This is an exact copy of a Briefing Note given by CADMUS to Transport Canada

This is an exact copy of a Briefing Note given by CADMUS to Transport Canada, the Transportation Safety Board of Canada and the Air Transport Association of Canada in early February 1997. When, on 24 August 1999, the author requested access to TC records relating to polyimide wire, this Briefing Note could not be found. One document that was released by TC made a reference to this Briefing Note, indicating that it had been both seen and reviewed.

Aviation Safety Concern: Polyimide Film Insulation

Since 1965, the commercial and military aircraft industry has made extensive use of polyimide (pronunciation: poly im’-ide or poly- i- mid’) film insulation on aircraft wiring due to its thinness and light weight. However, the potential for electrical arcing and catastrophic fire caused by damaged polyimide film insulation, sold under the trade name Dupont Kapton® and Allied-Apical Apical®, have caused the US Navy to ban it from their aircraft and to initiate an extensive re-wiring and awareness program. The USN also took their concerns to the American Society for Testing and Materials (ASTM) where the problem was studied and discussed for 5 years. ASTM has since published a destructive test procedure under document ASTM D3032.

The USN contracted the VEDA Corporation to produce a 12-minute video outlining USN concerns about polyimide film insulation and how these problems should be managed.

Although the USN is taking aggressive steps to manage this situation, the FAA has not yet seized itself of the problem with equal vigour. Indeed, the FAA aircraft wiring expert contacted was unaware of the USN initiative. Steps have since been taken to bring the USN video to the attention of the FAA.

The 12-minute video explains two phenomena, Arc Tracking and Topcoat Flaking, endemic of, but not exclusive to, polyimide wiring.

VIDEO HIGHLIGHTS:

Background

· Wiring is absolutely critical to safe aircraft operations -- for power, control, navigation and communication;

· Polyimide wiring under the trade names of Dupont Kapton® or Allied-Apical Apical® have been used extensively in the aircraft industry since 1965;

· Polyimide wiring is thin and light weight – both attractive qualities for aircraft use;

· It is tape-wrapped in two crosscut biases and then shrink wrapped with heat. A colored top coat is then applied for ease of marking; and

· Owing to problems encountered with electrical arcing and fires, US Navy policy prohibits the use of polyimide wiring in US Navy aircraft.

· The primary problem with damaged polyimide wiring is Arc Tracking - where a small electrical short or arc caused by a damaged wire will burn the insulation material, leaving a carbon deposit (or track) which is highly conductive to further arcing. Repeated arcing and tracking can lead to a catastrophic fire (flash over) with extremely high temperatures (up to 5,000^oK) ;

· A secondary problem is Topcoat Flaking, where the coloured and marked topcoat flakes off, making inspection more difficult. It can give the untrained eye a false appearance of problems and also makes wire identification difficult, frustrating effective troubleshooting.

Many risk scenarios with potentially catastrophic results are possible. Aggravating factors include aging aircraft, component changes, exposure to de-icing fluid, inexperienced maintenance staff, and lack of appreciation by pilots, airline managers and regulators. Possible consequences could be as severe as those experienced 11 May 1996 by ValuJet 592. The aircraft in question was known to have had serious electrical problems. Although the National Transportation Safety Board is looking at the illegally packaged and marked oxygen generators as the ignition source for the fire, a scenario with equal or greater plausibility would have arc tracking as the ignition source for the fire which was then greatly intensified by presence of the oxygen generators.

US Navy Preventive Measures

·      Eliminate polyimide wiring wherever possible

·      Where this is not possible;

                       ·      Do not reset circuit-breakers, except when essential, then only once;

·      Inspect wiring to check for damage and to ensure:

· bundles which are designed to move do so in a twisting motion rather than a bending motion;

· appropriate routing away from severe weather and moisture problems (SWAMP) as well as heat, vibration; and

· Conduct an awareness program for pilots, technicians and maintenance managers.

Recommendations

· Transport Canada should either convene or charge an existing industry working group to collect, analyze and share data on polyimide wiring;

· Transportation Safety Board of Canada should conduct a review of electrical failures of aircraft using polyimide insulated wiring in comparison with aircraft not so wired to identify any significant trends;

· Transport Canada’s Research and Development Centre should conduct additional research, based on extenuating Canadian factors such as long term exposure to temperature and humidity extremes, aircraft de-icing and anti-icing fluids, etc.;

· Transport Canada should, immediately inform aviation industry of the problems with polyimide wiring found by the USN and by subsequent research and the action being taken to monitor and remediate the situation; and

· TSBC and TC staff should exercise vigilance concerning polyimide wiring in the course of their duties.