Aviation Investigation Report A98H0003

4.1.10  Material Flammability Standards

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

The investigation's continued research into material flammability standards has revealed several safety deficiencies that pose unacceptable risks to the flying public. On 28 August 2001, the TSB issued ASRs (A01-02 through A01-04) (STI4-28) (STI4-29) (STI4-30) detailing its concerns regarding inadequacies that exist with respect to flammability standards for certain materials; testing and certification of aircraft wiring; and the requirements when conducting system safety analyses, which should also include the analysis of potential system failures that could be created by on-board fires.

The TSB believes that the use of a material, regardless of its location, type, or quantity that sustains or propagates fire when subjected to realistic ignition scenarios, constitutes an unacceptable risk, and that, as a minimum, material used in the manufacture of any aeronautical product should not propagate or sustain a fire in any realistic operating environment. Therefore, the TSB made the following recommendation:

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 fire. A01-02 (issued 28 August 2001) (STI4-31)

Regardless of efforts to design, install, and maintain an aircraft's wiring system to a high standard, deficiencies with wires will likely persist and present the potential for wire failures. While all wires will arc under certain circumstances, the dynamics of how a particular wire fails during an arcing event is highly dependent on the composition of the wire insulation. Understanding the dynamics of how a wire will fail under realistic conditions would be valuable, given the known consequences of the failure of an energized wire. While the FAA endorses several failure tests (e.g., the dry arc-tracking test procedure), it does not require any failure tests as a basis for wire certification.

Therefore, given the incidence of aircraft wire failures and their role as potential ignition sources, the absence of a certification requirement that measures a wire's failure characteristics, and that specifies performance standards under realistic operating conditions, constitutes a risk. Therefore, the TSB made the following recommendation:

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 the risk of ignition. A01-03 (issued 28 August 2001) (STI4-32)

All aircraft systems are subject to a system safety analysis as part of their certification process. Notwithstanding, for most systems this analysis does not ascertain how the system will perform in a fire-in-progress situation. Systems, such as oxygen, conditioned air, and hydraulic systems can exacerbate such a situation. The TSB believes that a fire-induced material failure in some aircraft systems has the potential to augment the combustion process and exacerbate the consequences of an in-flight fire. Therefore, the TSB made the following recommendation:

As a prerequisite to certification, all aircraft systems in the pressurized portion of an aircraft, including their sub-systems, 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. A01-04 (issued 28 August 2001) (STI4-33)  Transport Canada

TC agrees with the TSB's recommendations and agrees that more must be done to ensure appropriate regulations with respect to material flammability standards. TC intends to coordinate its actions with both the FAA and JAA in order to harmonize their respective regulatory environments.  United States Federal Aviation Administration

The FAA agrees with the thrust of the TSB's recommendations that material flammability standards must be improved. The FAA is confident that its previously announced Flammability of Materials in Inaccessible Areas and Improved Flammability Requirements for Thermal/Acoustic Insulation programs, in addition to its Test Methods for Evaluation of Low Heat Release Materials program, will address the concerns raised in TSB Recommendation A01-02.

With respect to the issue raised in A01-03, the FAA feels the arc fault circuit breaker (AFCB) program enhances the protection of aircraft wiring. In addition, the FAA has given the Wire Systems Harmonization Working Group the task of revising the standards for wiring performance and test requirements. The FAA advises that this effort may result in the development of a technical standard order for wiring. This group is also reviewing FAR 25.1309 in order to develop recommendations for the new Wire Systems Rule to address potential wire failures and in-service conditions.

Finally, the FAA believes that the existing regulations dealing with fire protection and prevention of critical systems (e.g., oxygen) are sufficient to deal with the system fire-hardening concerns raised in A01-04. The FAA's position is that current regulations, coupled with the results of FAA initiatives, such as the AFCB program, will mitigate the risks of fire-induced failures.  The Boeing Company

On 18 May 2001, Boeing issued SB MD11-35-021 entitled "OXYGEN - Control and Distribution - Modify Crew Oxygen Supply Line Installation." The purpose of the SB was to inform MD-11 operators of an FAA-approved modification procedure that replaces the aluminium components of the crew oxygen supply line system with steel components as a fire-hardening measure.

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