Interesting article.
October 18, 2009
Scientists says Air Canada flight that plunged, injuring 10, hit wave of air
John Cotter, THE CANADIAN PRESS
EDMONTON - Experts say an Air Canada jet that plunged more than 1,000 metres without warning last year, injuring 10 people, rose up and then dove down a massive invisible wall of smooth air called an internal wave.
The official cause of what happened to Flight AC-190 on Jan. 10, 2008, over a rugged area near the B.C.-Washington border is still under investigation by the Canadian Transportation Safety Board.
The internal wave transformed the routine flight from Victoria to Toronto into a terrifying roller-coaster ride that violently tossed passengers, crew and heavy drink carts around the cabin before the pilot made an emergency landing in Calgary.
Scientists not connected with the official investigation say the Airbus 319 hit an atmospheric phenomenon that can loom up out of nowhere and doesn't show up on weather or aircraft radar.
"This Air Canada plane was just unfortunate enough to encounter one of these waves," said Bruce Sutherland, a University of Alberta professor of earth and atmospheric sciences.
"This isn't something that occurs all the time. You are just really unlucky if you happen to capture a wave that has gotten to this large amplitude and has not yet broken. If a plane showed up behind them 10 minutes later, that wave would have overturned and broken and that plane would have encountered lots of turbulence, but they wouldn't have been carried up a thousand feet in seconds."
Internal waves occur in the atmosphere and the ocean. They're not unlike the disturbance caused when water flows over a rock. In the case of the atmosphere, wind blows over a mountain and creates a wave of air behind it that flows both up and down.
The size of the wave depends on how fast the air is flowing and on how much the air temperature changes with altitude.
"The wind is always blowing over the mountains. But every now and then it is just right to create these very large amplitude waves," Sutherland said. In some cases, such waves can be 20 kilometres wide.
Immediately following the Air Canada jet's troubles there were unconfirmed reports that the plane with 88 people aboard might have encountered "wake turbulence" - flying through the wake of a passing aircraft, perhaps a U.S. military stealth jet or a heavily loaded 747 cargo aircraft. There was also speculation about internal waves.
Canadian investigators at the time would only say there was a high probability the sudden plunge was caused by an external force.
Thomas Peacock, an expert on internal waves at the Massachusetts Institute of Technology, said he also believes the Air Canada jet hit an internal wave.
Peacock said he is working on a documentary on ocean and atmospheric internal waves for the Discovery Science Channel and hopes to interview the Air Canada pilots for the program that is to be broadcast next year.
"The Air Canada flight encountered a particularly strong internal wave which is why it had such a dramatic effect," he said. "It must have been a very strong event that took place there."
Radar helps aircraft avoid rough air currents. But an internal wave is a much smoother disturbance that can't be seen or detected in a cloudless sky, he said.
"These radar systems can detect the highly intense activity of turbulent air, but a nice smooth disturbance consistent with an internal wave cannot be picked up. But that disturbance has the dramatic effect that if a plane encounters it, it is swept up or swept downwards.
"You want to have your seatbelt on when that happens."
The Canadian Transportation Safety Board declined comment other than to say that a draft report into what happened to Flight AC-190 is being circulated to Air Canada and other organizations. The board's final report is expected to be released by March.
Air Canada officials also declined comment. The Air Canada Pilots Association did not respond to interview requests.
Sutherland said specific internal waves are difficult to predict without launching many weather balloons within an area, so the most effective way to deal with them is to ensure that flight crews are trained to respond quickly.
If a plane suddenly lifts without warning in areas prone to such internal waves, pilots may have only seconds or perhaps a minute or two to prepare for the plunge down the other side of the wave, he said.
"Even if they couldn't detect it in advance, they would see the signal of it when they hit one side of it and they could at least fix the problem before they hit the other side."