On August 8, 2000, about 1830 eastern daylight time, a Bell OH-58C helicopter, N9142S, registered to and operated by the Hillsborough County Sheriffs Office, was substantially damaged when it collided with the ground during an attempted autorotation landing at Vandenberg Airport in Tampa, Florida. The helicopter-rated certified flight instructor (CFI) and one helicopter-rated commercial pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local instructional public use flight being conducted under Title 14 CFR Part 91. The flight originated from Tampa, Florida, at 1800. According to the certified flight instructor, they were practicing 180-degree autorotation landings on runway 5 while performing a 90-day pilot competency ride as required by the Sheriff's Office. The maneuver was initiated from a left downwind for runway 5 from 600 feet above ground level (agl) at 60 knots of airspeed. The collective was lowered to the full down position and the throttle was rolled to the flight idle position and a left turn was initiated. Sixty knots of airspeed was maintained and at approximately one quarter of the way through the turn, the rotor speed began climbing out of the high side of the green arc. Collective pitch was adjusted to maintain rotor speed in the center of the green arch. At approximately 200 feet agl, full engine power restored and the rotor and turbine needles rejoined. After completing the 180-degree turn, an unusual vibration was felt and rotor and turbine RPM started to decay. The helicopter touched down on the runway in a level attitude, bounced once, then settled back onto the runway. The examination of the helicopter disclosed that the tailboom was deformed and cracked but remained attached to the helicopter. Examination of the helicopter revealed no mechanical faults or failures. According to Bell Helicopter, the damage to the helicopter tailboom is normally associated to low rotor induced oscillation of the tail boom while the aircraft was still in flight. Examination of the helicopter failed to show structural damage associated with a hard landing. An Operations Safety Notice, published by Bell Helicopter Textron on September 13, 1984, stated that tailboom and aft fuselage damage can occur during an autorotation landing if the main rotor RPM is allowed to decay below 70 percent. Applying collective pitch in excess of that required could result in excessive flapping of the main rotor during or after touch down. This can result in damage to the tailboom and/or aft fuselage.