Like the veins that carry blood to vital organs in the human body, wires perform a critical function in bringing electrical power and information to the various systems in an aircraft. The miles of wiring in an aircraft are increasingly being viewed as a system by airlines, cargo operators, U.S. regulatory agencies and military services.
In the past decade, an evolution has occurred in the way the miles of wiring inside an aircraft are viewed. Wires and cables were previously seen as components that support aircraft systems, and thus didn't need to be maintained and inspected as regularly-the "fit and forget" attitude. In many cases, including wiring to avionics boxes, the view was that the initial wiring installed did not need to be replaced over the life of the aircraft.
But "fit and forget" has been largely discarded in commercial and military wire maintenance, replaced by the viewpoint that wiring should be treated as a system, and require similar maintenance, training and testing to that which is performed on flight-critical systems, as well as within avionics units.
"People's mindset has changed," explains Kent Hollinger, chairman of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC). This philosophical shift has led the commercial transport industry and the military to implement more training specific to wiring maintenance and inspection; uniform guidelines for cleaning, crimping and proper separation of wires; and to increasingly use advanced equipment such as automated wire testing devices. The new thinking also has contributed to changing how wiring malfunctions should be recorded and documented, as evidenced in a recent study of U.S. Navy aircraft and efforts to add more detailed categories involving wiring hazards into maintenance logbooks.
FAA is in the midst of reworking a comprehensive Notice of Proposed Rulemaking (NPRM) that will set new requirements for airplane wiring, including the design, modification and maintenance of wiring systems. Initially published in October 2005 and based on recommendations from ATSRAC, the NPRM is titled "Enhanced Airworthiness Program for Airplane Systems and Fuel Tank Safety (EAPAS/FTS)." The NPRM defines guidelines for the maintenance and inspection of electrical wiring interconnection systems (EWIS), as well as fuel tank wiring. While FAA has issued numerous airworthiness directives (ADs) over the past decade to address specific wiring issues in specific aircraft types, the NPRM, once it is adopted as a final rule, will provide overall guidelines for wiring design, maintenance and inspection. FAA has received numerous comments to its NPRM from airlines, industry associations, and the U.S. National Transportation Safety Board (NTSB), among others.
But the agency has pushed the adoption of the final rule, at least for the EWIS-related portions, further into the future. As noted in the Federal Register on July 7, 2006, FAA doesn't anticipate issuing a final rule "that will respond to those [industry] comments for several months."
While the agency said it will move ahead with its original plan to require operators to comply with new fuel tank system maintenance and inspection regulations by Dec. 18, 2008, FAA "has decided not to maintain the alignment of the fuel tank system and EWIS compliance times," in the NPRM. Instead, it will publish an updated compliance date for the EWIS regulations when it issues the final rule. NTSB added momentum to FAA's effort in a June 29, 2006, press release. While the safety board commends FAA for safety improvements made over the last decade, Chairman Mark Rosenker noted that progress on the NPRM is moving too slowly.
FAA created ATSRAC in the late 1990s in response to the July 1996 mid-air explosion of TWA Flight 800, a Boeing 747. NTSB later determined that accident to have been caused by a wiring failure, leading to a spark that ignited in the fuel tank. That disaster-and the 1998 crash of a Swissair MD-11 in Nova Scotia-provided the initial impetus to begin reshaping commercial wiring policies. The Swissair disaster is believed to have been caused by electrical arching originating from an in-flight entertainment cable.
But other cases involving wiring have reinforced the need for stronger regulations. Some of these accidents and incidents have been linked to wires connected to an avionics unit, which accounts for a significant amount of an aircraft's overall wiring structure-more than half by some estimates, depending on aircraft type and number of avionics systems. Examples of avionics-related wiring causing potential hazards can be seen in the following three non-fatal, but nevertheless alarming, incidents reported in our sister publication, Aviation Safety Week (see May 13, 2002 and Oct. 17, 2005).
In November 1998, wire arcing on board a Delta Air Lines L-1011 led to the failure of 33 systems, including the autopilot, cabin pressurization, auto-spoilers and thrust reversers.
