On November 26, 2015, about 1145 central standard time, a Robinson R44II, N7512N, was substantially damaged during a separation of the tail rotor drive shaft during a power check at the Destin Executive Airport, Destin, Florida. The commercial pilot and 3 passengers were not injured. The helicopter was being operated under the provisions of 14 Code of Federal Regulations Part 91, as a local air tour flight. Visual meteorological conditions prevailed in the area at the time of the accident, and no flight plan was filed for the local flight. The flight was originating at the time of the accident.The pilot stated that he performed a preflight inspection of the helicopter and the passengers then boarded the helicopter. The engine was started and ground run-up checks of the engine were satisfactory. While doing a power check which involved applying up collective, he heard a loud "pop." He responded by immediately lowering the collective and secured the helicopter. Examination of the helicopter revealed the steel tail rotor drive shaft part number (P/N) C196-1 was circumferentially fractured at the weld of the forward flange; a portion of the intermediate flex plate P/N A947-2, remained bolted to the flange. The remaining portion of the flex plate remained bolted to the clutch shaft yoke, P/N C195. The tailcone attachment frame P/N C237-1, and the damper assembly mount angle was fractured. There was no report of any damage to any frame or interior of the tailcone aft of the damper. The helicopter time at the time of the accident was reported to be 1,770 hours. A section of the tail rotor drive shaft assembly containing the fracture, separated flange with attached portion of flex plate, and clutch shaft yoke with remaining portion of flex plate were retained for examination by the NTSB Materials Laboratory. According to the NTSB Materials Laboratory factual report, the fracture between the tail rotor drive shaft and flange was consistent with torsional overstress. The report also indicated that a portion of flex plate attached to the flange also exhibited features consistent with overstress. Inspection of the bolted connection between the clutch shaft yoke and the portion of the flex plate revealed all of the hardware was in the correct sequence with the exception of the omission of a washer between the flex plate and the clutch shaft yoke. The washer omission was consistent on both of the connections between the clutch shaft yoke and the flex plate. The hardware stack-up for the connection between the flex plate and the portion of the drive shaft flange was correct. Review of the maintenance records revealed an entry dated November 12, 2015, at helicopter total time of about 1,759 hours, indicating in part that the sprag clutch was removed and replaced. Review of the maintenance manual revealed that with respect to the C-195 yoke removal and installation, a caution indicates, "There must be one AN960-416L or one AN960-416 washer between each arm of C195 yoke and A947-2 flex plate…." The maintenance manual also references Section 7.330, which pertains to intermediate flex plate installation and shimming. That section contains measurements to determine proper shimming required at the intermediate flex plate. The mechanic who performed the clutch removal and installation stated that the measurements he took at the intermediate flex plate during and after his repairs were exactly the same. He also indicated that when they installed new hardware, he obtained the same dimensions. According to the maintenance record entry returning the helicopter to service after the accident, there was no record that the governor was removed, replaced, or repaired. According to a representative of the helicopter manufacturer, if misalignment of the clutch shaft, sheave, and intermediate flex coupling was not corrected during installation, there could be significant preload or tension in the entire length of driveshaft between the main rotor gearbox and tail rotor gearbox. The representative also indicated that the effect of a missing washer would have caused fatigue type failure of the tailrotor drive-line, and they had not previously seen a failure of the tail rotor driveshaft at the flange as had occurred in the accident helicopter.