Rep. Bart Gordon asked:

“I particularly must raise questions as to why, when the military has already taken steps to address questionable wiring in its planes, the airlines, in whose craft many more persons fly, have been allowed to continue operating with wiring even FAA experts say should not be allowed.”

Gordon was referring to a January 2008 study, called the Aircraft Wiring Degradation Study, which was performed by a team at the FAA’s William J. Hughes Technical Center in Atlantic City, NJ. The study is referenced throughout Gordon’s heavily-footnoted letter.

The dynamite portion of the study is in its Appendix C, some 161 pages into the 275-page study, where this statement exists, almost without any preamble or follow-up explanation:

“PVC [polyvinyl chloride] and fluorinated ethylene propylene/polyimide (Kapton) wire insulation materials should not be used in airborne applications.”

There is a saying in journalism, to the effect that one does not “bury the lead.” In other words, the most important item in a story is not left lurking in the body of the text, but rather it is placed high up, in the lead paragraph. That is not the case here, where the only mention of these two insulation types in the Executive Summary does not say they shouldn’t be used:

“The purpose of this initial research program was to evaluate the aging characteristics of three types of aircraft electrical wire: polyimide (PI) [Kapton], polytetrafluoroethylene/polyimide composite (CP), and polyvinyl chloride/nylon (PV). … The results demonstrate that PI and PV aircraft wires that are present in high-moisture areas will have a higher risk of aging or degradation.”

Just three such high-moisture locations on an aircraft are the wing leading and trailing edges and the wheel wells for the landing gear. Wires in these three areas are often exposed to the elements, to say nothing of increased wear from vibrating in the slipstream.

Both PVC and Kapton wires are found in hundreds of airliners now flying. Rather than ask why these wire types are not removed and replaced with a safer type, Gordon asks what technologies the FAA has deployed to reduce the chance of a wire-induced fire. 

One technology the FAA hoped could be used to assess in situ wire health is called time domain reflectometry (TDR), in which a pulse of current is sent down a wire and breaches in the wire insulation could theoretically be gauged to the inch.

However, the Degradation Study poured cold water on TDR, saying:

“TDR experts stated that using ‘standard’ TDR techniques to detect wiring insulation degradation, such as chafing and cracks, is not just difficult, but virtually impossible. This type of degradation is exactly what the TDR testing was intended to reveal.”

The study does not suggest the wholesale replacement of wiring at a certain point in an airplane’s service life, saying only that “the wire types studied are typically designed to exceed 10,000 hours of service life at rated temperatures when stressed with specific mechanical and electrical factors.” The study does not remind the reader of the Transportation Safety Board (TSB) of Canada’s concern that wiring is not subject to realistic conditions when it is qualification tested for use on airplanes. The TSB complained that the FAA only requires the 60º flame test to certify wiring. In other words, there are no FAA tests for durability in service, with wire bent (which puts the insulation under strain) and under high humidity (which causes the insulation to break down).

The Degradation Study itemized the numerous factors that put stress on wire in an airplane:

The Degradation Study observes that these factors are inversely proportional to the usable service life of a wire. That is, the higher the level of stress – from one or a combination of factors – the faster the material will age.

What’s really interesting is the fact that this study was snatched from relative obscurity to feature prominently in a Congressional letter to the FAA. The FAA has not imposed any requirements that would disqualify PI (Kapton) or PV (polyvinyl chloride) wire insulation, and Gordon seeks assurances that these wire types do not find their way into new aircraft that are going to be in service for the next 30 years or more. (The Aircraft Wiring Degradation Study can be found at www.tc.faa.gov/its/worldpac/techrpt/ar082.pdf)