The Systems Engineering of Aircraft Wiring
A Fit and Forget Item or a System to be Maintained?
By Jim Jenkins — Systems Engineering Scholar
According to myth among aircraft maintenance professionals, aircraft wiring was not a maintenance issue. It was thought of as a fit-and-forget item. If the interfaces worked and all the components worked, all must be well. (NAVAIR, 2006c)
It was a myth until the horrible aircraft tragedies of TWA 800 on July 17, 1996, and Swissair 111 on September 2, 1998, which killed 230 and 229 people, respectively.
After thorough investigations, The National Transportation and Safety Board ruled that the most likely cause of the explosion of TWA 800 minutes after departing New York’s Kennedy International Airport was a short circuit arcing event outside the center wing tank mixing with fuel vapor. (Loeb, B., 1998)
The Transportation Safety Board of Canada determined the Swissair 111 fire and subsequent crash …
“most likely started from an electrical arcing event that occurred above the ceiling on the right side of the cockpit near the cockpit rear wall. The arcing event ignited the flammable cover material on nearby metallized polyethylene terephthalate (MPET) covering on the thermal acoustic insulation blankets. As the fire spread across the surface of the insulation blankets, other flammable materials became involved, including silicone elastomeric end caps, hook-and-loop fasteners, foams, adhesives, thermal acoustic insulation splicing tapes, and metallized polyvinyl fluoride (MPVF) insulation blanket cover material. The fire progression was rapid, and involved a combination of these materials that together sustained and propagated the fire.” (Transportation Safety Board of Canada, 1999)
These two mishaps were the catalyst for the creation of new integrated product teams across the commercial industry and military aviation arenas, chartered to propagate new standards, technologies, maintenance techniques, training procedures and policies, and cultural changes regarding the health and maintenance of an aircraft electrical wiring interconnect system (EWIS).
According to Boeing, the 747 aircraft contains 150 miles of wire that make up its EWIS. (Monroe Aerospace Blog, 2020)
“Today’s aircraft rely more and more on sophisticated electrical and computer systems, placing a premium on the reliability of wiring, power feeder cables, connectors and circuit protection devices. It’s time to stop treating wire as a ‘fit and forget’ item and begin treating it as a system,”
Explained Jerome Collins, former Branch Manager for the Wiring Systems Branch at the Naval Air Systems Command (NAVAIR).
The need to maintain an aircraft EWIS is not a myth.
Environment and Stakeholders
The environment in which EWIS operates may have been thought of as benign, but nothing could be further from the truth. Aircraft EWIS lives in tight, hard-to-reach (and maintain) areas, often in environments with extreme temperature fluctuations and high vibrations. These temperature fluctuations and vibrations rapidly break down the wire insulation (polyimide), causing it to be brittle and exposing bare wire. Often, vibrations associated with aircraft operation, specifically where the wiring bundles are tied down via p-clamps, string, or zip ties, also cause chaffing and exposed bare wire. Where there is bare wire exposed, arcing, sparking, and fires are sure to follow.
The stakeholders are literally every air traveler in the world and every aircraft manufacturer and makers of copper wiring products. To narrow it down a little, this article focuses on the investigatory and engineering organizations involved in examining the crash and charged with finding solutions to the problem. To pinpoint one stakeholder’s journey, this article follows NAVAIR and how it addressed its own wiring maintenance issues.
NAVAIR’s Aging Aircraft Integrated Product Team (AAIPT) was formed to address the increasing burden of aircraft wiring systems maintenance and impact on reliability of aging wiring, to coordinate the efforts of the Department of Defense and government agencies in optimizing the service life of aging aircraft fleets, to maximize aircraft material safety, to minimize aircraft operating costs and duplication of effort, and to sponsor and conduct exchanges of information on these efforts. (INCOSE-Project Planning Process, ch. 5.1, 2015)
The NAVAIR AAIPT comprised engineers and logisticians divided into teams focused on logistics, avionics, wiring, Diminishing Manufacturing Sources and Material Shortages (DMSMS)/Obsolescence, and air vehicle concerns. Aging became an issue in the 1990s because budget shortfalls forced military aircraft systems that were used to being entirely upgraded and replaced with newer systems on a 20-year cycle. In other words, aircraft systems that were designed to last 20 years are now having to last at least 20 more years beyond their initially designed life cycle. To emphasize the need for attention, AAIPT Head, Bob Ernst, liked to tell people that the mother of the last pilot of the B-52 wasn’t born yet. The continued cost of operating and maintaining these aircraft systems is rising at unprecedented levels.
