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MultiScan™ Radar Windshear
Figure 6-32 Non-Turbulent versus Turbulent Targets
WINDSHEAR
WINDSHEAR EVENT
A windshear event, such as a microburst, is a condition in which the
wind abruptly changes its speed or direction (or both) over a small
distance (♦page 5-22, ♦page 5-26). It can be associated with frontal
systems occurring over a large area, or with thunderstorms occurring
over a small area. Windshear can occur at altitude without the presence
of clouds, referred to as clear air turbulence, or near ground level where
it has an impact on the takeoffs and landings of aircraft. It is a specific
type of windshear occurring near the ground, referred to as a down
burst, that presents the greatest danger to aircraft.
A down burst can be detected by a Doppler weather radar. This is
possible because of the atmospheric moisture content associated with
these events. The echo returns are analyzed for characteristic velocity
and direction patterns called signatures. The “velocity signature” of a
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Windshear MultiScan™ Radar
down burst is defined as an area of positive and negative velocities
existing over a short distance.
During a down burst, strong head winds of the event will produce a
positive Doppler frequency shift, while the strong tail winds will produce
a negative Doppler frequency shift. The distance between these two
events is determined by the amount of elapsed time between the
respective echo returns. If the velocity signature meets the minimum
criteria, aural and visual alerts are provided to the flight crew (♦page
4-50) .
The velocity gradient is used to establish a hazard factor. The hazard
factor used to define a windshear event is defined by NASA and is
shown in figure 6-33.
Figure 6-33 NASA Hazard Factor for Windshear
The term V/As represents down draft. The velocity of the down draft
(V) is by definition negative. Therefore, - V/As is a positive number and
is added to Wh/g to provide the hazard factor.
In figure 6-33, the hazard factor is the total of the time rate of change of
the horizontal wind (Wh/g) and the down draft measure (V/As). By
setting V/As = 0 we can determine the time rate of change of the
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MultiScan™ Radar Windshear
horizontal wind that would be required for a hazard factor of 0.13:
F = 0.13 = Wh/g + V/As
Set V/As = 0
Then F = 0.13 = Wh/19.06 kts/sec
Solving for Wh = 2.47 knots/sec
Thus, if the down draft is equal to zero, it would require a 2.47 knots/sec
rate of change of the horizontal wind to activate the cockpit windshear
alerts.
Likewise, if we set rate of change of the horizontal wind (Wh/g) equal
to zero and assume an approach airspeed of 150 knots, then we
can calculate the down draft that is required to activate the cockpit
windshear alerts:
F = 0.13 = Wh/g + V/As
Set Wh/g = 0
Then F = 0.13 = V/150 knots
Thus, if the rate of change of the horizontal wind is equal to zero and
the aircraft approach speed is 150 knots, then it would require a 19.5
knot down draft to activate the cockpit windshear alerts.
An actual windshear event will contain both horizontal and vertical
hazards.
AUTOMATIC ENABLE
Windshear alerts are active in the cockpit below 1,200 feet AGL.
However, the WXR-2100 radar actually enters the windshear scanning
mode at 2,300 feet AGL to provide time for the system to power up
(if necessary) and update the displays before the aircraft reaches the
1,200 feet AGL level. During windshear scan, the radar automatically
reduces the sweep of the antenna to a 120° (+/- 60°) area directly
ahead of the aircraft (♦page 4-45). This feature provides an increase
in the system update rate to the display during the takeoff and landing
phases of flight. The weather display is updated once every 6 seconds
(as opposed to once every 8 seconds) as the weather scans are
alternated with the windshear scans. This implementation provides
rapid weather updates and also allows the radar to continually display
weather behind the windshear icon.
Windshear detection scanning is automatically activated when the radio
altimeter reports an altitude less than 2,300 feet and the aircraft is on
the runway or in the air (not in the maintenance hanger). Automatic
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activation occurs even if the radar is turned off in order to provide
detection and warning of microburst events during the landing and take
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Collins Weather Radar operator’s guide(41)
