What happened
On 29 April 2019, a Boeing 767-322, registration N657UA, was climbing through 15,000 feet after departing from London Heathrow Airport for Chicago O’Hare. During the climb, cabin crew notified the commander of an electrical burning smell, haze, and smoke appearing near Row 20.
The crew declared a PAN to London Air Traffic Control and received clearance to return to Heathrow. While performing the 'smoke and fumes' checklist, the crew switched the utility bus switches to the off position, which caused the visible smoke and haze to disperse, though the smell of burning persisted. Due to the nature of the flight, the commander prepared for an overweight landing. Following the landing, the aircraft stopped at a remote stand to allow the airport fire service to inspect the cabin. There were no injuries to the 86 passengers or 11 crew members on board.
The investigation
Following the incident, the operator inspected the area near the reported smoke. An examination of the cabin ceiling lighting harness above the left side of Rows 19 and 20 revealed evidence of heat damage and soot on the wire insulation. One passenger also reported seeing a brief flash of light above the overhead panels during the climb.
Investigators found that the circuit breaker protecting the circuit had tripped. The lighting ballast and the circuit breaker were removed and underwent functional testing by the manufacturer, both of which yielded successful results.
Findings
- The smoke and haze were produced by electrical arcing between two insulated wires within a cabin ceiling lighting harness.
- This electrical event generated enough heat to cause localized melting of the copper strands within the wiring.
- The specific cause of the initial electrical event could not be determined.