Incident Overview
A pilot reported that while approximately 4 feet above the ground during a landing approach, he applied power and felt an unusual vibration in both tail rotor pedals and the helicopter's airframe. He increased power and hovered at 10 feet to reposition about 20 feet away. During repositioning, the vibrations and noise worsened. On the second landing attempt, he heard a clanking sound and immediately lost all tail rotor authority. He reduced power and executed an autorotation. The helicopter spun approximately 200 degrees to the right before settling upright into a 1-foot deep layer of snow.
Passenger Observations
A passenger stated they were landing on a down slope. As the helicopter approached the landing zone, he noticed the nose elevate slightly and then felt something at the rear of the helicopter strike the snow. The pilot lifted the helicopter about 10 to 15 feet as it began to spin, and they touched down facing west. The passenger observed a mark in the snow where the pilot first attempted to land, approximately 30 feet away and about 10 feet long. He later watched the pilot walk directly through that mark.
Post-Accident Examination
Visual inspection revealed a 3-inch by 4-inch tear on the aft right side of the fuselage, and the tail rotor shaft was fractured. No tail rotor blade damage was noted. A Boeing representative reported that the tail rotor drive shaft had a rotational/torsion fracture near fuselage station 170.0, exhibiting a degree of shaft wind-up normally associated with a sudden stoppage to the tail rotor system.
Metallurgical Analysis
A metallurgist examined the fractured tail rotor drive shaft and found that the fracture surfaces exhibited extensive torsional deformation on both sides of the aft fracture. The indicated direction of torsion, as viewed looking aft, was as if the forward portion of the shaft rotated clockwise relative to the aft portion. The fracture was largely circumferential and located where the exterior surface of the shaft contained mechanical rubbing and rotational contact damage. Several other areas of rotational damage were noted on the shaft surface between the forward and aft fractures. Detailed examination showed clean fractures on a 45-degree slant plane, typical of recent overstress fracture under tension or tearing loads. No evidence of preexisting fracture such as fatigue cracking was observed.