Background
Prior to the flight, the pilot obtained an outlook briefing that included information about widely scattered thunderstorms and rain showers along the planned route.
Weather Information
At the time of the accident, a convective SIGMET (significant meteorological information for convective weather) was in effect for the route and was broadcast by air traffic control (ATC). Several aircraft in the vicinity were deviating around the weather. Communications between the pilot and the ATC controller indicated that the accident airplane's weather radar was functioning and possibly providing more detailed information than the ATC weather radar.
Flight Sequence
The pilot initially declined a deviation query from ATC but later accepted one. Shortly after accepting the deviation, the pilot was unable to maintain the assigned altitude of 28,000 feet mean sea level (FL 280). The airplane subsequently impacted terrain in a manner consistent with a vertical descent.
Weather Radar Observations
At the accident time and altitude (FL 280), weak to moderate weather radar echoes were present. Very strong to intense weather radar echoes existed around FL 200. The ATC facility was equipped with NEXRAD-derived weather displays offering four altitude settings: below FL 240, between FL 240 and FL 330, above FL 330, and from sea level to FL 600. The controller's display was set to show echoes above FL 240, revealing weak to moderate returns at that level. The strong echoes at FL 200 were not visible on the controller's display because that altitude was not monitored for traffic. The investigation could not determine whether the pilot was aware of the stronger intensity echoes below his altitude or whether the airplane's weather radar depicted them.
Mechanical Examination
Examination of the wreckage found no evidence of preimpact mechanical malfunctions.
Probable Cause
The official probable cause is the pilot's inadvertent encounter with thunderstorms.