From Air Safety Week:

Wiring Woes History Already Written

A history of aircraft wiring problems has been written already and it could save a government-industry task force the effort of having to prepare a similar document from scratch.

At the recent meeting of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), the task of writing a history of the wiring problem for inclusion in the preface of a brace of advisory circulars (ACs) now in preparation was deferred to the Federal Aviation Administration (FAA). The idea was to ensure that the histories will be "comparable" across all the relevant documents produced as a result of the ATSRAC effort (see ASW, July 15).

However, one member of Working Group 8, which dealt with wiring maintenance practices, prepared a history. It was written by wiring expert Ed Block, who claimed that the working group, whose title was Enhanced Training Program for Wire Systems, unanimously endorsed the history he prepared. To be sure, Block is known for his astringent, sometimes vitriolic, calls to improve the spate of wiring in the fleet. However, the history he wrote is a calm, straightforward exegesis of the problem. Block's draft does not use the term Electrical Wiring Interconnections System (EWIS), which was adopted for use by the ATSRAC, but this term could easily be incorporated.

At least two reasons come to mind as to why Block's suggested background has not been received more favorably. First, ATSRAC members are mindful that the TWA Flight 800 crash has been used to justify action regarding fuel tank safety; once used, a particular accident cannot be used to justify action on related and corollary safety problems (see 'Cost benefit hurdle' in ASW, July 15). Second, Block's treatment of the history addresses the limitations attendant to visual inspections and the need for more comprehensive non-destructive evaluation (NDE) to assess the state of installed wiring. The ATSRAC, however, is calling for an enhanced zonal analysis procedure (EZAP), which will be limited to visual inspections, albeit with hand-held mirrors and of all wiring within arm's reach - but still short of NDE (see 'Enhanced zonal inspections' in ASW, July 15).

Below is an abridged version of the history Block wrote:

"Safety concerns about aging wiring systems in airplanes were brought to the forefront of public and governmental attention by a fatal accident involving a Boeing Model 747-131 airplane on July 17, 1996 [TWA Flight 800]. (See ASW, July 15.) The National Transportation Safety Board (NTSB) determined that the probable cause of the accident was an explosion in the center wing fuel tank (CWT) resulting from ignition of the flammable fuel/air mixture in the tank. Although the source of ignition energy for the explosion could not be determined with certainty, the NTSB believes the most likely source was a short circuit outside of the CWT that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system (FQIS). In its investigation, the NTSB found several potentially unsafe conditions in and near the electrical wiring of the airplane, including cracked wiring insulation, metal shavings adhered to a floor beam along which FQIS wires would have been routed, other debris, and sulfide deposits. The NTSB also found evidence of several repairs to the accident airplane that did not comply with the guidelines in Boeing's Standard Wiring Practices Manual.

"Noncompliant repairs included the use of an oversized strain relief clamp on the terminal block of the No. 1 fuel tank compensator, which did not adequately secure the wires, numerous open-ended - rather than sealed - wire splices, which exposed conductors to possible water contamination, several wire bundles containing numerous wire splices on adjacent wires at the same location, and excessive solder on the connector pins inside the fuel totalizer gauge, which had connected the pins/wires from the right wing main fuel tank and the CWT FQIS.

"Most alarming is the NTSB's finding that deterioration, damage and contamination of aircraft wiring and related components, and noncompliant repairs, were common in the airline transport airplanes that it inspected during the investigation. Therefore, the NTSB concluded, "The condition of the wiring system in the accident airplane was not atypical for an airplane of its age and one that had been maintained in accordance with prevailing industry practices [the widespread existence of such conditions also was corroborated by Boeing's service letter 747-SL-20-048 dated Jan. 25, 1995, which detailed similar conditions found in numerous 747s].

"The NTSB found the unsafe conditions of aircraft wiring systems especially disturbing because [they] revealed general shortcomings of the current visual wiring inspection criteria ... the NTSB found that a large portion of aircraft wiring is difficult, if not impossible, to inspect because of its inaccessibility, and that wire damage ... may not be detected, even on visible and accessible portions of aircraft wiring.

