On October 7, 1999, approximately 1800 central daylight time, a Bell 206B helicopter, N21166, registered to MidTex Investments Inc., of Austin, Texas, and operated by the Texas Department of Public Safety, was substantially damaged when the helicopter nosed over during a practice autorotation at the Slaton Municipal Airport near Slaton, Texas. The flight instructor and the commercial pilot receiving instruction were not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 public use training flight. The local flight originated from Lubbock, Texas, at 1620. In the NTSB Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), the flight instructor reported he was conducting 180-degree left turn autorotations at 500 feet agl, 400 feet agl., and 300 feet agl, all with power recoveries. During the 300-foot agl practice 180-degree autorotation, approximately 90 degrees into the turn, the pilot allowed "a high sink-rate to develop", and began to pull pitch to arrest the descent. The pilot started to "roll the throttle" open and he observed the N2 and Nr needles join at 92 percent. At this point, the left 180-degree turn was completed, and the aircraft rolled out parallel to and west of the runway with approximately 40 knots indicated airspeed. The pilot continued to roll on throttle and increase the collective in order "to hold the helicopter about 3 feet above the ground." However, the "rotor rpm decreased too low", and the helicopter settled to the ground and slid approximately 60 feet. The left skid toe and lower wire cutter dug into the ground, and the helicopter rocked forward onto the toes of the skids. Subsequently, the main rotor blades contacted and severed the tail boom, and the helicopter rolled over, and came to rest on its left side. In the section of the NTSB Form 6120.1/2 entitled "Recommendation (How Could This Accident Have Been Prevented)", the operator stated that "this maneuver could be entered from a higher agl altitude, this would have prevented the accident." According to the FAA inspector, who responded to the accident site, the main rotor blades were destroyed, the left side of the helicopter was crushed, and the tailboom was separated.