On December 8, 1998, a Hughes 369D (registration N1096L) was involved in an aviation accident near Taholah, WA. Investigators recorded the probable cause as: 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. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter sustained substantial damage during external load operations after a loss of engine power, autorotation, and forced landing. Postaccident inspection revealed excessive blending and a sharp notch on the first stage turbine wheel.
Incident Summary
During external load operations, the helicopter experienced a loss of engine power. The pilot performed an autorotation and forced landing, during which the helicopter sustained substantial damage.
Witness Report
A witness reported hearing a loud bang, followed by the engine spooling down. The witness also observed smoke coming from the engine as the helicopter descended through trees. The helicopter landed hard on a tree stump located under the pilot's seat area.
Inspection Findings
Postaccident inspection of the turbine engine revealed excessive blending in the web of the first stage turbine wheel. Within the blended area, a notch exhibited a radius sharper than allowed by blend limits. The manufacturer's metallurgists believed the notch was created by a hand-held grinder. It could not be determined whether the manufacturer of the turbine wheel or an overhaul facility made the grinding marks.