What happened
During an en route descent to its destination, a passenger transport encountered severe turbulence. The flight forecast for the area had indicated the development of thunderstorms in the vicinity of the destination airport. While the crew noted that the destination ATIS reported severe turbulence on approach, they did not recall receiving any specific ATC advisory stating that the turbulence was severe.
In response to a request from ATC to reduce speed for traffic separation, the pilot slowed the aircraft to a velocity below the recommended speed for turbulence penetration. At the time the aircraft entered increasing turbulence, it was performing a turn to avoid clouds and virga, with the speedbrakes extended and the autopilot engaged. During the encounter, the airplane pitched up 20 degrees and rolled to 45 degrees. The pilot successfully righted the aircraft and ceased the descent 500 feet below the assigned altitude.
Findings
An analysis of the flight parameters indicated that the extension of speedbrakes increases the roll rate of the aircraft. Additionally, it was noted that when using the autopilot in severe turbulence, use is recommended only in the pitch-hold mode.