On August 24, 1997, at 1515 central daylight time, a Robinson R-22 Beta helicopter, N8057Z, was substantially damaged after impacting terrain following a partial loss of engine power and autorotation landing to an open field 4 miles west of Terrell Municipal Airport (TRL), Terrell, Texas. The student pilot, sole occupant, received serious injuries. Visual meteorological conditions prevailed for the solo cross country flight which was conducted under Title 14 CFR Part 91. The flight originated from the Garland/DFW Heloplex Heliport (T57) in Garland, Texas, at 1400. The pilot reported to the FAA representative that the cross country flight was from Garland to the Terrell Municipal Airport, then on to Ennis Municipal Airport (F41), Ennis, Texas and returning to Garland. The pilot stated that the helicopter began experiencing engine vibrations with a loss of engine RPM and he decided to perform an autorotation. Approximately 40 feet above ground level (AGL), he initiated the landing flare pulling up on the collective at the same time. He stated that he noticed "the aircraft feeling funny" and reduced some of the collective. At about 8 feet AGL, the helicopter "just dropped" to the ground. The helicopter came to rest in the upright position. The pilot reported the following weather conditions at the time of the accident: clear skies, visibility 10 miles and winds from the south/southeast at 10 knots. According to the Pilot/Operator Aircraft Accident Report (NTSB form 6120.1/2), the pilot had accumulated a total of 638 hours, of which 89 hours were in the R-22. Examination of the helicopter at the site by the FAA inspector revealed that the rotor mast and shaft separated from the helicopter. The main rotor blades were bent upward. The tail rotor and aft 2 feet of the boom section separated from the helicopter. The right skid was bent and broken. The right fuel tank was full and the left tank was 3/4 full. On September 25, 1997, the engine was examined at Air Salvage of Dallas, Lancaster, Texas. No abnormalities or deficiencies were noted. The engine was mounted on a test stand the same day and operated for 15 minutes. The engine operated normally throughout all parameters.