GAA17CA132 · Bell Helicopter Textron 206 L1

The flight instructor on the controls of the high skid-equipped-landing-gear helicopter reported that he was the pilot-in-command (PIC). The PIC reported that he and another flight instructor were performing simulated emergency procedures during the flight. He reported that he attempted to demonstrate a simulated fixed-pitch (right stuck antitorque pedal) emergency procedure. He reported that, during the maneuver, the nose of the helicopter was about 40° nose right of centerline. He reduced the throttle, and the nose corrected to about 20° nose right of centerline. The helicopter touched down on taxiway delta with minimal forward airspeed, and it then bounced about 5 ft above the ground and yawed right about 1 3/4 turns. The helicopter touched down a second time about 65 ft south of the taxiway centerline and rolled onto its left side. The helicopter sustained substantial damage to the firewall, main rotor drive system, and tail rotor drive system.

A METAR at the time of the accident reported that the wind was from 110° at 08 kts. The flight instructor seated in the right seat reported that the wind at the time of the accident was from 090° at 08 kts.

When the PIC was asked by the National Transportation Safety Board investigator-in-charge if he placed the collective in the full-down position after touchdown or if he increased the collective after the initial touchdown, he responded that he could not remember. When asked if he applied full left pedal to combat the right yaw, he said that he did not because the event happened quickly.

According to the Federal Aviation Administration Helicopter Flying Handbook (FAA-8083-21A), the Helicopter Instructor's Flying Handbook (FAA-8083-4), and Advisory Circular (AC) 90-95 "Unanticipated Rapid Right Yaw in Helicopters," the loss of tail rotor effectiveness is a critical, low-speed aerodynamic flight characteristic that can result in an uncommanded rapid yaw rate that 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 and states that the tail rotor vortex ring state occurs when the wind is from 210° to 330°.

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

AC 90-95, Section 10, "Recommended Recovery Techniques," (page 8), states:

a. If a sudden unanticipated right yaw occurs, the pilot should perform the following:

(1) Apply full left pedal. Simultaneously, move cyclic forward to increase speed. If altitude permits, reduce power.

(2) As recovery is effected, adjust controls for normal forward flight.

b. Collective pitch reduction will aid in arresting the yaw rate but may cause an increase in the rate of descent. Any large, rapid increase in collective to prevent ground or obstacle contact may further increase the yaw rate and decrease rotor rpm.

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

Verbatim NTSB analysis and probable cause from the NTSB dataset