Primary finding
Probable cause
The in-flight disengagement of the tail rotor extension shaft from the tail rotor gearbox, resulting in a loss of tail rotor control.
Investigator assessment
Analysis narrative
The pilot reported that as he approached a pre-planned landing zone, he reduced engine power for landing. The helicopter yawed to the right, then back to the left. The helicopter yawed again, and the pilot felt unusual vibrations, so he decided to perform an autorotation. The helicopter landed hard on a gravel road and sustained substantial damage. Examination of the wreckage revealed impact damage to the aft tail boom from main rotor contact. The aft-most section of the tail rotor drive shaft was missing and was not recovered. It is likely that, during the hard landing, a main rotor blade struck the tail boom and liberated the drive shaft section. Manipulation of the tail rotor shaft input at the tail rotor gearbox revealed a lack of continuity to the tail rotor hub and blades. The tail rotor did not rotate when the universal joint, located at the forward end of the extension housing tube, was turned. Subsequent examination showed that the tail rotor gearbox was operational; however, the tail rotor extension shaft was found disconnected from the input coupling to the tail rotor gearbox. Rotational scoring inside the extension shaft housing indicated excessive play in the shaft that was present before the accident, which would have resulted in the loss of tail rotor control.
Source record
Factual narrative
On August 31, 2013, about 0950 eastern daylight time, Continental Copters, Inc. 47G, N6356X, was substantially damaged during a forced landing near Sandy Hook, Kentucky. The commercial pilot and one passenger were not injured. The helicopter was registered to Sierra Aeronautics LLC and was operated by the pilot under the provisions of 14 Code of Regulations Part 91 as a positioning flight. Day, visual meteorological conditions prevailed and no flight plan was filed. The flight originated at Morehead, Kentucky (M97) about 0930. As the helicopter was established in the traffic pattern for landing at a pre-established landing zone, at 300 feet above ground level (AGL), the pilot turned onto final and made a power reduction. The helicopter began to yaw to the right, and then back to the left. The pilot raised the collective and the helicopter yawed again to the right and the pilot felt unusual vibrations. The pilot then attempted an autorotation. He avoided a tree and a parked car and the helicopter landed hard on a gravel road. An inspector with the Federal Aviation Administration responded to the accident site and inspected the wreckage. There was evidence of main rotor contact with the aft section of the tail boom. The tail boom and main rotor blades sustained structural damage. The most aft section of the tail rotor drive shaft was missing; a search of the area failed to locate the missing component. Manipulation of the tail rotor shaft input at the tail rotor gearbox revealed a lack of continuity to the tail rotor hub and blades. When the universal joint, located at the forward end of the extension housing tube, was turned, the tail rotor did not rotate. Disassembly of the tail rotor gear box revealed that the internal gears were normal in appearance and well lubricated. The tail rotor extension shaft was found disconnected from the input coupling to the tail rotor gear box. The splined end on the aft section of the shaft exhibited signatures of overheating and rotation while out of the input coupling. There were scoring marks inside the extension housing tube, indicative of excessive play in the shaft and center bearing housing.