Incident Response
The flight crew detected a burning odor in the flight deck and initiated appropriate procedures. By arming the forward cargo fire suppression system, electrical power was removed from the recirculation fans. This prevented further arcing and damage to the structural carbon fiber beam, support brackets, and wiring. Although significant soot and electrical arcing were present, manually de-energizing the circuit before sufficient current went to ground prevented the electrical load control unit from tripping. Once the electrical current was deactivated via the forward cargo fire switches, the smoke likely ceased. Discharging the fire bottles into the forward cargo space, while procedurally correct, had no effect because the arcing source was in the sealed zone between the cargo ceiling panel and the passenger floor compartment, not in the cargo compartment itself.
Post-Incident Inspection Findings
A subsequent inspection of the aircraft revealed that an electrical wiring harness (W5279) was positioned incorrectly. This misplacement allowed one of the forward cargo ceiling liner retainer screws to chafe on the wires. The resulting electrical current from the chafed wire dispersed through the passenger floor carbon fiber beam at about body station 508. The current generated significant heat, causing heat damage to 14 of the cargo ceiling polyetheretherketone resin standoff brackets and chafing and delamination in several areas of the structural carbon fiber beam. The smoke from the arcing migrated through the forward cargo ceiling liner into the forward cargo compartment, activating the forward cargo fire detection system.
Related Events
This incident was the fifth reported case of wire bundle damage in the forward or aft cargo compartment of a Boeing 777 aircraft caused by chafing on a ceiling liner screw or nutplate. It was the first event that triggered the cargo fire warning and the only one detected in flight. A subsequent investigation by Boeing determined that wire bundle W5279 had been incorrectly routed, likely during aircraft manufacture, and not installed per design drawings. Slight variations in its position allowed the bundle to run directly above the screw and nutplate, leading to chafing over time.