From: "Matt Whiting"
Ron McKinnon wrote:

"Matt Whiting" wrote in message
...Why? It is acceptable to "polish the frost smooth" per FAR 91.527.

Interesting. It seems a rather dubious practice to me. It would indeed
appear to be legal in the US - but, FWIW, it isn't legal in Canada.

Why? Smooth frost is less of an aerodynamic issue that a thick layer of
dirt, bugs and dust...

Be that as it may, arguing that its not as bad as something else (that you
shouldn't fly with either), isn't any argument at all for the acceptability
of flying with smooth frost.

Frost, smooth or otherwise, or other contaminants arguably affect the
aerodynamic behaviour. How much does it change with a given
level of contamination?, what level of contamination is significant?
or how significant is such a change? how much is too much? how
smooth is smooth enough? in what way will such change affect the
aerodynamic behaviour of the plane? (does it change the critical
angle of attack for the wings? the control surfaces?, what are
the new critical angles (what's the new stall behaviour? is a
tailplane stall more likely? will it spin easier? how does it change
rudder effectiveness?) for instance) are critical questions. But
these are not questions that a typical pilot is in a position to answer
operationally. This a job for computer simulations, and lab analysis
in controlled test conditions, and for test pilots in controlled test
situations.

Critical surfaces should be clean.