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According to a Gleim study guide,

"Generally speaking, smooth ice on top of the wing is more dangerous than heavy accumulated icing on the leading edge."

Is this true? If this is true, why?

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    $\begingroup$ Wings are designed with very specific shapes, and adding ice throws it off. Not only is the flow messed up, but it also makes it difficult for spoilers and other flaps to work. With ice on the leading edge only, I would assume the slats would still function and while the flow would be affected, it should still stay on the wing as designed. Plus, there's the issue of evacuation and such, where ice on the wing is a slipping hazard (but at this point it's the least of everyone's concerns). $\endgroup$
    – Pheric
    Apr 25, 2019 at 6:17
  • $\begingroup$ Make that an answer and I will upvote it. Also, clear ice adds a lot more weight than rime. $\endgroup$
    – Michael Hall
    Apr 25, 2019 at 19:03
  • $\begingroup$ @Pheric I vaguely remember read in some place Fokker 100 was particularly prone to problems with ice formation in their thin short wings but cannot find a reference and it caused a couple of crashes but cannot find a reliable reference $\endgroup$
    – jean
    Apr 25, 2019 at 19:58

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This seems to be from an older Gleim study guide. It references a now-cancelled AC 91-51A:

(AC 91-51A) Ice on the leading edge, while seemingly more dangerous, can be removed using deicing systems. Ice on the upper surface is much more difficult to clear from the structure, thus making it more dangerous.

This AC has been replaced by 91-74B, which repeatedly mentions the dangers of ice on nonprotected surfaces, but also points out that:

Ice on an airfoil can have other effects not depicted in these curves. Even before airfoil stall, there can be changes in the pressure over the airfoil that may affect a control surface at the trailing edge.

and, discussing roll upsets:

If the flaps are extended, do not retract them unless it can be determined that the upper surface of the airfoil is clear of ice. Retracting the flaps will increase the AOA at a given airspeed.

So current guidance seems to be that ice on the upper surface of the wing is dangerous because of:

  • The upper surface of the wing generally not featuring an ice protection/removal system, and
  • The possibility of control surfaces being affected.

but it doesn't go so far as to say it is "more" dangerous.

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