DEN05LA056 · Hughes 369HS

According to the pilot, while approximately 4 feet above the ground, he applied power for the landing and felt an "unusual" vibration in both tail rotor pedals and the helicopter's airframe. He stated that he increased the power and brought the helicopter up to a 10-foot hover to reposition approximately 20 feet away. While repositioning, the vibrations and noise became worse. As he attempted the second landing, he heard a "clanking" sound. He immediately lost all tail rotor authority. He decreased the power and executed an autorotation. The helicopter spun to the right approximately 200 degrees as it settled upright into a 1foot deep layer of snow. One of the passengers stated that they were landing on a down slope. As they approached the landing zone, he noticed the nose of the helicopter "elevate slightly" and then he felt something in the rear of the helicopter "hit" the snow. He said that the pilot lifted the helicopter up about 10 to 15 feet as it began to spin and then they touched down facing to the west. He stated that he noticed a mark in the snow where the pilot first attempted to land. The mark was approximately 30 feet away and was about 10 feet in length. He stated that he watched the pilot walk away from the helicopter, in the direction of the mark, and he watched him walk directly through the mark in the snow. A visual examination revealed a 3 inch by 4 inch tear on the aft right side of the fuselage, and that the tail rotor shaft was fractured. No tail rotor blade damage was noted. According to the Boeing representative, the tail rotor drive shaft had a "rotational/torsion fracture" near fuselage station 170.0, and exhibited a degree of "shaft wind-up" normally associated with a "sudden stoppage to the tail rotor system." According to a metallurgist who examined the fractured tail rotor drive shaft, the fractured 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 of the shaft. This fracture was largely circumferential and was at a location 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. A detailed examination of the fracture surfaces in the shaft revealed 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 noted.

Verbatim NTSB analysis and probable cause from the NTSB dataset