WHILE the cause of last week鈥檚 plane crash in New York City is still unknown,
it seems likely that violent wake turbulence thrown up by another plane was at
least a factor in the disaster. Now observers are wondering why, decades after
wake vortices were first identified as a danger to aircraft, this threat to air
safety remains so poorly understood.
Wake vortices are horizontal tornadoes that stream corkscrew-like from the
flaps and wing tips of aircraft. Wakes either side of the plane rotate in
opposite directions and spin at up to 350 kilometres per hour.
The US National Transportation Safety Board, the Washington DC-based
organisation investigating last week鈥檚 crash, says its enquiry is focusing on
why the tail fin of the Airbus A-300 sheared off three minutes after take-off.
Flight recorders indicate that it encountered wake turbulence caused by the
plane ahead of it, the board says.
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A key issue is whether turbulence could contribute to the loss of a tail.
Airbus says it carries out no flight tests on newly designed aircraft to see how
they handle wake vortices from other planes. And 快猫短视频 knows of no other
plane makers who do carry out such tests.
Aerospace experts claim that vortices are hard to simulate, and subjecting
planes to them would be hugely expensive and produce inconclusive results.
Vortices can extend for miles, and it鈥檚 not possible to build wind tunnels to
model them at full-size, says Michael Harris at Qinetiq, the British defence
research lab in Malvern, Worcestershire.
It is possible to produce scaled-down wake vortices in wind tunnels. But
Harris argues that such vortices can only be sustained for very short periods.
鈥淵et in the atmosphere, they can remain for some time,鈥� he says.
Harris also questions whether measurements taken from vortices created by
scale models in wind tunnels accurately reflect how they behave in reality. For
example, he and his colleagues recently discovered that aircraft with four jet
engines can produce concentric vortices鈥攐ne inside another. They only
spotted this unexpected effect using a lidar鈥攍aser-based
radar鈥攖rained on real planes. The effect has never been observed in a wind
tunnel.
Despite this, the threat to big jets from wake turbulence in the real world
has been all too clear since 1972, when a wake from a widebody American Airlines
DC-10 caused a smaller Delta Airlines DC-9 to crash in Texas. Such incidents
have led to the enforcement of separation standards for planes鈥攂ut even
these may not always be enough. In his 1999 book, The Tombstone Imperative: The
truth about air safety, aviation expert Andrew Weir says: 鈥淎voiding wake
vortices is difficult to regulate for, because their precise mode of operation
and how long they persist in the air is little understood.鈥�
Separation will be one focus of the Flight 587 enquiry. Different-sized
planes have varying separation distances, ranging from 6.5 to 11 kilometres,
says National Air Traffic Services, which controls British air traffic. 鈥淲e are
very strict about departure clearance,鈥� a NATS spokesman says. In New York, a
Japanese 747 took off 1 minute and 45 seconds before Flight 587. Normally, a
two-minute gap is needed to guarantee a 6.5-kilometre separation.
Assuming air travel recovers from its slump, pressure to reduce separations
may increase. If that happens, say observers, a better understanding of wake
turbulence will be crucial.
