Accident Overview
A Part 135 on-demand cargo flight operating in instrument meteorological conditions experienced a loss of control following a second missed approach. The pilot informed air traffic control (ATC) of plans to divert to an airport approximately 36 miles to the southeast. About 7 minutes after the missed approach, the pilot's radio communications became unintelligible, described by ATC as very garbled and difficult to understand, "almost hypoxic." ATC instructed the pilot to use oxygen.
Communications and Maneuvers
Approximately 19 minutes after the second missed approach, ATC noted the airplane had descended 1,600 ft and was not following assigned course vectors or instructions. When asked if everything was alright, the pilot did not respond. ATC attempted to relay communications through other aircraft in the area, receiving only an unintelligible response from the accident airplane. Subsequent transmissions urging oxygen use and to open a window went unanswered.
Witness Account
A witness reported seeing the airplane descend out of a low overcast cloud layer at a high rate of descent. The airplane abruptly transitioned into a steep climb before re-entering the clouds. Seconds later, the witness heard the airplane impact terrain and responded to the accident site.
Probable Cause
According to the investigation, the sudden change in communications indicated possible pilot impairment. Combined with ATC data and the witness account, and the lack of mechanical irregularities or malfunctions, it is likely the pilot had difficulty controlling the airplane due to impairment. Although the pilot was at increased risk for an acute cardiovascular event, the limited medical evidence could not quantify that risk or confirm whether such an event occurred. The probable cause was determined to be an in-flight loss of control as a result of pilot impairment, the cause of which could not be determined.