On March 24, 2005, about 1207 mountain standard time, a Robinson R22 Beta, N225G, experienced a mechanical malfunction on base leg to the Ernest A. Love Field, Prescott, Arizona. The certified flight instructor (CFI) took the flight controls from his student and performed an autorotation. The helicopter touched down hard, rolled over, and was substantially damaged. Neither the CFI nor the student pilot was injured during the instructional flight. Guidance Helicopter, Inc., Prescott, operated the helicopter. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight was performed under the provisions of 14 CFR Part 91, and it originated from Prescott about 1200. The CFI reported to the National Transportation Safety Board investigator that he monitored his student as he performed the preflight inspection of the helicopter. No evidence of any problem was detected. The CFI reported that there were no outstanding airworthiness issues with the helicopter. The accident occurred while he was preparing his student for the private pilot certification check ride. The lesson plan/maneuver that he intended the student to practice was a run-on landing. On base leg to runway 21R, while cruising about 60 knots at 300 feet above ground level, the helicopter's nose suddenly yawed left and then right. Concurrently the helicopter shook, and the clutch light illuminated. The CFI additionally reported that he took the controls from his student and entered an autorotative descent. About 20 feet above ground level he commenced a cyclic flare for the landing. The underlying terrain was soft sand, and the helicopter rolled over. Upon exiting the helicopter, the CFI observed that the V-belts were shredded. The helicopter was recovered from the accident site. It was examined by the operator's personnel while under the direction of a Federal Aviation Administration (FAA) aviation safety inspector. In summary, on March 30, 2005, the operator's director of maintenance (DM) and the FAA inspector verbally reported to the Safety Board investigator the results of the examination. The DM reported that 2 days (and about 4.3 helicopter operation hours) before the accident he had performed a 100-hour inspection on N225G. Evidently, during that inspection he had loosened the self-locking nut on the belt actuator housing assembly. Thereafter, he had rotated the down-limit stop screw upward until the screw bottomed out in the actuator housing in the full up position. The DM further reported that, evidently, upon completing the helicopter maintenance, he had forgotten to reposition the stop screw and tighten the nut. The Robinson Helicopter factory participant indicated to the Safety Board investigator that, if the referenced self-locking nut on the belt actuator housing assembly was incorrectly positioned in the manner described by the DM (as evidenced during the wreckage examination), the main drive V-belts could have become excessively loose during shutdown on the flight previous to the accident, and remain so until start up on the accident flight. An excessively loose belt could fail to enter its proper sheave groove on start up, which could lead to a subsequent belt failure.