Bell 206L 1 N206KK

The pilot reported that, during cruise flight, he heard a loud pop and that, immediately after, the engine lost power. The pilot subsequently performed an autorotation. During the autorotation, the left front door opened, and the cabin then filled with black smoke. The pilot subsequently landed the helicopter upright in an open area, and he and the passengers egressed. The pilot stated that, upon exit, he saw fire originating from about 2 ft aft of the right forward door; the fire consumed most of the helicopter. Examination of the helical power train drive assembly revealed that the No. 4 bearing rollers were exposed to intense heat and that one of the rollers exhibited evidence of flat spotting, consistent with a skidding roller. The No. 4 bearing separator was fractured through one roller pocket at three locations, and the fracture surfaces were oxidized, which prevented the identification of a specific fracture mode. Signatures observed on the No. 4 bearing and the pinion gear were consistent with damage generated from the bearing spinning on the pinion gear. It is likely that the failure of the No. 4 bearing generated heat, which caused the expansion and softening of the aft end of the power train pinion gear and resulted in the gear disconnecting from the turbine shaft-to-pinion gear coupling splines. As a result of this disconnection, the load on the power turbine was suddenly removed, which resulted in an overspeed of the 3rd- and 4th-stage turbine wheels and the subsequent separation of their shrouds and blades, some of which were liberated from the engine. Several exit holes caused by high-energy debris were found in-line with the fuel control unit and power turbine governor, which likely was the source of the in-flight fire. Oil filters that had been retained by the operator's mechanic during previous troubleshooting were examined, and they contained particles consistent with two different alloys used to manufacture the pinion gear and bearing races, respectively, indicating that a problem existed before the accident flight. About 15 years before the accident, the manufacturer issued a service bulletin (SB) that outlined the replacement of both the Nos. 3 and 4 bearings with updated bearings before the end of 2002, which was 11 years before the accident. During engine disassembly, the No. 3 bearing was found to have been replaced with the updated bearing as outlined in the SB; the maintenance logbooks did not indicate when this bearing was replaced. However, the No. 4 bearing was found to have the same serial and part numbers as the original bearings installed in the engine about 17 years before the accident. If the No. 4 bearing had been replaced in accordance with the SB, it likely would not have failed in flight, and the accident could have been prevented.

Quoted verbatim from the NTSB record.