Originally Posted by
FrontRunner
YouTube - 1, 2, 3, 4, 5 
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So what makes anyone think that "the terrorists" can not torture the PIN out of you?
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I think the idea is that you can give them
any pin when they torture you, and they won't know if it's the real one or not (until it's too late).
Of course this relies on pilots preferring to be shot down rather than take the chance that the hijackers might land and negotiate.