In January 1998, a United Airlines Boeing 767 sustained arcing damage to dozens of wire bundles in the avionics bay, leading to numerous display errors and tripped circuit breakers, as well as forcing a precautionary landing at London's Heathrow.
In May 1996, Martinair Flight 631, also a B767, experienced numerous electrical power disruptions that caused all four primary electronic flight instrument displays to go blank and the navigation system to fail.
The ATSRAC committee's recommendations formed the basis of the NPRM. At the top of the list is that additional training needs to be incorporated to address EWIS-related inspections.
NTSB expands on the need for additional training in its NPRM response, noting that wiring training "should apply to all maintenance personnel and not just EWIS technicians."
The NPRM creates new inspection criteria related to the environment that a wire is placed in, and lays out an enhanced zonal analysis procedure (EZAP), which treats wiring as a separate system and emphasizes close inspections of potential trouble areas. Essentially, technicians will be required to think about a physical failure, such as a wire shorting out, and what effects it could have on the aircraft, i.e., leading to a fire or causing a flight display or instrument to shut down.
Proper separation of wire and cable bundles is also important to safety. For instance, wires should not be placed right next to a power feeder cable, and should be kept away from oxygen and hydraulic lines. There is also more emphasis on cleaning of wire and cable bundles, as accumulated dust and dirt could potentially ignite if a wire shorts out.
United Airlines noted in its February 2006 response to the NPRM that "Many procedures exist to explain how to clean wire bundles once they are found to be contaminated, but there is no written requirement to clean the bundles."
Detailed clamping and crimping procedures are defined in the NPRM. FAA also seeks to establish minimum guidelines regarding wiring in manufacturers' standard practice manuals, and a uniform way of organizing the material, so that technicians working on different aircraft have a consistent means of handling wiring maintenance.
ATSRAC also found that aging in wires isn't necessarily just related to time, but can be accelerated by maintenance issues such as improper care, exposure to the elements, or even a faulty installation, which could cause the wire to rub against other structures.
"We found wiring in very old, retired airplanes that was in very good shape. We also found wiring in young planes that wasn't so good," Hollinger says.
To this effect, the NPRM defines aging as encompassing more than just the physical and chemical degradation of wire and cable systems, but also considers maintenance and design.
"If you use good design and maintenance practices, the wiring will, in fact, last for the life of the aircraft and not need replacement," Hollinger explains.
Testing for a fault in a wire connected to an avionics unit can be tricky, but advanced techniques, such as automated wire test equipment and arc-fault circuit breakers, are helping to speed up the process. Several test equipment companies are working on non-destructive testing tools, which send a pulse down the wire to find the location of a possible short or failure.
In the past, for instance, when a pilot reported that a multifunction display was "flickering," or shutting off and coming back on intermittently, it could take several inspections to link the problem to faulty wiring. Mechanics might swap out the screen, and the action might fix the problem for a couple flights, before the display ultimately begins flickering again. A similar process could occur with switching out the box controlling the display and the circuit breaker before finally a short in the wire is discovered.
"If all it's doing is making your display flicker, that's one thing," Hollinger says. But if the faulty wire is causing an electrical arc to form, there is the potential that an actual fire could start while the aircraft is still in service during the repeated swapping out of the avionics boxes and circuit breakers.
While efforts to finalize a new set of guidelines for commercial aircraft are ongoing, the U.S. military is continuing to study the effects of wire failure in aircraft systems. An Aug. 29, 2006, press release from the Naval Air Systems Command (NAVAIR) notes that based on maintenance records and fleet surveys, incidents of aircraft wiring failures are "significantly under reported" and should be treated with heightened scrutiny.
NAVAIR notes that in the past, maintenance personnel were too often using Mal Code 160, which has a generic description about why a wire has failed, in describing a wire or cable failure-a "one size fits all" mentality. The research also found that many components labeled with a Mal Code 799 problem, which indicates a failure that cannot be duplicated, "were actually experiencing wire failures, but not necessarily related to the component." Instead, the wiring failure contributed to the component's failure, causing technicians to believe the component itself was the problem, and leading to a series of "swaptronic" events, or swapping out the component several times. "The actual wire failure was never traced, so very expensive components were swapped back and forth ... [and] receiving maintenance they didn't need," the study found, causing maintenance personnel to waste time and money.
