On August 10, 2002, about 1205 mountain standard time, a Boeing B75N1, N17678, experienced a partial loss of engine power during initial climb from the Casa Grande Municipal Airport, Casa Grande, Arizona. The pilot made a forced landing in a field. During rollout, the airplane rolled into a ditch and was substantially damaged. Neither the airline transport certificated pilot/flight instructor, nor the pilot receiving instruction was injured. The flight instructor was operating the restricted category airplane under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight originated from Casa Grande about 1200. The pilot reported that 35 gallons of fuel were in the airplane's tanks when he initiated the accident flight. The engine power loss occurred between 400 and 500 feet above ground level, and the airplane began descending. The pilot slowed the airplane to its maximum glide airspeed and reversed course in an effort to return to the departure airport. Unable to reach the airport, the pilot landed in open desert terrain. During rollout, the airplane slowed to between 20 and 30 miles per hour and impacted the upslope side of a ditch. According to the pilot, the engine was subsequently examined and numerous deficiencies were noted. However, the specific reason that the engine lost partial power was not ascertained.