What happened
The aircraft encountered moderate turbulence while climbing through flight level 180 (FL180) within cloud cover. At the time of the incident, there were no weather radar returns indicating turbulent conditions, and neither Air Traffic Control nor other aircraft had reported any turbulence in the area.
During the event, one flight attendant located in the aft galley sustained minor injuries. A passenger seated nearby was thrown from her seat into the aisle after the male portion of her seat belt disconnected from its attachment point. The investigation confirmed that both the attachment point and the latching mechanism were intact at the time of the incident.
The investigation
Examination of the restraint system revealed that the seat belt hardware itself was not broken or deformed. The failure mode was identified as a disconnection between the male connector and its fixed attach point, despite both components appearing undamaged upon inspection.
Findings
Contributing factors included the encounter with moderate turbulence in clouds without prior warning from weather systems or air traffic control. The broken rib sustained by the passenger resulted directly from the sudden ejection from her seat due to the restraint system failure.