On April 27, 1998, at 1755 hours Pacific daylight time, a Robinson R-22 Beta, N805EH, rolled over during a practice autorotation to an open field near Sacramento, California. The aircraft sustained substantial damage; however, neither the instructor nor his student pilot were injured. The aircraft was being operated as an instructional flight by Sacramento Executive Helicopters at the time of the accident. The flight originated in Davis, California, about 1730. Visual meteorological conditions prevailed at the time and no flight plan was filed. The instructor and student were returning to Sacramento Executive Airport after a local training flight. Prior to contacting the control tower, the instructor decided to give the student a final practice autorotation. After reaching an open area, they entered the maneuver with both pilots on the controls. The instructor stayed on the controls with the student throughout the approach and recovery. The instructor did not notice anything unusual about the maneuver until he initiated a power recovery. As the power recovery progressed, the aircraft continued to descend rather than coming to a hover. The aircraft contacted the ground, in a near level attitude. After touching down, the aircraft slid forward in an area of soft mud. The toes of the skids sunk into the ground, the aircraft rocked forward onto its nose and then fell over on its right side. During the accident sequence, the main rotor blades struck both the ground and the tailboom. The tailboom was subsequently separated from the aircraft. Both the instructor and student were able to exit the aircraft through the left cabin door without assistance. The instructor attributed the unintended descent to high temperature and humidity near the surface, which reduced aircraft performance capabilities. He reported the temperature as 85 degrees Fahrenheit. The dew point is unknown. The elevation of the accident site was approximately 24 feet msl. The aircraft manufacturer's representative concluded that performance degradation due to density altitude, by itself, does not adequately account for the event. It was his opinion that rate of descent, trim, and airspeed were also factors that effects the aircraft's ability to transition to a hover. Robinson Helicopter Safety Notice SN-22, dated July 1986 and revised June 1994, cautions pilots to always reduce rate of descent before reducing airspeed. It states that as the pilot who has not reduced his rate of descent but has reduced his forward airspeed raises his collective and flares to stop his rate of descent, he flares in his own downwash, increasing the power and collective pitch required. The aircraft begins to enter the vortex ring state and a hard landing occurs. This can occur during a steep, power off approach.