On April 27, 2007, about 1345 eastern daylight time, a Hiller UH-12C helicopter, N92993, registered to and operated by Tampa Bay Helicopters Inc., as a Title 14 CFR Part 91 instructional flight, made a forced landing in a field following loss of engine power near Plant City, Florida. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight. The commercial-rated flight instructor was not injured, and the student received serious injuries. The flight originated at Vandenberg Airport, Tampa, Florida, the same day, about 1300. The flight instructor stated that he was conducting a discovery flight with a student, and all of a sudden, while the helicopter was in cruise flight at an altitude of 600 feet, the engine shook violently, shaking the whole helicopter. He said this lasted for several seconds, and then the engine ceased operating. He said he immediately entered autorotation and turned the helicopter to a westerly heading into the wind. He saw that he was not going to reach the large open field he wanted to reach, so the pilot said he selected a secondary field which was surrounded by trees of varying heights. As he was descending below the trees the pilot said the wind stopped, and it appeared that the helicopter was going to impact the northernmost tree perimeter, so he turned the helicopter. By the time he was clear of the trees the pilot said they were at an altitude of about 5 to 6 feet, and "there was nothing left to cushion the descent", so the helicopter fell to the ground, incurring damage. Postcrash examinations by an FAA licensed airframe and powerplant mechanic, under the supervision of an FAA airworthiness inspector revealed that the right magneto impulse coupling had failed and caused the impulse coupling to impact the stud used for starting. According to the inspector, this resulted in gear train stoppage, and the engine ceasing to operate. The maintenance log revealed that the helicopter had last been given an annual inspection on June 5, 2006.