A98H0003 - Interim Aviation Safety Recommendations for Flight Recorder Duration and Power Supply

Independent Power Source

When aircraft power to the SWR 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 information being recorded. This lack of recorded information has hampered the accident investigation.

Power interruptions have resulted in flight recorder information not being captured during the last minutes of several other recent aircraft occurrences. These include ValueJet (Miami, Florida; DC-9-32; 11 May 1996), TWA flight 800 (East Moriches, New York; Boeing 747-131; 17 July 1996), SilkAir (Palembang, Indonesia; Boeing 737-300; 19 December 1997), Delta Air Lines (Cork, Ireland; MD-11; 08 October 1998), and Delta Express (Orlando, Florida, Boeing 737-232; 15 December 1998).

In modern aircraft, flight data and other data from multiple sources are used by the aircraft systems and by the flight crew to operate the aircraft. To record the parameters it needs, the FDR simply monitors the data flowing through data buses. If electrical power to a particular sensor or data bus is lost, FDR information pertaining to that sensor or data bus will no longer be available. In the event of a total loss of electrical power, essentially there would be no data to record. There may be merit in independently powering the FDR and its flight data acquisition unit in order to capture whatever data are available during partial electrical failures. However, as a minimum, the TSB believes the CVR and its cockpit area microphone must continue to be powered for short periods regardless of the availability of normal aircraft electrical power. This independent power source would allow the continued recording of the acoustic environment of the flight deck, including cockpit conversations and ambient noises, for a specific period.

With maintenance-free independent power sources, it is now feasible to power new-technology CVRs and the cockpit area microphone 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 Board recommends that:

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)

Transport Canada's Response:

Transport Canada supports this recommendation with the provision that U.S. and Canadian requirements are harmonized. In their response to a similar recommendation made by the NTSB, the FAA indicated that they would be introducing a Notice of Proposed Rulemaking to amend Technical Standard Order (TSO) 123(a) to address the requirement for a 10 minute independent power supply for CVRs. This TSO is based on a standard developed by the European Organisation for Civil Aviation Equipment, of which Transport Canada is a participating member. The progress of this standard will be monitored and, when appropriate, consideration will be given to introducing this requirement into Canadian legislation.

Separate Electrical Buses

In the current configuration of the MD-11, the FDR and CVR installations are both powered from generator AC Bus No. 3. The MD-11 emergency checklist, dealing with smoke/fumes of unknown origin, requires the use of the SMOKE ELEC/AIR switch. 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 switch is placed in the first (3/1 OFF) position, generator AC Bus No. 3 and No. 1 air conditioning packs are turned off, thereby simultaneously disabling the FDR and the CVR. Additionally, if the smoke/fumes are cleared in this first position, the SMOKE ELEC/AIR switch is to remain in this position for the duration of the flight, which means that the CVR and FDR both remain inactive while there are data to be recorded. Although it has not been established whether the recorders on SWR 111 stopped as a result of deteriorating electrical systems or the selection of the SMOKE ELEC/AIR switch, the fact that both recorders can be disabled by a single switch selection poses an unnecessary risk of losing critical recorder information.

The Federal Aviation Administration's FAR 25.1457 (CVR) and FAR 25.1459 (FDR), Transport Canada's Canadian Aviation Regulations Standards Part V--Airworthiness Manual, Chapter 551, Articles 551.100 and 551.101, and European Civil Aviation Electronics (Eurocae) specifications require that recorders be installed so that they receive power from the electrical bus that provides the maximum reliability for operation without jeopardizing service to essential or emergency loads. With both the CVR and the FDR on the same generator bus, however, a failure of that bus or the intentional disabling of the bus (as could result from checklist actions in an emergency) result in both recorders losing power simultaneously.

To enhance the capture of information needed for the identification of safety deficiencies, the Board recommends that:

Aircraft required to have two flight recorders be required to have those recorders powered from separate generator buses. (A99-04)

Transport Canada's Response:

Transport Canada, in accordance with CAR 551.01, CAR 605.33 and Airworthiness Manual 551.100, requires the use of separate supply buses. Furthermore, the Transport Canada requirement is harmonized with the EUROCAE-ED-56A by reference.

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