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Two new acronyms have recently joined the aviation lexicon, EWIS (electrical wiring interconnection system) and EZAP (enhanced zonal analysis program). Both are intimately related to ICA (instructions for continued airworthiness).
EWIS was conceived on July 17, 1996, when TWA Flight 800 fell into the Atlantic 12 minutes after its New York departure. During its four-year investigation the NTSB never found what ignited the 747’s center wing tank, but it did find a number of potentially unsafe conditions nearby, including cracked insulation, open-ended splices vulnerable to moisture, and other repairs that didn’t comply with Boeing’s Standard Wiring Practices Manual.
The arcing IFE cables that likely brought down Swissair Flight 111 on September 9, 1998 reinforced the ATSRAC mission. After the Lear 35 carrying golfer Payne Stewart crashed in 1999, the NTSB urged the ATSRAC to look at all transport category aircraft. While the NTSB found no specific cause for the loss of pressurization, the system is controlled electrically.
Until this time, wiring rarely received any special maintenance or inspections, even though its failure causes delays, unscheduled landings, IFE system problems, and both nonfatal and deadly accidents. The ATSRAC recommended internationally harmonized certification requirements, standard wiring practices, and maintenance procedures to correct and prevent these consequences.
These recommendations led to NPRM 05-08, Enhanced Airworthiness Program for Airplane Systems/Fuel Tank Safety. Published on October 6, 2005, it christened EWIS and, for the first time, officially defined the electrical system and gave it a home, Subpart H of Part 25. The point is that safety depends on the reliable transfer of electrical energy, and that EWIS is equal to the critical systems it connects and controls.
Officially, EWIS is “any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy, including data and signals, between two or more intended termination points…. This includes electrical cables, coaxial cables, ribbon cables, power feeders, and databuses.” Don’t think wires, think wiring diagram. EWIS covers everything a mechanic can maintain, repair, or modify.
The final rule (dated December 2007, with compliance starting 39 months later) bred dozens of advisory circulars. AC 25,17001-1, Certification of Electrical Wiring Interconnection Systems on Transport Category Airplanes, guides the creation of the original system and its modification by companies like ASIG. A third of its guidance pages is dedicated to the qualitative and quantitative safety assessment and analysis required for original and supplemental type certification.
Operators incorporate the new instructions with an EZAP, which determines the appropriate inspection and cleaning procedures. Simply put, mechanics evaluate EWIS’s condition and the affect of nearby items, such as plumbing or control cables, to its safety. An EZAP is a logical addition to aircraft covered by a zonal inspection program. On non-ZIP aircraft the EZAP will identify EWIS-related tasks that must be consolidated in the inspection and maintenance programs. (Those who hold the design approval for these aircraft may find it worthwhile to create a ZIP in conjunction with an EZAP).
Ultimately, OEMs, STC holders, aircraft operators, repair stations, and anyone who provides maintenance need to evaluate their overall philosophy and specific maintenance tasks so that EWIS receives the same care and attention as any other system critical to aircraft operation.
Until next time, stay 5x5, mission ready, and Wired!
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