The Transportation Safety Board says passengers not wearing seatbelts, even after they were told to buckle up, led to the injury of 21 people on a turbulent Air Canada flight to Toronto from China in December 2015.
The Boeing 777 carrying 332 passengers hit severe turbulence at 33,000 feet over Alaska and was force to divert to Calgary so the injured could be treated.
The safety board says in its final report that the crew gave several warnings in multiple languages 35 minutes before the flight entered the area where turbulence was expected.
Still, the board says, people were thrown into the ceiling and back down to the floor because they weren't strapped in.
The board notes that the turbulence occurred while many passengers were sleeping and low lighting and blankets made it difficult for crew to verify seatbelts were fastened.
The board says what happened should serve as a reminder to air travellers to listen to seatbelt instructions.