The flight departed Burbank at 14:20 for Oakland and Sacramento. Following takeoff from Oakland at 16:56, the flight cruised five miles northwest of the Oakland Airport for seven minutes while waiting for receipt of an instrument flight clearance which provided for a cruising altitude of 4,000 feet to Sacramento. The flight then proceeded on course, and at 17:08 reported over the Richmond Radio Range Intersection, 16 miles northwest of Oakland, stating that it would be over the Fairfield Radio Range Station at 17:23. This was the last communication received from the flight. Proceeding along Amber Airway 8, the aircraft began to descend for unknown reasons. The aircraft struck a hill at an elevation of 782 feet or approximately 3,200 feet below the flights assigned cruising altitude. PROBABLE CAUSE: “Failure of the flight to fly at the assigned altitude on an instrument flight plan, which resulted in the aircraft striking a hill obscured by clouds.”