Interim Aviation Safety Recommendations - Thermal Acoustical Insulation Materials
Flammability Test Criteria
The flammability test for thermal acoustical insulation, insulation covering, and insulation blankets, as stated in Appendix F of FAR 25.853, necessitates a vertical flammability test of samples using an approved burner. The type of cover material on the insulation blankets installed on the Swissair aircraft had been subjected to this test and met the applicable flammability test criteria for FAA certification.
In-service fires of the metallized PET cover material, and inconsistent results from the vertical burn test method specified by FAR 25.853, prompted manufacturers to seek additional flammability test criteria. Subsequently, aircraft manufacturers developed a "cotton swab" test, which yielded more consistent results when testing the flammability characteristics of the various cover materials. This additional testing was adopted by several major aircraft manufacturers who subsequently modified their internal material specifications. In 1996, based on results of the "cotton swab" test, McDonnell Douglas advised its customers not to use metallized PET, and discontinued its use in production aircraft. In 1997 an FAA sponsored study confirmed that the "cotton swab" test was a more reliable and reproducible test method to assess the flammability characteristics of metallized PET cover material; however, the FAA did not amend FAR 25.853, Appendix F to improve test standard requirements.
As the incidents listed in Appendix B attest, the limitations of the FAR 25.853, Appendix F, test criteria may not be confined to its inability to accurately and reliably identify the flammability characteristics of metallized PET cover material.
On 14 October 1998 the FAA stated that the test criteria used to certify the flammability characteristics of thermal acoustical insulation materials were inadequate, and committed itself to conducting the research necessary to establish a more comprehensive test standard. At the same time, the FAA indicated that because materials containing polyimide film have performed well in preliminary flammability tests, these materials would be considered compliant under the new regulation. Until adequate flammability test criteria are available, it is not possible to determine whether polyimide film, or other materials, provide adequate protection against fire propagation. Thermal acoustical insulation materials are installed in aircraft as a system, including such related components as tape, fasteners, and breathers. The Board believes that thermal acoustical insulation materials for use in aircraft must be judged against more valid flammability test criteria, not as individual components, but as a system. Therefore, the Board recommends that:
On an urgent basis, regulatory authorities validate all thermal acoustical insulation materials in use, or intended for use, in applicable aircraft, against test criteria that are more rigorous than those in Appendix F of FAR 25.853, and similar regulations, and that are representative of actual in-service system performance. (A99-08)
Transport Canada's Response:
The authorities including Transport Canada (TC), have already recognized the propensity of certain types of thermal acoustical insulation material systems to spread fire under certain conditions, and the inadequacy of the present flammability test requirements to properly characterize such performance. The authorities have accordingly been working at developing new test standards which will quantify the flammability characteristics of thermal acoustical insulation material systems against realistic fire threats under the environment/conditions to which they are subjected in service, and reliably discriminate between safe and unsafe material systems.
TC has been actively cooperating with the United States Federal Aviation Administration (FAA), as well as Joint Aviation Authorities (JAA) States, in the development of these standards, and associated regulatory material. This work is expected to come to fruition within the next few months.
TC will continue the above work and plans, concurrently with other authorities, to arrive at internationally harmonized solutions to the issue. TC will then proceed with appropriate regulatory action to ensure that thermal acoustical insulation material systems on Canadian-manufactured and Canadian-registered aircraft present safe flammability characteristics.
Further, TC has offered to work with the TSB who plans to explore at the FAA Technical Centre certain thermal acoustical insulation fire propagation theories that could apply to the Swissair MD-11 accident configuration.
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