Electrical Wiring is an Aircraft System

Air Safety Week,  Feb 3, 2003  

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By Kent Hollinger, Chairman, Aging Transport Systems Rulemaking Advisory Committee (ATSRAC)

The basis of all recommendations made by ATSRAC comes from the realization that electrical wiring is an aircraft system and should therefore be treated in that manner. Modern aircraft depend upon wiring not only for power delivery, but also for several flight critical functions such as communications, navigation signal delivery, and flight control surface commands.

The delivery of electrons throughout the aircraft is such an essential function that wiring can no longer be considered as "fit and forget." Past practices such as using wire bundles as ladder rungs to reach higher areas, and hanging maintenance lights from wiring, can no longer be tolerated. Our detailed inspections of "in service" wiring has shown that problems are common to both large and small transport aircraft. Also, design criteria can be enhanced to improve wiring system safety and avoid unnecessary removals and replacements.

The concept of wiring as a system has not come easily to some segments of the industry. There are certain individuals who rely on outdated definitions and practices to declare that wiring does not perform an "aircraft function." While the Wright brothers weren't concerned with electricity on their 1903 Flyer, today's aircraft could not function without the capability to move electrons from place to place. Therefore, wiring does indeed perform an aircraft function and must be analyzed and treated as such. Appropriately, a definition of Electrical Wiring Interconnection Systems (EWIS) was developed (see ASW, July 15, 2002). An additional characteristic of EWIS is that its failure can do more than just prevent the performance of its function. The physical failure of wiring has caused damage to other aircraft systems and has ignited flammable material in close proximity. These potential physical failures require a separate system safety assessment for the wiring system.

ATSRAC identified enhanced training as the greatest area of need with regards to wiring systems. A detailed training curriculum has been recommended with varying levels of instruction for mechanics, inspectors, engineers, aircraft cleaners, flight crew and cabin crew. Also recommended was training of aircraft manufacturing personnel and students at Part 147 aviation maintenance schools. Additional recommendations were made for training of regulatory and industry personnel performing wire system safety assessments. Once the level of awareness of the criticality of wiring systems has been raised, further safety improvements in maintenance and design will be possible.

(Note: ATSRAC reached its conclusions after performing non-intrusive inspections of 81 in-service large transport aircraft and 39 in-service small transport aircraft, along with detailed intrusive inspections of 6 retired large aircraft that then had wiring samples removed and analyzed in the laboratory. Service history reviews were also performed on large and small transport models. Information from ATSRAC meetings, as well as all of the final reports and recommendations submitted to the FAA, can be found at http://www.mitrecaasd.org/atsrac/index.html )

Byline: Kent Hollinger is the chairman of ATSRAC and is currently Principal Engineer at MITRE/CAASD. His involvement in aging wiring systems began during his previous positions as Chief Engineer at Northwest Airlines and Vice President of Quality Assurance and Engineering at America West Airlines.

A Brief History of ATSRAC

Following the crash of TWA Flight 800 in July 1996, President Clinton established the White House Commission on Aviation Safety and Security. The commission, chaired by then Vice President Gore, issued 50 recommendations, including:

In cooperation with airlines and manufacturers, the FAA's Aging Aircraft program should be expanded to cover non-structural systems.

In early 1998, FAA inspectors and Boeing representatives inspected the wiring on several older aircraft that were in storage in the desert. The intent of the aircraft inspections was to obtain first-hand information on the state of aged systems. In March 1998, Boeing held a meeting to brief the airlines on the results of the aircraft inspections.

A follow-on meeting was held in mid-April, which included FAA representatives who were seeking industry input prior to formulating an aging systems plan. The team agreed that the top four potential aging system concerns were wiring, connectors, grounds and circuit breakers.

In June, industry and FAA agreed to form an Aging Systems Task Force, comprised of a lead airline representative from each of the fleet types that were over 20 years old, the airframe manufacturers and the FAA. Eight aircraft model-specific Task Force Working Groups were formed to develop detailed inspection plans for each fleet type. It was also agreed that airlines and manufacturers would share "best practices" concerning maintenance of aircraft systems.