Flight Planning and Departure
The airline transport pilot delayed the scheduled departure of a night cargo flight due to thunderstorms along the route. Before departing, the pilot explained to the flight follower that if he could not get through the thunderstorms along the planned route, he would divert to the alternate airport.
En Route Communications
While en route, the air traffic controller advised the pilot of a "ragged line of moderate, heavy, and extreme" precipitation along the planned route and stated that he did not see any breaks in the weather. The controller cleared the pilot to descend at his discretion from 7,000 ft mean sea level (msl) to 3,000 ft msl and later suggested a diversion to the northeast for about 70 nautical miles to avoid the most severe weather. The pilot responded that he had enough fuel for such a diversion but concluded he would "see what the radar is painting" after descending. About 1 minute 30 seconds later, as the airplane descended through 7,000 ft msl, the controller stated that he lost the airplane on radar, possibly due to the weather. About 40 seconds later, the pilot advised that he intended to deviate to the right of course, and the controller permitted turns as needed. Shortly thereafter, the pilot stated he was going to turn around and proceed to his alternate airport. The controller cleared the pilot direct to the alternate and instructed him to maintain 3,000 ft msl. The pilot acknowledged, and the controller offered any altitude. The pilot said he would try to climb back to 3,000 ft msl. The controller recommended a heading of 180°, and the pilot responded, "alright 180." There were no further communications.
Loss of Control
Radar data showed the airplane enter a right turn that continued through about 540°. During the turn, its airspeed varied between 198 and 130 knots, and estimated bank angles were between 40 and 50°. The airplane's last recorded altitudes were around 3,500 ft msl.
Wreckage Examination
Examination of the wreckage indicated that the airplane experienced an in-flight breakup at relatively low altitude. The symmetrical nature of the breakup, damage to the outboard wings, and damage to the upper fuselage were signatures indicative that the left and right wings failed in positive overload almost simultaneously. All fracture surfaces had a dull, grainy appearance consistent with overstress separation. There was no evidence of pre-existing cracking or any mechanical anomalies that would have prevented normal operation.
Weather and Conditions
Review of base reflectivity weather radar data showed that while the pilot was maneuvering to divert, the airplane operated in an area of light precipitation that rapidly intensified to heavy precipitation. During this time, the flight was likely operating in clouds along the leading edge of the convective line, where the pilot most likely would have encountered updrafts and severe or greater turbulence. The low visibility conditions at night and in instrument meteorological conditions, coupled with the turbulence, were conducive to the development of spatial disorientation. The airplane's maneuvering during the final moments was consistent with a loss of control due to spatial disorientation. The pilot's continued flight into known convective weather conditions and his delayed decision to divert directly contributed to the accident.
Operator Procedures
Although the operator had a system safety-based program, the responsibility for the safe outcome of the flight was left solely to the pilot. Written company policy required completion of a flight risk assessment tool (FRAT) before each flight by the assigned flight follower; however, a FRAT was not completed for the accident flight. The flight followers responsible for completing the FRATs were not trained to complete them for night cargo flights, and the operator's management was not aware that the FRATs were not being completed for night cargo flights. Further, if a FRAT had been completed, the resultant score would have allowed the flight to commence into known hazardous weather conditions without further review. If greater oversight had been provided by the operator, it is possible that the flight may have been cancelled or re-routed due to the severity of the convective weather conditions along the planned route.