Accident Overview
A pilot conducting a postmaintenance test flight in day visual meteorological conditions failed to maintain altitude and collided with terrain. The flight was intended to break in six recently installed engine cylinders and was expected to last 3.5 hours. Recorded automatic dependent surveillance-broadcast (ADS-B) data indicated that the aircraft operated at altitudes below 500 ft mean sea level for most of the flight. The data ended approximately 3 hours after takeoff, with the aircraft located about 23 miles from the accident site. No witnesses were present, and the accident occurred in a remote area. When the aircraft did not return, the operator reported it as overdue to the Federal Aviation Administration. Searchers later discovered fragmented wreckage submerged in a swift-moving river, about 40 miles southeast of the departure/destination airport.
Flight Details
According to the operator, the purpose of the flight was to break in six recently installed engine cylinders. The flight was expected to last 3.5 hours. ADS-B data showed that for the majority of the flight, the aircraft operated at altitudes of less than 500 ft mean sea level. The data ended about 3 hours after takeoff, with the aircraft located approximately 23 miles from the accident site. The operator reported the aircraft overdue when it did not return.
Wreckage and Toxicology
Search teams found the fragmented wreckage submerged in a swift-moving river about 40 miles southeast of the departure/destination airport. Postmortem toxicology tests on the pilot's blood revealed a carboxyhemoglobin level of 21%. The pilot was a nonsmoker; nonsmokers typically have no more than 3% carboxyhemoglobin. There was no evidence of postimpact fire, suggesting the elevated level resulted from acute carbon monoxide exposure during the approximately 3 hours of flight before the accident. The pilot did not notify air traffic control or the operator's home base of any problems, indicating he was likely unaware of the carbon monoxide presence.
Carbon Monoxide Exposure
Early symptoms of carbon monoxide exposure can include headache, malaise, nausea, and dizziness. Carboxyhemoglobin levels between 10% and 20% may cause confusion, impaired judgment, and difficulty concentrating. While the exact symptoms experienced by the pilot cannot be determined, it is likely he had symptoms that could have been distracting and impaired his judgment and concentration. Given the low altitudes at which he was operating, there was little margin for error. The carbon monoxide exposure likely adversely affected the pilot's performance and contributed to his failure to maintain clearance from the terrain.
Mechanical Findings
The aircraft was equipped with a "winter heat kit" that modified the original cabin heat system. This modification added an exhaust/heat shroud system to provide increased cabin heat during winter operations. Maintenance records showed that the modification was not installed in accordance with Federal Aviation Administration field approval procedures. Examination of the recovered wreckage did not reveal any preexisting mechanical anomalies that would have precluded normal operation. Inspection of the right-side exhaust/heat exchanger found no leaks or fractures that could have introduced carbon monoxide into the cabin. The left-side exhaust/heat exchanger was not recovered, so it could not be determined whether it was the source of the carbon monoxide.
Probable Cause
The probable cause of the accident was the pilot's failure to maintain altitude, which resulted in collision with the terrain. Contributing to the accident was the pilot's impairment from carbon monoxide exposure in flight. The source of the carbon monoxide could not be determined because the wreckage could not be completely recovered.