A major effort to improve wiring safety in airliners does not specify a set or minimum distance that one wire must be separated from another. However, the criteria by which wires should be separated to minimize, for example, the dangerous effects of electrical arcing, are laid out in general terms.
Details are contained in a notice of proposed rulemaking (NPRM) on aircraft wiring and an associated draft advisory circular, AC 25.17XX, both of which were released by the Federal Aviation Administration (FAA) for public comment on Oct. 6.
The wiring initiative is an outgrowth of the TWA Flight 800 and Swissair Flight 111 disasters, both of which were attributed to faulty wiring, as well as inspections of in- service and retired aircraft, which revealed numerous discrepancies indicating deficiencies in wire system design and maintenance. The problems were serious enough to prompt the FAA to convene the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), which provided recommended changes to wiring system design, certification and maintenance that have culminated in the publication of the NPRM and numerous associated advisory circulars.
In its investigation of the TWA Flight 800 explosion, attributed to a current arcing from a fuel flow gauge to a fuel quantity indication system (FQIS) circuit, which caused the fuel tank explosion in the B747, the National Transportation Safety Board (NTSB) noted the lack of generally accepted separation criteria for wiring that would guide the design or modification of wiring systems in an aircraft.
Similarly, the Transportation Safety Board (TSB) of Canada, in its investigation of the electrically stoked in-flight fire that downed the Swissair jet, an MD-11, noted, “There is no guidance material … in situations where physical/spatial wire separation is not practicable or workable, such as in confined areas.”
The ATSRAC wrestled with the problem of establishing a minimum separation distance. In considering such a separation interval for both large and small aircraft, the ATSRAC concluded that it would not be practical, especially for small aircraft, where the physical space to separate wires would not be available.
Thus, the NPRM and associated draft AC opted for general principles of wire separation instead of minimum distances. The NPRM proposes a new Section 25.1709 of the Federal Aviation Regulations (FARs), titled “Systems Separation” that, as explained in the NPRM, “Would require applicants to design EWIS [electrical wiring interconnection systems] with appropriate separation to minimize the possibility of hazardous effects upon the airplane or its systems.”
The NPRM noted that “existing regulations fall short.” The NPRM explained:
“As used in the proposed rule, the term ‘separation’ is a measure of physical distance. The purpose of separation is to prevent hazards of arcing between wires in a single bundle, between two or more bundles, or between an electrical bundle and a non-electrical system or structure.
“In some cases, the proposal would allow separation to be achieved with a barrier or other means shown to be at least equivalent to the necessary physical distance. However, distance separation is preferred because service experience shows that the use of barriers such as conduits can cause wire damage or lead to maintenance errors. In some cases, wire bundle sleeving is used to provide separation, although the sleeving itself is susceptible to the same types of damage as wire insulation.
“Determining the necessary amount of physical separation distance is essential. However, the proposed rule does not mandate specific separation distances because each system design and airplane model can be unique, and because manufacturers have differing design standards and installation techniques. Instead, it requires that the chosen separation be adequate so that an EWIS component failure will not create a hazardous condition.”
To this end, the NPRM and the associated draft AC outlined a number of factors to be considered. The listing does not mention wire orientation, which is to say that a wire or bundle running vertically will experience more damage from arcing than if running horizontally. Indeed, the listing, with its emphasis on arcing, covers only part of the threat. In addition to arcing, surface current leakage conduction through moisture absorbed in surface dust, salt particles and other material deposited over time from air in the aircraft, can cause electrical “noise” in line replaceable units (LRUs). This electrical noise is particularly troublesome, as the phenomenon is frequently intermittent in nature. However, the listing hints at the large number of variables affecting required separation. The result of meticulous consideration of these factors can still be a failure in service, as unusual or abusive actions on wire, cable and bundles can exceed the safety factor applied.
In any event, comments are solicited on wire separation and other factors in the NPRM, and the debate over wire separation is not over.
Wire Separation: Factors To Be Considered As Propounded By The FAA
1) The electrical characteristics, amount of power, and severity-of-failure condition of the system functions performed by the signals in the EWIS and adjacent EWIS.
2) Installation design features, including the number, type, and location of support devices along the wire path.
3) The maximum amount of slack wire resulting from wire bundle build tolerances and other wire bundle manufacturing variabilities.
4) Probable variations in the installation of the wiring and adjacent wiring, including position of wire support devices and amount of wire slack possible.
5) The intended operating environment, including amount of deflection or relative movement possible and the effect of failure of a wire support or other separation means.
6) Maintenance practices as defined by the airplane manufacturer’s standard wiring practices manual
7) The maximum temperature generated by adjacent wire/wire bundles during normal and fault conditions.
Source: FAA, NPRM, Oct. 6, FR Doc 05-19419