Windshear reporting

[cossack]

"Windshear is a sustained change in the wind velocity along the aircraft's flightpath, which occurs significantly faster than the aircraft can accelerate or decelerate".

So says the ATCOs Manual Part 1.

I believe that some aircrews do not report windshear accurately. To be more precise, I think they report conditions that they believe to be windshear but are in fact purely unsustained gusts and lulls. Phrases such as "plus and minus 5 knots all the way down the approach" when the surface wind is 270 25G45 spring to mind.

If an aircraft encounters a shear it has either passed from an area of low to high windspeed or vice versa, or encountered a rapid and sustained change in direction which has the effect of either incresing or decreasing airspeed. The key words are "rapid" and "sustained." If it isn't rapid and sustained it isn't, by definition, windshear.

Until all UK airports get windshear alerting systems, we will continue to rely on pilot reports as the sole method of windshear reporting. Please think carefully about whether or not what you experience is windshear.

I know some aircraft have winshear alerting equipment installed and give very accurate information regarding the shear, but most still do not and rely on pilot perception.

When you report windshear to ATC please try to include altitude, the speed gain or loss and any tendency to drift sideways. This information will then be passed on to others by both RT and ATIS.

If you fly into an airport that has a windshear report on the ATIS and you either experience a different shear or no shear, let us know. There is nothing worse than us pedalling out of date or inaccurate information.

[sprucegoose]

Windshear has been defined in my past experience as a sustained change in indicated airspeed of +/- 15kts, a change in rate of climb/decent of greater than 500 feet/min without a pitch change input from the pilot, an uncommanded change in pitch attitude of +/- 5 degrees.

You are quite correct in stating that pilot reports of +/- 5 kts all the way down final as a windshear report is incorrect. especially if any of the reported conditions are not sustained as would be the case in gusty conditions. The intentions are almost always well meaning. It might be argued that a smaller aircraft might report windshear when conditions are less severe than those that I have listed, after all a report of turbulence of any intensity must be considered in the context of the aircraft type that reports it.

[Bally Heck]

Cossack,

I think most pilots would disagree with the ATCO manual definition. The Boeing manual states "Severe windshear is that which produces airspeed changes greater than 15 knots." It doesn't have to be beyond the aircraft's performance capability to give you a fright.

It also states the following as indicating uncontrolled changes from the steady state condition ie windshear
Changes in excess of
15kts IAS
500fpm vertical speed
5 degrees pitch attitude
1 dot displacement from the glideslope
Unusual thrust lever position for a significant period of time.

I have the advantage that when I'm hand flying you would think you were in windshear the whole time. (doh)

"Is that a boy doggy Johnny?"

[cossack]

Bally Heck,

The Boeing manual definition is for "severe windshear" ours is for "windshear" so ours could be used to define any measureable, sudden and sustained change whilst Boeing's puts a figure on what it considers to be "severe."

The point I am trying to make isn't so much about the degree of windshear, but whether or not it is actually windshear that is being experienced and reported.

How do you tell the difference between a gust increase of 20kt and a windshear increase of 20kt? A gust will only last a few seconds at most, whereas a shear will continue much longer.

Can the atmosphere produce multple shear lines each with a change in speed of 5kt? I don't think so. What is being felt, especially in smaller aircraft, are the normal changes in wind speed to be expected when flying in proximity to the earth's surface. Some people would call this turbulence rather than windshear.

[Bally Heck]

Cossack

Your absolutely right. I have often heard windshear reports of "plus or minus 10 knots" That is of course turbulence.

Windshear however does not have to have a certain magnitude threshhold to be windshear. A loss of ten knots low down could ruin that greaser. It does not have to happen faster than the aircraft can accelerate. just faster than we poor buggers can react to it.


Off to the pub now chaps it's POETS day