Aircraft wiring is like the body's nervous and circulatory systems combined, distributing power, signals and data throughout the platform. It is the foundation upon which avionics functions are built. Even the flight stability of some military fighters depends on control signals carried over the wiring system. Though invisible to the eye, the miles of wire and cabling in modern aircraft form a complex web whose reliability must be monitored to ensure safety. This approach differs fundamentally from the traditional viewpoint that wires are not a system but a support function subordinate to individual avionics boxes.
Although treating wire like a system sounds obvious, this concept is relatively new, driven by catastrophes like the TWA Flight 800 fuel tank explosion accident in 1996 and the Swissair MD-11 crash off Nova Scotia in 1998. The occurrence of the TWA and Value Jet crashes so close together in 1996--although the Value Jet disaster had nothing to do with wiring--led to the formation of the FAA's Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) and on to the notice of proposed rulemaking (NPRM) issued this fall (see Safety column, page 45).
In commercial aviation the idea of wiring as a system was proposed in 2001. When ATSRAC advanced this concept, it met resistance from system safety analysts, recalls Kent Hollinger, ATSRAC chairman. They thought of systems as performing aircraft functions like propulsion or landing, he says. Traditional safety analysis would say that if in-flight entertainment (IFE) wiring fails, there is no effect on safety, as IFE is a passenger convenience. But what about Swissair Flight 111, where arcing from an IFE cable occurred in the area where the fire most probably originated? The new thinking about wiring looks not just at a failure's effect on a system, but on wire failures in themselves, as these may affect much more than a single system. This article surveys some of the initiatives springing from this perspective in the U.S. Air Force Research Lab (AFRL) and Aeronatical Systems Center (ASC).
Treating wiring like a system means approaching it more systematically, says George Slenski, principal technologist in AFRL's Materials Directorate. That means looking at proper separation, modeling wire failure mechanisms, and forecastting expected system life before installing wiring in the aircraft. A wiring system, according to some experts, encompasses not only the wiring, but also connectors, circuit breakers, power distribution panels, and even motors and generators.
Serious wiring events happen more frequently than one might expect. A search through incident and "service difficulty report" databases maintained by the National Transportation Safety Board (NTSB) and FAA, respectively, identified 233 wire arcing and smoke events over a five-year period, 1999 to 2004. The events were serious enough to require the aircraft to turn back, divert or make emergency landings. (An arc occurs when current jumps from wire to wire or from wire to ground. The electrical discharge can cause a fire when converted to thermal energy.)
Over the past few years work on diagnostics and components has been funded in the United States by FAA, NASA and the military.
One center of activity is the Air Force's Wire Integrity Program, which has led the effort within the service to develop a "howmal," or "how malfunction," code. It enables the collection and storage of more specific wire problem and repair information for tracking problems and analyzing trends.
"Before, if an avionics box failed on an aircraft, a technician would look at the box," explains Mark Ragland, program manager for avionics with the Aging Aircraft Systems squadron. "If it wasn't the box, itself, but a wire associated with the box, the failure would get coded to the box."
The advantage of a specific wiring code is that technicians can record wiring problems and corrective actions in more detail, populating a database. Technicians will use a software application that allows them to describe the problem, e.g., a short on a certain wire in a certain bundle going to a specific system.
The technicians also can say how they repaired the problem--e.g., by removing or splicing the wire--leaving a trail for future maintainers to follow, as well as a data source for spotting trends. AT&T Government Solutions, which conducted the research leading to the establishment of the howmal code, is scheduled to deliver the software application, an interface to the Air Force's Core Automated Maintenance System (CAMS), in April 2006. Ragland intends to implement the system first with the F-16, a wire-intensive aircraft.
The Aging Aircraft Systems squadron also is looking at arc fault circuit breakers (AFCBs) to replace older units on the F-15, which is scheduled for rewiring in 2009. The office is revising the current mil spec to address electronics repair and test issues. AFCBs add electronics to help detect anomalies and disconnect circuits more rapidly than today's thermal breakers can do. Thermal breakers shut down a problem area when they sense a certain heat level.
Ragland expects a full competition for the F-15 components after completing a proof-of-concept program on the aircraft. Among the candidate approaches are full electronic breakers programmed to understand what a thermal fault is and hybrid units that add electronics to traditional thermal breakers.
Better inspection and troubleshooting equipment is also a major focus. FAA and the U.S. Defense Department (DoD) have examined more than 30 technologies for nondestructive inspection. One promising troubleshooting candidate is time domain reflectometry (TDR), which measures the time a signal (sine wave) takes to travel to an anomaly in the wire and reflect back to the source. From this it is possible to identify and locate problems like opens and shorts.
A challenge is that "you have to have controlled impedances and resistances in the wire to measure the transmission speed, Slenski says. When that's not controlled, it's tougher to make an accurate measurement.
The Air Force has been using TDR for years, but it is complicated to operate and usually requires a dedicated technician. Now, however, the technology is being packaged in smaller, handheld units, about the size of a paperback novel, Ragland says. The new units offer improved electronics, lighter weight and (sometimes color) liquid crystal displays.
TDR's accuracy, however, is still less than desired. "We're probably within 6 feet [1.8 m]," Ragland says, meaning +/- 6 feet from the location of a fault. That's good enough for some aircraft, but not for small, tight ones like the F-16.
Ragland also reports a promising wiring analysis tool. About the size of a small suitcase, the troubleshooting aid can check thousands of wires for opens and shorts in the course of a minute, he says. The tool is still in development, and the designer is building cables to test not only twisted shielded pair, but wiring associated with secondary power, flight control, stick grips and primary power.
Researchers also are developing new materials. The authors of a recent technical paper presented at a Society of Automotive Engineers conference report that polymer wire promises to offer wire conductors with lower weight, higher break strength and longer flex life than copper. Polymers such as polyacetylene, polyaniline, polythiophene and polypyrrole have demonstrated conductivity near that of copper via chemical and electrochemical doping. Such materials, however, are expensive and have problems, such as environmental instability and "poor" mechanical properties, according to the paper.
Embedded sensors to monitor the health of wiring, as part of an integrated health monitoring system (IHMS), would help move the industry along the road from reactive to proactive maintenance, a long-held goal. The authors anticipate that micro electromechanical systems (MEMS) devices could be used to improve the reliability and decrease the cost of embedded sensors for detecting arcs, shorts, opens, intermittents and faulty connections. Data collected by the embedded sensors, they say, could be used to improve predictive models and eventually to develop health monitoring systems for aircraft electricaI power systems that would be able to incorporate generator prognostics and interface with aircraft maintenance and troubleshooting systems.
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