What happened
During a Part 135 on-demand cargo operation, the pilot was flying in instrument meteorological conditions when the flight encountered difficulties following a second missed approach. After the missed approach, the pilot notified air traffic control of plans to divert to an airport situated approximately 36 miles to the southeast. Shortly thereafter, the pilot's radio transmissions became garbled and difficult for controllers to understand, leading air traffic control to suggest the use of supplemental oxygen.
Approximately 19 minutes after the second missed approach, air traffic control observed that the aircraft had descended 1,600 feet and was failing to follow assigned vectors or instructions. The pilot did not respond to inquiries regarding the aircraft's status. Although controllers attempted to relay messages through other nearby aircraft, the only response received from the flight was unintelligible. Controllers issued multiple instructions for the pilot to use oxygen and open a window, but no further contact was established.
A witness observed the aircraft descending rapidly from a low overcast cloud layer. The aircraft then performed an abrupt, steep climb before disappearing back into the clouds, followed seconds later by the sound of a terrain impact.
Findings
Investigation into the incident noted that the pilot's communications changed suddenly, suggesting a possible impairment. Data from air traffic control and eyewitness accounts, combined with the absence of any reported mechanical malfunctions, suggest the pilot likely experienced difficulty controlling the aircraft due to impairment. While the pilot faced an increased risk of an acute cardiovascular event, available medical evidence was insufficient to confirm if such an event occurred or to quantify the specific medical risk.