20050203001489A

(-23) Pilot states that during a mission to perform avalanche control in high mountainous terrain, 10,000' to 11,400' at some point he was hovering at about 4' when he detected some unusual vibrations in both pedals. He decided to reposition to a better landing area and while doing so the vibrations worsened and he heard a "clanking sound" followed by a spin to the right, he immediately performed a hovering autorotation. The aircraft settled into the snow upright and with no damage. The pilot and two passengers exited the aircraft and were eventually taken from the site by another helicopter. The pilot inspected the aircraft and determined the tail rotor drive shaft had sheared. The cause of the shear is to be determined after the aircraft is recovered. Because of the altitude and problems finding a helicopter capable of slinging the aircraft out the FSDO has not been able to inspect the aircraft. The plan is that this will happen during the week of feb. 22-26, 2005. After access to the aircraft is possible this report will be amended to reflect an on-site investigation and if new information is obtained that will also be noted. (-23 amendment) while landing at a snow covered lz at approximately 10,300 feet the pilot inadvertently allowed the tailrotor to dig into the snow during the flare to land. The pilot was certain that he did not have a tailrotor strike however, witness, lab and photographic evidence indicates the tailrotor went about 6 inches into the snow at a point consistent with the point of the landing flare. The pilot likely experienced a height depth perception illusion caused by the light and snow conditions which led him to believe he was at a safe height during the landing flare.