has a width of 5.38 inches, a length of 12.41 inches and a height of 7.62 inches. 
     Turning now to FIGS. 4-6, a prior art CVR  50  is shown installed in the tray  10 . The standard (“conventional”) CVR shown in FIGS. 4-6 is 4.88 inches wide and fits within tray  10 . 
     The CVR  50  has a gripping handle  52 , a bracket  54  for a pinger (not shown), a pair of L-shaped brackets  56 ,  58 , and a covered headset connector  60 . The CVR  50  is installed by sliding it on the rails ( 28 ,  30  in FIGS. 1-3) until two holes in the rear (not shown) of the CVR engage the tapered alignment rods ( 34 ,  36  in FIGS. 1-3) and the female electrical connector (not shown) in the rear of the CVR couples with the male electrical connector ( 32  in FIGS.  1  and  3 ). 
     The remaining components depicted in FIGS. 4-6, identified by reference numerals  12 ,  14 ,  16 ,  20 .,  22 ,  24 ,  38  and  40 , are identical to the corresponding numbered components referred to and described hereinbefore with reference to FIGS. 1-3. 
     After the CVR is in position, the two locking nuts  42 ,  44  are used to secure it to the tray. In particular, the wing nuts are rotated until they hit the stops and the collars are pulled back against the wing nuts, the pivoting threaded member is lifted and the collars are slid over the L-shaped brackets  56 ,  58 . The wing nuts are then tightened. 
     As mentioned above, the independent power supply of the invention is designed to fit into the existing CVR tray shown in prior art FIGS. 1-6. 
     Turning now to FIGS. 7-10, the independent power supply  100  of the preferred embodiment of the present invention preferably has a footprint which is substantially the same as the existing CVR ( 50  in FIGS.  4 - 6 ). This again is a preferred design for the power supply set forth for illustrative purposes only; but is not required to practice the invention per se. 
     For example, a power supply with a footprint of one half the size of a conventional CVR may be combined with a reduced size CVR that when combined with the power supply has approximately the footprint of and occupies approximately the same space as a conventional CVR. This and many other variants of the invention designed to fit the CVR and power supply combination within approximately the same space as a conventional CVR (in an existing rack), all come within th scope and spirit of the invention. 
     Reference should now be made to FIG. 9 to best see one example of how the rear end of independent power supply  100  can be designed to couple with an existing connector (like connector  32  of FIG. 3 shown in a prior art tray). 
     In particular, as best seen in FIG. 9, the rear end of independent power supply  100  has a pair of holes  102 ,  104  adjacent to a female electrical connector  106 , all three of which are configured to interface with an existing tray in the same manner as an existing CVR. Similarly, as seen best in FIGS. 8 and 10, the front end of the independent power supply  100  has two L-shaped brackets  108 ,  110  which are configured to act in the same manner as the L-shaped brackets of the existing CVR. 
     The remainder of the independent power supply  100  according to a preferred embodiment of the invention includes a generally L-shaped chassis  112  within which various circuits (described below) are contained including a means for storing a charge, for example, a capacitor or battery (not shown) which is the power source for the independent power supply  100  and means for shutting off the power supply after a predetermined period, e.g., 10 minutes. 
     A male electrical connector  114  with a plurality of keying guides  116  is mounted on the interior of the upstanding rear wall  118  of the chassis  112 . This connector mirrors connector  32  that the CVR would normally “see” being placed into a conventional tray. A pair of parallel guiding rails  120 ,  122  extend from the rear wall  118  defining a platform for receiving a CVR according to a preferred embodiment of the invention, as will be described hereinafter with reference to FIGS. 11-13. A pair of ratchet nuts  124 ,  126  are hingedly coupled by pivot pins  128 ,  130  adjacent to the front ends of the respective guiding rails  120 ,  122 . 
     Turning now to FIGS. 11-13, the independent power supply  100  of the invention fits into the existing CVR tray  10  as described above and as shown in these figures. 
     More particularly, the chassis  112  lies between the rails  28 ,  30 . The guide holes  102 ,  104  (FIG. 9) are engaged by the tapered guide rods  34 ,  36 ; and the male connector  32  (FIGS. 1 and 3) is coupled to the female connector  106  (FIG.  9 ). The lock nuts  42 ,  44  are secured over the L-shaped brackets  108 ,  110 . 
     The remaining components depicted in FIGS. 11-13, identified by reference numerals  12 ,  14 ,  16 ,  20 ,  22 ,  24 ,  28 ,  30 ,  38 ,  114  and  116 , are identical to the corresponding numbered components referred to and described hereinbefore with reference to FIGS. 1-10. 
     As mentioned above, the independent power supply  100  of the preferred embodiment of the invention is intended to be used with a cockpit voice recorder (CVR) which is designed to mate with and be mounted in the power supply. FIGS. 14-16 illustrates such a CVR  200  mounted in and electrically coupled to the power supply  100 . 
     The CVR  200  generally includes an L-shaped chassis s  202  housing the electronics of the CVR  200 . The chassis  202  fits between the guide rails  120 ,  122  of the power supply  100 . 
