On March 17, 2000, about 1352 eastern standard time, a Hiller UH-12B, N5304V, registered to a private owner, crashed while attempting a emergency landing at Sanford Airport, Sanford, Florida, while on a Title 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter received substantial damage and the commercial-rated pilot received minor injuries. The flight originated from Sanford, Florida, a few minutes before the accident. The pilot stated he was performing practice autorotations with power recoveries. While departing into the pattern, he began to experience a severe one per revolution vibration. He was climbing through about 50 feet. He immediately lowered the collective and tried to maintain an approach to the surface away from construction workers and equipment. As he got closer to the surface the vibrations intensity increased substantially to the point where he could hardly hold onto the controls or see outside the helicopter. He attempted to raise the collective as he neared the surface and felt as if there was no response or control of the helicopter at that point. After the helicopter hit the surface and rolled a few times, he unbuckled the seat belt and got out of the helicopter. Postcrash examination of the crash site and helicopter was performed by a FAA inspector. The inspector stated the initial impact point of the helicopter was about 300 feet from the final stopping point, indicated by the two tail rotor blades lying where they had initially struck the ground and became separated from the tail rotor hub. There were skid marks from this point to the final spot where the helicopter rolled onto it's right side. There were no indications of more than one roll and no indication of a complete roll, only 90 degrees to end resting on the right side. The main rotor blades had been shattered from the impact, but the steel spar straps were intact and still attached to the rotor hub. One control paddle had been broken off at the hub from the impact. Control continuity was established for the main rotor and engine controls, but the tail rotor cables had been separated during the crash. They were still connected at the pedals and pitch change horn. No obvious mechanical defects could be detected. The "P" lead from the left magneto was hanging loose from the magneto. The threads on the "P" lead cap were not damaged and the magneto had no impact damage. A witness located in a construction area on the airport, near the crash site, stated he observed the helicopter circle overhead practicing landings west of runway 18-36. He watched the helicopter land and then takeoff again. During takeoff and climb the helicopter sound changed and he watched the helicopter descend from about 200 feet, in level flight but coming down fast. The helicopter hit the ground and flipped over.