Bell 47G 3B 1 N3078G accident near Mosby, Montana

Probable cause

The pilot’s failure to account for environmental conditions conducive to a loss of tail rotor effectiveness, which resulted in a loss of tail rotor effectiveness and subsequent loss of control. Contributing to the accident was the pilot’s inappropriate application of power during the recovery.

NTSB analysis

The pilot had refueled the helicopter, bringing the total fuel on board to about 58 gallons, and departed with two passengers for a 15-mile flight to a private ranch. After arriving at the destination, the pilot circled the area and determined the wind direction. He established a normal approach path into the wind to land near a building. While about 30 to 40 feet above ground level, the helicopter entered an uncommanded yaw to the right. Despite the pilot’s control inputs, which included application of full left pedal, the helicopter continued to yaw to the right. The pilot attempted to recover and applied forward cyclic to gain airspeed to go-around. The helicopter continued to yaw right and ascended above the building as described by the two passengers and a witness located near the building. During the recovery attempt, the pilot recalled initially lowering the collective when the yaw first occurred, but did not recall raising or lowering it again. The pilot lost control of the helicopter, which subsequently impacted the ground in a silage pit about 250 feet north of the intended landing area.

An engine test run and airframe examination revealed no mechanical failures or malfunctions with the helicopter that would have precluded normal operation.

During the approach, the helicopter was operating in a high-power demand condition due to high density altitude, high gross weight, and while near the out-of-ground effect boundary and at a slow airspeed. These conditions reduced the amount of yaw control effectiveness available to counter an uncommanded right yaw. Additionally, the pilot’s selected approach path placed the helicopter on the leeward side of a building and likely exposed the helicopter to disturbed air that disrupted the airflow entering the main rotor and tail rotor and initiated the uncommanded right yaw/loss of tail rotor effectiveness.

During the recovery attempt, the pilot likely increased the collective while applying full left pedal, which resulted in the ascent described by the passengers and witness. The added power from the increased collective would also have resulted in an increased right yaw rate and the subsequent loss of control.

Text from the NTSB. Open the NTSB docket for WPR19LA194 for the full record.