On July 2, 2001, at 1426 Pacific daylight time, a single engine Robinson R22 Beta helicopter, N4052K, experienced a hard landing and rolled over after making a 180-degree autorotational approach at Hayward, California. The helicopter sustained substantial damage. The certified flight instructor and commercial pilot/flight instructor candidate received minor injuries. The helicopter was registered to a private individual, and the Sierra Academy of Aeronautics of Oakland, California, was operating the helicopter as an instructional flight under the provisions of 14 CFR Part 91 at the time of the accident. The local flight departed Oakland at 1330. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. According to a written statement provided by the flight instructor, the objective of the flight was to increase the student's proficiency in "full-down straight in" and "180-degree power recovery" autorotations. After the student completed full-down straight in autorotations, the instructor had him practice the 180-degree power recovery autorotations. The flight instructor indicated that the first attempted maneuver was uneventful, and the second maneuver entry was "accurate with regard to airspeed, altitude, rotor rpm, and rate of turn and trim." The airspeed was stable at 62-63 knots, with the rotor rpm "pegged in the middle of the green region." After approximately 130 degrees of turn, a "high rate of descent developed," while the airspeed and rotor rpm remained stable. The instructor "joined" the student on the controls and initiated a progressive rollout of the turn while simultaneously applying aft cyclic, raising the collective, and applying more power for a power recovery to level flight. However, the engine did not respond to the throttle input. Due to the rate of descent, the flight instructor elected not to attempt an "aggressive flare fearing a tail rotor ground strike." He continued with aft cyclic input and collective manipulation to reduce forward airspeed and rate of descent for a run-on landing. The helicopter contacted the ground in a "near skids level attitude" approximately 30 knots, with the rotor rpm in the "high end of the green region." The helicopter became airborne approximately 8-10 feet above the ground. The flight instructor indicated that the throttle was still unresponsive. He raised the collective to cushion the second impact and the low rotor rpm warning light and horn came on prior to set down. The helicopter impacted the ground again approximately 20 knots, the left skid collapsed on contact, and it rolled to the left coming to rest on its left side facing approximately 180 degrees from the approach heading. Once the helicopter came to rest, the flight instructor realized that the student was not positioned in his seat "due to the failure of his seat belt." The flight instructor kicked out the right side Plexiglas canopy and exited the helicopter with his student. The instructor then turned off the main fuel valve, and the battery and alternator switches. The tail boom was found fractured forward of the tail rotor danger sign and both main rotor blades sustained substantial damage. Both blades were curved upward and one blade was bent up approximately 90 degrees, 1.5 feet outboard of the main rotor hub. Photographs taken at the accident site showed the left seat restraining system's male clasp load bar failed. The Federal Aviation Administration (FAA) inspector, who responded to the accident site, indicated that he had confirmed flight control continuity from the collective an cyclic to the pitch change links; however, the post-accident movement was limited due to bent push-pull rods. The antitorque system was disconnected at the tail boom separation point. No anomalies were noted with the main or tail rotor systems. The auxiliary fuel tank, which gravity feeds into the main fuel tank, was found empty, and approximately 3.5-4.0 gallons of fuel were found in the main fuel tank. The carburetor fuel bowl was found approximately 1/2 full. The FAA inspector indicated that oil contaminated the inlet air filter area and two of the bottom spark plugs from laying on its side. He removed the filter and cleaned the two spark plugs and reinstalled them. The inspector rotated the engine crankshaft by manually rotating the cooling fan. He did not note any binding or anomalies. The gascolator drain was opened and clean fuel came out with no evidence of contamination. There was no evidence of leaks in the fuel line. The inspector attached a battery and set the engine to a full rich mixture setting and primed the engine twice by turning the throttle. The engine started without difficulty and ran until the engine temperature rose. The inspector then performed a magneto check and found that they performed within limits. No anomalies with the engine were noted during the test run. At the time of the accident, the helicopter had accumulated a total of 8,028.9 hours of flight time. Review of the helicopter's maintenance records revealed that the last 100-hour inspection took place on June 16, 2001, at a total time of 7,981.8 hours. The helicopter underwent its last annual inspection on January 15, 2001, at a total time of 7,683.0 hours. According to the the flight instructor's written accident report, he reported having accumulated a total of 1,760 rotorcraft flight hours, of which "1,000+" hours were in the same make and model as the accident helicopter. The commercial pilot student accumulated a total of 174 rotorcraft flight hours, of which 111 hours were in the same make and model as the accident helicopter.