What happened
A test flight intended to break in six new engine cylinders ended in a fatal accident in a remote area. The flight, conducted under visual meteorological conditions, was expected to last approximately 3.5 hours. Surveillance data indicated that the aircraft type was flying at altitudes below 500 feet mean sea level for most of the mission. Approximately three hours after departure, tracking data ceased while the plane was roughly 23 miles from the crash site. After the aircraft failed to return, the operator notified the FAA that the flight was overdue. Search teams later located fragmented wreckage submerged in a fast-moving river, situated about 40 miles southeast of the departure and arrival airport. The accident resulted in 1 fatality.
Findings
Postmortem toxicology results for the pilot revealed a carboxyhemoglobin level of 21%, significantly higher than the typical 3% found in nonsmokers. Because there was no post-impact fire, investigators believe the pilot suffered from acute carbon monoxide exposure during the flight. The pilot did not report any issues to air traffic control or the operator, suggesting the exposure may have been unnoticed. Such high levels of carbon monoxide can cause confusion, impaired judgment, and physical symptoms like nausea or dizziness. Operating at very low altitudes left the pilot with minimal margin for error if performance was compromised.
Investigation into the aircraft's systems focused on a "winter heat kit" that had modified the standard cabin heating system. Records showed this modification, which added an exhaust/heat shroud, was not installed according to FAA field approval procedures. While the recovered right-side exhaust/heat exchanger showed no leaks or fractures, the left-side component was not recovered for examination. No other mechanical failures were identified in the wreckage that would have prevented normal flight operations, leading investigators to conclude that carbon monoxide exposure likely caused the pilot to lose terrain clearance.