What happened
While operating in cruise flight at flight level 400, the crew of the Boeing 747 observed the right wing overheat light illuminate. Following the completion of the appropriate checklist, the fire loop indicators for engine number three and engine number four B illuminated approximately five minutes later.
Shortly after these indications, 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 within the fire detection card file and extinguished it using a handheld fire extinguisher. The aircraft subsequently diverted to a nearby airport, where the crew completed an uneventful precautionary landing. There were no injuries reported.
The investigation
Post-event examination revealed that the fire detection card for engine number four B Loop fire detection (card A16) and its associated socket connector had sustained severe thermal damage, including partial destruction.
Subsequent testing and examinations of the A16 card, the card file, the card file protection circuit, and other aircraft systems failed to identify any specific anomalies. Investigators found no evidence that a lightning strike served as the initiating event. The aircraft has remained in service since the incident without further maintenance discrepancies related to the fire detection card files.