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What is the cause of the widely known danger of snow on the wings of the airliner? Why does it require deicing before taking off?

I cannot believe it just makes the plane too heavy, and it is not obvious if it could alter the wing shape for this to be a problem. Control surfaces may not move as commanded? Possible shape and balance differences between left and right sides when that snow starts leaving the wing?

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    $\begingroup$ SK 751 is an example of what can happen (though specific to tail-mounted engines). $\endgroup$ – Henning Makholm Jan 14 '17 at 17:00
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    $\begingroup$ Snow and ice on the tail can actually be heavy enough to tip the plane over onto its tail, plus the wings and fuselage have a very large surface area so that weight actually does add up. That being said, the larger dangers are the aerodynamic effect from making the wing surface uneven and rough which increases drag and reduces lift significantly (I'm sure that someone will expand with a more detailed and technical answer, but this is it in a nut shell.) $\endgroup$ – Lnafziger Jan 14 '17 at 17:30
  • $\begingroup$ On Skeptics.SE: Can a sandpaper-thick layer of ice reduce lift by 30 percent and increase drag up to 40 percent? $\endgroup$ – ChrisW Jan 14 '17 at 18:07
  • $\begingroup$ Related: aviation.stackexchange.com/questions/25367/… $\endgroup$ – Peter Kämpf Jan 14 '17 at 18:39
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    $\begingroup$ If you've ever touched an aircraft wing with your hands, you'll notice they're exceptionally smooth. Under a magnifying glass, the effects of snow and ice on the wing's shape become very obvious. $\endgroup$ – kevin Jan 15 '17 at 12:05
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it is not obvious if it could alter the wing shape for this to be a problem

It may not be obvious but it is so.

Plain old snow will just blow off as soon as the plane picks up speed, but if the wing was above freezing when the snow first fell on it, the snow will probably conceal a layer of rough ice, which will not blow away.

Anything adhering to the wing surface will change the lift force. According to this article even a thin coating of ice can disrupt an airfoil's boundary layer airflow, causing a premature aerodynamic stall and dramatically increased functional drag.

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    $\begingroup$ Excellent answer, +1. You might expand it by saying that during and shortly after liftoff the wing is pushed to its limits, which are greatly affected by small disturbances especially near the nose. See here or here for the influence of bugs and rain on glider wings which pose a very similar situation. $\endgroup$ – Peter Kämpf Jan 14 '17 at 18:38
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    $\begingroup$ Another classic example is that even a thin layer of frost (not ice) can destroy a third of the wing's potential lift, significantly increase stall speed (and thus safe liftoff and approach speed), etc.; and an equally large danger is the pilot(s) have no way of knowing how much effect today's snow/ice/frost is having at any given time-its much easier to simply deice and move on. $\endgroup$ – nexus_2006 Jan 14 '17 at 21:08

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