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TSB Raises Bar On
Flammability Standards

FRANCES FIORINO/NEW YORK

The Transportation Safety Board of Canada last week released the fourth set of safety recommendations--these focusing on material flammability standards--stemming from its ongoing probe of the Sept. 2, 1998, crash of Swissair Flight SR111 into the Atlantic Ocean off Peggy's Cove, Nova Scotia.

The investigation to find the cause of the inflight fire that is suspected of downing the MD-11, killing 215 passengers and 14 crew onboard, is "by far and away the most complex and challenging" one faced by the board, said TSB Chair Benoit Bouchard. It has proven to be "a puzzle of outstanding magnitude." Since initiating the probe in 1998, the board issued recommendations related to firefighting measures, power provided to flight recorders and recording capacity, thermal acoustical blankets and flammability test criteria ( AW&ST Dec. 11, 2000, p. 85; Nov. 23, 1998, p. 41).

The latest set of recommendations are "intended to address deficiencies in regulatory requirements and industry standards and practices which compromise safety." Specifically, they are related to materials used in the manufacture of aircraft, the testing of wiring and the potential for systems to exacerbate an inflight fire:

  • Material flammability standards (Recommendation A01-02). For the pressurized portion of an aircraft, flammability standards for material used in the manufacture of any aeronautical product be revised, based on realistic ignition scenarios, to prevent the use of any material that sustains or propagates a fire.

    VIC GERDEN, SWISSAIR 111 investigator in charge, said no material should be used if it sustains or propagates fire in the operating environment, and that "an airplane should not crash as a result of one ignition source." He points out the board recommended removal of thermal acoustical blanket material, identified earlier in the probe as a factor in the SR111 fire. He adds that other materials, such as elastomeric substances, which are not required to pass rigorous certification tests, were in the aircraft. Indications from ongoing analysis are that such materials burned off earlier and likely augmented propagation of that fire.

    • Material flammability test requirements for aircraft wiring (A01-03). A certification test regime be mandated that evaluates aircraft electrical wire failure characteristics under realistic operating conditions and against specified performance criteria, with the goal of mitigating risk of ignition.

      Gerden said aircraft wiring testing should be far more stringent as wiring can be an active ignition source. "We know that fire could have been ignited by an electrical arc from a wire, and here's a sobering fact--an aircraft of this type contains about 250 km. of electric wire." The only test currently mandated for certification is the "60-sec. Bunsen burner" that is performed on a single, unpowered wire and designed to determine if insulation material will burn and for how long.

    • System evaluation: fire-hardening considerations (A01-04). As a prerequisite to certification, all aircraft systems in the pressurized portion of an aircraft, including subsystems, components, and connections, be evaluated to ensure that those systems, whose failure could exacerbate a fire in progress, are designed to mitigate the risk of fire-induced failures.

    Bouchard said all aircraft systems should be evaluated in terms of impact on inflight fire. If it is determined they would exacerbate a fire, redesign should be considered.

    Gerden said many parts of the same system could have dissimilar properties and some parts may fail before others. Most of the oxygen lines in the MD-11 are made of stainless steel but have aluminum caps/fittings. A failure disconnecting the lines could lead to a large amount of oxygen propagating a fire.

    When asked about industry costs involved in implementing the recommendations, Gerden said it was "hard to put a price tag on it at this point," but estimated a world fleet cost of $1 billion for removal of mylar material. But first, he said, "an assessment would have to be done, and that is a function of the regulatory authority."

    He said there is still investigative work ahead, and the TSB's final report on the crash of Swissair Flight SR111 is expected "sometime next year."

    September 3, 2001 The McGraw-Hill Companies Inc.


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