Government/Military Trends
June 2005
Wiring: A Critical Issue in Aging Aircraft
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As an aircraft ages, its components decline at different rates. The airframe may last for several decades, but other parts must be replaced after only a few months of service. Alarmingly, however, the most critical system within the aircraft, the electrical wiring interconnection system (EWIS), is the most difficult one to analyze, maintain, and repair. Some aircraft currently in service were designed for “fit and forget,” the idea that the system would last as long as the craft. Yet more and more aircraft are pressed into longer terms of service as the deterioration of the EWIS accelerates.
Numerous assaults work to weaken the wiring in today’s aircraft, from the conditions of flight, such as severe temperature fluctuations, vibration, chafing, and contaminants, to the properties of the wire itself as its insulation becomes dry, brittle, and cracked with age. Two to three hundred miles of wire snake through the frames of some large transport category aircraft. Recent investigations of aircraft with more than 20 years of service found as many as five breaches in the insulation for every thousand feet of wire. The average age of an aircraft in the military fleet is 23 years old. Each breach is a possible flashpoint for an arcing incident such as those implicated in the well-known commercial disasters of TWA flight 800 in 1996 and SwissAir flight 111 in 1998.
The military has lost aircraft as well, and it also feels the weight of gargantuan costs and diminished readiness because of wiring failures. As reported in 2004, the Navy has seen as many as 1,400 mission aborts, 540 of them in flight, caused by wiring problems, and roughly 2 to 3 percent of its fleet grounded for the same reasons.
Another issue to be confronted in the years ahead is the threat of tin whiskering. The Restriction of Hazardous Substances (RoHS) directive, which comes into force in July 2006, has caused a shift toward use of pure tin solder. The phenomenon of “whiskering” is the spontaneous growth of elongated single crystals of pure tin that can cause a short. Though the military has been exempted from the RoHS, lead-free commercial components purchased for military use may pose an additional and unpredictable danger to EWIS integrity.
A diligent effort has been mounted to address the growing crisis of aging EWIS in aircraft. However, discovering whether a problem exists and where it lies is exceedingly difficult. Today, there is no “silver bullet,” but several technologies are in use or in development.
Among the tools available for diagnosis of wiring problems and defects are visual inspection, impedance testing, and reflectometry. Visual examination is a painstaking art in itself. Some breaches in insulating materials are no larger than the head of a pin, some are hidden within bundles and beneath clamps, and vast stretches of wiring are inaccessible without dismantling the aircraft. Impedance testing measures resistance from one end of the wire to the other. Nondestructive resistance tests, using voltage of 28V or less, are available, but the technique requires frequent disconnection and reconnection of cables. Several types of reflectometry have become vital instruments in the suite of tools for diagnosing wiring failures. Reflectometry, sending a pulse down a wire and then studying the peaks and waves of its reflection as it returns, can detect many issues, but it can’t detect looming problems, such as frays, and it is a fairly difficult technique to master. All of these methods are less than perfect in both their effectiveness and practicality, but newer versions are under development.
The best hope for solutions to aging wiring systems lies in the immediate future. Live-wire testing of aircraft EWIS during flight will detect intermittent faults that cannot be located during maintenance on the ground. Arc Fault Circuit Interrupter technology will provide additional safety measures when a fault occurs. Nanoscale sensors embedded within emerging smart wire systems will detect and correct faults in real time. Fiber optics and wireless technologies will eventually replace sections of bulky wiring systems. While these and other techniques are being developed and tested, fleets must rely on diligent application of the array of available diagnostic technologies.