On May 25, 2005, about 0945 Pacific daylight time, a Robinson R-22 Beta II, N192SH, collided with a boundary fence and impacted the terrain during a practice 180-degree autorotation near the Jean Airport, Jean, Nevada. Silver State Helicopters was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and the private pilot undergoing instruction (PUI) were not injured; the helicopter sustained substantial damage. The cross-country instructional flight departed the North Las Vegas Airport, Las Vegas, Nevada, about 0850, with a planned destination of Jean. Visual meteorological conditions prevailed, and no flight plan had been filed. Both the CFI and PUI submitted a written statement. They stated that they completed several straight-in autorotation approaches and three 180-degree autorotations. They conducted the last 180-degree autorotation from 3,500 feet mean sea level (about 600 feet above ground level). As the helicopter rolled out of the 180-degree turn on short final, the CFI and PUI noticed that the engine rpm began to decay. The PUI then increased the throttle in an attempt to go around, and the CFI took the controls. At this point the CFI noticed that the engine rpm was at 92 percent and the helicopter was descending. The CFI was unable to stop the decent rate, and the helicopter collided with a boundary fence and pitched forward into the terrain. In a written statement, the CFI reported that the temperature was 32 degrees Celsius, the dew point was negative 15 degrees Celsius, and the altimeter setting was 29.84 inHg. The density altitude was approximately 5,590 feet. The CFI and PUI both stated that the helicopter and engine had no mechanical failures or malfunctions during the flight.