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
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
USN contracted the VEDA Corporation to produce a 12-minute video outlining USN
concerns about polyimide film insulation and how these problems should be
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.
12-minute video explains two phenomena, Arc
Tracking and Topcoat Flaking,
endemic of, but not exclusive to, polyimide wiring.
· 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
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 5,000oK) ;
damaged to wires which can lead to arc tracking can
be caused by:
chafing of wires or wire bundles;
stress of bending wires or wire bundles; and
wire or bundle handling during inspection or equipment changes.
Arc tracking can cause flash-over -- with extremely
Arc tracking can be aggravated by moisture,
chemicals and high temperature.
· 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.
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:
proper clamping and clearances;
anti-chafing measures are taken;
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
· minimum bend radii are observed; and
Conduct an awareness program for pilots,
technicians and maintenance managers.
· Transport Canada should either convene or charge an existing industry working group to collect, analyze and share data on polyimide wiring;
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;
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.;
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.
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