Aviation Investigation Report A98H0003
1.18.4 Wire-Related Issues
In the MD-11, the overhead CB panel contains six electrical buses that supply power to many of the aircraft's systems: four emergency buses, the battery bus, and the battery direct bus. The battery direct and the battery bus feeds are routed together, making five separate cable runs into the cockpit overhead panel from the avionics compartment. The five cable runs are spatially separated from each other until they reach an area approximately 31 to 46 cm (12 to 18 inches) aft of the housing that is located behind the overhead CB panel. They are then bundled together and enter the overhead CB panel through an oval opening on the right aft side of the housing.
One of the emergency bus feed power cables found in the wreckage, identified as a section of the left emergency AC bus feed, exhibited an area of melted copper consistent with an arcing event in a location between 10 and 15 cm (3.9 and 5.9 inches) outside the housing. This particular cable was insulated with BXS7008, a XL-ETFE type insulation, whereas the remaining cables were constructed from BXS7007, a polyimide-wrapped film and a meta-aramid fibre paper outer cover. This general area above the cockpit had experienced heat damage from the in-flight fire. Other wires from the same general location, but in a different wire bundle, were also found to have arced. As indicated in Section 1.18.8 of this report, the known sequence of events does not support the hypothesis that the arcing of the wires in this area was related to the initiating event. That is, the arcing in this area took place later in the failure sequence and was the result of fire-related heat damage to the wires.
The left emergency AC bus lost power shortly before the flight recorders stopped recording. It is unknown whether any or all of the remaining emergency or battery buses were eventually affected, but it is known that none of them were affected at the same time as the left emergency AC bus. However, because the cables were brought into such close proximity in the overhead panel housing area, they would all have been exposed to the same threat, such as heat, fire or arcing event.
It could not be determined whether power sources other than the left emergency AC bus were lost to the overhead panel; however, an assessment was completed to determine the effect on aircraft systems if a total loss of electrical power occurred. Because it was known that primary electrical power was still available until the time of impact, the assessment focused on determining what functions would remain available from primary power sources to provide basic information, such as attitude, altitude, airspeed, and heading. The primary flight controls do not require any electrical power to operate and so were not part of this assessment.
DU 2 would remain powered from primary power and would be reconfigured to a PFD with DEU 3 providing the inputs. The PFD would provide attitude and heading information. Altitude and airspeed would be available from the standby instrument. Engine control would be available, but no engine and alert status information would be available. All radio communication would be lost. Numerous other warning lights, such as the master warning and caution lights, would not be functional.
The MD-11 wire installation design was assessed as part of the investigation to determine whether it met the regulatory requirements for wire separation, and whether there were any associated safety deficiencies.
The following FARs relate to wire separation and routing:
- FAR 25.1309(b) states that failure analysis must consider that "no single failure shall prevent safe flight and landing";
- FAR 25.903(d) states that turbine engine installation design must minimize hazards to wires in case of rotor failure;
- FAR 25.631 requires that wiring, necessary for continued safe flight and landing, must be protected against a bird strike hazard; and
- FAR 25.1353(b) deals with the separation of essential system wiring and heavy current-carrying cables.
Specifically, this assessment focused on the area where the power cables are bundled together just before they enter into the overhead switch panel housing. This bundle was in close proximity to hundreds of other circuit wires that were also bundled together as they entered or exited the housing. When assessing the MD-11 overhead panel housing area, the most relevant regulation is FAR 25.1353(b), which states "Cables must be grouped, routed and spaced so that damage to essential circuits will be minimized if there are faults in heavy current-carrying cables." This requirement calls for the spatial separation of cables to avoid damage to essential circuits. Essential loads, as defined by the MD-11 manufacturer, are those that are essential to maintain controlled flight in zero visibility. The regulation requires that a potential threat be minimized; it does not require that a potential threat be eliminated. The term "minimized" is not defined; however, according to the FAA, the term has an element of reasonableness associated with it.
To meet the requirement of FAR 25.1353(b), the electrical cables in the MD-11 that run to the overhead panel are spatially separated from each other until they enter the overhead panel housing. Once the cables were positioned together, spatial separation no longer existed. To minimize the risk of wire insulation chafing, the manufacturer fitted the edge of the oval opening in the housing with a nylon grommet. For added mechanical protection, the cables that are between 8 AWG and 00 AWG in size are protected by an extra (third) wrap of polyimide insulation, plus an outer jacket (meta-aramid fibre paper). The wires between 24 AWG and 10 AWG have two wraps of polyimide insulation. The three right emergency AC bus cables were also wrapped in a silicone elastomer-coated glass fibre braided electrical sleeving to provide additional protection. The additional mechanical protection provided on the cables in the bundled area met the FAA's interpretation of minimization and reasonableness. Because of the lack of specific quantitative information in the guidance material, it could not be determined what criteria was used to determine the level of mechanical protection needed to satisfy the requirements of FAR 25.1353(b) for situations where adequate spatial separation is impracticable.
The design of the MD-11 wire routing into the overhead switch panel housing was based on the design used in the DC-10. The service histories of both aircraft were reviewed to determine whether any problems, such as chafing, were reported; none were found.
The predominant general wire insulation used on the occurrence aircraft was the MIL-W-81381, polyimide-film-type wire insulation. However, other types of insulations were also used depending upon requirements. Wires with these different insulation types were sometimes routed in the same wire bundles. While there are no regulations pertaining to the mixing of wire insulations with dissimilar properties, FAA ACs 25-16 and 43.13-1B provide guidance. These guidelines rely on the aircraft manufacturer or subsequent modifier to establish the compatibility of various wire insulation materials through satisfactory in-service performance history, additional tests, or both.
When the DC-10 entered revenue service over 25 years ago, some problems were encountered with the premature failure of clamps and wires at clamping points in areas of high vibration (pylons, wing, and tail engine). The MD-11 manufacturer developed a wire compatibility test procedure to represent the most severe vibration environment on the aircraft that replicated the failures experienced on in-service aircraft. Testing allowed the MD-11 manufacturer to assess the compatibility of various wire insulations, to evaluate new wire insulation types, and to develop containment parts (clamps, nylon tie-wraps, etc.) and materials including protective sleeving and tubing. The MD-11 manufacturer advises that high vibration and wire-to-wire abrasion testing has shown that, when properly installed, the mixing of different approved insulation types has not been a problem. Typically, it was found that the wear patterns are similar regardless of whether there is a mix of insulations. The wire compatibility test developed by the MD-11 manufacturer has become their standard for evaluating and developing new wire insulation types, containment parts and materials, and protective sleeving and tubing. In addition, the FAA has indicated that there is no systemic problem associated with the use of mixed wire types that are properly installed and maintained.
 Information provided by Boeing Materials & Producibility Engineering.
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