What happened
During an instrument flight rules (IFR) flight, a Beech C99 Airliner was operating at 10,000 feet MSL under visual flight rules (VFR) conditions. The aircraft was being operated by a single ATP-rated pilot. Shortly after receiving a hand-off from Los Angeles Center to Oakland Center, the aircraft's position was no longer visible on radar at 0239 hours.
Analysis of available radar data showed that in the four minutes preceding the loss of contact, the aircraft's altitude rose to 10,500 feet. Following this climb, the plane entered a left-hand descending turn with a maximum diameter of approximately 2.1 nautical miles. The final radar returns tracked the aircraft descending through 5,600 feet MSL, noting an extremely high descent rate of roughly 18,000 feet per minute as it continued the turn.
Findings
Investigation into the radar data provided no evidence of any mechanical failure involving the engines, propellers, or the airframe itself. It was noted that the aircraft was not equipped with an autopilot system.