What every pilot should know about aircraft wiring safety

by Stephen Pope

In the typical new business jet, with its abundance of advanced avionics and networked in-flight entertainment devices, literally hundreds of thousands of feet of tightly bundled wiring wend throughout the cabin and cockpit, creating the “electrical logic” of the aircraft. Most pilots of newer jets probably never give much thought to the condition of all that insulated copper. For aviators flying older aircraft, however, most of which have been subjected to years of harsh temperature and humidity fluctuations, wiring safety is likely higher on the list of concerns, particularly with the added attention the subject has been getting in the wake of a few high-profile accidents and incidents.

FAA officials, faced with mounting public anxiety over the safety of electrical wiring in aging airliners, are finding themselves walking a thin line between keeping the air transportation system in motion and making sure older aircraft are safe. Safety experts, however, say the fears are valid, and they are cautioning the agency against downplaying the potential dangers of cracked and exposed wiring in civil airplanes.

In the aftermath of the TWA Flight 800 and Swissair Flight 111 accidents, both of which are believed to have been caused by short circuits in the airliners’ electrical systems, attention has been directed squarely to the potential hazards associated with cracked and brittle wiring in older airplanes. During congressional hearings in October, around which swirled national media attention, FAA officials were criticized for the slow progress the agency is making in addressing such concerns. The officials defended themselves by pointing out that an agency task force is close to issuing findings of recent studies on wiring safety, and also said significant advancements are being made toward developing a new type of circuit breaker that is better at detecting arc faults in wiring than today’s breakers, which won’t trip in response to smaller spikes of voltage associated with arcing.

The NTSB has said it believes this type of arcing may have caused the explosion aboard TWA Flight 800. If not remedied, wiring deficiencies could lead to more catastrophes, according to the Safety Board. As a result, it has recommended that the FAA conduct design reviews of wiring on all jet models.

Alarming Findings

An FAA spokeswoman said the task force, which was supposed to have issued a report in October, will now likely do so early next year. While results of the task force’s testing have yet to be made public, officials have admitted that wiring in jets built more than 20 years ago is starting to show signs of wear. In a Lockheed L-1011 that was recently retired from airline duty, for example, investigators found at least four cracks exposing bare wire in every 1,000 ft of wire that was tested. Experts say that discovery is alarming.

During October’s hearings FAA spokespeople in Washington sought to assure the public that despite the recent studies, which found cracks in aircraft wiring insulation aboard older airliners, including the Boeing 747, DC-9 and DC-10, “a fleetwide problem does not necessarily exist.” Cracks found in wires do not represent “a safety concern by themselves” because other causal factors have to be involved, another FAA spokeswoman told reporters the week of the hearings.

Statements such as these, from the agency responsible for studying and addressing the potential safety hazards of aging aircraft wiring, have triggered sharp criticism from some safety experts. Dr. Armin Bruning, president of Lectromechanical Design in Washington, and one of the chief engineers who studied wire-arcing scenarios during the TWA 800 accident investigation, said even small cracks in aircraft wiring can lead to arcing or intermittent electrical faults that can cause a fire to break out in flight or even ignite a catastrophic explosion. While such cases are extremely rare, Bruning said the potential threat to safety should not be understated.

“An arc fault is completely different from a chemical fire,” he said. “The arcing can be as hot as 10,000 degrees Fahrenheit and can quickly cause a fire to start. I have personally inspected enough airplanes and talked with enough pilots to tell you this is occurring. To ignore or minimize the problem is nothing short of foolish.”

Bizjets Not Immune to Problems

In an incident that the NTSB’s director of aviation safety, Bernard Loeb, says illustrates the need for improved wiring safety guidelines and reviews in all jets, a six-year-old Cessna Citation III operated by Mercury Communications was heavily damaged by fire on April 3, 1997, after an electrical arc ignited hydraulic fluid in a nearby hydraulic line. Descending through 4,000 ft for landing in Buffalo, N.Y., the crew smelled smoke in the cockpit. Almost immediately the captain’s FMS display went blank and all radio communications were lost. Realizing they had a potentially serious problem on their hands the pilots made a quick decision to get the airplane on the ground as soon as practical. The fact that the airplane was at low altitude and near an airport at the time of the fire, and the crew’s decision to land immediately, the Safety Board later decided, probably saved the lives of all on board.

