Airwest Helicopters MD Helicopter 369D accident

The helicopter pilot was flying passengers and equipment to a landing zone on a peak with an elevation of 9,993 feet. He made one high pass around the summit and assessed the wind as light and variable. He made a right circling approach and slowed from 60 knots to 40 knots. The airspeed suddenly decreased to 20 knots and the helicopter yawed 90 degrees to the right. He temporarily stabilized the situation, but then the helicopter went into a violent and uncontrollable right spin. It spun down vertically until it collided with terrain, rolled onto its right side, and slid about 75 yards before coming to a stop facing uphill. One of the passengers held a rotorcraft/helicopter pilot certificate. He indicated that on the first landing attempt the pilot appeared to be running out of left pedal. The second attempt was at a different angle to the landing zone, but once again the pilot seemed to run out of left pedal and the helicopter began to spin. The pilot could not recover during the attempt and the helicopter made three turns before colliding with the terrain. The Rotorcraft Flying Handbook refers to unanticipated yaw that does not subside on its own as a loss of tail rotor effectiveness. It notes that, if not corrected, this can lead to a loss of control. It may occur in all single-rotor helicopters at airspeeds less than 30 knots. A common cause is insufficient tail rotor thrust for a given power setting at higher altitudes, where tail rotor thrust and efficiency are reduced. Another potential cause is encountering a quartering tailwind from certain directions.

Verbatim NTSB analysis and probable cause from the NTSB dataset