What happened
While cruising at flight level 370 en route from New York to Switzerland, the crew of a Boeing 767 detected the smell of electrical smoke in the cockpit. Shortly after, thick, acrid smoke and intense heat, described by the crew as resembling an arc welder, surged from behind the glaushield on the first officer's side. An Engine Indication and Crew Alerting System (EICAS) message for a forward equipment overheat was also displayed.
In response, the crew declared an emergency and requested the nearest airport. Moncton Centre directed them to Sydney, Nova Scotia. The crew initiated an emergency descent and instructed the flight attendant to prepare the cabin, though they noted that smoke levels appeared to stabilize after the initial burst. The aircraft landed safely at Sydney airport 23 minutes after the smoke was first detected. Emergency services met the aircraft on the runway, and passengers deplaned through the main cabin door without incident.
The investigation
Investigators examined the aircraft and found no evidence of fire or smoke damage within the forward equipment bay. However, a closer inspection of the right front windshield revealed significant damage. A terminal block on the lower outer corner of the window was heavily heat-damaged, and the inner glass ply had cracked.
Technical analysis of the window components revealed that the inner glass ply had experienced melting and a crater-like depression directly beneath the J5 terminal. The high temperatures associated with the arcing were sufficient to melt the glass and cause the thermal stresses that cracked the windshield. While the power connector showed some heat-related changes, such as resolidified cadmium, there was no evidence of arcing at the connector itself.
Engineers also examined the copper braid conductors. Evidence showed that some ends of the remaining copper braid were melted, which is consistent with the arcing observed by the pilots. Although the exact cause of the initial failure could not be definitively isolated due to the extent of the damage, investigators noted that the copper braids likely had to separate to initiate the arcing. It was speculated that the braids might have been nicked during the manufacturing process, potentially allowing them to flex and eventually separate.
Findings
- An electrical failure occurred under the J5 terminal block on the right front window, resulting in localized arctuating.
- This arcing caused the combustion of the epoxy terminal block, which produced the smoke observed in the cockpit.
- The intense heat from the arcing caused the inner glass ply of the windshield to melt and crack.
- The circuit breakers for the window heat circuit did not trip, and power to the system was not disconnected until after the aircraft had landed.
Safety action
Boeing issued an In Service Action Report to the airline industry, providing a summary of the event and the findings from the component teardown. Additionally, Boeing suggested that operators should remove power to the window heat circuit immediately upon detecting any smoke or arcing to prevent further heat generation and to preserve the components for more effective forensic analysis.