Accident Summary
An aircraft crashed during an uncontrolled descent after a turbulence upset at flight level 370. The upset occurred due to clear air turbulence associated with a VIP level 6 thunderstorm located within 7 miles of the aircraft's last known position. Following the upset, the aircraft penetrated and descended through the thunderstorm cell, which contained lightning, extreme turbulence, and severe icing. During the upset and descent, both engines flamed out. The crew's attempts to recover the aircraft were unsuccessful due to the conditions within the cell.
Flight and Weather Encounter
The crew had received an improper briefing from the Flight Service Station (FSS) and reported to the deputy controller that their weather radar was malfunctioning. SIGMET and AWW information was not provided during the briefing. As the aircraft exited the bottom of the thunderstorm cell at 4,000 feet, the crew overstressed the airframe. The overstress caused the left main landing gear door to separate, which then struck the left horizontal stabilizer, causing the stabilizer to separate due to overload. The aircraft impacted terrain at an angle of 82° nose down.
Investigation Findings
The investigation determined that the nature of the aircraft's radar malfunction could not be determined. The official findings listed multiple occurrences, including: failure of the weather radar system; continued operation with known equipment deficiencies; encounter with a thunderstorm, clear air turbulence, gusts, and downdraft; dark night conditions; improper preflight briefing by FSS personnel; hazardous weather advisory not issued; in-flight weather advisories not issued; unclear information from FSS; excessive workload on the pilot and copilot; loss of engine power (total, nonmechanical); and a subsequent uncontrolled descent leading to collision with terrain.