Aeronca 7AC N84580 accident near Ellsworth, Nebraska

Probable cause

The pilot's impairment due to carbon monoxide poisoning from a known cracked engine exhaust muffler, which resulted in a loss of aircraft control. Contributing to the accident was the pilot's decision to continue flying the airplane without properly repairing the exhaust muffler.

NTSB analysis

The private pilot departed for a local personal flight on a winter day with an outside air temperature of about 6°C. About 1 hour after takeoff, the pilot's brother saw the airplane maneuvering near his home, which was in a rural area about 31 miles from the departure airport. The airplane did not return to the departure airport, and the accident site was located in an open field 2 days later, about 4 miles from the pilot's brother's home. Examination of the accident site revealed wreckage and impact signatures consistent with the pilot losing control of the airplane. Examination of the engine's exhaust muffler revealed cracks in several locations, and the muffler's shroud contained a layer of exhaust residue. Six months before the accident, the pilot and the mechanic who had previously performed an annual inspection on the airplane became aware of a crack in the muffler near a weld that the pilot had performed. The pilot had purchased a replacement muffler, but it was not installed before the accident. A carbon monoxide detector was not on board the airplane.

Toxicology testing of the pilot's blood revealed a carbon monoxide level of 40%, which was more than enough to severely impair the pilot. The carbon monoxide likely entered the airplane's cabin because of the cracked engine exhaust muffler. The toxicology testing also revealed several non-impairing medications and two potentially impairing medications (temazepam and buspirone). According to the pilot's medical records, he was being treated for anxiety with temazepam and buspirone, and he may have been fatigued from insufficiently treated sleep disorders (insomnia and obstructive sleep apnea). However, it could not be determined whether the pilot's anxiety, the medications used to treat it, or fatigue contributed to his poor judgment in flying the airplane with known cracks in the exhaust muffler.

Text from the NTSB. Open the NTSB docket for CEN16FA130 for the full record.