Incident Overview
A helicopter flight instructor was providing instruction to a student pilot during a cross-country flight. The instructor asked the student to perform a land-as-soon-as-possible emergency procedure. The student pilot approached the selected hillside landing site from the south. The wind was out of the west.
Descent and Loss of Control
During the approach, the helicopter descended and decelerated below effective translational lift (ETL). According to the FAA Helicopter Flying Handbook (FAA-8083-21A), ETL occurs between 16 and 24 knots.
The student pilot reported: "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: "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. It touched down on the skids and rolled onto its left side.
Damage and Mechanical Condition
The helicopter sustained substantial damage to the tail rotor drive shaft, the main rotor blades, and the tail rotor blades. The pilot reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.
Background Information on LTE
The Federal Aviation Administration Helicopter Flying Handbook (FAA-8083-21A), the Helicopter Instructors Flying Handbook (FAA-8083-4), and Advisory Circular (AC) 90-95 describe Loss of Tail Rotor Effectiveness (LTE) as a critical low-speed aerodynamic flight characteristic that can result in an uncommanded rapid yaw rate which does not subside on its own and, if not corrected, can lead to loss of aircraft control. AC 90-95 Section 7.d.3 notes that tail rotor vortex ring state occurs when the wind is out of 210° to 330°.