CHI00LA277

The helicopter impacted the terrain following a loss of tail rotor effectiveness. The pilot reported he just completed a turn in order to begin a swath run when he experienced a loss of tail rotor effectiveness. He attempted to control the helicopter, however it continued to rotate until it impacted the terrain in a clearing in a wooded area. Post accident examination revealed a failure of the tail rotor drive gear assembly. The drive gear shaft was fractured at the underside of the bevel gear. Metallurgical examination revealed fatigue was found over 90 to 95 percent of the shaft cross sectional area. The fatigue cracking origin was on the cylindrical portion of the shaft surface, adjacent to the keyway slot.

On August 31, 2000, at 0930 central daylight time, a Bell 47G-2, N67083, collided with the terrain following a loss of control while maneuvering during an aerial application flight in Spring Brook, Wisconsin. The pilot was not injured and the helicopter was substantially damaged. The 14 CFR Part 137 flight was operating in visual meteorological conditions. The flight originated in Spring Brook, Wisconsin, at 0925 central daylight time.

The pilot stated he was going to spray a field and had just made a turn to enter his first swath run when he experienced a loss of tail rotor power. He stated his attempts to return to normal flight were unsuccessful resulting in rotation of the helicopter and airspeed dropping to zero. He reported the helicopter settled into a clearing in a wooded area at the end of the field. The helicopter impacted some small trees and the terrain, which resulted in the separation of the main rotor and substantial damage to the helicopter.

Post accident inspection of the helicopter revealed a failure of the tail rotor drive gear assembly. The assembly was removed from the helicopter and sent to the NTSB Materials Laboratory in Washington, D.C. The metallurgical inspection of the drive gear assembly revealed the drive gear shaft was fractured at the underside of the bevel gear. The examination revealed, "The fracture surface was on a flat transverse plane and contained crack arrest positions over most of the fracture, indicative of fatigue cracking." The metallurgist's report also stated, "The origin area was located on the cylindrical portion of the shaft surface, directly adjacent to the keyway slot. The origin area contained a single ratchet mark, indicating initiation from at least two origin sites in the origin area. Fatigue crack features visible on the scanning electron microscope extended from the origin area through about 90 percent to 95 percent of the shaft cross sectional area." The report defined a ratchet mark as a small step in the fracture surface that separates fatigue origin areas on slightly offset planes.