Wiring Matters: An Overview of the Aircraft Wiring Issue

Air Safety Week, June 19, 2000

A commentary by the editors of Air Safety Week

Over the past couple years, the potential hazard posed by bad aircraft wiring has generated a tremendous amount of activity within the industry. Consider: the hunt for electrically-related potential ignition sources in fuel tanks continues, with a recent airworthiness directive from the Federal Aviation Administration requiring operators to inspect B767 wiring in fuel tank conduits for evidence of chafing and potential arcing (see ASW, June 5). The action is being taken to prevent a fuel tank explosion of the type that destroyed TWA Flight 800 in 1996.

Meanwhile, a team of experts from the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) is conducting a series of intrusive inspections of the electrical wiring on various aircraft. The findings, we are informed, are sobering. Last week, the White House Office of Science and Technology hosted a meeting of a task force convened to assess the wiring situation across all modes of the transportation industry; a report reportedly is due out next October. This week in Indianapolis, Indiana, the U.S. Navy-sponsored Aircraft Wiring and Inert Gas Generator (AWIGG) working group holds another in a series of meetings to further the state of knowledge about aircraft wiring and what can be done to mitigate the hazard posed by wire failures and wire-induced in-flight fires.

Pilots are concerned, as evidenced by the recent call from the Air Line Pilots Association (ALPA) to the FAA for an updated policy on resetting circuit breakers (see ASW, March 6). Nor is this a U.S. story. Britain's Air Accidents Investigation Branch (AAIB) hopes to publish a report by the end of this year of its investigation into the electrical arcing that caused a United Air Lines [UAL] B767 to divert its planned Trans-Atlantic flight eastward and land at Heathrow in early 1998. Molten conductor was found in the avionics bay underneath the flight deck.

Carriers can assess their own situation, perhaps by asking how many returns to the gate, take off and returns, and unscheduled landings have occurred in, say, the past 30 days due to electrical malfunctions?

This being said, we offer below a somewhat broader perspective of the aircraft wiring issue, including a contrarian view to the search for ever thinner and lighter wire insulation:

Wiring 101

* The amount matters. Modern jets contain 100-200 miles of wiring running into every nook and cranny of the airplane. To borrow a biological metaphor, the wiring is akin to the body's nervous system.

* The trend matters. New jets feature more wiring carrying more current. The cabin area of a new-production jet, for example, features wiring for such things as in-flight entertainment systems. A measurement the electric power generating capacity of 1st, 2d, and current generation jets of comparable passenger-carrying capability would show a steady increase in aircraft electric power generating capability.

* Protection matters. Fire detection and suppression is inadequate. Enough electric power for a medium-size office building is concentrated in the electrical and equipment (E&E) bay located under the cockpit. The E&E bay has neither fire detection nor suppression. A runaway electrical fire downed Swissair Flight 111 in Sept. 1998; a month later a Delta Airlines L-1011 experienced an electrical fire behind the flight engineer's panel, in a location where hand extinguishers were virtually useless. With about 100 miles remaining on a flight from Hawaii to California, the crew effected an emergency landing at San Francisco. This airplane could easily have been "another Swissair," but this time a tragedy involving an airplane of U.S. registry.

* Age matters. Wiring is not immortal; it ages in service. Over time, the insulation can break, exposing conductor. Exposed conductors create a fertile field for ticking faults, spurious signals and, worse, full-blown electrical arcing. The 1998 inspections of wiring in fuel tank conduits on 737's revealed the strong correlation between aircraft age and the degree of wire chafing (an almost linear relationship, some wiring experts point out). Any carrier with a significant population of its aircraft with ten or more years' service has an aging wire problem.

* Location matters. Wiring is subject to changes in temperature, moisture, vibration and chafing. In some areas of the aircraft, such as in the leading/trailing edges of the wing, the landing gear wheel wells, etc., the physical stresses are higher than in more protected areas (e.g., the cabin).

* Installation matters. Sharp bend radii, improperly supported wire bundles, mixed insulation types in the same bundle, routing high and low power circuits in the same bundle, to name a few sins, can exacerbate the known environmental effects. Arcing in a vertically oriented bundle is more hazardous than in one running horizontally. One might suggest that large wire bundles indicate an electrical wiring philosophy based on ease of installation during manufacture, not necessarily ease of maintenance for the operator.