Incident Overview
A flight returned to its departure point after encountering apparent electrical problems, including circuit breakers open for the center tank right pump and the right tank forward pump. After landing, the crew performed a normal shutdown. During taxi in, they noted an electrical smell, which they originally attributed to use of the galley ovens. When they elected to disembark the passengers, passengers leaving the airplane who had been seated mid-cabin noted that they had smelled electrical smoke.
Post-Landing Troubleshooting
The airplane was repowered by maintenance personnel after shut-down and disembarking of passengers. During the trouble-shooting process, additional systems and indicators were affected, including right wing anti-ice and right wing overheat. Technicians isolated an electrical short to the forward cargo compartment. When they touched the panel around station 410, they saw some sparks. Power was removed from the aircraft once the damage was found.
Inspection and Findings
The technicians opened the ceiling panel to gain access to the area. When they touched the bottom of the raceway clamp, it fell apart. FAA and NTSB investigators documented the damaged wiring bundles at station 420 above the forward cargo compartment ceiling on the right side of the airplane, and had them sectioned for further inspection and testing. The wire bundle channel and clamp were also removed for inspection and analysis. A total of 113 conductors were reported to have been subjected to the electrical arcing event.
Wire and Component Analysis
Investigators were unable to determine the cause of electrical arcing that burned numerous wires of the subject bundles in two. One of the nine Teflon insulated wires, W336-003-10, exhibited evidence that it might have been damaged prior to the electrical arcing. W336-003-10, and two other 10 gauge wires in the bundles, were conductors for the three-phase 115VAC system A electric hydraulic pump. Electrical arcing had damaged approximately twelve inches of insulation on wire W336-003-10. The exposed conductors at one end of the wire had sustained arc damage at two locations. The arcing burned several wire strands in two, which became unraveled from the rest of the strands. There was approximately one third of the insulation left in this area of the wire. A small area of the edge of the insulation appeared to have been cut prior to the occurrence of the electrical arcing.
Channel and Clamp Assessment
The wire bundle channel (bracket) and clamp were also analyzed. Evidence indicated that an unidentified wire that was involved in the arcing/burning event had been out of place, along the side of the bracket between the cushioning foam and the plastic (nylon) frame before or during the event. Analysis of these two parts could not determine what type of wire had caused the damage to the side of the cushion clamp or the nylon channel.