On May 19, 2005, at 1541 mountain daylight time, a Robinson R22 Beta, N983KC, piloted by a private pilot under the supervision of a commercial-certificated flight instructor, was substantially damaged when it collided with terrain during a practice autorotation near St. George, Utah. Visual meteorological conditions prevailed at the time of the accident. The local instructional flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot and flight instructor sustained minor injuries. The flight originated at St. George, Utah, approximately 1500. The instructor said they had been airborne approximately 45 minutes prior to the accident. During that time, they had performed three straight-in autorotations and four 180-degree autorotations, and that the aircraft and all systems were operating normally. They entered the autorotation normally and completed a 180-degree turn at an altitude of approximately 250 feet agl. There was a noticeable lateral vibration of the airframe and a rapid increase in descent rate. The instructor took control, advanced the throttle, and noticed that the rotor and engine rpm needles were "joined" just below 100 percent rpm. He said he did not understand why the helicopter had a descent rate greater than 1,500 feet per minute when the engine was at near full power and rotor rpm was almost at 100 per cent rpm. Neither pilot heard the LOW ROTOR RPM warning. The instructor attempted to flare but there was insufficient time, and the helicopter impacted terrain. The tail rotor was dragged along the ground approximately 30 feet, then the skids struck the ground and the helicopter rolled over. The tail was severed from the aircraft, the landing skids were torn off, the windscreen was shattered, and the skin was deformed. A mechanic who examined the helicopter said it appeared all systems were operating normally prior to the accident, and no mechanical anomalies were found. A witness located 1 mile north of the airport said that the day of the accident was the first day of increased dust devil/wind shear/microburst activity common to the region during the summer months. The instructor said he also noticed dust devils in the area after the accident occurred. The helicopter's rate of descent during autorotation, as reported by the pilot, was equal to the rotor downwash velocity of approximately 1500 feet per minute. The power had been at idle, but was increased at the onset of the vibration. The instructor said that the airspeed never dropped below 50 knots, and that during the autorotation, the power remained off. A spokesman for Robinson Helicopter confirmed that the translational lift velocity for this helicopter was approximately 20 knots. The FAA Rotorcraft Flying Handbook states that following conditions are linked to vortex ring state: a vertical descent of at least 300 feet per minute, the engine power setting between 20-100 percent, and a horizontal velocity slower than the effective translational lift velocity.