In order to address these issues, NAVAIR removed Mal Code 160 from the Naval Aviation Maintenance Program and introduced 63 new wiring malfunction codes to the manual, specifically designed to identify the root causes of wire failures. There were previously only six codes that could link wiring as a problem in maintenance reports. NAVAIR is asking aircraft maintainers to assist in efforts to help everyone who deals with wiring systems, including designers and analysts, better document specific types of wire failures, so that engineers can understand what specific failure occurred and identify solutions more rapidly.
"We need to find out if the wire is chafed, if it's misrouted, if it's a bad solder joint, crimp or whatever," says Bob Ernst, director of NAVAIR's Aging Aircraft Integrated Product Team (AAIPT), in the Aug. 29 release. "We need to understand, not only what the problem is, but what [part of the aircraft] it is in, also."
In their comments to the proposed rule, NTSB, some airline and cargo operators have questioned the methodologies for FAA's cost-benefit analysis. The agency predicts some $192 million in efficiency benefits and $563 million in safety benefits, or $755 million total, over a 25-year period, as a result of the rulemaking.
NTSB notes that the analysis does not include indirect EWIS-related accidents, such as a COPA Airlines Boeing 737 that crashed in Panama in June 1992. The probable cause of that accident was determined to be the fraying and shorting of a gyroscope wire, which led to incorrect instrument displays and pilot loss of control, resulting in 47 deaths. NTSB "believes that the number of EWIS- related accidents and incidents that can be prevented will exceed [FAA predictions]," the response stated.
Others have rejected the idea that more regulations are needed, saying that the renewed emphasis on wiring in the past decade has led to voluntary changes in maintenance and inspection, and that FAA already has means of enforcing wiring guidelines. For instance, in its response to the NPRM, cargo operator UPS notes that the agency already has authority through ADs to require compliance from manufacturers and operators. "As 'known unsafe conditions' are already regulated, aviation safety is not further benefited by granting the FAA the unprecedented authority to retroactively impose new regulations onto manufacturers and operators of existing airplanes," UPS wrote, adding that the agency should justify the need for specific EWIS rules "for each aircraft fleet, comparing safety benefit to compliance cost, with the existing AD process."
Hollinger notes that the industry's viewpoint has changed profoundly since the ATSRAC committee began its work seven years ago. There is an increased awareness that wires are a contributing factor to aircraft safety. Manufacturers such as Airbus and Boeing, as well as many airlines and cargo operators, have voluntarily incorporated wiring best practices, including cleaning, crimping and proper separation, into their regular maintenance programs.
"The Wright brothers may not have worried so much about transmitting electrons," Hollinger says. But in today's environment, "the transmittal of electrons through an airplane is an aircraft function," he continues, noting that the more prevalent use of advanced avionics and fly-by-wire technology in aircraft design will increase the number of wires.
In its February 2006 response to FAA's Notice of Proposed Rulemaking, the National Transportation Safety Board identified three main issues that it says should be addressed in the final rule:
The need to improve maintenance personnel training. "The NPRM provides only guidance and recommendations for operators to implement; it does not propose mandatory training. Such training should apply to all maintenance personnel and not just EWIS technicians."
The need for better reporting of potential unsafe wiring conditions. "Industry does not have a sufficient data collection and retrieval system in place that would enable the FAA to ensure that its proposed regulatory actions are effective. The existing method of researching service problems [FAA's Service Difficulty Reporting System (SDRS) and Continued Airworthiness Surveillance System (CASS)] is unsatisfactory."
The need to integrate advanced nonvisual techniques, such as arc fault circuit breakers and automated wire test equipment, into regular maintenance and inspection.
While the NPRM describes an enhanced zonal analysis procedure (EZAP) in which more detailed visual inspections and nonvisual inspections can be used, it does not require the use of nonvisual techniques. The incorporation of automated wire test equipment "will enhance safety as the technology itself continues to mature," NTSB notes, adding that some operators have already implemented such techniques in critical aircraft functions, such as braking.
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