WIRING MATTERS:

The National Transportation Safety Board (NTSB) lent added urgency to the need for wiring inspections with its late June press conference, timed shortly before the 10th anniversary of the TWA Flight 800 disaster, to reinforce and restate the Board's concern about fuel tank safety and aging, cracked and deteriorated wiring. Recall that the accident airplane, an old B747-100, blew up shortly after takeoff from New York's JFK International Airport on July 17, 1996, for an overnight flight to Paris.

All 230 aboard were killed when flammable vapors in the center wing fuel tank exploded. Electrical arcing in a bundle of wires outside the fuel tank produced a surge of current that passed down a fuel quantity indication system (FQIS) wire. As the Board noted in its press release of June 29, "The ignition of the flammable fuel/air mixture in the tank was attributed to an electrical failure."

Chafing the Dominant Problem

To be sure, numerous airworthiness directives (ADs) have been issued since the TWA disaster, mandating wiring and other modifications to ensure electrical system safety. While the FAA does not have good records on the incidence of wire failures in the commercial industry, the U.S. Navy has amassed considerable information and insight (see box). Navy data suggests that as many as one million manhours are spent annually in troubleshooting, isolating, locating and fixing wiring faults. Naval Air Systems Command (NAVAIR) data suggests that nearly as many hours are spent on unscheduled wiring maintenance as on scheduled maintenance.

Further, the data collected by NAVAIR indicate that chafing contributed to more than a third (37%) of all wiring failures on Navy aircraft during the period 1980-1999. Moreover, despite the fact that chafing, or the erosion of insulation and the exposure of conductor, is a known problem, and the tools to resolve it are available, analysis of data from the years 2000 to 2004 show that chafing remained the leader of all wire failure modes on Navy aircraft.

Perhaps the closest to an industry-wide measure for the commercial side comes from the fleetwide inspections mandated by the FAA for fuel system wiring on the B737 fleet in 1998. The inspections were directed after fuel was observed leaking from a conduit for wiring that had been opened by electrical arcing. All B737 operators were required to report their findings to the FAA. The inspections revealed a clear relationship between aircraft age and the severity of the problems found. Fully 30% of aircraft with more than 70,000 hours were found with severe chafing and bare wires. That is twice the percent found on B737s with fewer than 70,000 hours.

Some commercial operators have raised awareness of good wiring husbandry and practices to be avoided. For example, United Air Lines has widely distributed a poster outlining the do's and don'ts for wiring maintenance (see poster p. 32).

United's laudable effort notwithstanding, 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 advent of wireless systems is reversing this trend). 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, 2nd, 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," 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. Any carrier with a significant population of its aircraft having 10 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.
• Type matters. Certain types of wire insulation, notably aromatic polyimide, have known properties of hardness, vulnerability to cracking, and the tendency to arc spectacularly. Indeed, the carbonized insulation under arcing conditions itself becomes a conductor, spreading the danger literally with the speed of lightning.
• Maintenance matters. Wiring can be damaged during maintenance of other aircraft components, largely because technicians are unaware of the potential hazard created by stepping on a bundle or yanking it in such a way that brittle insulation is damaged further. Another major problem is unrelated maintenance damaging the wire. For example, drilling into aluminum structure creates shavings, called swarf. If those bits of swarf fall onto wire, they can eventually cut or wear through insulation, giving rise to intermittent (or worse) electrical failures. To be sure, it takes time to put a cover over the wires while drilling, then folding up the covers and removing them from the airplane. But it may take less time than involved in finding swarf-related faults in the wiring weeks or months later,
• The military's experience matters. Some industry officials believe the U.S. military's experience is not relevant to the airline industry because military jets are exposed to higher maneuvering loads and to harsher operating environments. On the other hand, the military's experience with a jet designed with a 6,000 hour service life may be highly relevant to an airliner with a design service goal of 60,000 hours. The airliner is exposed to lower extremes over an order of magnitude longer period of time. In this respect, the military's experience may be considered a form of accelerated aging from which the commercial side of the aerospace industry could learn much.
• Inspection types matter. Visual inspections are not enough. Eyeballing the wiring in a jet may uncover only a third or less of the insulation breaches exposing conductor. Yet technologies can be mobilized to quantify the state of wiring in an airplane, and to assess the amount of life remaining. These techniques can be used to target a cost-effective program of selective wire replacement.

A Broad View

The airline industry may be at a place with respect to wiring that it was a decade ago with aging structure. The physical structure of an airliner now is built to be damage tolerant. That is, the airplane is designed such that structural components feature sufficient residual strength to withstand the weakening effects of fatigue cracking, say, from a tiny flaw that may lurk unseen somewhere in the structure from the day it leaves the factory. Recall that when damage tolerant structure was being debated, the manufacturers worried the added weight would drive them out of the airplane building business and into the manufacture of railroad rolling stock.

