On January 29, 2015, about 1700 central standard time, a Bell/Watson 47-G helicopter, N43762, was substantially damaged after impacting terrain during landing at North Little Rock Municipal Airport (ORK), North Little Rock, Arkansas. The flight instructor and student pilot were not injured. The helicopter was registered to Shamu Aviation; North Little Rock, Arkansas; and was operated by a private individual. Day visual meteorological conditions (VMC) prevailed at the time of the accident and a flight plan had not been filed for the 14 Code of Federal Regulations Part 91 local instructional flight. According to the flight instructor, he was manipulating the controls and the student pilot was "following along" as the flight instructor was demonstrating a power off touch-down autorotation to a large open paved ramp area. While flaring for touchdown the flight instructor allowed the rotor RPM to get too low, the tail skid made contact with the asphalt and the helicopter bounced into a nose low attitude, then "fell through" and had a hard landing. During the accident sequence the main rotor blades struck the pavement and the tail boom which resulted in the complete separation of the tail rotor assembly, the partial separation of both main rotor blades, and hard landing damage to the skid gear. The helicopter rotated to the right about 180 degrees from the landing direction and came to rest upright. The flight instructor shut down the operating engine, and both occupants exited without assistance. The flight instructor reported that the rotor "seemed to have run out of energy" but the rotor tachometer indications did not indicate a reduction in rotor RPM. After the accident, the dual tachometer indicator and the rotor tach generator were removed from the helicopter and examined at a Federal Aviation Administration approved repair station. The examinations showed that the rotor tachometer indicator was operating nominally, but was slightly out of calibration. The pointer for the engine tachometer indicator was free floating and evidence showed it had recently detached from its main shaft hairspring. The generator output for the four-pole rotor tach generator was also nominal. Examination showed that it matched specified calibration data at 50 rotor RPM and at 300 rotor RPM, and the phasing and frequency was correct, but the voltage output was slightly low. Except for the broken engine tachometer indicator pointer no preaccident anomalies were found which would have precluded normal operation. Clinton National Airport (LIT), Little Rock, Arkansas, was located about 7 miles at 171 degrees from the accident site, at an elevation of 257 feet msl. At 1653 the surface weather observation site at LIT reported wind from 340 degrees at 15 knots with gusts to 20 knots, visibility of 10 miles, few clouds at 3,500 feet, scattered clouds at 15,000 feet, broken clouds at 25,000 feet, temperature 11 degrees Celsius (C), and dew point 1 degree C, with an altimeter setting of 30.31 inches of Mercury.