Rime ice weighs less than clear ice, but rime ice may seriously distort airfoil shape and
thereby diminish lift.
Mixed ice is a combination of rime and clear icing. It has a very rough appearance and it
forms when snow or ice particles become embedded in clear ice.
Frost is light feathery deposits of ice crystals which form when water vapor contacts a
cold surface. Frost occurs when the temperature on the surface of the aircraft is below
freezing at the time condensation takes place. Frost can form on airborne aircraft if the
aircraft flies from a region where the temperature is below freezing to a region where
the temperature is slightly warmer and the air is moist.
Aircraft icing is classified into four intensities for reporting purposes. Table 1-12 contains
the four intensity categories and a brief description of each.
Rate of accumulation slightly greater than sublimation. Ice becomes
perceptible. Even if de-icing/anti-icing equipment is not used, non-
hazardous unless encountered for more than one hour.
The rate of accumulation may create a hazard if flight is prolonged in this
environment (over 1 hour). Occasional use of de-icing/anti-icing
equipment removes/prevents accumulation. It does not present a hazard
if the de-icing/anti-icing equipment is used.
The rate of accumulation is such that even short encounters become
potentially hazardous and the use of de-icing/anti-icing equipment or flight
diversion is necessary.
The rate of accumulation is such that de-icing/anti-icing equipment fails to
reduce or control the hazard. Immediate flight diversion is necessary.
Table 1-12 -- Four intensity categories of aircraft icing