SEA99LA171

The lower surface of the 90-degree gearbox exhibited fracture surfaces of the six attachment studs. Fretting was observed on the lower surface where the gearbox quill assembly was attached. The fracture surfaces for the six attachment studs each showed one or two relatively flat regions with smooth curving boundaries typical of fatigue. Multiple origin sites were identified on each stud, with observed ratchet marks. All of the origin areas were circumferentially aligned with the center of the gearbox, with origins on both sides of each stud, except for one stud, which had an origin area on only one side. In each case, the larger fatigue region had origins on the counter-clockwise side of the stud (as viewed from above, from the center of the gearbox).

On September 21, 1999, approximately 1405 Pacific daylight time, a Garlick UH-1H helicopter, N32800, sustained substantial damage when it collided with terrain after a loss of control while engaged in external load logging operations near Usk, Washington. The commercial pilot, the sole occupant, was uninjured. No flight plan was filed for the flight, which was conducted in visual meteorological conditions. There was no report of an ELT actuation. The flight was conducted under 14 CFR 133. There was a post-crash fire, which was reported to have caused minimal damage.

The pilot stated that he had a load of logs engaged which he "punched off" when he encountered what he believed was a loss of tail rotor effectiveness. FAA inspectors reported that it appeared that the 90 degree gearbox mounting studs had sheared, and the gearbox assembly had remained with the aircraft until ground impact.

In a written statement, the operator stated that the aircraft was approaching the log landing with the load of logs when the pilot heard a loud boom in the aft section of the aircraft. He slowed the aircraft down and started a right turn, and believed he had lost his tail rotor. "The aircraft spun once rapidly, the pilot closed the throttle [to] slow the rate of turn and proceed[ed] to autorotate from about 150 feet. He had set the logs down on the edge of the landing. The aircraft stopped spinning and the pilot continued the autorotation to the best landing spot available, in a lightly treed area. The pilot jettisoned the long line, pulled pitch to slow entry into the trees and came down vertically inside the group of trees. The slowing rotor blade stuck the trees, pulling the whole aircraft forward, swinging the tail boom to the left and twisting the aircraft sideways where it landed." The pilot exited through the crew side door. There was a small fire which was immediately extinguished.

The tail rotor and 90-degree gearbox were found within about ten feet of the tail-boom at the wreckage site. Two of six gearbox studs with nuts were recovered at the accident site. Six segments of the 90-degree gearbox studs remained secure in the gearbox casting. The tail rotor drive shaft (between the 42 degree and 90 degree gearboxes) had rotational contact signatures, and had separated. The 90 degree gearbox quill shaft and a section of the associated drive shaft were missing and were not recovered. The leading edges of both tail rotor blades exhibited leading edge crushing and compression. Both pitch links remained intact, and the tail rotor blades swung freely on their pivot hinge.

The tail rotor blades and hub with attached 90-degree gearbox, the upper portion of the vertical fin, upper aft portion of the tail-rotor driveshaft, and two studs with attached nuts, were inspected and analyzed by the NTSB materials laboratory. (See attached report number 00-057.)

The lower surface of the 90-degree gearbox exhibited fracture surfaces of the six attachment studs. Fretting was observed on the lower surface where the gearbox quill assembly was attached. The fracture surfaces for the six attachment studs each showed one or two relatively flat regions with smooth curving boundaries typical of fatigue. Multiple origin sites were identified on each stud, with observed ratchet marks. All of the origin areas were circumferentially aligned with the center of the gearbox, with origins on both sides of each stud, except for one stud, which had an origin area on only one side. In each case, the larger fatigue region had origins on the counter-clockwise side of the stud (as viewed from above, from the center of the gearbox).