Accident Overview
On July 13, 2019, about 1430 mountain daylight time, a Bell 47G-3B-1 helicopter, registration N3078G, was substantially damaged during an approach near Mosby, Montana. The pilot and one passenger sustained serious injuries, while another passenger received minor injuries. The helicopter was operating under Title 14 Code of Federal Regulations Part 91 as an aerial observation flight.
Flight Details
Before departing on the 15-mile flight, the pilot fueled the helicopter, bringing total fuel to about 58 gallons. Upon arriving at the destination, the pilot circled the landing area to assess wind conditions, concluding that the wind was light and from the west. He then entered a normal approach into the wind to land near a building where the helicopter would be stored.
Sequence of Events
During the approach, the helicopter entered an uncommanded yaw to the right. Despite the pilot's control inputs, including full left pedal application, the yaw continued. The pilot attempted recovery by applying forward cyclic to gain airspeed and perform a go-around. He recalled lowering the collective when the event began but could not recall adjusting it during the go-around. The controls felt ineffective, and the helicopter descended straight down, impacting the ground hard.
One passenger reported the helicopter pitched side to side, then the tail came around twice. The pilot tried to climb, but the helicopter continued spinning. Another passenger recalled the helicopter swinging right and ascending above the building. A witness near the building observed the helicopter approach from the east, appearing normal until it was about 30–40 ft above ground. The witness stated the helicopter turned right, left, then right again before ascending and flying overhead. A photograph taken by the witness captured the helicopter as it began the right yaw.
Postaccident Findings
The helicopter came to rest in a level attitude in a silage pit about 250 ft north of the intended landing area, at an elevation of 2,856 ft above mean sea level. All major components were found in the debris area.
Postaccident examination of the airframe revealed no preaccident mechanical malfunctions or failures that would have precluded normal operation. The engine was test run at a Rolls-Royce authorized facility and operated normally with no malfunctions.
Pilot Information
The pilot had about 1,092 hours of flight experience in reciprocating engine helicopters. After being hired, he began training on the turbine-powered Bell 47G-3B-1, receiving 18 hours of ground instruction, 4 hours of flight training, and 6 hours of solo flight time. He accrued an additional 15.5 flight hours on aerial observation flights before the accident.
Weather Conditions
An automated weather station at Lewiston Municipal Airport, about 84 miles west, reported wind from 080° at 7 knots, 10 statute miles visibility, clear skies, temperature 31°C, dew point 12°C, and an altimeter setting of 30.00 inches of mercury. Calculated density altitude at the accident site was about 5,406 ft msl. Another station about 5 miles northeast reported wind from 160° at 7 knots, temperature 31°C, dew point 15°C.
Loss of Tail Rotor Effectiveness (LTE)
The FAA Helicopter Flying Handbook describes LTE, or "uncontrolled right yaw," as a condition occurring when airflow through the tail rotor is altered, affecting antitorque capability. Factors include out-of-ground-effect operations with high-power demand below 30 knots airspeed at low altitudes. Recovery technique involves applying full left pedal, forward cyclic to increase speed, and reducing power if altitude permits; autorotation may be considered if ground contact is imminent.