Flight and Initial Symptoms
The flight departed Oklahoma City, Oklahoma, with en route stops planned at Amarillo, Texas, and Tucumcari, New Mexico. At Tucumcari, the occupants reported feeling 'staggering a little' and attributed the sensation to exposure to high altitude (the en route altitude was 6,500 feet; Tucumcari elevation is 4,065 feet).
Incapacitation During Climb
After leveling at 9,600 feet on the next leg of the flight, the right front seat occupant became nauseous, vomited, and fell asleep. The pilot began feeling sleepy and subsequently passed out due to carbon monoxide. The aircraft entered a circling descent.
Attempts to Revive and Passenger Intervention
Efforts by the back seat occupants to revive those in the front seats were unsuccessful, except momentarily when the pilot asked, 'Where is the runway?' A 15-year-old passenger in the back seat took control of the aircraft by reaching between the seats. During an emergency flare-to-touchdown, the aircraft struck a fence.
Post-accident Findings
Inspection revealed multiple exhaust cracks and leaks in the heater-muffler. The aircraft had received its annual and 100-hour inspections shortly before the accident. The mechanics reported that exhaust airworthiness directive (AD) inspections were performed. The owner had performed some of his own maintenance but had allowed operation with several discrepancies.