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Author Topic: question about wind shear alert.  (Read 7399 times)
Аэрофлот Jr.
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« on: January 22, 2009, 08:47:26 PM »

In what general condition makes aircraft give out wind shear alert ?  Is there specific wind speed that triggers wind shear alert ? or how does it work ? 
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sincerely, Rae
jsapyta
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« Reply #1 on: January 22, 2009, 10:39:15 PM »

Take a look here.

http://en.wikipedia.org/wiki/Wind_shear

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N7XLQ
cessna157
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« Reply #2 on: January 23, 2009, 03:35:35 AM »

GPWS or LLWAS?
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CRJ7/CRJ9 F/O, Travel Agent
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« Reply #3 on: January 23, 2009, 06:47:43 AM »

GPWS or LLWAS?

 GPWS please
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sincerely, Rae
cessna157
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« Reply #4 on: January 23, 2009, 08:44:40 AM »

Basically, there are 2 types of Winshear alerts.  There is a Windshear Caution (sometimes called a Windshear Alert).  This level is an indication of improving aircraft performance (entering the front edge of a microburst, for example).  One would think "If the aircraft is doing better, then why have an alert?"  Well, usually, what occurs next is why.  A Windshear Warning is an indication of rapidly deteriorating aircraft performance.

To put it easily, if the airspeed starts increasing very rapidly, that is an Alert.  If it starts descreasing rapidly, that is a Warning.  In actuality, what triggers each level is based upon many factors, including airspeed, vertical speed, and height above the ground (see my attached picture).

In general operation, in a Windshear Alert, and amber "WINDSHEAR" is presented on my attitude indicator.  The flight director will continue normal flight unless the pilot pushes the TOGA buttons to tell the flight control computers that they will go around, then windshear escape guidance is brought up.
A Windshear Warning puts a red WINDSHEAR on my attitude indicator, along with an audibel alert (see the other thread for that sound), along with automatic windshear escape guidance, along with stall protection pitch limits, along with the autopilot automatically disconnecting.

But, like any automatic system, the GPWS can be fooled into thinking there is windshear just by a gust of wind.  False windshear alerts are seen occasionally.  False windshear warnings usually are true, but not always.  "According to the book" a windshear warning must always be followed, but a windshear alert can be disregarded at the agreement of both pilots that a microburst situation does not exist.

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CRJ7/CRJ9 F/O, Travel Agent
cessna157
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« Reply #5 on: January 23, 2009, 08:47:20 AM »

Here is the Collins description of the eGPWS windshear system:

Mode 7 - Windshear Detection and Recovery Guidance System
Mode 7 provides windshear warnings and alerts between 10’ and 1,500’ AGL during the initial
takeoff and final approach phases of flight when the level of windshear exceeds predetermined
threshold values. The EGPWS computes a total shear signal and compares this value to present
aircraft configuration and windshear threshold data to determine if an excessive windshear
condition exists.

Windshear Alerts

Windshear Alerts are provided for increasing headwind (or decreasing tailwind) and vertical
updrafts typically associated with the leading edge of a microburst windshear. Alerts will activate
the amber WINDSHEAR caution message on the PFD (flashes then comes on steady). The caution
message will remain on the PFD for as long as the aircraft is exposed to the caution alert threshold.
During a windshear alert, the amber pitch limit indicators (alpha-margin indicators) will appear on
both PFDs for as long as the windshear alert is active (minimum of 60 seconds). The pitch limit
indicators display the amount of pitch attitude change that can be made before the aircraft reaches
stall angle of attack.

Windshear Warnings

Windshear Warnings are provided for decreasing headwind (or increasing tailwind) and vertical
downdrafts that exceed a defined threshold. These are characteristic of conditions within or exiting
an encounter with a microburst. Warnings will activate the red WINDSHEAR warning message
and the amber pitch limit indicators on the PFDs. An aural “siren”, followed by the aural message
“WINDSHEAR, WINDSHEAR, WINDSHEAR” is generated. The warning message will remain
on the PFD as long as the aircraft is exposed to conditions in excess of the warning alert threshold.
The aural message will not repeat unless another event occurs. The autopilot is automatically
disengaged two seconds after windshear warning (if autopilot not previously disengaged). During
these two seconds, the autopilot will follow the windshear escape guidance.

Flight director escape guidance is available for both windshear alerts and warnings. For alerts, one
of the TOGA (Takeoff/Go-around) switches on the thrust levers must be pressed to activate escape
guidance. For warnings, escape guidance is activated automatically or when the TOGA switches
are pressed. Command bars will appear on both PFDs and the FMA will display the GA/WS (Goaround/
Windshear) mode. The command bar is dynamic, responding to pitch and low speed limits
for that configuration. The positions of the command bar and other cues are calculated using angle
of attack data. Windshear Warnings take priority over all other aural alerts and warnings, except a
stall warning.
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CRJ7/CRJ9 F/O, Travel Agent
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« Reply #6 on: January 23, 2009, 09:10:28 AM »

Very interesting reading, Cessna - thanks for that.
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cessna157
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« Reply #7 on: January 23, 2009, 09:26:06 AM »

Very interesting reading, Cessna - thanks for that.

I aim to please!
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CRJ7/CRJ9 F/O, Travel Agent
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« Reply #8 on: January 23, 2009, 03:30:51 PM »

Thank you very much 'Cessna' (:
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sincerely, Rae
djmodifyd
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« Reply #9 on: January 23, 2009, 10:01:21 PM »

thanks
very interesting

i wish we still had FAM flights....i would love to see what goes on at the other end...and what exactly a pilot has to do when i give them instructions
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