Event Description
While in cruise flight at flight level 400, the right wing overheat light illuminated. The crew executed the appropriate checklist. Approximately five minutes later, the engine number three and engine number four B fire loop indicators illuminated. Shortly thereafter, the flight crew detected an acrid odor and observed smoke emanating from behind the P-6 circuit breaker panel. The crew identified a small fire in the fire detection card file, which they extinguished using a hand-held fire extinguisher.
Crew Actions and Diversion
After extinguishing the fire, the crew diverted the aircraft to a nearby airport and completed a precautionary landing without further incident.
Investigation Findings
The post-event investigation revealed that the fire detection card for engine number four B Loop fire detection (card A16), along with its associated socket connector, had sustained severe thermal damage, including partial destruction. Subsequent tests and examinations found no evidence of any specific anomalies in the A16 card, the card file, the card file protection circuit, or any of the aircraft's systems. Additionally, there was no evidence indicating a lightning strike as the initiating event. The aircraft has continued in service since the incident without generating any other maintenance discrepancies known to be directly related to the fire detection card files.