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I am aware that wind shear is the geostrophic wind between two pressure levels and am also aware of the catastrophic effect microbursts can have.

If informed, can the crew avoid wind shear? And if not, how do they deal with it?

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When an aircraft experiences changing winds as it climbs or descends, this is technically "windshear" and it can have consequences (changes in airspeed and turbulence), but the much more dangerous phenomena is the microburst, and discussions of windshear avoidance and recovery are mostly concerned with that.

There are some good tools available for avoidance: major airports typically have Doppler radars that are designed specifically to detect microbursts and significant windshear events, and tower controllers broadcast those warnings. Airliners and other aircraft with modern radar systems have similar capabilities, looking ahead into their flightpath for the next few miles.

Without those capabilities, windshear detection was essentially accomplished by comparing groundspeed (from an INS or GPS) with indicated airspeed, and noting any changes in that relationship. So if your GS was 120 and your IAS was 100 (the difference due to combined effects of the wind plus the delta between True and Indicated Airspeed), then suddenly you had 130 knots GS and 95 knots IAS, you've suddenly picked up some tailwind, and if that trend continues, the loss in indicated airspeed could become catastrophic, and so an escape is necessary.

On a more basic level, some cues are typically published that may indicate windshear on approach:

  • ± 15 knots indicated airspeed
  • ± 500 fpm vertical speed
  • ± 5 degrees pitch attitude
  • 1 dot displacement from the glideslope
  • unusual thrust lever position for a significant period of time

    (Source)

When you receive a warning from Tower or your onboard systems, aircraft that haven't started taking off (or are very early in the takeoff roll) stay on the ground, and aircraft on approach will discontinue the approach (go around). Since the effects of a microburst are much more pronounced close to the ground, the difference between encountering the microburst at 3000' AGL instead of 500' AGL may be the difference between a bumpy ride instead of a truly dangerous event. Also, if the microburst can be localized, either from radar or visually by observing virga or a storm cell, then turning away from it is often appropriate.

What is also practiced (in the simulator!) is what is referred to as a windshear escape maneuver, which is the aircraft manufacturer's (and possibly, operator's) best guidance for your best chance of surviving a windshear encounter. This would typically involve steps like advancing the power to the maximum available, retracting speedbrakes (if extended) to reduce drag, rolling wings level, and climbing as quickly as possible. At least in Boeing aircraft, it is not advised to change the flap configuration because doing so would increase your stall speed, and it's also not advised to retract the landing gear, because even though this would reduce the drag, it would also make any contact with the ground far more damaging. Each aircraft has its own procedure, but that's a general case.

A couple of links:

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  • $\begingroup$ I'm curious, does a "wind shear" AIRMET indicate the possibility/probability of microbursts? Like you said, any change in wind is technically "wind shear", but I'm wondering if there are any actual warnings for it except in the case of microburst activity. $\endgroup$ – Joel M. May 4 '15 at 17:02
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    $\begingroup$ To add to your excellent answer, at least from a historical standpoint, in the 1990s in 747-100 and -200 simulator work, we were taught to raise the nose until we got the stick shaker and then fly out of the condition with the stick shaker. I have no idea whether they still teach it that way. $\endgroup$ – Terry May 4 '15 at 19:04
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    $\begingroup$ @Terry, the line in our book is to "respect the stick shaker" -- it's not something to be avoided at all costs, but also not something we're trying to stay in. Our Flight Directors have windshear escape guidance with TOGA, which will try to keep us right on the optimal AOA, which may be a step up from what the 747-100 and -200's had. In the sim, you can get a couple moments of the stick shaker during an escape and that's considered normal. (Stick shaker = approach to stall, not yet IN a stall, for those not familiar.) Thankfully haven't gotten to see this in the actual airplane so far! $\endgroup$ – Ralph J May 4 '15 at 19:13
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    $\begingroup$ @RalphJ It's a giant step up from what we had. Our flight directors had no such capability. Like you, I never experienced in the airplane windshear of the severity that required such a maneuver. $\endgroup$ – Terry May 4 '15 at 19:48

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