Either it's really early for next winter, or really late for last winter, but the FAA has issued a notice of proposed rulemaking that would disallow pilots from taking off with so-called "polished frost" on their aircraft's wings. Part of the reason is the ambiguity of when frost can be considered smooth enough to be safe. The FAA had previously recommended that manufacturers propose procedures for smoothing frost on their aircraft's wings, but no manufacturers had done so. The FAA action implies that the agency has determined that even smooth, polished frost presents too great a risk. The proposed rule would apply to airline operations under Part 125, for-hire flights under Part 135 and some private flights under Part 91.