What happened
The aircraft was cleared to descend from FL410 to FL2D30, but an air traffic control controller later amended the clearance to FL270 to provide the flight with a smoother ride. Shortly after the aircraft leveled off at FL270, approximately 25 NM east of Grand Rapids, Michigan, the flight encountered sudden, moderate turbulence.
At the time of the encounter, a passenger was exiting the lavatory. The passenger fell during the turbulence and sustained serious injuries. Approximately five minutes prior to the event, the seat belt sign had been illuminated, and the captain had announced to passengers that they should expect bumps during the approach. The aircraft's lavatories are equipped with a flight attendant call button, a public address speaker, and a 'return to seat' sign that illuminates whenever the cabin seat belt sign is active.
Findings
The flight crew reported that the onboard weather radar did not indicate any weather within an 80-mile scan. However, data from the company's dispatch and weather department indicated that the aircraft had been flying through an area containing 3/10ths coverage of level three and four thunderstorms.