Then, there's the theory (I've read this somewhere, but I honestly can't remember where,) that the formula is incorrect if, upon touchdown, there is no initial wheel spin-up, i.e. a real greaser of a landing on a runway that is contaminated with water after a heavy rain storm. In this case, you may never realize any braking action.
Which is why we have learned that you should make a positive landing (i.e. not a greaser) when there is a possibility for aqua planing.
