getting to grips with surveillance(97)

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6.1.4 – Reactive Wind Shear. The PWS provides alerts when the aircraft is ahead of the wind shears for avoidance purposes. The Reactive Wind Shear provides alerts when the aircraft is in the wind shears for escape purposes.
The PWS detection is based on the Doppler principle. A wind shear is an area where wind velocities of opposite directions exist on a short distance. The PWS triggers alerts when wind shears exceed an threshold called Hazard Factor.
6.1.3.4.1. Hazard Factor F
The Hazard Factor F, developed by NASA, measures the losses of altitude and airspeed due to wind shears. It is defined as:
Wh: Rate of airspeed loss (kt/s)
Wh V
G: Gravitational acceleration (19.06 kt/s)
F  =. with
g As  V: Vertical downdraft (kt)
As: Airspeed (kt)

Wh V
 represents the loss of airspeed, and . the loss of altitude.
g  As

The velocity of the vertical down draft V is defined as a negative value. . is
As
then positive. Wind shear alerts are triggered when F > 0.13.
Considering the loss of airspeed only (i.e. V = 0), a variation rate of 2.48 kt/s
would be required to trigger the wind shear alerts.
Considering the loss of altitude only (i.e. Wh = 0), a downdraft of 18.2 kt would
be required to trigger the wind shear alerts, with an airspeed of 140 kt.

6.1.3.4.2. PWS Automatic Activation

The PWS mode automatically activates when the aircraft is below 2 300 ft RA at take-off or landing, even if the weather radar is OFF (provided that the PWS switch is on AUTO). The PWS triggers wind shear alerts below 1 200 ft RA.
To prevent inadvertent alerts, the PWS mode automatically activates if the following conditions are met:
.  
Honeywell RDR-4B – Autotilt and Rockwell Collins WXR 701: One transponder is on AUTO or ON, and one of the engines 2 or 3 is running.

.  
Rockwell Collins WXR 2100 – Multiscan:

o  One of the engines 1 or 2 is running

o  One of the following conditions is met:

- 
The ground speed is above 30 kt, or

- 
The longitudinal acceleration is above 0.07 g.

 

For both weather radar models, the controls of TILT and GAIN are automatic in PWS mode.

For both weather radar models, when the PWS detects a wind shear when the weather radar is OFF (PWS on AUTO), the weather radar automatically switch to:
-WX+T or WX/TURB mode when the ND range is less than 60 NM,
-WX mode when the ND range is more than 60 NM. When the aircraft passes the wind shear, the weather radar automatically switches back to OFF.
Note: When one weather radar fails (e.g. WXR 1), the PWS function is not available until the weather radar switch is set on the second weather radar (e.g. WXR 2).
6.1.4. REACTIVE WIND SHEAR
According to the aircraft model, the Reactive Wind Shear is supported by:
　
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