On Sunday, June 27, 1993, at 1400 eastern daylight time, an Enstrom F 28A, N9555, registered to Hier Aviation, Inc., and piloted by Daniel R. Hieronimus, sustained substantial damage during a forced landing in Ironton, Ohio. The certified flight instructor and student pilot were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The flight was being conducted under 14 CFR 91. The pilot stated, "While flying at approximately 15-1800' MSL the engine began to miss causing violent yaw. [I] immediately wrapped the throttle off and lowered the collective and advised [the student] that we were going ... to land." The pilot performed an autorotation to a yard near a house. During this maneuver, the pilot had to modify the flight path in order to avoid power lines and personnel on the ground, and the tail struck a tree. Mr. Stanley P. Faske, Principal Maintenance Inspector for the Federal Aviation Administration, conducted an examination of the helicopter. In his report, Mr. Faske stated: At the accident site, the engine was inspected and nothing was found which would have prevented the engine from producing power. On August 24, 1993 N9555 was test run.... The engine ran normally and produced power throughout all RPM ranges.