By the time the jet touched down on the runway at Buffalo an intense fire was burning a hole through the Citation’s fuselage, and was about to engulf much of the airplane. Within seconds of coming to a stop on the ramp the pilots and passengers scrambled out of the burning aircraft to safety. The airplane’s fuselage and interior, however, were substantially marred by the fire. The damage was so severe, in fact, that the incident was officially classified as an accident by the FAA and NTSB.

The Safety Board’s subsequent investigation of the blaze quickly revealed that the fire had ignited when a wiring bundle contacted a hydraulic line and short-circuited. In reviewing the engineering drawings for the Citation III, investigators immediately saw problems in the airplane’s design, which they say should have been detected during the business jet’s certification program.

FAA guidelines state that under no circumstances should electrical wiring be routed within half an inch of a hydraulic line, and any wiring routed within two inches of a hydraulic line should have clamps installed to ensure positive separation. But the design drawings for the Citation III showed that wiring was routed within half an inch of hydraulic lines and did not incorporate the required clamps, an error that was somehow overlooked by both Cessna engineers and the FAA during design certification.

After the NTSB released its findings in May 1997, the FAA issued an AD requiring installation of clamps around the Citation III’s wiring to ensure proper separation. In investigating the Citation accident, the Safety Board also learned of three incidents involving Boeing 767s in which electrical wiring routed too close to oxygen lines arced and caused fires. In view of these four incidents, the Board recommended wiring reviews of all aircraft to ensure adequate clearance around electrical wiring, in accordance with the FAA’s own guidelines.

While a March 1998 response from the FAA to the NTSB seemed to indicate the agency agreed with the Safety Board’s recommendations, the FAA one year later sent a letter to the Board stating that it had found aircraft manufacturers were in compliance with the wiring guidelines, even though the recommended design reviews were never conducted.

The NTSB’s Loeb later told members of a House subcommittee investigating aging wiring concerns that the FAA’s response to its recommendation “was not consistent with the Safety Board’s findings.” Since then the NTSB has issued other wiring-safety recommendations to the FAA, none of which has yet been adopted by the agency.

A Disaster Waiting in the Wings?

Through the first 10 months of last year there were 964 reported events of smoke or fire aboard aircraft operating in U.S. airspace. In 359 of these cases unscheduled landings had to be made. A study of these incidents showed that more than half were related to problems with aircraft electrical systems.

Despite the fact that there were no fatalities last year in the U.S. related to electrical arcing or bad wiring, it may be only a matter of time before another catastrophe occurs, said Bruning.

In his view, top-level executives at the nation’s major airlines are ignoring tertiary evidence that clearly shows aging wiring presents an imminent threat to safety. Each airliner in service has about 150 mi of wiring running through its fuselage. The cost to replace all that copper is conservatively estimated at more than $1 million per airplane.

Without hard evidence showing the dangers of aging wiring, airline CEOs seem to be avoiding the question of whether bad wiring represents a disaster waiting to happen, said Bruning.

“It’s human nature, I think, to want to say there is no wiring problem,” Bruning said. “The CEOs and presidents of major airlines are good people. I’ve met and spoken with many of them. In a way it’s understandable that when they don’t see conclusive evidence they want to say there is no problem. On average, there’s one unscheduled landing every day because of smoke and fire, but there are no dire circumstances, and so we go on with our heads buried in the sand. I’m not saying the sky is falling, but clearly there is a real safety issue here.”

So What Should Be Done?

The situation, although potentially dire, is not hopeless, Bruning said. A new type of circuit breaker that will trip when even small spikes of voltage occur is “90 percent there. We’ve just got to find a way to make it smaller.” The FAA plans to begin full-scale flight testing of the components, called arc-fault circuit breakers (AFCBs), in mid-2002. If the AFCBs prove to be effective in preventing arcs and begin appearing in older airplanes, much of the problem could be solved, Bruning said.

