What happened
The pilot was operating an aircraft under instrument flight rules at an altitude of 12,000 feet. Seeking to escape instrument meteorological conditions, the pilot contacted air traffic control and requested a higher altitude. The controller cleared the pilot to climb and maintain 14,000 feet. The pilot acknowledged this clearance verbally at 18:55, but was not heard from again on radio communications. Radar contact with the aircraft was subsequently lost at approximately 19:00.
Approximately one hour prior to these events, the pilot had been advised of a significant meteorological advisory (SIGMET). This advisory warned of moderate to severe turbulence below 18,000 feet, characterized by strong updrafts and downdrafts. Despite this warning, the pilot did not report any mechanical problems or distress to air route traffic control centers during the period when the aircraft's heading and ground speed deviated from the planned flight path.
The investigation
Analysis of air traffic control radar data revealed significant anomalies in the aircraft's performance immediately following the altitude clearance. The aircraft's ground speed dropped sharply from 110 knots to 70 knots at approximately the time the pilot acknowledged the climb instruction. Shortly after this decrease, the ground speed increased to 90 knots.
In addition to the erratic ground speed, the aircraft changed its heading by approximately 90 degrees to the right of its original westerly course. The aircraft continued on this new northerly track until it disappeared from radar coverage. Examination of the air traffic control automation system confirmed these trajectory and speed deviations occurred without any reported pilot input regarding equipment failure or emergency.
Findings
The deviation in ground speed and heading suggests the aircraft encountered significant atmospheric disturbances consistent with the previously reported SIGMET. The loss of radio contact shortly after acknowledging the clearance, combined with the radar data showing uncommanded course changes and speed fluctuations, indicates the pilot may have been incapacitated or unable to maintain control due to the severe turbulence.
The absence of any distress call or report of problems to air traffic control during the critical period implies that the situation developed rapidly or that the pilot was unable to communicate. The weather conditions described in the SIGMET provided a plausible explanation for the physical forces acting on the aircraft.