Finding Fault – Locating Hidden Hazards on Aircraft Wiring

As today's military and commercial aircraft age past their teen years, the many kilometers of wiring buried deep within their structures begin to crack and fray. Once thought to be rare and benign, such faults are found by the hundreds in a typical aircraft. Unlike obvious cracks in a wing or an engine, damaged wire is extremely difficult to detect, but the resulting arcing and electromagnetic emissions can be just as deadly: faulty wiring has been implicated in the downing of Swissair 111 near Nova Scotia in 1998 and of TWA 800 off New York's Long Island in 1996. Indeed, any densely wired system is vulnerable--the space shuttle, nuclear power plants, subways and railroads, large industrial machinery, homes and business buildings, communication and power distribution networks, and even the family car.

Public scrutiny has prompted strongly worded recommendations from the likes of NASA, the U.S. Federal Aviation Administration, and the National Transportation Safety Board (NTSB). "The safety of the nation's wire systems is an issue of major importance to us all," noted a White House report issued in 2001. Several months earlier, the NTSB concluded its lengthy investigation of TWA 800 with the verdict that a short circuit sparked an explosion in the center wing fuel tank. The condition of the wiring, it noted, was "not atypical for an airplane of its age." Among the NTSB's recommendations was to incorporate into aircraft "new technology, such as arc-fault circuit breakers and automated wire test equipment."

Failing the test of time

Typically, a copper conductor (from 1 to 10 mm in diameter) is covered by a thin outer insulation (from 0.5 to 2 mm thick). Damaged insulation can expose the copper, giving rise to arcs, shorts, and electromagnetic emission and interference. As the wire ages, the insulation may become brittle and crack. Chafes appear as wires vibrate against each other, a tie-down, or any other hard surface. Maintenance can also be hard on wires, as they may be nicked by workers' pliers, or bent beyond their tolerable radius, or sprinkled with metal drill shavings, chemicals or water, or even used as stepladders in hard-to-reach places. Even simple moisture condensation can spell trouble, particularly in conjunction with polyimide insulation, which breaks down when exposed to moisture and heat, not a good scenario for a vehicle that must contain drip loops in the wiring, because it is normally wringing wet after each flight! Moisture creating a short circuit between compromised wires can cause a tiny arc, gradually carbonize the insulation, and finally result in flashover and fire. And it isn’t just old planes that have problems. In areas such as the wheel well, nearly 1/3 of all planes will have wiring faults within the first year.

The hazard of these pervasive “wet arcs” has prompted the development of arc fault circuit breakers. Ordinary circuit breakers are heat-sensitive bimetal elements that trip only when a large current passes through the circuit long enough to heat the element. This power may be on the order of 1000 percent of the rated current for 0.35 to 0.8 seconds. By comparison, a single arc fault may last only 1.25 ms, and a series of events may last 20–30 ms. Too fleeting to trip the circuit breaker, these arc faults can nonetheless cause catastrophic local damage to the wire. Fires have been known to break out with the breaker still intact.

Arc-fault circuit breakers contain sophisticated electronics to sample the current on the wire at submillisecond intervals. Both time and frequency domain filtering are used to extract the arc-fault signature from the current waveform. This signature may be integrated over time to discriminate, by means of pattern-matching algorithms, between a normal current and a sputtering arc-fault current. And so ordinary transients, due to, say, a motor being turned on and off, can be distinguished from the random current surges that occur with arcing.

Arc-fault breakers are already required in new home wiring in the United States and are now being miniaturized for use on aircraft. One of the most significant problems that is limiting the adoption and implementation of arc-fault breakers is lack of a method for locating the tiny damage left on the wire after the breaker has tripped. The figure below shows the damage left after a traditional thermal circuit breaker has tripped, damage that is clearly visible or that could be found with today’s test methods, and damage that could have started a fire if flammables had been near the fault when it occurred. The figure on the right shows the damage left after the arc fault circuit breaker has tripped, damage that is so small that the wire is still fully functional, has an impedance discontinuity of less than an ohm, and damage that would be extremely difficult or impossible to locate.

LiveWire Test Labs can locate these faults – a critical enabling technology for the deployment of arc fault circuit breakers.
Figure 1: Samples of polyimide wire that have been tested for wet arcs. Two radial cracks were made ¼” apart, and several drips of saline solution were dripped over these cracks when the system was energized with three-phase 400 Hz 115V power. The wire on the left shows damage typical of today’s thermal breakers, and the wire on the right has damage typical of protection by an arc fault circuit breaker.

Not to panic

If you happen to be flying this week, do not panic. Few wiring problems end in disaster. There is cause for concern, though, as the air fleet continues to age, and our reliance on air transport grows. While an aircraft's other major systems undergo preflight testing and regular inspection and maintenance, its central nervous system--wiring--has been long neglected. Sorely needed are new maintenance methods that account for the aging of wires, as is done for aging structural and computer systems.

Diagnosis is good. Prognosis is better. And prevention is better still. This last may require a new way of thinking for electrical engineers, who tend to be more at home with obsolescence than geriatrics. For aging aircraft wiring, the dream of smart systems that can detect and locate the intermittent faults before they cause disasters like TWA 800 and Swissair 111 is on the horizon. LiveWire Test Labs is proud to be making a difference in how the most basic neuron in our electrical nervous system, the wire, is understood and maintained. Soon, this tiny neuron will have a mind all its own, whether it is in an aging airplane, train, ship, skyscraper, nuclear power plant, communication network, or even in your family car.

For more information on how Livewire's technology has direct application to your product, please contact us.