Aviation Investigation Report A98H0003

4.1.2  Flight Recorder Duration and Power Supply

  1. - Transportation Safety Board of Canada
  2. - United States National Transportation Safety Board
  3. - United States Federal Aviation Administration
  4. - Transport Canada
  5. - The Boeing Company  Transportation Safety Board of Canada

Shortcomings related to the duration of cockpit voice recorder (CVR) recordings and the supply of electrical power to the flight data recorder (FDR) have been identified during this and other aircraft accident investigations. Consequently, on 9 March 1999, the TSB issued four ASRs (A99-01 through A99-04) (STI4-2) (STI4-3) (STI4-4) (STI4-5) to TC and the Joint Aviation Authorities (JAA), dealing with CVR duration, independent power supply, and the use of separate electrical buses.

A lack of recorded voice and other aural information can inhibit safety investigations, and delay or prevent the identification of safety deficiencies. Given the need for longer periods of recorded sound to capture the initiating events of aviation accidents, and the availability of two-hour CVRs, the TSB believed that such recorders should be mandated by regulatory authorities worldwide. However, it also recognizes that a period of several years may be reasonably required for manufacturers and operators to implement this change. Therefore, for newly manufactured aircraft, the TSB made the following recommendation:

As of 01 January 2003, any CVR installed on an aircraft as a condition of that aircraft receiving an original certificate of airworthiness be required to have a recording capacity of at least two hours. A99-01 (issued 9 March 1999) (STI4-6)

In addition, the TSB believes that, with appropriate lead time, a retrofit program is warranted for aircraft already in service. Therefore, the TSB made the following recommendation:

As of 01 January 2005, all aircraft that require both an FDR and a CVR be required to be fitted with a CVR having a recording capacity of at least two hours. A99-02 (issued 9 March 1999) (STI4-7)

When aircraft power to the SR 111 flight recorders was interrupted at 10 000 feet, the FDR and CVR stopped recording. The aircraft continued to fly for about six minutes with no on-board information being recorded. This lack of recorded information hampered the accident investigation.

With maintenance-free independent power sources, it is now feasible to power new-technology CVRs and the cockpit area microphone (CAM) independently of normal aircraft power for a specific period of time in the event that aircraft power sources to the CVR are interrupted or lost. Therefore, to enhance the capture of CVR information needed for accident investigation purposes, the TSB made the following recommendation:

As of 01 January 2005, for all aircraft equipped with CVRs having a recording capacity of at least two hours, a dedicated independent power supply be required to be installed adjacent or integral to the CVR, to power the CVR and the cockpit area microphone for a period of 10 minutes whenever normal aircraft power sources to the CVR are interrupted. A99-03 (issued 9 March 1999) (STI4-8)

At the time of the occurrence, FDR and CVR installation in MD-11 aircraft were both powered from AC Generator Bus 3. The Smoke/Fumes of Unknown Origin Checklist (see Appendix C – Swissair Smoke/Fumes of Unknown Origin Checklist) requires the use of the SMOKE ELEC/AIR selector. This switch is used to cut power to each of the three electrical buses in turn in order to isolate the source of the smoke/fumes. The nature of this troubleshooting procedure requires that the switch remain in each position for an indeterminate amount of time, typically at least a few minutes. When the SMOKE ELEC/AIR selector is in the first (3/1 OFF) position, alternating current (AC) Generator Bus 3 is turned off, thereby simultaneously disabling the FDR and the CVR.

With both the CVR and the FDR on the same generator bus, a failure of that bus, or the intentional disabling of the bus (e.g., the result of checklist actions in an emergency), will result in both recorders losing power simultaneously. To enhance the capture of information needed for the identification of safety deficiencies, the TSB made the following recommendation:

Aircraft required to have two flight recorders be required to have those recorders powered from separate generator buses. A99-04 (issued 9 March 1999) (STI4-9)  United States National Transportation Safety Board

Coincidently, the NTSB issued recommendations A-99-16 through A-99-18 to the FAA, which contain the same elements as the TSB recommendations. The NTSB also recommended that aircraft be fitted with two combination CVR/digital flight data recorder (DFDR) recording systems. As described in Section, the FAA has yet to begin NPRM action in response to the NTSB recommendations. As of 25 July 2001, the NTSB regarded as unacceptable the amount of progress made in the two years since the recommendations were issued. The NTSB continues to urge the FAA to act expeditiously on these recommendations but remains sceptical that the dates for final action can be met.  United States Federal Aviation Administration

The FAA agreed with the intent of the NTSB recommendations and indicated that it would initiate NPRM action by the end of summer 1999. By August 1999, the FAA advised the NTSB that because of competing priorities, the NPRM would be delayed until March 2000. Responding to an update request from the NTSB dated June 2000, the FAA announced in April 2001 that rulemaking based on the CVR/FDR recommendations would be further delayed until the end of 2001. As of this writing, the FAA advises that NPRM action will take place in the spring of 2003.  Transport Canada

TC responded to the TSB's recommendations with respect to flight recorders and power supply by indicating that it was TC's intention to harmonize its position with the JAA and address the FAA's NPRMs at an appropriate Canadian Aviation Regulation Advisory Council meeting. Therefore, TC's implementation timetable is linked to the FAA schedule.  The Boeing Company

Boeing published SB MD11-31-101 on 19 December 2001, which allows MD-11 recorders to be powered by separate buses. Incorporation of the SB will result in the CVR being powered by the right emergency bus, and the digital flight data acquisition unit/DFDR by the Engine 1 AC generator bus.

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