on A/C Wiring/Air Safety Week
Intrusive Inspections of Aircraft Wiring Show Deterioration Over Time
New information provides compelling evidence that aircraft wiring is not immortal. Its degradation over time, particularly breaches in insulation that can lead to dangerous electrical arcing, indicates that operators may need to implement wire husbandry programs similar to the activities now well established to assure the integrity of aging aircraft structure.
Recent detailed electrical wiring inspections of six transport aircraft with 20 or more years of service revealed one to five breaches in the insulation for every 1,000 feet of wire. A breach in the insulation can open a pathway for arcing and the attendant hazard of an electrical fire. The inspections were conducted as part of a government-industry task force charged with assessing the integrity of internal systems like wiring in aging jetliners and recommending programs to assure their safe functioning.
Inspection teams, working under the aegis of the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), have conducted general visual inspections (GVI) of the wiring on 81 old jetliners and found more than 3,000 discrepancies requiring corrective maintenance action on these jets.
These visual inspections were followed by more detailed, or intrusive, inspections of six old jetliners. In these inspections, connectors, wire bundles and such were physically disassembled to probe for flaws that would not be revealed by the visual inspections. The wires also were subjected to various non-destructive testing (NDT) protocols.
Look harder, find more The results of these inspections are still being assessed, but the preliminary conclusion is that visual inspections reveal only a fraction of the wiring flaws that may lurk in an old jet.
"We are finding about four to five times more flaws than the GVI effort,"
Dr. Chris Smith told the full ATSRAC at its meeting last week. Smith directs the aging transport programs at the Federal Aviation Administration's (FAA)
William J. Hughes Technical Center in Atlantic City, New Jersey. He is leading the intrusive wiring inspection effort.
In the aircraft selected for intrusive inspections (2 DC-9s, a DC-10, a B747, an L-1011 and an A300), anywhere from 2,000 to 8,000 feet of wiring was selected for intrusive examinations. The specimen types included:
Wiring and bundles inside and outside of the fuselage pressure vessel.
Bilge and crown areas.
Wiring located in areas of high and low maintenance activity (to gauge the impact of damage to wiring as a result of maintenance on other aircraft components).
Straight runs and complex harnesses.
Small and large bundles.
Small gage and large gage wire (power feeders).
As Smith explained, the locations were selected to correspond with areas likely to be inspected visually. Fuel tank wiring was not included, Smith said, as that is part of a separate industry effort. In addition, the aircraft were selected on the basis of the type of general purpose wiring installed, to include aromatic polyimide (Kapton) and PVC glass nylon.
Although PVC glass nylon is no longer in fashion, Smith said, its longevity needs to be understood. "There's a lot of it out there in old planes," he said.
The wires were subjected to NDT before removal and then again in the laboratory prior to detailed analysis. This procedure ensured that any damage during removal was taken into account. The thousands of feet of wire removed from the aircraft will be subjected to a battery of testing techniques.
First impressions The effort is far from complete. This much can be said, according to Smith's presentation: most of the visual findings were configuration-related (e.g., improper routing, bad clamping). Clearly, intrusive inspections reveal far more in terms of physical damage to wire, such as breaches in insulation not discovered in visual inspection, and damage that may be buried deep inside a thick bundle of wire. Smith showed a photograph of a disassembled bundle where the insulation was broken in three places on one wire. This case illustrated what Smith described as "a wire dependent flaw."
The bundle in the photo was of Kapton wire.
The information from the intrusive inspections will provide the grist for a threat assessment of the various wire flaws, to include wire insulation type and aggravating or contributing factors. The threat assessment analysis, according to one of Smith's briefing slides, "will focus on plausible, hypothetical situations which we believe may not have been given adequate consideration during the certification process." Final results are weeks, if not months, away.
Records review In addition, Smith's team undertook a review of nearly 50,000 Service Difficulty Reports (SDR's) submitted on aircraft with operating service ranging from 6-30 years. That review revealed a steady increase over time in the number of SDR's per aircraft per year where the reports referenced wiring and electrical cabling. The difference in the rate of SDR generation is some 600 percent between an aircraft with six years of service and one with 30 years of service. Nothing close to his dramatic difference was shown for other systems (e.g., actuators, valves, hydraulic/pneumatic lines, and fittings).
When presented with this data, ATSRAC co-chairman Kent Hollinger remarked, "If we'd looked at this data two years ago (the ATSRAC started its work in Jan. 1999), the conclusion is that we should look at wire."
"Yes," Smith replied.
Wire Hazards Not Considered Safety Threat if Maintenance Would Have Caught Them
Despite exclamations that the wording was "completely misleading," a government-industry task force assessing the safety of wiring in aging airliners has approved a report indicating that an average of more than 30 significant items were found on the airplanes whose wiring systems were subjected to detailed visual inspections.
The report concluded that the findings of chafed, abraded or otherwise damaged or worn wiring found in so-called "non-intrusive" (e.g., visual) inspections of the wiring on 81 airliners with 20 or more years service did not pose an immediate fleet-wide safety problem.
The report was endorsed by the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), the task force convened in January 1999 to assess the safety of internal systems on geriatric jets. The committee's work is to be completed by January 2001. The first major work-product of the task force was a report of visual inspections of the state of wiring on old jets.
Preliminary results were presented earlier this year (see ASW, Jan. 24). The final results, presented last week, increased the preliminary count of some 3,100 notable items to a final count of 3,215, a 3 percent increase over what was reported earlier - for an average of about 39 "non-routine" findings per aircraft.
The term "non-routine" was described as "something the airline didn't expect to find during scheduled maintenance." Of these 3,200 items, 187 were judged to be "significant," but none were considered a fleet-wide "safety of flight" concern, as determined by the definitions used in the report.
Of the 187 "significant" items, under further analysis 177 of them have been deemed not serious enough to affect safety of flight. The remaining five are still under evaluation.
Various ATSRAC members challenged the logic of the report's assessment that no safety of flight issues were uncovered in these inspections. One member observed that the detailed inspection reports contained numerous mentions of open breaks in the wire insulation. An insulation breach can open the door to arcing and a potential electrical fire. Defenders of the report argued that none of the findings reached the threshold for mandatory Service Difficulty Reports (SDR's), such as overt evidence of arcing (blackened insulation, melted wiring, etc.). However, one ATSRAC member challenged the assurance that no fleet-wide problem exists on the basis that "reporting of other instances was not required."
Perhaps the most controversial aspect was the proposition that a significant finding did not pose a fleet-wide safety hazard if it would be caught during scheduled maintenance. The case in point involved chafed battery power feeder cables on two of one operator's B747's. The battery is located in the avionics bay behind the flight engineer in an area amenable to inspection. Yet the chafed cabling was not caught until the ATSRAC team's special visual inspection. A chafed cable could lead to arcing, possibly an explosion of the battery and resultant fire/acid in the vital avionics space. According to one of the individuals involved in the assessment of the inspection findings, the conclusion that the chafed feeder cable did not present a potentially fleet-wide hazard was based on the following: Over the years, the operator in question missed three opportunities to catch the chafed cable during scheduled maintenance. Since other operators could be expected to catch this item, "On that basis, it was determined not to be a fleet-wide problem."
The report was unanimously accepted on the basis that it contained a wealth of unprecedented information about the state of wiring on older jets.
Terms of Reference
Significant item: A defect which may require design changes or notification for enhanced inspections, based on Potential hazard (e.g., fire, bundle damage) or...
Frequency of occurrence at a specific location.
Safety of flight item: A discrepancy or safety of flight concern requiring immediate fleet-wide action. Impending critical failure seen in the sample.