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Icing Conditions

Appendix C Icing Conditions

Appendix C (14 CFR, Part 25 and 29) is the certification icing condition standard
for approving ice protection provisions on aircraft. The conditions are specified in
terms of altitude, temperature, liquid water content (LWC), representative droplet
size (mean effective drop diameter [MED]), and cloud horizontal extent.

Forecast Icing Conditions

Environmental conditions expected by a National Weather Service or an

−approved weather provider to be conducive to the formation of inflight icing

on aircraft.

Freezing Drizzle (FZDZ)

Drizzle is precipitation at ground level or aloft in the form of liquid water drops
which have diameters less than 0.5 mm and greater than 0.05 mm. Freezing drizzle
is drizzle that exists at air temperatures less than 0_C (supercooled), remains in
liquid form, and freezes upon contact with objects on the surface or airborne.

Freezing Precipitation

Freezing precipitation is freezing rain or freezing drizzle falling through or outside
of visible cloud.

Freezing Rain (FZRA)

Rain is precipitation at ground level or aloft in the form of liquid water drops which
have diameters greater than 0.5 mm. Freezing rain is rain that exists at air
temperatures less than 0_C (supercooled), remains in liquid form, and freezes upon
contact with objects on the ground or in the air.

Icing in Cloud

Icing occurring within visible cloud. Cloud droplets (diameter < 0.05 mm) will be
present; freezing drizzle and/or freezing rain may or may not be present.

Icing in Precipitation

Icing occurring from an encounter with freezing precipitation, that is, supercooled
drops with diameters exceeding 0.05 mm, within or outside of visible cloud.

Known Icing Conditions

Atmospheric conditions in which the formation of ice is observed or detected in

Because of the variability in space and time of atmospheric conditions, the existence
of a report of observed icing does not assure the presence or intensity of icing
conditions at a later time, nor can a report of no icing assure the absence of icing
conditions at a later time.

Potential Icing Conditions

Atmospheric icing conditions that are typically defined by airframe manufacturers
relative to temperature and visible moisture that may result in aircraft ice accretion
on the ground or in flight. The potential icing conditions are typically defined in the
Airplane Flight Manual or in the Airplane Operation Manual.

Supercooled Drizzle Drops

Synonymous with freezing drizzle aloft.

Supercooled Drops or /Droplets

Water drops/droplets which remain unfrozen at temperatures below 0 _C.
Supercooled drops are found in clouds, freezing drizzle, and freezing rain in the
atmosphere. These drops may impinge and freeze after contact on aircraft surfaces.

Supercooled Large Drops (SLD)

Liquid droplets with diameters greater than 0.05 mm at temperatures less than
0_C, i.e., freezing rain or freezing drizzle.