What happened
The aircraft was operating at flight level 225 during nighttime conditions when it encountered moderate to severe turbulence. The incident occurred as the plane passed momentarily through clouds. A flight attendant sustained a broken ankle during the encounter.
Prior to entering the cloud layer, the captain reported that onboard radar did not indicate any weather ahead. The flight crew activated the seat belt warning light approximately 15 seconds before entering the clouds, which was the moment they first visually observed the cloud formation.
The investigation
The investigation focused on the meteorological conditions and the aircraft's radar capabilities at the time of the encounter. The captain's report confirmed that the weather radar showed no indications of precipitation or storm activity in the immediate path prior to the turbulence event.
Findings
The primary factors contributing to the injury were the unexpected nature of the turbulence and the limitations of visual detection during nighttime operations. The flight crew attempted to mitigate risk by illuminating the seat belt sign upon visual confirmation of clouds, but the onset of turbulence was rapid enough to cause injury before all passengers could be secured.
Safety message
This incident highlights the potential for clear air turbulence or unforecast weather cells to occur even when radar displays are clear. It underscores the importance of keeping seat belts fastened while seated, particularly during night flights where visual cues for weather may be limited or delayed.