On June 13, 1997, at 1935 central daylight time, a Robinson R22 Beta helicopter, N4041W, registered to the Charlie Lima Corporation and operated by Helicopter Experts as a Title 14 CFR Part 91 instructional flight, was substantially damaged during a practice autorotation at the Stinson Municipal Airport near San Antonio, Texas. Visual meteorological conditions prevailed, and a flight plan was not filed for the local flight. The certificated flight instructor and student pilot were not injured. The flight originated from the San Antonio International Airport, about 35 minutes before the accident. The flight instructor reported to the investigator-in-charge that he was instructing the student pilot in straight-in power recovery autorotations to the Stinson Municipal Airport's center sod. The student initiated the maneuver from an altitude of 1,100 feet MSL, and an airspeed of 65 KIAS, with the rotor RPM in the "green." During the autorotation, at 40 feet AGL, the student initiated a cyclic flare. "As the flare continued, the descent rate slowed." The student "started to apply forward cyclic to level the helicopter when it started a left yaw." The student told the flight instructor that he tried to roll the throttle on after leveling the helicopter; however, there was no engine response. The flight instructor got on the controls and aligned the aircraft with the landing direction, and the low rotor RPM warning horn activated. He then "rolled on the throttle and the helicopter yawed quickly to the right." After he corrected for the yaw, the helicopter "fell approximately 10 to 15 feet straight down." It "impacted [the ground] hard to where the skids were spread out and the seats were collapsed down." The flight instructor also reported that the fuel sample he took from the fuel tank had some water. A fuel sample was again taken after the accident and water was found in that sample. Examination of the helicopter by a FAA inspector revealed the skids were "stressed beyond allowable limits," and their attaching points were twisted. Both upper and lower welded frame assemblies sustained damaged. Examination of the helicopter by the manufacturer revealed that there was no water in the fuel tank or carburetor, and continuity was established to all of the valves. For practice autorotation with power recovery (below 4,000 feet), the helicopter's flight manual states, "At about 8 feet AGL, apply forward cyclic to level ship and raise collective to stop descent. Add throttle as required to keep RPM in green arc."