The NAVAIR AAIPT conducted operational logistics actions where they concurrently tuned the aging systems of interest (SOI) and their enabling systems during their operational life to ensure continued effective and efficient delivery of the system capabilities. (INCOSE, 2015-Maintenance Process-4.13) The AAIPT conducted a wiring study where they performed a failure analysis and trend analysis on three aircraft type — H-1, P-3, and E-6 — and all their maintenance histories and found that the wiring systems were “eating their lunch.” For the P-3, the analysis found that 5 of the total 17 systems (lighting, miscellaneous wiring, engine electrical, electrical systems, and AC power) that make up the P-3 was causing 74% of the total maintenance man-hours (MMH) expended. For the H-1, they found that 4 of the total 15 systems (pilot section electrical system, miscellaneous wiring, gunner electrical, and lighting system) made up 82% of the total MMHs expended. For the E-6, they found that 6 of the total 25 systems (AC power, DC power, aircraft wiring, DC power control panels, landing gear, and lighting system) made up 79% of the total MMHs expended. (Anteon, 2006)
The AAIPT existed for approximately 10 years and tackled wiring issues from multiple angles and tried to offer several solutions. They scoured Maintenance Action Forms and conducted fleet surveys that revealed aircraft wiring failures were being significantly under reported because maintenance personnel were over-using malfunction code 160, which has a very generic description of why the wire has failed. It was sort of a one size-fits-all mentality. They also found that many components described as having a Mal Code of 799 — a failure that cannot be duplicated — were experiencing wiring failures, but not necessarily related to the component. The wiring failure simply affected the component causing maintainers to believe it was a component that failed. These components were usually swapped out for another component, which may have worked until the initial failure reoccurred, causing a series of “swaptronic” events to happen. The actual wire failure was never traced, so very expensive components were swapped back and forth, going in and out of intermediate level shops receiving maintenance they didn’t need. A lot of money and maintenance man-hours were wasted. So, the team introduced 63 new wiring malfunction codes designed to neck down how failures on wiring were occurring. The new aircraft wiring malfunction codes were designed for more accurate aircraft wiring failure data collection so that the Navy’s decision makers have a better understanding of the actual degradation conditions of the Navy’s aging aircraft wiring problems. (NAVAIR, 2006c)
The team also spent many man-hours consolidating the wiring maintenance manuals for the Navy/Marine Corps, Air Force, and Army. All three services used different manuals with, at times, conflicting information for the maintenance of an aircraft wiring system. Even though the aircraft might be different, the way a maintainer handles the wiring system should all be similar, to help efficiency. (NAVAIR, 2006a)
The team worked with all depot-level maintenance shops and class desk engineers to encourage complete overhauls with replacement of the wring systems with polyimide wiring and worked with electrical engineers at NAVAIR to develop the arc-fault circuit interrupter (AFCI). The AFCI equipment provided significant safety and circuit protection against arcing conditions. However, the AFCI had to be fielded with common test equipment and procedures to be fully effective.
