SNOHOMISH, WA, USA
N8491E
Bell 47G-3B
The pilot was in a commercial helicopter pilot upgrade training program. He was not yet rated for helicopters, but did hold an instructor's endorsement granting helicopter solo privileges. At the time of the accident, he was aborting a practice landing in wind conditions which were conducive to an unanticipated right yaw (also known as loss of tail rotor effectiveness, or LTE) event. As he completed a turn to downwind, the helicopter made an uncommanded right turn. Despite the application of left pedal and forward cyclic by the pilot, the helicopter 'continued to rotate violently until impact on the left side.' FAA inspectors, who examined the helicopter at the accident site, reported that they found no evidence of a mechanical malfunction; they stated that the tail rotor system was intact.
On May 15, 1997, approximately 1610 Pacific daylight time, a Bell 47G-3B helicopter, N8491E, crashed approximately 1 1/2 miles southeast of Harvey Field, Snohomish, Washington. The helicopter was substantially damaged and the pilot, who held a commercial certificate with an airplane category rating and had solo privileges for helicopters, received minor injuries. The local 14 CFR 91 instructional solo practice flight, which the pilot was conducting as part of a commercial helicopter pilot upgrade training program, originated at Harvey Field. Visual meteorological conditions existed and no flight plan had been filed for the flight. The pilot reported that he was attempting a practice landing on an island in the Snohomish River at the time of the accident. He stated that he got high on his approach, to the point where a steep descent to his intended landing spot would be required, and decided to terminate the approach. As he accelerated through about 40 MPH at about 40 to 50 feet above ground level, he began a turnout to the left, toward downwind. The pilot reported that after completing the downwind turn, the helicopter made an uncommanded turn to the right. The pilot stated that despite the application of left pedal and forward cyclic, "the helicopter continued to rotate violently until impact on the left side." FAA inspectors responded to the accident site and performed an on-site examination of the helicopter wreckage. They reported to the NTSB that the on-site examination revealed no evidence of a mechanical malfunction in the helicopter, and that the tail rotor was found intact. In a telephone conversation with the pilot on May 15, 1997, the pilot stated that at the time of the occurrence, he was heading "parallel with the trees" (roughly southeast.) The Seattle Sectional Aeronautical Chart depicts the Snohomish River coursing approximately to the south-southeast in the accident area. The pilot further reported on his accident reported that winds in the accident area were from 310 degrees at 8 to 12 knots. The reported relative wind direction and velocity is within a region conducive to a "weathercock stability" unanticipated right yaw (also referred to as loss of tail rotor effectiveness, or LTE) event as defined in FAA Advisory Circular (AC) 90-95, "Unanticipated Right Yaw in Helicopters." The AC states that unanticipated right yaw events can result in loss of control of the helicopter if the pilot does not apply timely corrective action (consisting of simultaneous application of full left pedal and forward cyclic, and if altitude permits, a reduction in power and collective pitch, followed by adjusting controls for normal forward flight as recovery is effected.)
the pilot's encounter with of a loss of tail rotor effectiveness (LTE) situation, and his subsequent inadequate remedial action. Related factors included: wind conditions that were conducive to an unanticipated right yaw event, and the pilot's limited experience in helicopters.
Source: NTSB Aviation Accident Database
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