Flight Release and SIGMET Updates
Approximately three hours before departure, a dispatcher approved a flight release containing SIGMET Whiskey 8, which forecast occasional severe turbulence from 14,000 feet to flight level 280. The turbulence area overlaid the departure airport and the planned en route climb, but the top of descent (TOD) and destination airport were clear. The dispatcher then went off duty.
When the pilot printed the flight release, SIGMET Whiskey 8 had been replaced with SIGMET Whiskey 9. The turbulence box had shifted east of the departure airport, and the TOD and destination remained outside the forecast area. The release also included a single pilot report of severe turbulence from a Boeing 737 at flight level 240 within the defined turbulence area.
Prior to departure, after the pilot had signed the release, the flight release was updated again, this time with SIGMET Whiskey 10. The turbulence box moved further east to cover both the TOD and the destination airport, encompassing a region stretching from Ottawa, Canada, to Florida, to Cleveland, Ohio.
Preparation for Descent
Nearing his destination, the pilot descended into the turbulence box defined by both SIGMET Whiskey 9 and Whiskey 10. Before entering the area, the pilot turned on the seat belt sign, asked the flight attendant to be seated, and made an announcement for passengers to remain seated, as they were within 30 minutes of the destination airport.
Encounter with Severe Turbulence
While descending through 17,800 feet, the flight encountered severe turbulence. The airplane was not equipped with ACARS (Aircraft Communications Addressing and Reporting System). According to operator procedures, flight crews were required to monitor the dispatch frequency for updates and were encouraged to obtain weather updates while en route. Weather updates were accomplished through direct radio contact between the dispatcher and pilots, or by pilots accessing FAA facilities during the flight.
Broader Context
The operator had approximately 100 flights operating within the turbulence box defined by the SIGMETs. None of those flights were cancelled due to forecast turbulence, nor did any other flight report encountering severe turbulence.