NTSB Identification: NYC91IA133.
The docket is stored on NTSB microfiche number 45046.
Scheduled 14 CFR operation of UNITED AIRLINES
Incident occurred Monday, April 29, 1991 in CHURCHVILLE, NY
Probable Cause Approval Date: 05/03/1993
Aircraft: BOEING 747SP, registration: N148UA
Injuries: 102 Uninjured.

NTSB investigators used data provided by various sources and may not have traveled in support of this investigation to prepare this aircraft incident report.

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.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

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

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