GAA16CA201 · Helicycle/Morrisey RR RM-1

According to the pilot, after a local flight in his experimental amateur built helicopter, he rejoined the pattern at the towered airport that he departed thirty minutes prior. The runway in use was 08, and the pilot requested to land on runway 17 in order to practice hovering. The pilot reported that he executed multiple approaches to runway 17, and requested a wind check from the tower. During the accident approach tower reported, "Wind 090 at 03." The pilot affirmed that as he made his turn from base to final his airspeed was slow. He reported that during his descent at slow airspeed, with a direct, left crosswind, the helicopter yawed right. He asserted that he lowered the collective, and applied forward cyclic which had no effect on the uncommanded rapid right yaw. Finally, the pilot reported that "application of left pedal throughout made no difference in correcting the aircraft heading." The helicopter made several 360 degree turns and just prior to the helicopter ground impact; the pilot reported that he increased the collective. The helicopter sustained substantial damage to the frame and windscreen.

The pilot reported that the accident occurred about 1600 mountain daylight time (MDT). The Automated Terminal Information Service reported that wind conditions at 1547 (MDT) were 120 degrees true at 10 knots. The pilot remarked that after exiting the helicopter, as he walked away from the wreckage, he noticed that the wind was variable and gusting.

The pilot reported that there were no mechanical failures or anomalies with the airplane prior to or during the flight that would have prevented normal flight operation.

According the Federal Aviation Administration (FAA) Helicopter Flying Handbook (FAA-8083-21A) section describing Loss of Tail Rotor Effectiveness (LTE), and more specifically FAA Advisory Circular (AC) 90-95 Unanticipated Rapid Right Yaw (pg. 1, para. 4.a.):

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.

7. b. Although specific wind azimuths are identified for each region, the pilot should be aware that the azimuths shift depending on the ambient conditions. The regions do overlap. The most pronounced thrust variations occur in these overlapping areas.

c. These characteristics are present only at airspeeds less than 30 knots and apply to all single rotor helicopters. Flight test data has verified that the tail rotor does not stall during this period.

d. The aircraft characteristics and relative wind azimuth regions are:

(1) Main rotor disc vortex interference (285" to 315"). (See figure 1.)

(a) Winds at velocities of about 10 to 30 knots from the left front will cause the main rotor vortex to be blown into the tail rotor by the relative wind. The effect of this main rotor disc vortex is to cause the tail rotor to operate in an extremely turbulent environment.

(3) Tail rotor vortex ring state (210" to 330").

(a) Winds within this region will result in the development of the vortex ring state of the tail rotor. As the inflow passes through the tail rotor, it creates a tail rotor thrust to the left. A left crosswind will oppose this tail rotor thrust. This causes the vortex ring state to form, which causes a nonuniform, unsteady flow into the tail rotor. The vortex ring state causes tail rotor thrust variations which result in yaw deviations. The net effect of the unsteady flow is an oscillation of tail rotor thrust. This is why rapid and continuous pedal movements are necessary when hovering in left crosswind.

Verbatim NTSB analysis and probable cause from the NTSB dataset