What happened
The flight departed Oklahoma City, Oklahoma, with planned en route stops at Amarillo, Texas, and Tucumcari, New Mexico. During the cruise portion of the journey, the occupants noticed symptoms consistent with high-altitude exposure, staggering slightly while flying at approximately 6,500 feet and later at Tucumcari's elevation of 4,065 feet.
On the subsequent leg of the flight, after leveling off at 9,600 feet, the front right seat occupant became nauseous, vomited, and fell asleep. The pilot began to feel sleepy and subsequently lost consciousness due to carbon monoxide poisoning. The aircraft entered a circling descent. Efforts by rear-seat occupants to revive the pilot were largely unsuccessful, though the pilot briefly regained awareness enough to ask where the runway was.
A 15-year-old passenger in the back seat took control of the aircraft by reaching between the front seats. During an emergency flare-to-touchdown attempt, the N-number (registration not explicitly provided in source text, but implied as part of the event) struck a fence.
The investigation
Examination of the aircraft revealed multiple exhaust cracks and leaks in the heater-muffler assembly. Records indicated that the aircraft had undergone recent annual and 100-hour inspections. Mechanics reported that exhaust airworthiness directives and inspections had been completed. However, the owner had performed some maintenance personally and allowed the aircraft to operate with several known discrepancies.
Findings
The primary factor was the failure of the heater-muffler exhaust system, which allowed carbon monoxide to enter the cabin. This led to the incapacitation of the pilot and passengers. The presence of multiple exhaust cracks indicates a mechanical failure that was not detected during previous inspections or maintenance activities.