Aging Aircraft Conference Focuses on Electrical Systems

Air Safety Week, Sept 27, 1999

1. How many bare flight-critical wires are in the air today?

2 .How many are on your aircraft?

In addition, the NTSB's recommendations of April 1998 to separate and segregate key electrical components in 747 airplanes could be applied to all aircraft models (NTSB Recommendation No. A-98-38 of April 7, 1998). Indeed, the MD-11 offers a classic illustration of a potential single point failure, with multiple power sources routed through the same bundle.

Update the regulations

FAA regulations (FAR part 25.1353) require that electrical systems be installed such that wires are separated to prevent a single-point failure. However, according to one presenter at the aging airliner conference, "we have wire bundles that are composed of AC power, DC power, ground wires and signal wires...and bundles that have different bus power sources contained in them."

Multiple wire insulation types, mixed in the same bundle, can also lead to differential wear and the potential for "ticking faults," which this presenter described as an "aging wire time bomb." Yet regulations in place since 1964, and last updated in 1978, do not prohibit the use of different wire types in the same bundle. On the other hand, an FAA Advisory Circular (which does not have the force of regulation) was issued in 1991 (AC 25-16) providing specific guidance on wire installation and the use of circuit protection devices. This guidance, it is argued, could readily be incorporated into FAR Part 25.1353, bringing the regulations up to date with the current reality.

A life limit?

A more profound issue is on the horizon. It deals with the service life of wire and cable. "When is wire and cable considered 'dead'?" asked Dr. Yogesh Mehrotra. Absent a definition, Mehrotra surmised, "How can we predict remaining useful life?"

Dr. Chris Smith, manager of aging nonstructural systems research at the FAA's technical center in New Jersey, confessed that "wire service life is an extremely sensitive area." For one thing, he explained, "there is no agreement on what constitutes an unacceptable condition." Wire life depends on the type of insulation, the type of installation and the in-service environment. Nor is that all. An insulation breach might be clear enough, but another measure of aging, conductor resistance, would be threshold-dependent. Not least, Smith added, establishing a service life for wiring "has regulatory implications." >> Anderson, e-mail jim.anderson@delta-air.com; Eaton, e-mail deaton@nps.navy.mil; Brown, e-mail mbrown@grci.com; McGuire, tel. 425/234-3434; Teal, e-mail CrispyTeal@aol.com; Scott, e-mail scottg@squared.com; Mehrotra, tel. 203/874-3100 <<

The Airlines' Wiring Problem is Not Unique to the Aerospace Industry

Example: Space Shuttle Wiring Problems

* A potentially life-threatening short in the July flight of the Shuttle Columbia. Arcing caused failure of power to 2 of the 3 engines.

* 64 cases of wiring problems found on the shuttle fleet.

* 38 location on Shuttle Endeavor

* 26 locations on Shuttle Discovery

* Sites of problems show nicks of exposed wiring.

* 100 miles of wire in each of 4 space shuttles to be inspected, causing several week delay of next launch.

Source: CNN.com, Sept. 6, 1999

Example: B-1 Bomber Kapton Wiring Issue

* Kapton wire is called out for use on B-1B wiring harness drawings.

* The requirement to use Kapton needs to be removed and substituted with the requirement to use Tefzel wire.

* OC-ALC (Oklahoma City, Air Logistics Command) is authorized to substitute Tefzel for Kapton on any wiring harness they make.

* Kapton has been identified as a flammable substance. Electrical wiring reliability will continue to impact safety and mission reliability.