Aviation Investigation Report A98H0003

4.3.3  Aircraft Wiring Issues

  1. - Material Flammability Test Requirements for Aircraft Wiring
  2. - Limitations of FAR 25.1353 Electrical Equipment and Installations
  3. - Potential Limitations of MIL-W-22759/16 Wire  Material Flammability Test Requirements for Aircraft Wiring

In one of its recommendations regarding Material Flammability Standards (A01-03), the TSB explained the need to augment the certification test regime used in the approval of aircraft wires. Specifically, the certification criteria need to be expanded to include the determination of wire failure characteristics, using realistic operating conditions and specified performance criteria. The goal of such certification requirements would be to establish standards that would prevent the approval of any wire whose in-service failure could ignite a fire and minimize further collateral wire damage.

Regulatory authorities have advised the TSB that this issue is to be dealt with under the auspices of the FAA's Aging Transport Systems Rulemaking Advisory Committee (ASTRAC). The Board is aware that a Wire Systems Harmonization Working Group has been established to review the certification standards related to aircraft wiring systems. However, in evaluating the assignments of this working group, the Board was unable to identify a specific task that would initiate a review, based on the deficiency described in A01-03.

The Board appreciates that regulatory authorities are dealing with the larger issue of in-flight fires on several fronts, including improved material flammability standards and AFCB technology. While such activities have been beneficial and necessary, the Board is concerned that the deficiency identified in its A01-03 recommendation will not be corrected unless a specific regulatory review of certification requirements is undertaken to ensure the proper evaluation of aircraft electrical wire failure characteristics.  Limitations of FAR 25.1353 Electrical Equipment and Installations

During this investigation, the TSB found that there are limitations associated with the interpretation and application of FAR 25.1353(b). In aircraft design, it is not always possible to maintain physical separation between wires, especially in the cockpit area where, typically, space available for installations is confined. The guidance material does not specify what measures or criteria would be acceptable to meet the requirements of FAR 25.1353(b).

The Board has not issued a safety communication on this subject as it is aware that the FAA's ASTRAC (includes the JAA and TC) has been tasked with identifying the requirements for wire separation as they pertain to electrical equipment and installations. Specifically, the ASTRAC is to determine whether a comprehensive wire separation regulation needs to be included in a new wire system rule.

The ASTRAC's final recommendations on this matter have yet to be published; however, the Board is aware that Working Group 6 has declared to the FAA that the creation of separation standards is well beyond the scope of its tasking. Given this situation, it is unlikely that substantive change on the matter of wire separation will result from the current round of ASTRAC assignments. The Board remains concerned about the limitations regarding the interpretation of FAR 25.1353(b) and encourages the regulatory authorities to take follow-up action to research and resolve this matter.  Potential Limitations of MIL-W-22759/16 Wire

The primary wire type selected for the IFEN system installation was MIL-W-22759/16. This wire is commonly used by aircraft modifiers and the general aviation industry, although the wire is not used by major aircraft manufacturers, such as Bombardier and Boeing. The wire type is certified and used successfully without any record of inherent problems or adverse service history.

The Board is aware that on 22 March 2002, the United Kingdom Civil Aviation Authority issued Appendix 64 to its Airworthiness Notice 12, entitled Experience from Incidents. Appendix 64 deals specifically with MIL-W-22759/16 Electrical Cable and states, in part:

[P]articular care must be taken when selecting this cable type to ensure that it meets all installation requirements and is fit for its intended application.

The appendix lists several areas that must be addressed prior to the approval of MIL-W-22759/16 wire usage in the United Kingdom.

While the Board has not determined that this wire type is problematic, it remains concerned that, based on the Airworthiness Notice 12, the in-service performance of MIL-W-22759/16 wire may not be fully known.

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