HISTORY OF FLIGHTOn February 23, 2016, about 1145 Pacific standard time, a Robinson R66, N266RH, sustained substantial damage to the tailboom following a precautionary autorotation at Zamperini Field Airport (TOA), Torrance, California. The pilot, the sole occupant, was not injured. The experimental (research and development category) test helicopter was registered to and operated by Robinson Helicopter Company (RHC) as a test flight under the provision of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight that originated from TOA at 1145. The purpose of the flight was to test the auxiliary fuel system, low RPM horn, autopilot, and sprag clutch. The pilot reported that he had performed a takeoff to a hover, a hover taxi, and one hovering autorotation without incident. He then executed a maximum performance takeoff. About 200 feet above ground level, the pilot heard a noise and felt a momentary helicopter vibration. Subsequently, he performed an autorotation to a grassy area on the airfield. After the helicopter landed in soft grass with forward momentum, it experienced a fore and aft rocking motion, and the main rotor blades contacted the tailboom. AIRCRAFT INFORMATIONThe five-seat helicopter, serial number 0002, was manufactured in 2009. The operator reported that the helicopter's maximum gross weight was 2,700 pounds, and it weighed 2,047 at the time of the accident. The helicopter was powered by a 300-horsepower Rolls Royce 250-C300/A1 engine. The last annual maintenance inspection was conducted on October 15, 2015, at the total engine time of 363 hours. The airframe had a total of 400.04 hours at the time of the accident, as it had operated 20 hours since its last maintenance inspection. The engine was examined on March 4, 2016, at RHC, Torrance, California. AIRPORT INFORMATIONThe five-seat helicopter, serial number 0002, was manufactured in 2009. The operator reported that the helicopter's maximum gross weight was 2,700 pounds, and it weighed 2,047 at the time of the accident. The helicopter was powered by a 300-horsepower Rolls Royce 250-C300/A1 engine. The last annual maintenance inspection was conducted on October 15, 2015, at the total engine time of 363 hours. The airframe had a total of 400.04 hours at the time of the accident, as it had operated 20 hours since its last maintenance inspection. The engine was examined on March 4, 2016, at RHC, Torrance, California. TESTS AND RESEARCHA visual examination of the airframe components was conducted and no anomalies were found. There were no metal particles found in the oil filter and the chip detector. The Engine Monitoring Unit (EMU) data was downloaded and reviewed by a representative from Rolls-Royce. The EMU provided general information about the operation of the engine including time, cycles, faults and any exceedances of engine operating limits. The downloaded data corresponded to the entire accident flight. There were no exceedances noted in the data; however, at the time stamp 32.27.1 the power turbine (N2) peaked at 107.76% and the Torquemeter Oil Pressure (TMOP) peaked at 95.304% (275 ft/lbs). According to the Rolls Royce 300 Operational and Maintenance Manual, the limit for N2 is 110% for 15 seconds, and the torque limit during takeoff is 288 ft/lbs with a maximum transient of 430 ft/lbs. With no pre-accident anomalies noted with the engine, the engine was test run on the helicopter through various power settings. No anomalies were noted during the engine exam. The engine data was downloaded after the engine runs, and no faults were noted in the data. Following the engine run, the sprag clutch was inspected and it was revealed that the sprag clutch had moved between 45-90 degrees from its original position. The sprag clutch was then removed, and it was observed that it took about 550 pounds of torque to loosen the clutch assembly compared to the 300 pounds of torque used to tighten the clutch in the assembly process. The clutch was then inspected, and no anomalies were found. A review of the maintenance logbook revealed that the new Revision E sprag clutch had been installed on the helicopter before the accident flight. The Revision E sprag clutch was compared to the previously installed Revision D sprag clutch, and the wear patterns appeared normal and consistent with those observed in the Revision E clutch assembly. The complete engine report is appended to this accident in the public docket. The Pilot's Operating Handbook states that a vibration can be an indication of a drive system failure. However, a drive system failure is also accompanied by nose right or left yaw, and a decrease of the rotor RPM while N2 RPM increases. RHC Safety Notice SN-39, issued in July 2003, stated that an unusual vibration can indicate a main rotor blade crack. However, those vibrations are characterized as severe and the helicopter would be difficult to control.