Background
The pilot purchased the aircraft approximately one month before the accident. He had limited experience in that specific aircraft and limited familiarity with high-altitude flight and its associated weather phenomena.
Weather Briefing
The pilot obtained two weather briefings prior to departure. During both briefings, he was advised of forecast and pilot-reported conditions of moderate to severe mixed icing and turbulence associated with a mountain wave on the eastern side of the Sierra Nevada mountains.
Flight Path
Instead of following his intended route along the western side of the mountains, the pilot chose a direct route that placed the aircraft over the eastern side. Recorded radar data established the aircraft's track. The data showed the aircraft cruising at 16,500 feet until about one minute before the accident. It then climbed to 17,200 feet before radar contact was lost. The last radar target was observed approximately two miles from the impact location.
Accident Sequence
The aircraft impacted terrain in a steep, nose-down descent. All four occupants were killed.
Probable Cause
The official probable cause was an inflight encounter with forecasted severe icing and turbulence beyond the capability of either the aircraft or the pilot, which led to an inflight loss of control. Contributing factors included the pilot's lack of understanding of high-altitude flight aspects and meteorological phenomena.