ASN Wikibase Occurrence # 291598
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| Date: | Sunday 26 November 2006 |
| Time: | 17:30 LT |
| Type: |  Bell 206L-3 |
| Owner/operator: | Air Evac Ems, Inc. |
| Registration: | N805SB |
| MSN: | 51192 |
| Year of manufacture: | 1986 |
| Total airframe hrs: | 10604 hours |
| Engine model: | Rolls-Royce 250-C30P |
| Fatalities: | Fatalities: 0 / Occupants: 3 |
| Aircraft damage: | Substantial |
| Category: | Accident |
| Location: | Brownwood, Texas -
 United States of America
|
| Phase: | Unknown |
| Nature: | Ferry/positioning |
| Departure airport: | Abilene Regional Airport, TX (ABI/KABI) |
| Brownwood, TX | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
The helicopter pilot experienced a sudden "explosion" followed by an uncommanded left yaw while initiating an approach. During the autorotation the pilot was forced to increase collective pitch in order to clear a berm resulting in a loss of main rotor RPM. The helicopter landed hard and remained in the upright position. A review of the turbine engine's maintenance records and work orders revealed the engine had been repaired and inspected in December 2005 for "a sudden stoppage." At that time, the compressor, gearbox, and turbine were replaced. A teardown examination of the engine revealed that the aft end of the turbine-to-compressor coupling had fractured in the region where it mated with the turbine splined adapter. The coupling fracture resulted in a loss of continuity in the N1 drive train and an overspeed failure of the first stage turbine wheel. Metallurgical examination of the fracture surfaces of the remnants of the first stage turbine wheel revealed there was no fatigue fractures. Examination of the splines in the aft end of the coupling and on the pieces from the fractured section revealed fretting and a fractured spline tip in the area of contact with the turbine spline adapter. There was non-uniform wear on 16 consecutive external splines on the turbine splined adapter and there was also non-uniform wear of the splines in the area where the adapter mated with the aft end of the turbine-to-compressor coupling. When the engine was disassembled, there was a discrepancy noted between the thickness of the shims (used for alignment of the gas producer and power turbine drive trains) on the mounting pad between the exhaust collector support and the accessory gearbox than what had been entered in the engine logbook or what was etched on the exhaust collector support. In addition, the balance marks on the spur adapter gear shaft and the turbine-to-compressor coupling were not aligned. According to the Rolls Royce 250-C30 series Overhaul Manual, the balance marks were required to be aligned during re-assembly of the engine. The Overhaul Manual also stated that the cracks found on the vertical fire shield could have been as a result of "abnormally high engine vibration."
Probable Cause: The fracture of the turbine-to-compressor coupling and subsequent overspeed failure of the first stage turbine wheel as result of the improper re-assembly of the engine by maintenance personnel. A contributing factor was the lack of suitable terrain for the forced landing.
Accident investigation:
 |
|
Sources:
NTSB DFW07LA027
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 07-Oct-2022 16:40 | ASN Update Bot | Added |
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