What happened
The flight returned to its departure airport after experiencing several electrical issues, specifically the opening of circuit breakers for the center tank right pump and the right tank forward pump. Following a normal shutdown after landing, the crew noticed an electrical odor during taxi-in, which was initially thought to be related to the use of galley ovens. Upon disembarking, passengers seated in the mid-cabin reported smelling electrical smoke.
The investigation
Maintenance personnel attempted to repower the aircraft following the disembarkation of passengers. During troubleshooting, further system malfunctions were identified, including issues with the right wing anti-ice and right wing overheat indicators. Technicians eventually isolated an electrical short to the forward cargo compartment. While inspecting the panel near station 410, technicians observed sparks, prompting the removal of power from the aircraft.
Upon opening the ceiling panel, investigators found that a raceway clamp had disintegrated. FAA and NTSB investigators examined damaged wiring bundles at station 420, located above the right side of the forward cargo compartment ceiling. The investigation revealed that 113 conductors were affected by an electrical arcing event.
Analysis of the wire bundle showed that one Teflon-insulated wire (W336-003-10), which served as a conductor for the three-phase 115VAC system A electric hydraulic pump, had approximately twelve inches of damaged insulation. Evidence suggested this specific wire may have been damaged prior to the arcing event, as a small area of the insulation edge appeared to have been cut. Additionally, investigators found that an unidentified wire had been displaced along the side of the bracket between the cushioning foam and the nylon frame, though the exact nature of the wire or how it caused the damage could not be determined.