On June 18, 2010, about 2045 Pacific daylight time, a Robinson R-22 Beta helicopter, N2548S, impacted the terrain during an autorotational landing about one-half mile south of Portland-Hillsboro Airport, Hillsboro, Oregon. The private pilot and his passenger were not injured, but the helicopter, which was owned and operated by Hillsboro Aviation, sustained substantial damage to its rotor system and its fuselage. The local 14 Code of Federal Regulations Part 91 personal pleasure flight, which departed Portland-Hillsboro Airport about 38 minutes prior to the accident, was being operated in visual meteorological conditions. No flight plan had been filed. According to the pilot, who was executing a visual flight rules (VFR) approach to the airport, when he was about one mile from the helicopter landing pad, the helicopter's engine rpm suddenly began fluctuating between 60 and 100 percent. He therefore lowered the collective and rolled on the throttle, which initially seemed to help. But soon thereafter the condition resumed, with the rpm fluctuating to even lower levels, resulting in the activation of the rotor low rpm warning horn and light. Seeing that he was not going to be able to make it to the landing pad, the pilot elected to do an autorotational landing to a nearby open field. Although the helicopter initially touched down on the skids, it subsequently rolled over onto its side, and the rotating main rotor blades impacted the terrain. According to the pilot, earlier on the same day the helicopter was written up for fluctuating engine rpm, but Hillsboro Aviation maintenance personnel were unable to determine the cause of the fluctuation or to duplicate the reported problem. A post-accident inspection of the engine and the rotor control system was directed and monitored by the Federal Aviation Administration (FAA). As part of that process the magnetos and the engine rpm governor were sent out for further inspection and analysis. The inspection and testing process did not find any evidence of an anomaly or malfunction that would have kept the engine and rotor from producing and maintaining 100 percent rpm.