Shortly after reaching the cruising level of flight level 310, the aircraft encountered a severe downdraft. The seat belt sign had been on since DEPARTURE. However, one passenger was thrown into the overhead panel and fell back on to the seat arm rest, receiving five broken ribs. No fault was found with the seat belt or its buckle. The reason the passenger was not strapped in at the time of the occurrence could not be determined. Although the aircraft was in cloud at the time, no weather radar returns had been received. Severe turbulence had been forecast for the area.
