Bell BHT407 N937GR accident near Big Fork, Montana

Probable cause

The pilot’s failure to maintain helicopter control while maneuvering over mountainous terrain in strong up/downdrafts and possible mountain wave activity, which resulted in a loss of tail rotor effectiveness.

NTSB analysis

The pilot reported that, while conducting a reconnaissance orbit over mountainous terrain, he crossed over a ridgeline and then lost directional control of the helicopter. He applied full left pedal and lowered the nose and collective in an attempt to regain control. The helicopter subsequently impacted the uneven snow-covered terrain and rolled onto its right side.

Postaccident examination of the airframe and engine revealed no evidence of mechanical malfunction or failures that would have precluded normal operation. A weather study for the area around the accident site revealed that the temperature profile and windy environment supported strong up/downdrafts with the potential for mountain wave formation near the altitude at which the flight was maneuvering. It is likely that the pilot was unable to maintain control of the helicopter as a result of a loss of tail rotor effectiveness while operating in the high-altitude environment and in an area with strong up/downdrafts and possible mountain wave activity.

Text from the NTSB. Open the NTSB docket for WPR13TA205 for the full record.