N911SW

On May 27, 1994, approximately 1300 mountain daylight time (MDT), a Bell 204B, N911SW, impacted the terrain during Part 133 operations 38 miles north of Libby, Montana. The commercial pilot received serious injuries, and the aircraft sustained substantial damage. The long-line logging flight, which was moving felled trees to a loading area, was being operated in visual meteorological conditions at the time of the accident. No flight plan had been filed, and the ELT, which was activated by the impact, was turned off at the scene.

According to witnesses, just as the aircraft was lifting a "light load," a loud bang or pop came from the aircraft. Witnesses then saw the helicopter begin spinning to the right, and heard the engine begin to wind down. The aircraft then descended almost straight down and impacted the forested terrain.

An NTSB supervised teardown analysis of the 42 degree gear box revealed that a fatigue crack originated in the root radius of one of the pinion teeth. This crack had extended through the pinion gear itself, and then far enough around the pinion gear shaft fillet radius to cause the pinion assembly to fail.