Aircraft Accident/Incident Report

Gorst, Washington 98337
Friday, May 11, 2001 9:22 PDT

During an instructional flight, radar data identified the helicopter maneuvering in level flight before making a rapid descent below radar coverage. About 33 seconds later, the helicopter was picked up on radar in a climbing right turn, then a left turn before radar contact was lost at an altitude of 1,500 feet MSL (1,060 AGL). Witnesses in the area reported observing the helicopter make "some radical flight maneuvers" before it lost altitude. The witnesses reported that the helicopter then descended in a "nose down" or "wobbling" attitude while it spun to the ground. Prior to ground impact, several witnesses observed an object or objects separating from the helicopter before they lost site of it in the trees. Two of the witnesses reported seeing the tail rotors separate followed by the tail section. The helicopter then collided with trees. During the on-site and post-accident investigation, it was determined that the tail rotor blades made contact with the side of the tail boom. The empennage assembly was found about 180 feet away from the main wreckage. Both tail rotor blades separated from the tail rotor hub. Components that separated were due to overload. Although a pitch change link and teeter bumper stop were not recovered, metallurgical examination of the tail rotor pitch control assembly determined that they were attached at impact. There was no evidence that pitch change control was lost prior to the tail rotor blade striking the tail boom. No evidence of a mechanical failure or malfunction was found. Further investigation determined that abrupt application of full left pedal during a simulated power failure could result in excessive flapping of the tail rotor and possible tail rotor blade contact with the tail boom. At the time of the accident, the mean tail rotor blade angle was 21.5 degrees to 22.0 degrees. Approximately three months after the accident, the manufacturer issued a service bulletin to re-rig the tail rotor to reduce maximum blade angle at the left pedal stop and required the installation of a harder teeter bumper. The mean tail rotor blade angle was changed to 16.5 degrees to 17.0 degrees.

An abrupt application of the tail rotor/anti-torque pedal by an unknown pilot resulting in tail rotor contact with the tail boom. Tail rotor blades and empennage assembly separation, and trees were factors.