As it turned out, damage tolerant design added about 1,000 lbs. (454 kg) to the weight of a DC-10 while greatly extending its service life. Damage tolerant structure is now considered the norm.

Wiring, however, is not damage tolerant. As a weight saving measure, the thickness of the insulation has been shaved to a minimum. In some wires, the insulation is about the thickness of four human hairs laid side-by-side. Or, as one expert observed, the industry is about "four hairs from electrocution." Indeed, many of the problems of chafing, etc., elucidated above would not be the threats they are if the insulation was about four times thicker. Admittedly, this is kind of a brute-force approach, but by one estimate thickening the insulation would add about 200 pounds (91kg) to the weight of wiring in a widebody jet.

That's about the equivalent weight of magazines and catalogues in the seat-back pockets. Perhaps a philosophy of damage tolerant electrical system design is only a matter of time -- and certainly it is within the current state-of-the-art.

Other potential improvements are numerous. Heavier insulation could be made an available option during manufacture. High power and low power wires could be better segregated. Connectors could be better separated, too, and not all bunched together so that an electrical arc can jump from one to another. Longer-life circuit breakers could be installed as original equipment, saving considerable money over the long haul.

Fire detection and suppression in the electronics and equipment (E&E) bay, and other unprotected areas where electrical systems are concentrated, could be insisted upon. The reduced maintenance costs, higher dispatch reliability, and fewer precautionary landings would, over the life of the airplane, more than offset the purchase cost of such features and protections.

From the United "Wire Aid" poster, page 32, it is evident that wiring is no longer a "fit and forget" item, but rather an essential system that necessitates continuing maintenance.

Wiring Problems Affect All Sizes of Aircraft

The following description of burned wire insulation on a Mooney M20R, submitted by a repair station technician, aptly demonstrates the tremendous potential for significant failures in very small mistakes: "When troubleshooting the squawk `recognition lights - on all the time,' we found charred wire insulation in the wire harness feeding the overhead bank of switches that control the ship's exterior lighting. We traced the charred insulation to an overhead condition in the recognition light circuit. A sub-standard crimp on a wire terminal was repaired, and the overheat condition was eliminated. The squawk `recognition lights - on all the time' was due to the burned insulation allowing power directly to the recognition lights, circumventing the switch." Note: the electrical connector is located behind the flight panel on the copilot's side. Source: Service Difficulty Report

The Maintenance Issues of Bad Wiring on U.S. Navy Aircraft Are Staggering

By Jerome Collins and Gail Edwards

"Although the U.S. military's challenges with electrical wiring are not as well publicized as those from the commercial sector [e.g., the TWA 800 and Swissair 111 accidents, both attributed to a wiring failure], the issues still exist. For instance, during a recent 30-month period, the U.S. Navy lost two aircraft due to electrical wiring failures ...

"The problem is also evident in the roughly two and one-half electrical fires that occur on U.S. navy aircraft each month ...

"With the safety issue aside, the maintenance and reliability issues are staggering as well. The U.S. Navy expends approximately one million maintenance hours per year at the OMA [organizational maintenance activity, or squadron level] alone on wiring related repairs ...

"The problem with wiring failures are also seen in the approximately 1,400 mission aborts (540 in-flight aborts) and an average of 125 non-mission capable aircraft ... With only 4,700 aircraft in inventory for the U.S. Navy [roughly the size of the commercial U.S. fleet before 9/11], this means that between two and three percent of the fleet is out of service due to wiring problems at any one time.

"Wiring system maintenance on U.S. Navy aircraft has historically been viewed as a reactive maintenance task. In other words, `If it ain't broke, don't fix it.' To compound the problem, the validation of the integrity of the conductive path is only sought if all other maintenance actions have been ruled out. This includes the replacement of a possibly good Weapons Replaceable Assembly [WRA, or in commercial parlance, a line replaceable unit, LRU] into the system to validate the WRA's inoperability. If the maintainer replaces a `suspect' WRA with a `known good' from supply, both the `No Fault Found' rate and unnecessary man-hour rate increases. Again, these `No Fault Found' incidents that are caused by wiring is another reason that the metrics are believed to be underreported. ...

"Although the challenge has been great in solving problems with wiring on U.S. Navy aircraft, the Naval Air Systems Command has chosen not to ignore the problems but to move in a direction of finding innovative solutions to the problems. These solutions include better training for the maintainers, insuring that quality wiring components are in the supply system, and inserting technology into the hands of maintainers that will help them do their jobs better."

From: "The Naval Air systems Command's Initiatives for Aircraft Wiring Diagnostic Support Equipment for Multiple Maintenance Levels," by Jerome Collins and Gail Edwards, NAVAIR, 2005