Aviation Investigation Report A98H0003
2.13.4 Electrical Wire Separation Issues
- 188.8.131.52 - FAR 25.1353(b) Requirements
- 184.108.40.206 - Electrical Wire Insulation Mixing
- 220.127.116.11 - Reporting of Wire-Related Discrepancies
From a wire separation perspective, no linkage was found between the design of the MD-11 wire routing and the source of the ignition for the in-flight fire. However, because all six power bus feed cables are routed together near the overhead switch panel housing, the design provided an increased opportunity for all the services supplied by these cables to be lost as a result of a single-point failure. It was established that the loss of the systems associated with the left emergency AC bus feed cable resulted from the cable being damaged by the fire. The failure of this cable occurred late in the sequence of events when the fire was well developed. Although the loss of associated systems would have compounded the already significant challenges being faced by the pilots, and could have contributed to the loss of control of the aircraft, it is likely that the fire environment, and not the loss of the various aircraft systems, was the crucial factor in the eventual outcome. The design of the MD-11 is such that even if all of these cables become de-powered, there would be limited, but sufficient, capability remaining to allow the pilots to maintain control of the aircraft.
Federal Aviation Regulations (FAR) 25.1353(b) states that "[c]ables must be grouped, routed and spaced so that damage to essential circuits will be minimized if there are faults in heavy current-carrying cables." The objective is to minimize the impact of the failure of a heavy current-carrying cable on any essential system wiring. The wording implies that such minimization measures need only be taken when a wire bundle contains both essential systems wire or wires and heavy current-carrying cable or cables. The guidance material does not specify what measures would be acceptable to meet the requirements of FAR 25.1353(b). Neither does FAR 25.1353(b) specifically address, from a wire separation perspective, the acceptability of grouping or bundling power cables that may be deemed to be part of the essential system wiring. In addition, interpretation of this regulation is made more difficult because several terms, such as "essential circuits," are not defined by the regulator.
In aircraft design, it is not always possible to maintain physical separation between wires, especially in the cockpit area where, typically, space available for installations is confined. There are no clear guidelines about what would constitute an alternate means of achieving compliance when physical separation is not practical or possible. For the MD-11, the manufacturer used protective sleeving, and considered it capable of providing an equivalent level of safety to physical separation. As there has been no history of problems in the MD-11 or DC-10 fleet over many years of service, this method has evidently served the purpose; however, neither the MD-11 manufacturer nor the FAA has quantified the effectiveness of such protective sleeving.
The lack of clarity of the guidance material is highlighted by the difficulty in making compliance determinations about how the routing of the emergency and battery power bus feeds in the MD-11 should be viewed. That is, if FAR 25.1353(b) applies to the wires in question, it is unclear whether the wire bundles are permitted to contain essential system wiring along with the heavy-current-carrying cables, even for short lengths. For example, the wire run near the overhead switch panel housing might be interpreted as not complying with FAR 25.1353(b), because physical separation was not achieved; however, the FAA's interpretation of this installation was that the wire run complied. The basis for this interpretation is not clear in that there is no specified method of providing for an alternate means of compliance to FAR 25.1353(b) for cases where physical separation is not practicable or workable. A review of the regulations and guidance in this area would be appropriate.
The FAA recognizes that mixing of wires whose insulation materials have different hardness characteristics can cause damage, especially in high-vibration areas. While there are no regulations pertaining to the mixing of wire insulations, FAA ACs 43.13-1B and 25-16 provide some guidance. AC 43.13-1B constitutes a general guide that provides acceptable methods, techniques, and practices for aircraft inspection and repair, while AC 25-16 supplements AC 43.13-1B with respect to the topic of electrical fault and fire prevention.
AC 43.13-1B is clear on the issue of wire mixing in that it states the "routing of wires with dissimilar insulation, within the same bundle, is not recommended." AC 25-16 suggests that the mixing of wires with "significantly different" insulation hardness properties should be avoided. Beyond relative wire-to-wire hardness, AC 25-16 also indicates that consideration should be given to the hardness factor between wire insulation and insulation-facing material, such as clamps or conduits.
The FAA relies on the aircraft manufacturer or modifier to establish material compatibility through prior satisfactory service experience or tests. To this end, Boeing's material compatibility tests have established an inventory of wires and insulation-facing material that, in its view, are suitable for use in the same bundles. Boeing-manufactured MD-11 wire bundles were designed and installed in accordance with acceptable industry standards to minimize wire-to-wire chafing damage and damage from wire-facing materials, such as clamps. Although after-market installations, such as in-flight entertainment (IFE) systems, may use the same or similar wire types as used by the aircraft manufacturer, the wire-to-wire compatibility would depend on the quality of the installation.
At the time of this occurrence, there was no requirement to report wiring anomalies as a separate and distinct category of discrepancy. Consequently, in many cases, wiring discrepancies were attributed to a component or line replaceable unit within the associated aircraft system. Additionally, many wiring discrepancies are repaired in situ without a full appreciation of the consequences of the anomaly being revealed. The lack of a dedicated Joint Aircraft Systems/Components Inspection Code (enhanced Air Transport Association codes) limited the development of methodologies to collect, compile, and monitor data regarding wire problems for trend analysis. Although more specific wiring information is now being recorded by technicians and regulatory inspectors, and progressively more data are becoming available to facilitate the validation of potential wiring deficiencies, the previous lack of guidance material for reporting wire-related failures resulted in the capture of limited historical data, which continues to hamper the evaluation of the nature and extent of wiring-related safety deficiencies.
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