On April 20, 2003, about 1730 mountain daylight time, a Piper PA-38 Tomahawk, N9705T, sustained substantial damage during an off airport forced landing following a loss of engine power near Trident, Montana. The airplane is registered to Aero Flight School of Belgrade, Montana, and was being operated as a visual flight rules (VFR) personal/pleasure flight under the provisions of Title 14, CFR Part 91, when the accident occurred. The private pilot, the sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and no flight plan had been filed. The cross-country flight originated from Gallatin Field, Bozeman, MT (BZN), with a stop at Helena Regional, Helena, MT (HLN) and a planned return to BZN. During a telephone interview and subsequent written statement, the pilot reported that the purpose of the flight was to use up the aircraft's fuel in order to perform a weight and balance. The pilot also stated that he felt he had enough fuel on board for the round trip. Approximately thirty minutes after leaving Helena Regional (HLN), the engine experienced a complete power loss. Following the loss of power, the pilot elected to land in a nearby field. After a "hard" landing, and during the landing rollout, the airplane collided with a fence post resulting in substantial damage. Personnel who recovered the airplane reported that both fuel tanks were empty.