The helicopter flight instructor reported that he was providing instruction to a student pilot during a cross country flight. During the flight, the instructor asked the student to perform a land as soon as possible emergency procedure.

The student pilot approached the hillside, landing site from the south. The wind was out of the west and the helicopter descended and decelerated below effective translational lift (ETL). According to the FAA 8083-21A, The Helicopter Flying Handbook, pg. 2-20, para. 2, ETL occurs between 16 and 24 knots.

The student pilot reported that, "We descended below ETL, maybe 10 feet off the ground and still descending. By this point we were what I perceived to be straight, and the instructor took the controls. From what I could tell, he used forward cyclic and left pedal immediately. It was too late."

The instructor reported that, "I took the flight controls right as we slowed below ETL. The helicopter started to develop a hard right yaw and I immediately gave full forward cyclic."

The helicopter developed an uncontrollable rapid right yaw and spun about two revolutions. The helicopter touched down on the skids and rolled on to its left side. The helicopter sustained substantial damage to the tail rotor drive shaft, the main and tail rotor blades.

The pilot reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.

According to the Federal Aviation Administration Helicopter Flying Handbook (FAA-8083-21A) and The Helicopter Instructors Flying Handbook (FAA-8083-4) and Advisory Circular (AC) 90-95 Unanticipated rapid right yaw:

Loss of Tail Rotor Effectiveness (LTE) is a critical; low-speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside of its own accord and, if not corrected, can result in the loss of aircraft control.

AC 90-95 Section 7.d.3. (page 7) defines flight characteristics and wind azimuths associated with LTE. It states that tail rotor vortex ring state occurs when the wind is out of (210° to 330°).

1. Winds within this region will result in the development of the vortex ring state of the tail rotor.