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Old 15th Feb 2009, 21:52
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Mad (Flt) Scientist
 
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Originally Posted by Basil
MFS,
As an operational pilot, all our procedures permitted some ice/frost on lower surfaces but, of course none at all on the upper surfaces.
Now, since, on a conventional aircraft, the tailplane or horizontal stabilizer produces a downforce should not our procedures have permitted frost on top but required a clean surface underneath the tailplane?
Logically, yes, the lower surface of a tailplane is likely more critical than the upper, and so you'd be more concerned about that.

However, there are reasons to allow no frost at all on a tailplane. Firstly, how did it get there? There's no cold fuel, so it's can't be due to that. And if it's the airframe that's cold, then any frost on one side may mean fuel on the other. So having seen some frost, how sure are you that there's no more? Finally, there are times you need control power in both directions on a tail, which means frost on either side might be bad news.

The allowances for ice/frost on lower wings are really there because of the difficulty of preventing it when there's cold fuel. As an aerodynamicist, I'd like there to be none, period. But the practical risk of non-environmental frost is low enough to grit your teeth and bear it. (Note that here again, it's only non-environmental fuel frost - if the conditions themselves are frosty, it's usually not allowed, for the same "if its there it could be anywhere" reasoning)
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