All but three of the 21 passengers sent to Calgary hospitals by violent turbulence on an Air Canada flight on Wednesday have been released, the airline said on Thursday, as efforts began to explain why the plane was buffeted so badly in the skies above Alaska.

The Transportation Safety Board (TSB) continued its investigation of an incident in which passengers were thrown from their seats and objects hit the cabin ceiling during a harrowing few minutes that forced the Toronto-bound flight from Shanghai to stop in Calgary for medical help.

The plane's black box will come under scrutiny, as will the flight and cabin crew, said Jon Lee, the western regional manager for the TSB.

Story continues below advertisement

But as the organization and the airline refrained from speculating on what caused the disturbing episode, experts suggested some form of "clear air turbulence" was likely to blame.

Rod Hayward, an aviation expert at the University of the Fraser Valley, said sudden turbulence in clear skies can sometimes be caused when a plane flies through a jet stream, a kind of narrow, fast-moving current of air. That sudden increase in air velocity, sometimes reaching 100 knots, can cause aircraft to shake violently, he said.

Because jet streams are invisible, they can take pilots by surprise, Mr. Hayward noted. Forecasts can often predict the location of the air patterns, but they move and change direction quickly.

"Situations like this, you can't forecast it completely accurately," he said.

While pilots may be accustomed to the phenomenon, and airplanes are built to withstand turbulence of that kind, passengers who are not wearing seat belts can be badly hurt by the sudden jolts.

"The aircraft is stronger than the humans within," Mr. Hayward said.

Pointing to passenger reports of rapid up and down movement in the plane, Wayne Hocking, a physics professor specializing in atmospheric turbulence at the University of Western Ontario, said that breaking "gravity waves" were a more likely culprit. Those are undulating currents of air that can cause severe up-and-down turbulence for aircraft. They can be caused by a range of factors, including heat from the ground, or hot and cold air fronts colliding in the atmosphere.

Story continues below advertisement

But in this case, Prof. Hocking said, it is probable that any gravity wave would have been caused by air flowing over the mountains of Alaska below. When air goes over a mountain it continues moving in the pattern of the mountain peak rather than flattening out, and before moving as a wave into the upper atmosphere, Prof. Hocking explained.

"The oscillation up and down is an absolute indication that this is gravity waves," he said.

Aircraft damage caused by these waves is uncommon, he added, but it can be serious.

"It's pretty rare. The bottom line is, you might have a dozen occurrences a year. It's a bit like tornadoes. It doesn't happen very often, but when it does, you damn well want to know about it," he said.

This is not the first time severe turbulence has forced an Air Canada flight to be diverted to Calgary. In 2008, one of the airline's Airbus A319 jets bound for Toronto from Victoria plunged and shook at 35,000 feet, putting 10 people in hospital.

On Wednesday, Calgary paramedics assessed 25 passengers aboard the aircraft; 21 were taken to local hospitals by ambulance, including seven with potentially serious neck and back injuries, said Stuart Brideaux, spokesman for Calgary EMS.

Story continues below advertisement

Flight and cabin crew received high marks for their calm response from shaken passengers on the ground.

"It was nobody's fault. Not the pilot's. Everybody did the best they could," Connie Gelber said.

On Thursday, Air Canada declined to comment on the possible causes of the incident.

"At this point, the TSB is reviewing this incident and we are assisting, so we will refrain from speculation and await its report," airline spokesperson Peter Fitzpatrick said.

With a report from Jeffrey Jones