Background
A pilot was returning a group of skydivers to their home base after a weekend of skydiving. After flying several jump flights, the pilot stopped early in the afternoon to prepare the airplane for the return flight. The flight was planned into an area of clouds, turbulence, and icing, which the pilot had researched. He delayed the departure until he decided that he could complete the planned flight under visual flight rules (VFR).
Weather Conditions
The accident occurred at night with little illumination of the moon, and the airplane was in an area of layered clouds. A detailed analysis of the weather conditions revealed that the flight probably encountered broken to overcast layers both below and above its flight altitude. Satellite and sounding imagery suggested that the airplane was possibly in an area of mountain wave conditions, which can enhance icing.
Flight Profile
Recorded radar data indicated that the pilot was likely maneuvering to go around, above, or below rain showers or clouds while attempting to maintain VFR. The airplane likely entered clouds during the last 3 minutes of flight, and possibly experienced icing and turbulence. The airplane was turning when it departed from controlled flight.
Aerodynamic Stall
A performance study showed that the angle-of-attack was increasing rapidly at the time of the departure. The study determined that the departure from controlled flight was consistent with an aerodynamic stall.
Pilot Experience and Hypoxia
The unpressurized airplane was flying at over 14,000 feet mean sea level for more than 1 hour during the flight. It reached 15,000 feet just prior to the accident in sequential 360-degree turns while climbing and descending. Supplemental oxygen was not being used. At these altitudes, the pilot would be substantially impaired by hypoxia, but would have virtually no subjective symptoms, and would likely be unaware of his impairment. The pilot had logged over 2,000 hours of total flight time, with nearly 300 hours in this make and model of airplane. He was instrument-rated, but had only logged a total of 2 hours of actual instrument flight time. Company policy was to fly under visual flight rules only, and they had not flight-checked the pilot for instrument flight.