Windshear

[Three Bars]

Anybody have some information on undershoot/overshoot windshear that they could post here?

Thanks in anticipation.

[Checkboard]

I am afraid you need to be a little more specific than that!

Do you want information on chances of occurrence, aviodance strategies, technical definitions of what constitutes windshear, accident histories etc?

Posting information on all of the above takes a little effort, if you would like someone to spend that effort, at least take the time to define what it is you are after and the level of detail you need! ;)

[Genghis the Engineer]

Or you could read AIC 33/1997 Pink 140, cunningly entitled "Low altitude windshear".

G

[Black_Dawn]

Hi Genghis

is there any site that you are aware of hosting the AIC 33/1997 Pink 140

thanks
B_D

[Three Bars]

Just the definitions will do thanks.

[Manflex55]

Let's see...

WS is the variation in direction &/or spd of the local wind, with changes in ht & horizontal distance. It can (usually) B detected by monitoring the ASI: any sudden IAS change could B a WS indication

WS can B associated with :

- gust fronts associated with TS

- wet or dry microbursts (highly concentrated powerful downdraughts). They last from 1 to 5mn & may reach a peak intensity of 45kt (hence a difference in horizontal spd of as much as 90kt)

- frontal passage, especially when the spd of the front is 30kt or more. With the normal easterly trk of a depression, the danger areas for WS are both ahead of the warm front & behind the cold front (the latter being the most dangerous)

- low lvl inversions : @ night in SKC conditions, the sfc cools very quickly to quite low T° & the sfc W/V drops to calm (or light & VRB), whereas, above the sfc, the wind can B quite strong. This difference produces a shallow transitional lyr marked by an inversion & WS

- topographical factors, interrupting the flow around terminal areas (buildings, mountains...)

- lee wave rotors, associated with mountain waves, which produce both vertical & horizontal WS


On approach to land (or on clb after TO), the effects of WS depend on the headwind resultant:

* With a sudden DECREASE in headwind (or increase in tailwind), IAS, lift & pitch all DECREASE, causing your ROD to INCREASE => tendency to go under the glide path => some pwr should B added to compensate.

* With a sudden INCREASE in headwind (or decrease in tailwind), IAS, lift & pitch all INCREASE, causing your ROD to DECREASE => tendency to go above the glide path => need to reduce pwr a bit to compensate.

Basically, keep your hand on the throttles & the ASI within your scan.

MF

[Three Bars]

Thanks for the info MF

[Checkboard]

Severe windshear is sometimes defined as that windshear that produces changes in excess of: +/- 15 knots in IAS
+/- 500 fpm in Vertical Speed
+/- 5° pitch attitude
+/- 1 dot displacement from glideslope
+/- unusual thrust lever displacement for a significant period of time

[The Nr Fairy]

AIS Pink 140 can be found at http://www.ais.org.uk/Uk_aip/pdf/aic/4P140.PDF

and all AICs can be found at http://www.ais.org.uk/Uk_aip/html/aicpge.htm

[john_tullamarine]

One needs to keep in mind that ASI fluctuations may not be a good indicator of what the air is actually doing. The ASI is only a pressure diff gauge and is very very subject to static pressure fluctuations and internal plumbing problems. Thus, if the ASI is swinging wildly, this may have little to do with the variations in actual airspeed and lots to do with fluctuations in local static source pressure as eventually sensed by the instrument.

One ought to be a little circumspect in interpreting ASI readings in other than steady or slowly varying situations.

[ 17 October 2001: Message edited by: john_tullamarine ]