Details of the Flight
The pilot was operating a pressurized airplane at an altitude of 25,000 feet, with a cabin altitude of 10,000 feet. Over approximately 43 minutes before the accident, the Air Route Traffic Control Center (ARTCC) contacted the pilot about six times, requesting that he correct his altitude.
Communications
Between 2305 and 2311, two transmissions from the pilot were characterized by "slurred, and unclear speech." This was followed by a two-minute period during which over 30 hot microphone transmissions occurred, with heavy breathing audible in some. At 2318:20, the pilot's final transmission was: "ah Charlie Hotel, we, we've a little bit of a problem here. We're in a descent, we'll straighten it out in a minute."
Observations
Witnesses observed the airplane enter a spin and descend until it struck the ground.
Medical Findings
An autopsy of the pilot revealed moderate emphysema in the lungs, with the presence of air-filled bullae measuring up to 3 centimeters.
Previous Behavior
According to the aircraft owner, on the two flights preceding the accident flight, the pilot had slept for 2 hours out of the 4.5 hours of flight time. Several friends of the pilot reported that he was observed to "easily doze" off while on the ground, and that he did so more regularly and for longer periods while flying.
Probable Cause
According to the official investigation, the probable cause was the pilot's failure to maintain aircraft control due to his incapacitation for an undetermined reason. A contributing factor was the subsequent inadvertent stall/spin to the ground.