NTSB Identification: SEA99FA018.
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Accident occurred Tuesday, December 08, 1998 in TAHOLAH, WA
Probable Cause Approval Date: 09/28/2000
Aircraft: Hughes 369D, registration: N1096L
Injuries: 1 Minor.
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.
While conducting external load operations the helicopter sustained substantial damaged after a loss of engine power, autorotation, and forced landing. A witness reported hearing a loud bang, followed by the engine spooling down. The witness also reported seeing smoke coming from the engine as the helicopter descended through the trees, landing hard on a tree stump under the pilot's seat area. Postaccident inspection of the turbine engine revealed excessive blending in the web of the first stage turbine wheel. Within the blending, a notch exhibited a radius sharper than allowed by blend limits, which the manufacturer' metallurgists believed to have been created by a hand-held grinder. It could not be determined whether the manufacturer of the turbine wheel or an overhaul facility made the subject grinding marks.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: Turbine wheel failure as the result of a low-cycle fatigue crack that initiated in a beyond-limits grinding gouge. Factors include improper maintenance by unknown parties and tree stumps that prohibited a successful emergency landing. Full narrative available
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