On July 24, 1995, approximately 1715 mountain daylight time, an Air Tractor AT-301, N3660U, impacted the terrain and nosed over while maneuvering over an open field north of Havre, Montana. The airline transport pilot, who was the sole occupant of the aircraft, was not injured, but the aircraft sustained substantial damage. The aerial application flight, which departed Havre Airport about 20 minutes earlier, was being operated in visual meteorological conditions at the time of the accident. No flight plan had been filed, and there was no ELT activation. According to the pilot, the aircraft's engine experienced a total power loss, which was followed almost immediately by a wing tip being caught by the tall barley he was spraying. After catching the wing, the aircraft cartwheeled and came to a rest inverted in the field. Investigation revealed that an exhaust valve rocker box had failed at the point where the rocker pivot bolt passes through the box. An NTSB metallurgist made a visual inspection of the fracture surface and stated that it appeared to show evidence of fatigue.