LAX00LA089

During landing approach at the completion of a solo cross-country flight, the student pilot was slowing below 30 knots to enter a hover when the helicopter began spinning to the right. The student was unable to arrest the right yaw and entered an autorotation, which terminated in a rotating collision with terrain 137 feet short of the runway. He was attempting to land with a left crosswind and/or quartering tailwind from a direction and at a speed conducive to a tail rotor vortex ring state condition. The condition results in tail rotor thrust variations, which can require rapid and continuous pedal movements to maintain heading and cause unanticipated right yaw rates to develop. If the yaw rate is not controlled immediately, the helicopter can rotate into a wind azimuth region where weather cock instability will accelerate the right yaw. This condition will be aggravated at airspeeds below 30 knots when the loss of translational lift results in an increased power demand (more torque) and a corresponding increase in antitorque requirement. The student's total flying experience and pilot-in-command time was about 60.6 and 5.2 hours, respectively.

On January 30, 2000, about 0830 hours Hawaiian standard time, a Robinson R22B Beta, N62DF, operated by Mona Loa Helicopters, Kailua-Kona, Hawaii, experienced an in-flight loss of control on approach for landing. The helicopter touched down hard at the Hana Airport, on the island of Maui. The helicopter was substantially damaged, and the student pilot was seriously injured during the solo instructional flight. Visual meteorological conditions prevailed, and a visual flight rules flight plan was filed. The flight was conducted under the provisions of 14 CFR Part 91, and it originated from Hawi, Hawaii, about 0800.

The operator reported that, at the time of the accident, the pilot's total flying experience and pilot-in-command flight time was about 60.6 and 5.2 hours, respectively. The pilot's flight instructor reported that his student was in the final preparation process of applying for a private pilot certificate.

In the pilot's written statement, he did not report experiencing any mechanical malfunctions during the en route portion of the cross-country flight between the Big Island (Hawaii) and the island of Maui. He indicated that after arrival over the Hana airport, he entered the traffic pattern to runway 26 and noted the existence of a southerly wind. The pilot indicated that when he was between 50 and 75 feet above ground level, at less than 30 knots airspeed, the helicopter suddenly began spinning clockwise. The spinning continued despite his application of left pedal during his attempt at stopping the rotation. The rotation continued, and he then reduced the collective to enter an autorotation; however, did not reduce the throttle. This also had no effect at reducing the direction of the spinning. A moment prior to impacting the ground, the pilot increased the collective to flare, and the rate of spinning increased again.

The Federal Aviation Administration (FAA) inspectors responded to the accident site, examined the helicopter, and interviewed the pilot. The helicopter impacted terrain about 137 feet from the runway. Components separated from the helicopter and were located within approximately a 50-foot radius of the main wreckage. The FAA noted that as the pilot approached runway 26 he would likely have experienced a right crosswind and/or tailwind. The pilot indicated that his ground speed seemed fast. The reported wind about 30 minutes after the accident was from 150 degrees, at 8 knots, or 260 degrees relative bearing from the nose of the helicopter. At no time during the flight did the pilot report having felt any unusual vibration, or having heard any unusual engine sound or the low rotor rpm warning horn. The FAA reported observing no evidence of any preexisting mechanical malfunction with the helicopter.

The FAA publication, ROTORCRAFT FLYING HANDBOOK (FAA-H-8083-21), describes a Loss of Tail Rotor Effectiveness (LTE) phenomena called Tail Rotor Vortex Ring State. Winds coming from an azimuth range from 210 degrees to 330 degrees relative to the helicopter's nose at speeds from 8 to 12 knots can induce a tail rotor vortex ring state condition. The condition results in tail rotor thrust variations, which can require rapid and continuous pedal movements to maintain heading and cause unanticipated right yaw rates to develop. If the yaw rate is not controlled immediately, the helicopter can rotate into a wind azimuth region where weathercock instability will accelerate the right yaw. This condition will be aggravated at airspeeds below 30 knots when the loss of translational lift results in an increased power demand (more torque) and a corresponding increase in antitorque requirement.

Robinson Helicopter Company management verbally reported to the Safety Board investigator that the tail rotor is driven by a takeoff from the main rotor transmission, and if the main rotor rpm were allowed to decrease below operational limits, the tail rotor's thrust capability would be significantly decreased. Thereafter, if a pilot were to respond by increasing the throttle, the increased torque reaction during application of the throttle might produce a clockwise helicopter rotation.