What happened
The commercial airliner was operating in cruise flight conditions at flight level 310 when it encountered turbulence. The crew had been actively deviating around thunderstorms in an effort to avoid severe weather systems. Despite the seatbelt sign being illuminated and a prior passenger announcement regarding the potential for turbulence, the aircraft experienced a significant vertical acceleration event.
Data from the Digital Flight Data Recorder (DFDR) indicated that the airplane experienced a total 'g' excursion of 2.77 'gs'. The severity of the turbulence resulted in injuries to several individuals on board. Specifically, two passengers and one flight attendant sustained serious injuries. Additionally, five other flight attendants and sixteen passengers received minor injuries.
Following the encounter with turbulence, the crew decided to divert the flight back to the destination airport. The aircraft landed safely, and emergency medical services were available to assist those who had been injured during the incident.
The investigation
The investigation relied heavily on data retrieved from the Digital Flight Data Recorder (DFDR). Analysis of this data confirmed the magnitude of the vertical acceleration experienced by the aircraft. The recorded 2.77 'gs' excursion provided quantitative evidence of the turbulence intensity. Investigators also reviewed the status of the seatbelt sign and passenger communication logs to determine if adequate warnings were provided prior to the event.
Findings
The primary contributing factor was the encounter with clear air or convective turbulence while deviating around thunderstorms. Although the seatbelt sign was on and a warning had been issued, some individuals were not securely fastened or were in motion when the sudden vertical acceleration occurred. The severity of the turbulence was sufficient to cause significant physical trauma to those not properly restrained.
The diversion back to the destination airport was a prudent operational decision given the number of injuries sustained. The incident highlights the risks associated with weather avoidance maneuvers and the importance of strict adherence to seatbelt protocols even when signs are illuminated.