The student pilot was the owner/operator of the aircraft. He and the passenger were on a pleasure flight in an area of mountainous terrain. The aircraft was seen to turn into a valley where it subsequently collided with the ground at an elevation of about 5500 ft. During impact, the wings were bent down and forward; there was a 45 degree crush line on the leading edges of the wings. No ground scars were found to indicate movement after ground impact. The aircraft's last annual inspection was dated 7/22/87; however, the student pilot continued to operate the aircraft; he performed oil changes himself. No preimpact mechanical failure or malfunction of the engine was found. The student's last medical exam was dated 11/25/87. A toxicology exam of the student pilot's blood showed a saturation of 20% carboxyhemoglobin. A flight surgeon reported that this percentage could cause a headache, a state of confusion, dizziness and visual disturbance. The investigation did not determine if the aircraft heater system could have been the source of carbon monoxide.