Flight Details
The pilot, holding an Airline Transport Pilot (ATP) rating, was the sole occupant of a Beech C99 Airliner. The aircraft was operating on an Instrument Flight Rules (IFR) flight plan at an altitude of 10,000 feet mean sea level (msl) in Visual Meteorological Conditions (VMC). No autopilot was installed in the aircraft.
Loss of Radar Contact
Approximately 10 minutes after the pilot acknowledged a handoff from Los Angeles Air Route Traffic Control Center (ARTCC) to Oakland ARTCC, radar contact with the aircraft was lost at 0239 hours.
Radar Data Analysis
Review of radar data from the final four minutes of the flight revealed that the aircraft's altitude initially increased to 10,500 feet msl. Subsequently, the airplane began a left descending turn with a maximum diameter of about 2.1 nautical miles. The last radar returns indicated that the aircraft continued the left turn while descending through 5,600 feet msl at a rate of approximately 18,000 feet per minute.
Mechanical Condition
Examination of the aircraft, engines, and propellers found no evidence of mechanical malfunction.
Probable Cause
The official probable cause of the accident was determined to be loss of aircraft control at night by the pilot for unknown reasons.