     A female electrical connector (not shown) is provided on the rear wall of the chassis  202  and couples to the connector  114 ,  116  on the power supply. An impact protected solid state memory  204  is mounted on the bed  206  of the chassis  202 ; and a pinger bracket  208  is mounted on the housing of the memory  204 . A pair of L-shaped brackets  210 ,  212  are provided on the lower front end of the chassis. These brackets are engaged by the ratchet nuts  124 ,  126 . 
     The remaining components depicted in FIGS. 14-16, identified by reference numerals  10 ,  42 ,  44 ,  108  and  110 , are identical to the corresponding numbered components referred to and described hereinbefore with reference to FIGS. 1-13. 
     Comparing FIGS. 14-16 with prior art FIGS. 4-6, it will be appreciated that the combined power supply  100  and CVR  200  of the invention occupy approximately the same space as the conventional CVR  50  in an existing CVR tray  10 . Moreover, as mentioned above, the power supply  100  and the CVR  200  are designed so that when they are coupled to each other as shown in FIGS. 14-16, they have approximately the same center of gravity as the prior art CVR  50  (FIGS.  4 - 6 ). Furthermore, as mentioned above, the power supply  100  is “plug compatible” with the existing tray so that all of the related aircraft electronics behaves substantially the same was as if an existing CVR were in the tray. 
     It is said that the electronics behave “substantially” the same because, as described in more detail below, a certain modification has been made so that the CVR test switch in the cockpit of the aircraft can be used to test both the CVR  200  and the power supply  100  and to receive fault indications therefrom. 
     According to present specifications, well known to those skilled in the art, “pin  12 ” on the existing CVR is coupled to a test switch in the cockpit. When this pin is grounded by the test switch, built in test equipment on the CVR conducts a self test. A passing test result is signaled by the CVR by applying a 1 ma current across “pins  15  and  16 ”. In addition, according to present specifications, any other built in test result may be reported by grounding “pin  23 ”. 
     According to a preferred embodiment of the present invention, when the existing test switch for the CVR is activated in the cockpit, two self-tests are initiated one in the modular power supply  100  and another in the CVR  200  module. The result of the CVR test is reported via “pins  15  and  16 ” and “pin  23 ” is used t o report the test result for the power supply. 
     Referring now to FIGS. 3,  9 ,  10 ,  13  and  17 , the present specifications for CVR and CVR tray include the specification that “pins  7  and  8 ” in the tray socket  32  are “jumpered together”. This jumper signals to the CVR that it is in the CVR tray and prevents the CVR from switching from the record mode to the play mode. 
     When the jumper is removed, or the CVR is removed from the tray, the CVR switches from the record mode to the play mode. The present invention preserves the functionality of this jumper with regard to the CVR record/play mode and makes additional use of the jumper as illustrated in FIG.  17 . 
     As shown in FIG. 17, a preferred embodiment of an illustrative power supply  100  includes, among other things, a capacitor  103  and a relay  105 . (It will be appreciated that the relay  105 , though illustrated as an electromagnetic relay, may be an electronic switch.) The positive node of the capacitor  103  is coupled to one node of the relay coil and is passed through the socket  114 ,  116  (illustrated as D “pin  9 ”) to the CVR. 
     The negative node of the capacitor  103  passes through the “pin  7  and  8 ” shunt to the chassis ground  107  where it is passed via pin A (“pin  17 ”) of plug  114 ,  116  to the CVR. Pins B and C of the plug  114 ,  116  are connected to the switching terminals of the relay  105 . These pins (B and C) act as the record mode switch formerly performed by “pins  7  and  8 ”. 
     Those skilled in the art will appreciate that when the power supply  100  is in the tray coupled to the socket  32 , the relay  105  will be activated causing a jumper between pins B and C for the CVR. When the CVR is removed from the power supply, it will switch to play mode. Moreover, if the CVR and power supply are removed as a unit from the tray, the jumpering of pins B and C will also cease because the relay  105  will no longer be powered. 
     Furthermore, the “pin  7  and  8 ” jumper now also provides an additional function. It uncouples the power supply capacitor  103  from the chassis ground  107  when the power supply is removed from the tray. This safeguards the power supply  100  from potentially damaging or dangerous short circuit accidents. 
     There has been described and illustrated a retro-fittable cockpit voice recorder with removable/replaceable independent power supply. While a particular embodiment of the invention has been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. 
     For example, although modular power supply and recorder units have been used to illustrate the preferred embodiment of the invention (to facilitate, for example, testing, repair and maintenance of one unit without having to open and re-seal/re-test both units, etc.), an integrated power supply/recorder that fits substantially into the footprint of a conventional cockpit voice recorder comes within the teachings of the invention. 
     As a further example, although the above described power supply is preferably generally “L-shaped” and has a footprint which is substantially the same as an existing CVR; those skilled in the art will readily appreciate that so long as the independent power and cockpit voice recorder modules contemplated by the invention have, in combination, a footprint which is substantially the same as an existing CVR (fitting into an existing tray), the particular shape of the modules could be varied. 
     Furthermore, although the invention is described with reference to a preferred embodiment where a modular power supply is installed in an existing rack first (followed by a modular piece of avionics equipment being coupled thereto); those skilled in the art will readily appreciate that a reverse installation would come within the scope and teachings of the invention, i.e., where the avionics unit is modified to accept a plug in power supply and the avionics equipment is installed in an existing tray first, followed by the power supply. 
     It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the disclosed invention without deviating from its spirit and scope as claimed hereinafter.