Windshear explained

Moderator Location : UK Spotter Watch Member : No

Windshear, a term often heard when aircraft are on final approach. Explained initially by TONKS.

Wind shear is a complicated subject and can be around for several reasons eg:

A significant difference between wind at height and surface wind. A few week ago on approach the wind at 1000" was 40 kts and 50 deg different from the surface... this continued down to 500'... at some point it has to change and slow down and at that point you may get shear.

In close proximity to a thunderstorm you may get fast rising and descending air... some of these up and downdrafts can be of a very high velocity and have brought down ac.

High winds flowing over buildings can produce turbulence and signs of shear.

Rotor wind off high ground can produce wind shear.

A quick google search produces soon good diagrams

The accident here was caused by severe wind shear and it can be very difficult to predict and detect.

Moderator Location : UK Spotter Watch Member : No

And complimented by Razor 61.

The strong downdrafts (like that from a Thunderstorm outflow) is called a microburst and has been atributed to many accidents on approach (as has windshear in general).

Microburst Windshear will literally force the plane (no matter how big) into the ground. Thunderstorm downdraft microbursts are very powerful.

Windshear is common at some airfields (like Bristol) where by the airfield is located on a hill...or high ground. Wind coming from lower levels rushing up the hill will affect aircraft on approach (updraft).

Even tall buildings near the approach of airfields can cause eddy winds, as with hills whereby wind is sheared directionally from the hills or buildings, creating a funnel effect and can cause horizontal vortex to form (hills) and even vertical motion from buildings...
A direct result of a crash of an aircraft in the USA was caused when a 737 hit a horizontal vortex...

Loads off different varieties of shear... of which you can also get it at very high altitude...

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More from TONKS

The key to flying in any of these conditions which can generically be called wind shear is to recognise both the conditions in which they "may" exist and to do something about it earlier rather than later... we do a lot of simulator work on this both during conversion to type and on regular sim sorties, often putting it on without warning... you have to throw away an approach if it "looks" wrong. There are some indications that can show the presence of shear eg rates of climb with low power, increase in speed with reducing power etc, ie opposite as to what you expect. The inertial nav can also give useful warning below 1000' above ground...

Act early...

Moderator Location : UK Spotter Watch Member : No

And completed by Razor 61

You can also {censored} Wind Shear is 'there' if you are entering a Precipitation curtain from a CB (Cumulonimbus) whereby you know the Precipitation core holds a strong downdraft. (Microburst)

Likewise above or within the top of a CB, you will then be in a strong updraft, again more windshear which creates turbulance (mild to severe).

Weather radar showing in the cockpit is obviously a good provider of knowing where windshear will likely take place, especially with cells knocking around...