Wind shear value

1 Answers

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   awair on Apr 18, 2020

   That’s a really good question, that if you look back in 20-years you’ll appreciate that it’s not quite so simple as a single figure…

   Also, are you sure you’re not asking instead about regular wind and gusts?

   The definition of wind-shear refers to a shift in both the direction and magnitude of the wind. For wind-shear to be considered significant, a figure of ‘greater than 15 knots’ is sometimes quoted.

   Remember that there can be vertical shear that may not be identified by reported wind value, but is evident by an abrupt change in vertical speed.

   Boeing manuals (FCOM, QRH & FCTM) include several references that combine to give you the ‘big picture’:

   Limitations: demonstrated cross-wind & maximum cross-wind including gusts (company limit)
   Standard calls: PM to call: ‘Speed’ +10/-5 knots, ‘Sink-rate’ >1000 fpm
   Stable Approach Criteria: ‘not more than Vapp +10, not less than Vref’
   Wind-shear: > 15 knots

   These are the numbers that we all remember, however many of the newer pilots fail to recall the ‘small print’ associated with each of these definitions.

   For example: the Stable Approach Criteria excludes “momentary excursions“ which can be considered as a deviation lasting a few seconds; Wind-shear refers to uncontrolled changes from normal steady state flight.

   So armed with these clarifications, a mandatory go-around is not required for a 16-knot speed change.

   Personally, I would rather experience a wind of 20G30, than 0G15, a steady 45 knots is preferable to 20G35. However, it all depends on the location…

   Some airports have nasty mechanical turbulence, depending on the wind direction.

   …and your altitude!

   At 1000′, I’m willing to accept a 20 knot deviation (in already gusty conditions); 500’ that becomes maybe, and below 200′ noooo! This is all in the absence of any other cues, or restrictions.

   Is the runway wet? Is the field length limiting? How much fuel do you have? How is the alternate?

   In summary, it all boils down to experience. And that’s not just the ‘Sky God’ in the left seat. It’s no good if you have a passenger in the RHS. A thorough brief of what to expect, and what (and when) it would be considered an unacceptable deviation.

   I’ve flown in 35-45 knots (turboprop Vapp 90 knots), with so much positive shear that we couldn’t touch down in the first ⅔ of the runway – I was scared, Captain eventually went missed.

   I’ve flown in 45-55 knots, mostly headwind (737), with a nasty shear before flare, where it didn’t seem that the Captain was reacting quick enough. (But he was.)

   I flew a Wind-shear go-around where the airport (and winds aloft) were steady-ish at 25 knots.

   I flew another Wind-shear go-around caused by mechanical turbulence in the flare from a 45 knot wind. The next approach to a less x-wind favourable runway was completely smooth in the same conditions.

   Finally, another approach with no shear, but constantly gusting 20-30, was one of the roughest I’ve ever experienced.

   If you’re lucky, as I was, you’ll fly with a grumpy old (or young) skipper, who tells you you’re doing it all wrong, and shows you! For me, that was 30 years ago on a turbo-prop, and it’s worked ever since.

   Tips: always leave yourself a Plan B, and brief!

   Stay safe.

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