Bell UH 1H N107CH accident near Oakdale, California

Probable cause

The pilot's loss of control during a precautionary landing following an abnormal noise in-flight, the source of which could not be determined.

NTSB analysis

During a cross-country flight, the helicopter started to produce an unusual noise. The pilot initiated a precautionary landing and the noise became louder. About 5 ft off the ground before touchdown, the helicopter entered an uncontrolled climb and yaw. The pilot was unable to recover from the loss of directional control and the helicopter subsequently impacted terrain.

Examination of fractured pieces from the main rotor drive shaft and the swashplate assembly exhibited features consistent with fracture from overstress; however, it is likely that this damage was sustained during the impact. Although it is possible that one of the components may have failed from overstress in-flight, this would have required an abnormally high load input to one or more components. The exam did not reveal any evidence of mechanical malfunctions or anomalies that would have precluded normal operation of the helicopter, and the source of the noise reported by the pilot could not be determined.

Text from the NTSB. Open the NTSB docket for WPR18LA206 for the full record.