Incident Overview
A scheduled domestic passenger air carrier flight experienced severe turbulence while in cruise flight within an area covered by a convective SIGMET. The flight crew reported that they had maneuvered to pass to the north of convective activity. They observed a patch of blue sky to the right front of the aircraft, with no radar returns indicating precipitation or turbulence. Without warning, a large buildup appeared directly in front of the airplane with less than two seconds notice. The aircraft then encountered severe turbulence and small hail.
Sequence of Events
Just prior to the encounter, the captain had released the flight attendants to resume cabin service. Following the release, the 'A' flight attendant informed the flight crew that as the flight attendants in the galley were removing their seat belts, the 'B' flight attendant sustained an injury—a broken ankle. The exact timing of the turbulence relative to the seat belt removal was not specified.
Weather Conditions
Other aircraft operating in the vicinity reported no conditions greater than light turbulence, indicating that the severe turbulence was localized and sudden. The encounter occurred despite the crew's efforts to avoid known convective activity and the apparent absence of radar returns in the area they entered.
Response and Reporting
The air carrier reported the incident, detailing the sudden nature of the turbulence encounter and the resulting injury to the 'B' flight attendant. No other injuries or damage to the aircraft were noted in the report.
Remarks
The incident highlights the challenges of convective weather avoidance, where clear skies and lack of radar returns can still conceal rapidly developing hazardous conditions.