IMPROPER INFLIGHT PLANNING/DECISION BY THE PILOT, AND HIS FAILURE TO MAINTAIN SUFFICIENT ALTITUDE OVER MOUNTAINOUS TERRAIN. FACTORS RELATED TO THE ACCIDENT WERE: FAILURE OF THE PILOT TO OBTAIN (AND/OR UNDERSTAND) WIND ALOFT INFORMATION, HIS LACK OF FAMILIARITY WITH THE TYPE OF OPERATION (MOUNTAIN FLYING), A DOWNDRAFT, AND THE TERRAIN CONDITIONS.