GAA17CA237 · Brantly B 2B

The pilot in the helicopter reported that, after 30 minutes of "hover pattern work," he decided to depart the practice area. He reported that the wind at the time of the accident was from 150° between 5 and 15 knots with 30-knot wind gusts. The pilot exited the practice area. He made a right pedal turn at 25 to 30 ft above ground level to return to the practice area. About 120° after beginning the right pedal turn, he felt a high wind gust, and the helicopter began to rotate to the right, "which I could not terminate through the controls." The helicopter descended, struck the ground, and rolled onto its left side. The helicopter sustained substantial damage to the main rotor drive system and the tail rotor drive system.

According to the Federal Aviation Administration Helicopter Flying Handbook (FAA-8083-21A), 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.

According to FAA-8083-21A, Chapter 11-20, paragraph 2:

Weathercock Stability (120–240°)

In this region, the helicopter attempts to weathervane, or weathercock, its nose into the relative wind. Unless a resisting pedal input is made, the helicopter starts a slow, uncommanded turn either to the right or left, depending upon the wind direction. If the pilot allows a right yaw rate to develop and the tail of the helicopter moves into this region, the yaw rate can accelerate rapidly. In order to avoid the onset of LTE in this downwind condition, it is imperative to maintain positive control of the yaw rate and devote full attention to flying the helicopter.

Weathercock stability is defined as a region of loss of tail rotor effectiveness (120 degree - 240 degree tailwind) that will weathervane the helicopter, and if not prevented will result in a loss of helicopter control about the horizontal axis.

The handbook further states:

Pilots who put themselves in situations where the combinations above occur should know that they are likely to encounter LTE. The key is to not put the helicopter in a compromising condition but if it does happen being educated enough to recognize the onset of LTE and be prepared to quickly react to it before the helicopter cannot be controlled.

According to AC 90-95, Section 10. a. 1-2 (page 8), Recommended Recovery Techniques:

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 affected, 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.

Per the National Transportation Safety Board Pilot Aircraft Accident Report, the pilot reported that he should have exited the practice area to the west with an immediate left turn to the south, allowed the helicopter to establish translational lift, and increased airspeed and altitude before making a turn to return to the practice area. "My initial plan was flawed from the outset, in retrospect."

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