HISTORY OF FLIGHT On November 10, 2005, at 1300 Pacific standard time, a Bell 47D1, N16356, landed hard in an open field during an autorotation that was precipitated by a tail rotor vibration 5 miles south of Rancho Murieta Airport (RIU), Rancho Murieta, California. Rancho Rotors Helicopters operated the helicopter under the provisions of 14 CFR Part 91. The helicopter was destroyed. The student pilot, the sole occupant, sustained serious injuries. Visual meteorological conditions prevailed for the local area solo instructional flight, and no flight plan had been filed. The flight departed RIU at an undetermined time. The wreckage was at global positioning system (GPS) coordinates of 38 degrees 24.272 minutes north latitude and 121 degrees 05.296 minutes west longitude. A Federal Aviation Administration (FAA) inspector interviewed the student pilot. The pilot reported that during cruise flight she felt a vibration in the anti-torque pedals. She identified the vibration as an oscillation (helicopter moving from side to side), decided to make a precautionary landing, and turned into the wind to land. During the turn, she heard a loud noise and she entered into an autorotation. She input fore and aft cyclic, which did not seem to help with control of the helicopter. About 35 feet above the ground, at tree top level, she flared the helicopter. The helicopter started to spin to the right, and subsequently landed hard. The pilot stated that after touchdown the helicopter bounced back into the air where she saw the transmission fly over the cockpit. The helicopter then struck the ground in a nose down attitude. In the pilot's written statement provided to the National Transportation Safety Board after the interview was conducted by the FAA, she stated that during cruise flight at 800 feet above ground level (agl), the helicopter began to vibrate slightly. She checked the gauges and observed no indication of engine problems. She made a turn toward RIU. With the vibration intensity increasing, she turned the helicopter into the wind and set up for an emergency landing. The vibration turned into violent shaking and the pilot heard a loud noise. The shaking stopped and the helicopter "felt as if it was dropping." She entered into an autorotation and started to flare the helicopter at 90 feet. The helicopter began to spin, and the pilot tried to level the skids and pulled pitch just prior to touchdown. The pilot stated that there was no response from the helicopter and it struck the ground, bounced into the air, and struck the ground again. According to the maintenance supervisor for the Sacramento County Sheriff's Department Air Support Bureau, the tail rotor assembly, inclusive of the tail rotor gearbox, had separated from the tail rotor drive shaft. The tail rotor assembly was found about 80 feet from the main wreckage. PERSONNEL INFORMATION A review of FAA airman records revealed that the pilot held a combined student pilot and aviation medical certificate. The pilot held a third-class medical certificate issued on March 9, 2005. An examination of the pilot's logbook indicated an estimated total flight time of 61 hours. AIRCRAFT INFORMATION The helicopter was a Bell 47D1, serial number 536. A review of the logbooks revealed that the helicopter had a total airframe time of 8,061.6 hours at the last 100-hour inspection dated October 27, 2005. A review of the airframe logbook revealed the following entries: On November 7, 2004, a Franklin VO-335 had been installed on the helicopter. A corresponding entry in the engine logbook showed a total time of "unknown." On February 2, 2005, the tail rotor yoke was replaced, new tail rotor blades were installed, and a static balance of the tail rotor system was conducted. On March 3, 2005, both tail rotor short shafts were replaced and the tail rotor boots were greased. On June 27, 2005, a 300-hour inspection of the tail rotor blades took place to comply with Airworthiness Directive (AD) 70-10-08, a recurring AD. Additional logbook entries showed that the AD had been complied with. A logbook entry for October 27, 2005, a 100-hour inspection, recorded a total airframe time of 8,061.6 hours. The entry recorded a tail rotor hub assembly replacement, tail rotor pitch bearings replacement, tail rotor gear box assembly was overhauled, with a dye penetrant inspection conducted on the tail rotor gearbox. The tail rotor was reinstalled and a track and balance of the tail rotor system had been conducted. A logbook entry for November 7, 2005, at a recoded total airframe time of 8,076.3 hours, indicated that the forward tail rotor cable (part number NAS302R66-2954) had been removed and replaced. A test flight was conducted and the helicopter was returned to service. According to the FAA accident coordinator, an emergency landing due to a tail rotor problem was reported on November 3, 2005, 1 week prior to the accident. An inspection of the tail rotor assembly revealed that the tail rotor control cable had broken. TESTS AND RESEARCH The Safety Board investigator-in-charge (IIC), FAA airworthiness inspectors, and Bell Helicopter, a party to the investigation, examined the helicopter at Plain Parts, Pleasant Grove, California, on November 17, 2005. An inspection of the engine revealed no mechanical anomalies that would have precluded normal operation. The airframe inspection revealed that the tail rotor assembly separated from tail rotor gearbox. One blade remained attached to the hub but was bent at the butt of the blade. There was no leading or trailing edge damage to the tail rotor blade. The other blade also remained attached at the hub with the tip portion of the blade separated. Both pitch change links remained connected to the pitch change horn; however, one pitch change link had separated at the tail rotor blade attachment. Investigators noted that the bolt had separated with the shank portion of the bolt broken from the threaded portion. The castellated nut and cotter pin remained attached to the threaded portion of the bolt. The bolt, castellated nut, and cotter pin were shipped to the Safety Board Materials Laboratory in Washington, D.C., for further examination. According to the Safety Board mechanical engineer, the facture surface of the shank had two separate groups of arced lines that culminated in a thin, dark, hourglass-shaped region in the center of the two arced lines. The arced lines are commonly referred to as a fatigue-banking pattern. The symmetrical pattern of the arced lines indicated that each side of the shank had been subjected to a similar level of stress and number of load cycles consistent with reverse bending fatigue.