IT’S every air passenger’s nightmare: as the plane is about to land, sudden
turbulence flips it over and smashes it onto the tarmac. The cause is wake
vortices, powerful swirling streams of air billowing from a plane that landed
earlier. Now a pair of engineers from Boeing have patented a way of stifling
these deadly vortices. As well as making flying safer, it promises to ease
congestion at busy airports by reducing the gap between planes as they land.
Wake vortices are generated by the pressure difference between the top and
bottom of an aircraft’s wings and tail. Vortices from the wing tips are the
strongest, followed by those nearer the fuselage. Wind speeds around the core of
these swirling currents can reach 350 kilometres per hour, and they can extend
for many kilometres behind the aircraft.
The danger comes when a plane coming along behind hits one of these powerful,
invisible vortices. “The following aircraft can be rolled to one side, which is
particularly hazardous when flying close to the ground,” say Boeing engineers
Jeffrey Crouch and Phillippe Spalart in their patent (US 6082679), which was
granted early this month. “Several accidents have been attributed to wake
turbulence,” they warn. Such accidents happen out of the blue and are often hard
to recognise
(91av, 16 November 1996, p 28).
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Until now, the only way to avoid the problem of wake vortices has been for
air-traffic controllers to keep incoming aircraft up to 11 kilometres apart.
This limits the number of planes that can land each hour. Airlines complain that
this leads to delays at busy times as planes queue up to land.
Boeing set out to solve the problem by destroying the vortices at source. In
earlier wind tunnel tests, engineers found that by wiggling the plane’s control
surfaces they could produce a wave-like disturbance in the airflow over the wing
that broke up the vortices. In these attempts, engineers oscillated the inner
and outer ailerons out of phase. But this caused too much stress on the wings
for it to be used routinely.
The breakthrough came when Crouch and Spalart found they could make the idea
work if they combined small perturbations of the outer ailerons and the
spoilers. This makes the inner and outer vortices on each wing interfere with
each other, causing instabilities that destroy them both. They say in their
patent that a sensor on-board the plane will measure the strength of the
vortices it is leaving behind. A computer will then use this information to
calculate the strength and frequency of the movements needed for the spoilers
and ailerons of the following plane.
“Wake vortices are an immense problem,” says Neil Halsey, an aviation
specialist at Britain’s Meteorological Office. “Linking two vortices will
disrupt them more quickly, so it will be interesting to see how Boeing is doing
ٳ.”
Boeing’s is not the only attempt to tackle wake vortices. NASA and Britain’s
Defence Evaluation and Research Agency are developing laser systems to let
airports predict wake turbulence as planes come in to land: vortices are
strongest in calm air, but are quickly broken up in windier weather. This will
allow controllers to adjust the separation between planes depending on
conditions at the time.
Larger planes create stronger turbulence, so Boeing’s rival Airbus is
designing the wings for its giant A3XX super jumbo to make “short-lived
vortices”, says Airbus spokesman David Velupillai.
