On June 9, 1998, approximately 0545 mountain daylight time, an amateur-built Stephens Vortex gyroplane, N9261N, was destroyed when it collided with terrain in an uncontrolled descent shortly after departure from the Chinook, Montana, airport. The private pilot, who was the owner and the sole occupant of the aircraft, was fatally injured. Visual meteorological conditions prevailed at the time of the accident. The aircraft burned after impact. There was no ELT on board the aircraft. The gyroplane had recently been issued a special airworthiness certificate. The private pilot, who was the builder of the gyroplane, had logged about 21 hours of flight time in gyroplanes, but did not have a gyroplane rating. A witness, who was flying in an airplane at the time of the accident, observed the gyroplane's departure from the airport, taking off from runway 08. He stated that the gyroplane "climbed on the runway heading to about 300 feet and made a left 90 degree turn. He flew about 1/2 mile to the north then made a 90 degree turn to the right and flew east about 1/2 mile. At that point, he was approximately 400-500 feet AGL [above ground level]. Suddenly the nose pitched down slightly, then rapidly upward about 40 degrees, then down rapidly about 40 degrees below horizontal then back up as before. He then seemed to fall nearly vertically downward to the ground. The aircraft looked as if it was being jerked around, falling like a leaf. Due to the fact I was in my aircraft at the time, I could not hear the engine. Having been around aviation all of my life, it appeared there was some sort of catastrophic failure to the aircraft." Statements were provided to the Safety Board concerning a main rotor blade repair that had been completed prior to this flight. FAA inspectors found no evidence of inflight breakup, or a main rotor blade strike of the empennage, and no evidence of a failed main rotor blade repair.