Bell 47G-4A N124SH accident near Rochester, Minnesota

Probable cause

The total loss of engine power due to a high-cycle fatigue fracture of a first-stage compressor blade. The reason for the fracture could not be determined during postaccident examination.

NTSB analysis

A witness reported that the agricultural helicopter was refueled and that the hopper was loaded with chemical before departure. He reported that he saw the helicopter depart, that the engine sounded normal, and that the helicopter's departure and flightpath appeared normal. He then saw a "brown puff of smoke" come from the helicopter, and it subsequently descended and impacted a hill. A postaccident engine examination revealed that the compressor rotor assembly had failed. The metallurgical examination of the compressor rotor assembly revealed that a single first-stage compressor blade had fractured in high-cycle fatigue. The microstructure, hardness, and chemistry of the first-stage compressor wheel conformed to the requirements of the engineering drawing. No evidence of corrosion was found near the fracture origin. The engine was installed on the helicopter during the last annual maintenance inspection conducted 3 days before the accident. Evidence indicated that the engine was installed properly. The reason for the fracture could not be determined.

Text from the NTSB. Open the NTSB docket for CEN10LA431 for the full record.