What happened
The pilot had climbed to 5,500 ft MSL and trimmed the aircraft at cruise power settings. After setting the GPS for a direct flight, he engaged the autopilot and switched the fuel selector to the left auxiliary fuel tank. The pilot recalled passing Herrington, KS, and setting his radios for arrival in Topeka, KS. Following this point, the pilot had no further memory of the flight until he awoke in a flat hay field near Cairo, MO.
The aircraft touched down in a wings level attitude and slid approximately 525 ft before striking a fence and small trees. Upon waking, the pilot experienced confusion and initially believed he was still airborne. He eventually walked to a farmhouse, where he waited between 1 and 1.5 hours for an ambulance to transport him to a hospital.
The investigation
Toxicology tests performed on the pilot's blood revealed a 26.8% saturation with carbon monoxide. An inspection of the aircraft identified a crack around one of the seams of the right muffler. This defect allowed exhaust fumes to enter the cabin heater housing and subsequently into the cabin. Records indicated the airplane had recently been at a service center for an annual inspection, and the pilot had elected to fly the aircraft before the annual inspection had been officially signed off.