On September 29, 2000, at 1200 mountain standard time, a Hughes TH-55A single engine helicopter, N331SD, experienced a main rotor-tail boom strike at the termination of a practice autorotation at Phoenix-Deer Valley Municipal Airport (DVT), Phoenix, Arizona. The helicopter sustained substantial damage; however, the certificated flight instructor and the airplane-rated student were not injured. The helicopter was registered to and operated by the Maricopa County Sheriff's Office as a public-use instructional flight under 14 Code of Federal Regulations Part 91 when the accident occurred. The flight was returning from a round robin flight that departed DVT with stops at local airports before terminating at DVT at the time of the accident. Visual meteorological conditions prevailed at the time and no flight plan had been filed. The flight instructor reported that he was demonstrating a 180-degree practice autorotation in the north traffic pattern. He said that the entry, airspeed, rate of descent, deceleration, flare, and touchdown were normal. After touchdown he estimated that there was a ground slide of about 5 to 8 feet. As the aircraft stopped, he heard a loud sharp sound that was accompanied by sustained vibration and an uncommanded right yaw of about 30 degrees. He shutdown the engine by pulling the fuel valve and mixture control into the "off" position. After he and the student exited the helicopter, he noted that the main rotor blades had contacted the tailboom, damaging the tail rotor drive shaft, tail boom, as well as all the main rotor and tail rotor blades. The flight instructor stated that he did not perceive any action or inaction on his part that could be construed as detrimental to the safe outcome of the maneuver. According to the Federal Aviation Administration inspector who responded to the accident site and examined the helicopter, there were no signs of mechanical problems that would have contributed to the accident. The flight instructor had accumulated approximately 1,320 hours of helicopter flight time, of which approximately 800 hours were obtained in the same make and model as the accident helicopter.