On December 17, 2000, at 1140 central standard time, a Robinson R22 helicopter, N7187W, was substantially damaged when it impacted terrain while maneuvering near Sunnyvale, Texas. The flight instructor and private pilot receiving instruction were not injured. The airplane was registered to and operated by Sky Helicopters, Inc., of Garland, Texas. Visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local flight departed Mesquite, Texas, approximately 10 minutes prior to the accident and was enroute to Garland, Texas, when the accident occurred. During a telephone interview conducted by the NTSB investigator-in-charge, the instructor pilot stated that the student was practicing "low RPM recoveries, settling with power, normal and steep approaches, and emergency procedures." They flew from Garland to Mesquite, where they practiced 4 straight-in autorotations. While returning to Garland at 1,000 feet agl, the instructor gave the student a simulated engine failure over a field and instructed the student to make an autorotation with a power recovery. The student pilot made a gradual 360-degree turn while descending to the field. The instructor stated that the student leveled the helicopter on final approach. The instructor made one last search for obstacles, and looked back into the cockpit to find the airspeed was at 45 knots and the main rotor RPM was at the bottom of the green arc (approximately 97%). The helicopter was approximately 100 feet agl when the instructor pilot took over control of the helicopter and attempted a "high power recovery." He rolled on the throttle, pulled up on the collective, felt a "slight hesitation," and the helicopter then "snapped to the right." At that time, the instructor applied left pedal and heard the low rotor RPM warning horn. The instructor stated that he thought that the helicopter did not touch down level due to the rotation to the right. The helicopter touched down in the field, "hopped to the right approximately 40 degrees" and then became airborne again. The instructor stated that he thought that he still had either full throttle applied to the engine and/or the collective raised. As the helicopter became airborne, it rotated 360 degrees to the right, contacted the ground with the tail rotor, then the right skid. Subsequently, the helicopter rolled over, and came to rest on its right side. At 1153, the Redbird Airport's weather observation facility (located approximately 15 miles southwest of the accident site), reported the wind as variable at 3 knots, the visibility 10 statute miles, and clear skies. According to the FAA inspector, who responded to the accident site, the helicopter sustained structural damage to the right side of the fuselage, the vertical stabilizer, and horizontal stabilizer. The tail rotor blades were separated from the helicopter and the tail rotor gearbox sustained damage.