On March 11, 1998, at 1600 central standard time, a Bell 47D1 helicopter, N13012, registered to and operated by Penn Tex Helicopters, Inc., was substantially damaged during a forced landing following a loss of control near Jeanerette, Louisiana. Visual meteorological conditions prevailed, and a flight plan was not filed for the Title 14 CFR Part 91 instructional flight. The flight instructor was not injured, and the commercial pilot receiving instruction sustained minor injuries. The flight originated from the Penn Tex Helicopters Heliport, Jeanerette, Louisiana, about 45 minutes prior to the accident. The flight instructor reported to the NTSB investigator-in-charge that his student was practicing landings in the local practice area, and they had completed 4 to 5 landings. On the next landing, they were heading west, level at 350 feet AGL, with an airspeed of 35 mph. As the student lowered the collective to initiate a steep approach, a severe vertical vibration was encountered. About 2 to 3 seconds later, a severe lateral vibration began. The flight instructor took the controls and continued the descent towards the landing area. "At about 100 feet I saw that we were in [a] direct collision course with a ditch and so I moved the cyclic to the left. This resulted in a slight left turn which turned us away from the ditch." At about 10 feet the instructor tried to reduce forward airspeed and descent rate by moving the cyclic aft and raising the collective. "Neither of those inputs seemed to help a lot and we impacted hard." The helicopter rolled over and came to rest heading east. The operator/instructor reported no problems with tail rotor control. Examination of the helicopter by the FAA inspector revealed that the left skid and tail boom were found separated from the fuselage, the main rotor and tail rotor sustained damage. Further examination of the main rotor system revealed that the scissor hinge bolt which attaches the scissor to the swash plate was missing. He added that according to the maintenance records, the 5, 258 hour helicopter, underwent a 100 hour inspection on February 15, 1998, approximately 20 hours prior to the accident.