"The [TWA 800] investigation ... resulted in a heightened awareness of the importance of maintaining the integrity of aircraft wiring. The FAA began to investigate fuel tank wiring, and to strengthen its focus on aging wiring in general. Also, in 1997 the White House Commission on Aviation Safety & Security (WHCSS) issued the following recommendation to the FAA: (Cont'd on p. 10)

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

"In July 1998, the FAA issued the Aging Transport Non-Structural Systems Plan (hereinafter 'Aging Systems Plan') which addressed the WHCSS recommendation. The Aging Systems Plan focused specifically on wiring systems. In the [plan] the FAA described the results of its evaluations of five transport category airplanes deemed representative of the [aging fleet]. The FAA found conditions similar to those [of] the NTSB ... including:

Cracked wire insulation.
Deterioration of wiring and related components.
Stiff and cracked wire.
Wire bundles contaminated with metal shavings [known as 'swarf'], dust and fluids.
Corroded connector pins.
Improper wire installation and repairs.
"The FAA's Aging Systems Plan also detailed several tasks aimed at correcting these problems:

Improving wiring inspection criteria and providing more detailed descriptions of undesirable conditions.
Improving inspector training to ensure ... recognition and repair of aging wiring components.
Developing new methods of nondestructive testing of wiring.
"To address the issues identified in the Aging Systems Plan, in 1998 the FAA established the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC) ... The ATSRAC effort was divided into two phases: non-intrusive inspections which utilized only the current visual inspections, and intrusive inspections, which involved opening up the wire bundles and subjecting the wires to non-destructive test equipment and laboratory analysis. The ... non-intrusive (visual only) effort ultimately proved its inadequacy. The Non-Intrusive Inspection Final Report stated, 'Existing maintenance programs may not contain adequate wiring inspection detail.' This report went on to say that 'discrepancies found during the survey did not appear to be wire-type dependent.'

"These statements led to the formation of the Intrusive Inspection Group, [which] was to determine if visual inspections worked, and to assess the state of wiring in the fleet of aging aircraft.'

"The intrusive inspection group found that wire type did matter, that there are 'typical characteristic flaws for each type of wire.' The group also determined that mixing of different wire types within the same bundle could be hazardous: 'The inherent differences in the performance and chemistry of the wire insulation types should be ample reason to conclude that mixing of certain wire types in the same bundle could be hazardous to aircraft safety due to the potential for arcing and fire.'

"The assessment of visual inspection proved it was inadequate alone to discern the majority of degenerative conditions such as cracking and arcing. The report also concluded that polyvinyl chloride (PVC) wiring was considered a flammable material ... [The] general threat analysis (GTA) used 'plausible, hypothetical situations,' but the report also declared the results of between 220-1,810 breaches found per aircraft. This information was contained in the Specific Recommendations portion of the final report, [with the concluding statement that] ... 'all flaw types should be considered possible.'

"This advisory circular addresses only the training program. It does not attempt to deal with the condition of the fleet's wiring, [nor does it address the development of] performance tests for wiring. This AC captures, in FAA guidance form, the aircraft wiring training program developed by ATSRAC. [It] includes a training syllabus, curriculum, training target groups and a matrix outlining training for each ... group." >> Block, e-mail edwblock@aol.com <<

continued Air Safety Week:

'Inherent Differences - Could be Hazardous'
The word on mixing wire types (extracts)

"[U.S. Navy] tests ... demonstrated that wires of different insulation types, mixed in bundles, could experience changes in dielectric strength due to the wearing of the insulation.

"Based on all the FAA's documentation and the Navy's tests ... it is only prudent to carefully consider the various wire insulation types in all applications ... The inherent differences in the performance and chemistry of the wire insulation types should be ample reason to conclude that mixing of certain wire types in the same bundle could be hazardous to aircraft safety due to the potential for arcing and fire. [Emphasis added]

"The same size of aircraft with mixed insulation types [six] was too small to draw any definitive conclusions. There were, however, two cases noted from the 747 and the first DC-9 of wires chafing against one [another] with visible damage to wire insulation. In both cases, newer wire was chafing against older wire of a different type. The wires were not bundled together but crossed one another where there was opportunity for relative motion between them. Some of the wire bundles on the DC-10 had mixed insulation types. The inspectors saw no visual indication of wear due to wire-on-wire abrasion in the bundles." (ASW note: this last sentence points out one of the limitations of visual inspections. The admonition against mixing wire types stems from the fact that there are hard and soft outer coatings, and a hard insulation coating can "saw" through a softer coating.)

Source: Transport Aircraft Intrusive Inspection Project, Final Report, Dec. 29, 2000, p. 54

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