OTTAWA — Sudden and intense turbulence during an Air Canada flight on Wednesday threw passengers out of their seats, with some hitting the ceiling of the airplane, and resulted in numerous injuries.

The Transportation Safety Board of Canada is investigating why the jet, a Boeing 777-300ER airliner that was headed to Toronto from Shanghai with 351 people aboard, suddenly plummeted, injuring 21 passengers. The plane made an emergency landing in Calgary, Alberta, on Wednesday afternoon.

The airline said that eight passengers suffered “nonlife-threatening injuries” while the rest were examined and released from a hospital.

Passengers at the airport in Calgary told reporters that blankets and objects flew through the air during the turbulence, and some compared the experience to riding a roller coaster.