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Wake turbulence from other aircraft possible cause of plane drop last week: TSB

Jan. 17, 2008, Edmonton - Turbulence from the wake of a passing aircraft could have been what caused an Air Canada jet to suddenly descend last week, injuring 10 people.


January 17, 2008
By John Cotter

Jan. 17, 2008, Edmonton – Turbulence from the wake of a passing aircraft could
have been what caused an Air Canada jet to suddenly descend last
week, injuring 10 people.

The Transportation Safety Board of Canada suggests that wake
turbulence is a possible cause of the incident last Thursday
involving Air Canada Flight AC190 over northern Washington state.

“It could be that an external force may have caused the
incident, but we haven't gotten to the stage yet where we are ready
to test hypotheses,'' John Cottreau of the Transportation Safety
Board said from Ottawa on Wednesday.

“There are a world of hypotheses about what may have happened to
upset Air Canada Flight 190. Right now we are still collecting the
facts.''

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Following the incident Canadian Transportation Safety Board
officials requested air traffic flight data recordings for the area
from the U.S. National Transportation Safety Board.

The information, which has been provided, includes speed,
altitude and flight path data on any civilian and military aircraft
that were flying in the area just south of the Canadian-U.S. border
that morning.

The Globe and Mail quoted sources close to the investigation
saying a heavily loaded Boeing 747 crossed paths with the Air Canada
plane just moments before the incident occurred.

A Seattle air traffic controller ordered one of the planes to
change altitude, the Globe said.

Moments after the order was issued, the Air Canada plane went
into a series of dives and rolls.

Wake turbulence is caused by the wing tips of aircraft and can
persist for more than two minutes. The turbulence sinks to different
altitudes from the plane that produces it. The phenomenon can last
longer in calm wind conditions.

Transport Canada publications warn pilots to “avoid the area
below other aircraft'' and note that “aircraft flying into the core
of a wing tip vortex can experience a rapid vortex-induced roll.
Small aircraft encountering large vortices may not be able to
recover.''

U.S. Federal Aviation Administration publications say that while
wake turbulence is a particular concern for aircraft at low
altitudes, it can also be encountered at higher altitudes.

“Flight tests have shown that vortices from larger (transport
category) aircraft sink at a rate of several hundred feet per
minute. Pilots should fly at or above the preceding aircraft's
flight path, altering course as necessary to avoid the area behind
and below the generating aircraft.''

There were 88 people aboard the Airbus 319 that was flying from
Victoria to Toronto last Thursday morning when the aircraft suddenly
plunged thousands of metres, rolling sharply from left to right.

Passengers and crew were pitched violently around the cabin along
with dishes and heavy drink carts before the aircraft made an
emergency landing in Calgary.

The 10 injured passengers and crew were treated in hospital and
later released.

  Real Levasseur, the Transportation Safety Board of Canada's
chief of investigations, has said there was a high probability an
external force may have caused the incident.

Cottreau said that wake turbulence is just one possibility.

“There is a high probability that a number of other things could
have caused the incident as well,'' he said.

The safety board is also considering whether Air Canada Flight
AC190 hit another atmospheric phenomena known as a “mountain
wave.''

Researchers say mountain waves are the result of air rising up
the side of a mountain barrier and then sinking down the other side.
This forms a bounce of standing waves of air downstream from the
mountain that may extend for hundreds of kilometres.

Pilots have reported mountain waves at altitudes as high as
60,000 feet.

"There is a world of possibilities about what could be to
blame,'' said Cottreau, who declined to outline any other possible
causes of the incident.

"We are going to do an analysis to find out what exactly was to
blame. We are looking at everything.''

THE CANADIAN PRESS