Most icing encounters involve droplets about the size of a thin human hair -- defined in formal terms as a "median volumetric diameter" (MVD) between 15 and 50 microns). An aircraft certificated for flight into known icing can handle flight in stratus-type clouds with droplet MVDs up to 40 micrometers, and flight in cumulus clouds with droplet MVDs up to 50 micrometers.
Supercooled Large Droplets (SLD) can be up to 100 times larger, and their mass is so great that they can strike well behind the protected areas of an aircraft. Despite icing certification limitations, accidents and incidents in SLD conditions have been documented, especially following sustained flight in freezing drizzle or freezing rain.
Larger droplets have greater inertia, and are less influenced by the airflow around the aircraft. As a result, larger droplets adhere to more of the aircraft surface, and are more likely to move behind ice protected surfaces ("flowback").
To detect SLD, look for:
- Visible ice behind the active part of the de-icing boots.
- Granular dispersed ice crystals, or total translucent or opaque coverage of the unheated portions of the front or side windows.
- Unusually extensive coverage of ice, visible ice fingers, or ice feathers on parts of the airframe not normally covered by ice.
Additional SLD cues when temperatures are near freezing:
- Visible rain with very large water droplets.
- Droplets splashing or splattering on impact with the windshield. (Note: Droplets covered by icing certification envelopes are so small that they are usually below the threshold of detectability.)
- Water droplets or rivulets streaming on heated or unheated windows. (Note: Droplets or rivulets are an indication of high liquid water content).
- Weather radar returns showing precipitation.
Actions when encountering to SLD conditions:
- Disengage the autopilot, which can mask important cues or abruptly disconnect and present unusual attitudes or control conditions.
- Keep control inputs as smooth and small as possible.
- Advise ATC and promptly exit the condition. Find an area above freezing, substantially colder than the current temperature, or clear of clouds.
- Avoid rapid descents close to the ground.
- When icing conditions exist, help other pilots by submitting a PIREP.
