On August 27, 1993, at 1745 central daylight time, a Robinson R22, N9071S, registered to Charles E. Duchek of Pacific, Missouri, and operated by a commercial rated pilot experienced a loss of rotor RPM and impacted the terrain while turning final to runway 17 at Alton, Illinois. The helicopter sustained substantial damage. The pilot reported no injuries. The personal 14 CFR Part 91 flight was operating in visual meteorological conditions. No flight plan was on file. The local flight departed Alton, Illinois, exact time unknown. The pilot stated that on turning from base to final for runway 17, he noted a low rotor RPM indication and initiated an autorotation landing adjacent to the runway in a crop field. A port accident examination conducted by Federal Aviation Administration inspectors failed to reveal any pre-accident anomalies with the helicopter.