There
were many speculations that wind shear was one of the cause of the
Singapore Airlines Boeing 747-400 crash at the Taipei's Chiang Kai-Shek
International Airport (Tuesday evening, 31st October 2000).
However, preliminary evidence of the investigation revealed that it
was not so. In fact, it was reported that the pilot took off
from the wrong runway that was closed for repairs and it was
littered with digging equipment.
Nevertheless,
it would interest air travelers to know more of what wind shear is
all about.
Wind
shear is a sudden and drastic change in wind direction or speed over
a short distance along the flight path, usually associated with a
microburst that often occurs in the vicinity of thunderstorms or
typhoons.
Generally,
most wind will travel horizontally, but under certain conditions in
thunderstorms and frontal system, wind shear will travel in a
vertical direction, Microburst wind shear is an
extremely violent downward blast of air that hits the ground and
radiates outward with its sharp shifts in wind speed. It can
cause an aircraft to lose lift and crash, especially during take off
or landing when slower speeds and its proximity to the ground make
altitude correction very difficult.
Downdrafts from Microburst
Wind
shear can be either an overshoot shear where there is an increase in
headwind or decreasing tailwind resulting in an increasing speed of
the aircraft. It can also be an undershoot shear where there
will be an increasing tailwind or decreasing headwind with
consequent loss of speed of the aircraft causing it to sink to the
ground if no immediate action is taken by the pilot.
Since
1996, all US airliners have been required to be equipped with
instrument that provides the pilot with advance warning of wind
shear.
In
the Boeing 777, the aircraft is equipped with a device known as the predictive wind shear warning system.
This system
uses wind velocity data gathered by the weather radar system to
identify the existence of wind shear. The system is activated
automatically on the ground when the thrust levers are set to take
off position. It has an effective range of 3 nautical miles
ahead of the aircraft.
There
are two alerts, a CAUTION
and a WARNING.
When a caution alert is detected during take off or approach
(between half and three nautical miles ahead and 25 degrees left or right
of the aircraft heading), an aural alert, "MONITOR
RADAR DISPLAY !" is
announced. When the warning alert is detected on the ground
before take off, the aural alert "WIND
SHEAR AHEAD !" is
sounded. Pilots have been trained to abort
the take off
when this warning comes on.
Pilots
have learned to detect the presence of wind shear through the
knowledge that they are indicated by thunderstorms, micro bursts,
typhoons, virga activities (rain that evaporates before reaching the
ground) and by other fellow pilots reports. Certain airports are also
equipped with low level wind shear alerting system (LLWAS)
warning. Such information are transmitted to all pilots
through the air traffic information service (ATIS) at regular
intervals.
Pilots
have also been trained to stay clear of thunderstorms, heavy rain
and areas of known wind shear. If severe wind shear is
suspected, they are advised to delay
the take off or approach.