***This report was modified on February 20, 2018, and April 19, 2018. Please see the docket for this accident to view the original report.***

On June 9, 2017, about 0820 mountain daylight time, a Robinson R44 II helicopter, N590GG, impacted terrain near Douglas, Wyoming. The pilot and two passengers were not injured, the third passenger received serious injuries, and the helicopter was substantially damaged during the accident. The helicopter was registered to and operated by Wagonhound Land and Livestock Co, LLC, under the provisions of 14 Code of Federal Aviation Regulations Part 91. Visual meteorological conditions prevailed at the time.

The pilot reported that he topped the fuel off at the Converse County airport (KDGW); then he and one other person departed the airport with calm winds. They traveled about 25 miles to a lodge to pick up two additional passengers. The pilot shut the helicopter down, for the passengers to board; while there, he estimated there was a steady west wind at 12-15 knots. They then departed the lodge and did some sightseeing en route to a ranch/stockyard. The pilot reported that he climbed to 1,000 ft as they passed KDGW. The automated weather station located at the KGDW was reporting wind, variable at 3 knots.

As they approached the cattle stockyards, they looked at crop status, fields, building irrigation machinery, etc. During that time, the pilot looked at trees, pond surfaces, and ground track which indicated a calm wind condition. The pilot then selected an approach course for a landing, which would avoid a barn and work crews; aiming for clear spot near water tanks.

During the approach, the pilot noted that the power and glide path indications were normal, he noticed the last airspeed was at 30 kts and 100 ft agl. The pilot added that "somewhere below 50 ft agl" there was a sudden, abrupt, high rate of descent; he manipulated the controls and the helicopter's low rpm light illuminated and the warning horn sounded. He continued, saying there was no time or altitude to lower the collective and regain rpm. The helicopter impacted terrain and rolled to the side when the left skid failed. The pilot added that after the accident, the engine continued to run and there was no prior mechanical malfunction/failure with the helicopter. He noted that he observed that there was a steady northwest wind estimated at 15 kts, a downwind condition for the approach he had flown. He stated that he believed the high rate of descent was due to the helicopter encountering "an advance gust" or "wind shear" as the wind shifted from calm to the tailwind.

Based on the conditions at the time of the accident, the calculated density altitude was 7,015 ft, and the helicopter was near the maximum gross weight of 2,500 lbs.

The Helicopter Flying Handbook (FAA-H-8083-21A), Chapter 11, "Helicopter Emergencies and Hazards," with regard to "Settling with Power (Vortex Ring State)" states the following:

Vortex ring state describes an aerodynamic condition in which a helicopter may be in a vertical descent with 20 percent up to maximum power applied, and little or no climb performance. The term "settling with power" comes from the fact that the helicopter keeps settling even though full engine power is applied.

…Tip vortices generate drag and degrade airfoil efficiency. As long as the tip vortices are small, their only effect is a small loss in rotor efficiency. However, when the helicopter begins to descend vertically, it settles into its own downwash, which greatly enlarges the tip vortices. In this vortex ring state, most of the power developed by the engine is wasted in circulating the air in a doughnut pattern around the rotor.

…A vortex ring state may be entered during any maneuver that places the main rotor in a condition of descending in a column of disturbed air and low forward airspeed. Airspeeds that are below translational lift airspeeds are within this region of susceptibility to settling with power aerodynamics.

…Some of the situations that are conducive to a settling with power condition are: any hover above ground effect altitude, specifically attempting to hover [out-of-ground-effect] OGE at altitudes above the hovering ceiling of the helicopter, attempting to hover OGE without maintaining precise altitude control, pinnacle or rooftop helipads when the wind is not aligned with the landing direction, and downwind and steep power approaches in which airspeed is permitted to drop below 10 knots depending on the type of helicopter....

The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25B), Chapter 12, "Weather Theory, Low-Level Wind Shear, page 2-11 states the following:

"Wind shear is dangerous to an aircraft. It can rapidly change the performance of an aircraft and disrupt the normal flight attitude. For example, a tailwind quickly changing to a headwind causes an increase in airspeed and performance. Conversely, a headwind changing to a tailwind causes a decrease in airspeed and performance. In either case, a pilot must be prepared to react immediately to these changes to maintain control of the aircraft."