NTSB Identification: NYC90FA156.
The docket is stored on NTSB microfiche number 43197.
Nonscheduled 14 CFR
Accident occurred Tuesday, July 10, 1990 in TETERBORO, NJ
Probable Cause Approval Date: 11/23/1992
Aircraft: BELL 206 B3, registration: N73LM
Injuries: 3 Uninjured.

NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.

THE PILOT WAS HOVERING WHEN HE HEARD A WHINE IN THE ENGINE THAT INCREASED IN INTENSITY. HE SAW TORQUE FLUCTUATIONS, HEARD AN EXPLOSION, AND AUTOROTATED TO THE RAMP. INVESTIGATION SHOWED THE POWER TURBINE INNER SHAFT FAILED IN TORSION DUE TO CORROSION ON THE INNER SURFACE AND THIS ALLOWED THE THIRD STAGE TURBINE WHEEL TO GO INTO OVERSPEED AND FAIL. ALLISON ISSUED THREE SERVICE BULLETINS ON THE POWER TURBINE INNER SHAFT. NONE OF THEM USED CALENDAR TIME FOR COMPLIANCE, ONLY ENGINE TOTAL TIME. THE HELICOPTER WAS 11 YEARS OLD WITH 2146 HRS TT AIRFRAME AND ENGINE. THE FIRST SCHEDULED WORK ON THE POWER TURBINE OCCURS AT 3550 HRS.

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

FAILED POWER TURBINE SHAFT. FACTORS IN THE ACCIDENT WERE CORROSION ON THE POWER TURBINE SHAFT AND INADEQUATE MANUFACTURER'S SERVICE BULLETINS.

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