Incident Overview
On April 2, 2024, at approximately 1056 central daylight time, a Mooney M20E airplane with registration N86UM sustained substantial damage during a forced landing near Alpine, Arkansas. The pilot, who was the sole occupant, received minor injuries. The flight was operated under 14 CFR Part 91 as a personal flight.
Pilot Actions and Symptoms
According to the pilot, while cruising in a gradual descent, both carbon monoxide (CO) detectors activated. He closed the cabin panel vent, opened a small cockpit window, and four overhead vents. The pilot reported feeling "intense worry and fear" as he noticed lethargy, confusion, and graying peripheral vision. He attempted to open the cabin door unsuccessfully. Deciding not to continue to the nearest airport, which was about 30 miles away, he shut down the engine and initiated a forced landing to a field. After engine shutdown, his symptoms gradually subsided, and he did not lose consciousness.
Damage and Postaccident Findings
During the forced landing, the airplane impacted unsuitable terrain, nosed over, and sustained substantial damage to the forward fuselage. After the accident, the pilot photographed his iPad, which showed a warning: "Caution, Sentry CO level is 79 ppm."
Postaccident examination revealed an existing repair weld on the exhaust pipe containing multiple cracks and holes, with engine oil and exhaust sooting present. A cabin air scat tube nearby showed residual engine oil, black soot, and missing material. The muffler had a pinhole along a weld edge. Additionally, the cabin air system's junction box had two of three outlet ducts covered with sections from an aluminum beer can and hose clamps.
Maintenance History
Maintenance records indicated that a cracked exhaust pipe was replaced on August 16, 2019. Another record, dated November 18, 2021, showed the exhaust pipe was removed, repaired, and reinstalled at a tachometer time of 5,447.56 hours. The most recent annual inspection, about nine months before the accident, recorded a tachometer time of 5,478.49 hours.
Medical Evaluation
The pilot was evaluated at a hospital emergency department following the accident. He sustained minor injuries not requiring hospitalization, and no carbon monoxide poisoning was identified. Blood collected at 1248 on the accident date showed a carboxyhemoglobin level of 1% (normal) via arterial blood gas testing with co-oximetry. The pilot was a nonsmoker. No oxygen administration was documented.
Carbon Monoxide Background
Carbon monoxide is an odorless, tasteless, colorless, nonirritating gas produced during hydrocarbon combustion. Exposure typically occurs by inhaling smoke or exhaust fumes. CO binds to hemoglobin, forming carboxyhemoglobin, which impairs oxygen delivery. Nonsmokers normally have carboxyhemoglobin levels under 1 to 3%; heavy smokers may reach 10 to 15%. Symptoms of low-level exposure are nonspecific and variable, including headache, nausea, and tiredness. Higher levels can cause impairment or incapacitation, with neurocognitive, cardiac, or vision problems, and can be fatal above about 50% carboxyhemoglobin, though symptoms depend on individual factors. After exposure ceases, carboxyhemoglobin levels typically decrease by half in 4 to 5 hours, or 1 to 1.5 hours with high-flow oxygen. CO poisoning can be insidious and hard to recognize.
The Occupational Safety and Health Administration's 8-hour occupational exposure limit is 50 ppm. Acute exposure guidelines suggest irreversible effects or impaired escape ability at 420 ppm for 10 minutes, 150 ppm for 30 minutes, or 83 ppm for 60 minutes.