One of the biggest outputs to come out of this corrective action was the maintenance strategy involving the actual finding of the faults in the aircraft. (INCOSE, 2015-Maintenance Process, ch. 4.13) Based on feedback from ongoing monitoring from the fleet maintenance personnel, the AAIPT studies identified problems linking wire chaffing to a large percentage of wiring-related safety issues. The Navy was experiencing at least two fires a month related to chafed wires. (Arnason, P. (2006) The AAIPT Wiring Team worked with contractors to develop the Automated Wiring Analysis (AWA) tool as an advanced off-board diagnostics capability to enable a modular approach to automated wiring systems evaluation from single wiring harnesses to complete aircraft. The electronic analyzer system detects faults, and the single wave reflectometry handheld device allows for fault location and waveform analysis. This tool helps pinpoint a chaffed wire’s location enabling the maintenance team to create a strategy to get to it and fix it. (NAVAIR, 2006b)
NAVAIR formed the Aging Aircraft IPT to produce and coordinate effective and workable plans to decrease the burden of aircraft wiring systems maintenance and impact on reliability of aging wiring, to coordinate the efforts of the Department of Defense and government agencies in optimizing the service life of aging aircraft fleets, to maximize aircraft material safety, to minimize aircraft operating costs and duplication of effort, and to sponsor and conduct exchanges of information on these efforts. (INCOSE, 2015-Project Planning Process, ch. 5.1) In fulfilling this charge, the AAIPT successfully broke down communication barriers and the stove pipes that made up NAVAIR’s competency-aligned organization to perform a corrective action that attempted to help bring down maintenance costs and raise operational readiness for the Navy’s aging aircraft fleet.
My instincts, however, are to go back to the concept design phase in the life cycle and completely re-design how the wiring systems lay in the aircraft frame such that every inch could be visually inspected and easily accessible by maintainers. The cost of having to re-design the entire aircraft and re-tool factories based on accessible wiring would be enormous. Understandably, NAVAIR chose to use Quality Management to establish, implement, and continuously improve the effectiveness and efficiency of its aging SOIs and produced corrective actions based on detailed analysis of available maintenance data and fleet feedback using the most cost-effective methods to gain savings of operating and maintenance costs over the long life of the aircraft. (INCOSE, 2015-Quality Management, ch. 7.5)
References
Walden, D., Roedler, G., Forsberg, K., Hamelin, R., & Shortell, T. (2015). Systems engineering handbook: A guide for system life cycle processes and activities (Fourth). INCOSE.
Arnason, P. (2006). NAVAIR wiring system initiatives. Patuxent River; NAVAIR. (https://www.slideserve.com/zachery-ramos/navair-wiring-system-initiatives)
Monroe Aerospace Blog (2020). 6 facts about the Boeing 747. One Monroe Aerospace. (https://monroeaerospace.com/blog/6-facts-about-the-boeing-747/#:~:text=%232)%20150%20Miles%20of%20Electrical%20Wiring&text=Like%20all%20airplanes%2C%20the%20Boeing,would%20extend%20over%20150%20miles.)
Ernst, R. (2006). A new way of looking at old airplanes. Patuxent River; Joint Council of Aging Aircraft. (https://www.sae.org/events/dod/presentations/2005roberternst.pdf)
NAVAIR (Jenkins, J.) (2006a). Aging Aircraft team creates joint wiring manuals. Aging Aircraft. (https://www.navair.navy.mil/node/14226)
NAVAIR (Jenkins, J.) (2006b). Aging Aircraft team getting the tools the fleet needs. Aging Aircraft. (https://www.navair.navy.mil/node/8821)
NAVAIR (Jenkins, J.) (2006c). Aging aircraft wiring lacking much needed attention. Aging Aircraft. (https://www.navair.navy.mil/node/8681)
Loeb, B. (1998). TWA 800 overview. D.C.; National Transportation Safety Board. (https://www.ntsb.gov/news/events/documents/moriches_ny-TWA_800_Overview.pdf)
Transportation Safety Board of Canada. (1999). (rep.). Swissair 111 investigation report — Executive summary. Ottawa, Ontario. (https://www.tsb.gc.ca/eng/medias-media/fiches-facts/a98h0003/sum_a98h0003.html)
Anteon (2006). Aging Aircraft Wiring Study. NAVAIR Contract N00421–04-C-0121.