Cleveland-based Eaton’s experimental AFCB uses a miniature microprocessor that is about the same size as current circuit breakers, which aren’t sensitive enough to detect the minute discharge from an arcing wire.

The question in the meantime, however, is what, if anything, should be done to protect aircraft from electrical fires now?

After ruling that the explosion that brought down TWA Flight 800 was probably caused by a short circuit inside the Boeing 747’s center-wing fuel tank, the NTSB recommended that the FAA review the design specifications for wiring systems and their bonding inside fuel tanks in all U.S.-certified transport-category jets. Although the condition of the wiring system in the doomed 25-year-old airliner was not atypical for its age and had been maintained according to accepted practices, the NTSB said that, until recently, insufficient attention has been paid to the condition of aircraft electrical wiring.

Until the FAA task force issues its recommendation, the best course of action for corporate aircraft operators may be visual inspections of wiring in older jets. If maintenance technicians begin to find cracks in wiring, immediate corrective measures should be taken. Bruning cautioned, however, that wiring which looks good may in fact be bad and wiring that looks bad may be acceptable. It takes a careful, patient eye to find cracks in the hundreds of thousands of feet of wiring aboard an aircraft, he said. General visual inspections of wiring can find only a small percentage of breaks and cracks. For the tiny cracks or carbon buildup that may be impossible to find with the naked eye or a magnifying glass, experts recommend using sophisticated test equipment that can ferret out trouble spots. It was this type of circumspect investigation that led to decisions by Swissair and Federal Express to replace literally miles of wiring on board their jets.

For a typical airplane, said Bruning, it is possible to calculate where cracks will first occur. Cracking is not the result of the number of years an airplane has been in service, nor does it depend on takeoff and landing cycles. Rather, the temperature and humidity of the environment in which the wires are routed determine their useful lives. For example, said Bruning, it is far more likely an operator will find cracks in wiring in the wheel wells of an airplane than in the galley or cabin, because the wheel wells frequently become wet. In fact, said Bruning, mathematical models exist that can predict how long a given wiring bundle will last at given temperatures and relative humidity levels.

Short Circuits and Electrical Arcs

Wire is routed throughout an airplane in a series of bundles with clamps and connectors creating the electrical pathway through the aircraft. As newer airplanes add wiring to accommodate additional avionics and in-flight entertainment equipment, the shear mass of stranded copper that is routed through terminal connectors is growing rapidly. Safe routing practices include engineering by the OEMs to prevent wires from wear, abrasion, contamination and contact with other components. Gentle bends and turns in wiring, for example, must be incorporated by engineers in the design of new airplanes to prevent cracking of the wire insulation later.

When the protective sheath of insulation on a wire cracks or tears and the conductor is exposed, the potential for a short circuit or arc exists. A short circuit occurs when electricity takes an unintended path. For example, condensation or other conductive material sometimes found on wire bundles can bridge the gap between a wire conductor and an adjacent metal surface. When electrical current follows the unintended path to the metallic structure, a short circuit can cause overheating and possibly a fire.

Electrical arcing, on the other hand, is a type of short circuit in which high current crosses a gap, emitting sparks that include molten material that is instantly vaporized in the high-energy discharge and produces extreme localized heat. The arcing can ignite flammable materials, such as cabin insulation, in the vicinity.

During the NTSB investigation of Swissair Flight 111, which crashed into the Atlantic Ocean near Peggy’s Cove in Nova Scotia, Canada, on Sept. 2, 1998, killing all 229 people on board, fire- and arc-damaged wires were discovered in the ceiling of the cockpit and forward cabin area. Some of the damaged wires were associated with the airplane’s in-flight entertainment system and, as a precautionary measure, that equipment has been disabled in all Swissair MD-11s and Boeing 747s, the only airplanes with that particular IFE system.

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