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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 
FAA

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 
flight. 

Note

 

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 
(SCDD) 

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. 

Meteorology 

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