Transportation Safety Board (TSB) officials who investigated a turbulent Air Canada flight that left 21 people hurt are reminding passengers to wear seatbelts after it was determined that many onboard the plane diverted to Calgary had not buckled up when urged to do so.

The plane, which was flying from Shanghai to Toronto in Dec 30, 2015, had to be diverted to Calgary.

A TSB report released Monday says about five hours into the planned 13-hour and 40-minute flight, crew members noticed a meteorological bulletin warning of severe turbulence over the southern coastal mountains of Alaska, about 85 kilometres northeast of Anchorage.

Just over two hours later — about 30 minutes before the plane entered the turbulent airspace — the first officer ordered that in-flight service be stopped and the cabin secured.

Seatbelt signs were turned on and announcements were made in English, French and Mandarin that warned the aircraft was entering a turbulent area and passengers should buckle up. But the TSB investigation found many passengers did not fasten their seatbelts.

Twenty-one passengers, including three children, on flight from China taken to Calgary hospital after turbulence causes multiple injuries 2:30

The 21 people were injured when the plane bounced around. One of them was slammed to the floor and suffered a broken pelvis.

First aid was provided onboard.

"It was plenty of warning and that's what the gist of the report is about," said Jon Lee, the TSB investigator in charge of the incident.

"The type of acceleration and deceleration forces the aircraft went through when it went through the turbulence were quite significant."

Jet streams are "very common," said Lee.

"They form up between two air masses of different temperature," he said. "But like anything in nature, they're variable, they can go in different directions, have different intensities and speeds. In terms of this one travelling south to north over the coastal mountains … that particular direction was not common."