Incident Overview
A Boeing 747SP, after leveling off at flight level 350, began accelerating from Mach 0.81 to Mach 0.85. During the acceleration, a vibration was felt in both the cockpit and cabin. No abnormal engine readings were observed at the time. The vibration persisted for approximately two minutes, after which a loud bang was heard in the cockpit.
Engine Findings
Inspection revealed that the number two engine had a failed sixth rotating airseal. The airseal was subjected to metallurgical examination, which showed features typical of overstress. No indication of preexisting cracks was found.
Conclusion
The event was caused by the failure of the sixth rotating airseal in the number two engine, attributed to overstress without any preexisting cracks.