Safety of Aging Aircraft Electrical Systems Under Unprecedented Examination

Air Safety Week, July 19, 1999

Basically, about 4-5 discrepancies per aircraft type or model were identified, such as severely corroded battery lugs (i.e., fittings), chafed and cracked wires and loose wire clamps. "We haven't found a single thing that would be an airworthiness concern," Marcontell said. However, these discrepancies, now under investigation and slated to be reported on more fully by August 1, could trigger some number of ADs requiring inspection and repair. Recall that a single improperly installed wiring clamp led to chafed wires, arcing, and a severe in-flight fire last November on an L-1011 (see ASW, July 12).

Although not among the 4-5 discrepancies inspectors are finding on average by aircraft type, Marcontell said flammable lint blanketing electrical components on these older aircraft "is coming up a lot."

* Intrusive inspections. Despite the relatively good news from the visual inspections, there is a perceived need to delve deeper, with a more intrusive inspection regime. "There is the potential for damage in these electrical systems even with intense visual inspections," Marcontell said. For this reason, he has recommended the creation of an Intrusive Inspection team to physically disassemble wire bundles, connectors and other components of recently-retired aircraft to assess their condition. In fact, a list of aircraft going into retirement already has been identified. The goal is to examine all the applications of different aircraft wire insulation types, including polyimide wire insulation. The need to cast a net wide enough to cover all applications and wire types, Marcontell said, "may take us to airplanes less than 20 years old."

The recognized need to cover polyimide, known more generally as Kapton, is significant. This wire insulation type has had a checkered and controversial history of premature cracking, wear-through and arcing.

Some industry officials have maintained that Kapton's in-service problems relate to improper installation, rather than any inherent deficiencies in the insulation type. Comments from some ATSRAC members, however, reflected the real sea-change in attitude that has occurred over the past six months. The risk of Kapton is now on the table. A remark by Bohdan Goyaniuk, representing Transport Canada, pretty much reflected what appeared to be the prevailing consensus of ATSRAC members. "We need to do something about polyimide. Does it need to be a subject of this intrusive inspection? Yes," he declared.

The intrusive inspection subcommittee is to present a proposed inspection plan by the end of September. It is an aggressive schedule, with the actual gallery of intrusive inspections to be completed by the end of the first quarter of 2000.

These inspections, too, could uncover additional problems that could take the form of service bulletins and airworthiness directives.

In addition to this three-pronged effort, ATSRAC members last week wrestled with two additional problems - one is statistical and the other is definitional. Both are significant.