What happened
A night cargo flight was delayed prior to departure because of thunderstorms forecasted along the intended flight path. The pilot had previously notified flight followers that a diversion to an alternate airport would occur if the weather became impassable. During the flight, air traffic control notified the pilot of a line of intense precipitation, including heavy and extreme cells, with no visible breaks in the weather. The controller suggested a northeast diversion to avoid the most severe cells, but the pilot opted to descend from 7,000 ft msl to 3,000 ft msl to evaluate the radar imagery.
Shortly after the descent began, the aircraft disappeared from radar due to a loss of transponder signal, which the controller attributed to weather interference. The pilot later communicated an intention to deviate from the course and eventually decided to turn back toward the alternate airport. While being cleared for the diversion, the pilot attempted to climb back to 3,000 ft msl. Following a suggested heading of 180 degrees to exit the weather, the pilot ceased all radio communications. Radar tracking subsequently showed the aircraft performing a steep right turn, with bank angles between 40 and 50 degrees and fluctuating airspeeds. The aircraft underwent an in-flight breakup at an altitude of approximately 3,500 ft msl.
Findings
Investigation of the wreckage revealed that the wings and upper fuselage sustained damage consistent with a simultaneous structural failure of the outboard wings due to positive overload. There was no evidence of mechanical failure or pre-existing structural cracks. The aircraft was operating in an area where light precipitation rapidly intensified into heavy rain, likely placing the plane in the leading edge of a convective line characterized by severe turbulence and updrafts.
At night and in instrument meteorological conditions, the combination of low visibility and turbulence likely caused the pilot to experience spatial disorientation, leading to the loss of control. Contributing factors included the pilot's decision to continue into known convective weather and a delay in executing the diversion. Furthermore, the company's flight risk assessment tool (FRAT) was not utilized for this flight because the assigned flight followers had not been trained to complete assessments for night cargo operations, and management was unaware of this training gap.