What happened
While the aircraft was in cruise flight, the flight crew detected a burning smell within the flight deck. Shortly thereafter, the forward cargo compartment fire detection system was activated. In response to the situation, the crew performed the necessary procedures to manage the event, which included arming the forward cargo fire suppression system. This action effectively removed electrical power from the recirculation fans, preventing additional damage from further arcing. The crew subsequently diverted the aircraft to the nearest airport, where a safe landing was completed.
The investigation
Investigators examined the aircraft following the incident and discovered that an electrical wiring harness, identified as W5279, had been installed in an incorrect position. This misrouting, which likely occurred during the initial manufacturing of the Boeing 777, placed the wires in a position where they were subject to chafing from a retainer screw in the forward cargo ceiling liner.
As the wires chafed, electrical current began to disperse through the structural carbon fibre beam near body station 508. This process generated intense heat, resulting in the heat damage of 14 cargo ceiling polyetheretherketone resin standoff brackets and causing delamination and chafing to several areas of the structural carbon fibre beam. Although the crew discharged the fire bottles in the forward cargo space, this had no impact on the arcing, as the electrical fault was located in a sealed zone between the cargo ceiling panel and the passenger floor compartment rather than within the cargo compartment itself.
Findings
- The primary cause of the smoke and arcing was the incorrect routing of the W5279 wiring harness, which deviated from the original design drawings.
- The electrical arcing produced a significant amount of smoke that migrated through the ceiling liner to activate the fire detection system.
- This event represents the fifth instance of wire bundle damage in the cargo compartments of a Boeing 777 caused by chafing from ceiling liner hardware, though it was the first to trigger a cargo fire warning while in flight.