CEN24LA145 · Mooney M20E

During cruise flight in a gradual descent, the airplane’s two carbon monoxide (CO) detectors began alerting. The pilot closed the cabin panel vent and opened a small cockpit window and 4 overhead vents. The pilot’s concern “turned to intense worry and fear as [he] noticed a feeling of lethargy and confusion, sort of slow motion coupled with graying peripheral vision.” He attempted to open the cabin door but was unsuccessful. The pilot decided not to continue the flight to the nearest airport, which was about 30 miles from his position; he shut down the engine and performed a forced landing to a field. The pilot reported that after he shut down the engine, his symptoms gradually dissipated, and he did not lose consciousness. During the forced landing, the airplane impacted unsuitable terrain, nosed over, and sustained substantial damage to the forward fuselage. After the accident, the pilot took a photograph of his iPad, which displayed, “Caution, Sentry CO level is 79 ppm.” Postaccident examination of the airplane revealed the exhaust pipe contained an existing repair weld that had multiple cracks. The cracks contained engine oil and exhaust soot deposits, indicating the cracks existed before impact. In addition, a cabin air scat tube displayed residual engine oil, black soot, and missing material in an area next to the exhaust pipe repair weld. The exhaust cracks and damaged scat tubing would have allowed exhaust gases to enter the airplane cabin. The cabin air system contained a junction box, and two of three outlet ducts were covered with sections from an aluminum beer can and hose clamps. The sections of the aluminum beer can were not approved components. The last inspection of the airplane occurred just over 9 months before the accident; the investigation was not able to determine how long the exhaust cracks and damaged tubing may have been present. The pilot’s carboxyhemoglobin level of 1% that was measured less than 2 hours after the accident indicates that he was unlikely to have been experiencing CO poisoning during the off-airport landing or during his reported symptoms. Physiological responses to acute stress likely contributed to those symptoms. Although the pilot did not likely experience CO poisoning, the CO alerts, cracked exhaust pipe, and damaged cabin air tubing indicate he likely was exposed to abnormally high CO concentrations in the cockpit air during the accident flight. Whether CO exposure leads to CO poisoning depends on exposure magnitude and duration, which is why pilots are encouraged to take early protective action when exposure is suspected.

Verbatim NTSB analysis and probable cause from the NTSB dataset