On December 16, 1997, approximately 1415 mountain standard time, a Bell 206L helicopter, N206LT, collided with the terrain during a forced landing about 15 miles south of Idaho Falls, Idaho. The airline transport pilot and his passenger were not injured, but the aircraft, which was owned by Tundra Copters, Inc., of Fairbanks, Alaska, and operated by Briles Helicopters, Inc., of Van Nuys, California, sustained substantial damage. The 14 CFR Part 91 repositioning flight, which was being conducted in visual meteorological conditions, departed Idaho Falls about ten minutes prior to the accident, and was en route to Salt Lake City International Heliport. No flight plan had been filed, and there was no report of an ELT activation. The pilot reported that the aircraft had been sitting on the ramp for a little over two months prior to the initiation of this flight. He said that during that time, the fuel tank was 17 gallons short of being full, but that it had been topped off prior to the flight. He reported that fuel had been drained from the aircraft prior to starting the engine, and that after finding no contamination or water accumulation, the aircraft was run on the ground for about 20 minutes. After the ground-run, the helicopter was hovered over the airport ramp for about five minutes prior to calling for departure. The pilot said that after departure he remained at a low altitude because he had overheard the pilot of another aircraft announce that he was inbound from the direction in which the helicopter pilot was departing. He said that about ten minutes after departure, while cruising at 5,500 feet MSL (760 feet AGL) and 100 knots indicated airspeed, the engine suddenly lost all power. The pilot elected to autorotate to a nearby open farm field, where the helicopter touched down on muddy terrain and slid forward until contacting a small ridge of frozen dirt. Upon impacting the ridge, the skids collapsed, and the main rotor blades contacted the tail boom. A Federal Aviation Administration Inspector from the Salt Lake City, Utah Flight Standards District Office inspected the engine and fuel system, and reported that prior to the removal of the wreckage from the field, the engine was started and run without any noted problems. He also said that fuel was drained from the aircraft and no contamination or water accumulation was noted. No anomalies or any evidence of a malfunction in the aircraft's systems were noted.