Serious incident Boeing 747SP N148UA, Monday 29 April 1991

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Date:	Monday 29 April 1991
Time:	12:00 LT
Type:	Boeing 747SP
Owner/operator:	United Airlines
Registration:	N148UA
MSN:	21648/367
Year of manufacture:	1979
Engine model:	P&W JT9D-7ASP
Fatalities:	Fatalities: 0 / Occupants: 102
Other fatalities:	0
Aircraft damage:	Minor
Category:	Serious incident
Location:	Churchville, NY - United States of America
Phase:	En route
Nature:	Unknown
Departure airport:	Newark, NJ (KEWR)
Destination airport:	(KTYO)
Investigating agency:	NTSB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
THE BOEING 747SP HAD JUST LEVELED OFF AT FL350 AND WAS ACCELERATING FROM M.81 TO M.85. DURING THE ACCELERATION, A VIBRATION WAS FELT IN THE COCKPIT AND CABIN. NO ABNORMAL ENGINE READINGS WERE OBSERVED. THE VIBRATION LASTED FOR ABOUT 2 MINUTES AND WAS FOLLOWED BY A LOUD BANG HEARD IN THE COCKPIT. THE NUMBER TWO ENGINE REVEALED A FAILED SIXTH ROTATING AIRSEAL. METALLURGICAL EXAMINATION OF FAILED AIRSEAL SHOWED FEATURES TYPICAL OF OVERSTRESS WITH NO INDICATION OF PREEXISTING CRACKS.

Probable Cause: A FAILURE OF THE SIXTH ROTATING AIRSEAL WHICH LED TO AN UNCONTAINED ENGINE FAILURE.

Accident investigation: