The most hazardous Ice Crystal Icing conditions can be found in Nimbostratus clouds.
Ice Crystal Icing is very dangerous when flying large Cirrostratus clouds as they are entirely made of ice crystals.
Ice Crystal Icing conditions can be found anywhere, even at lower levels, as long as the cloud temperature is below the freezing level.
Convective cloud with high vertical extent such as a Cumulonimbus containing a high cloud ice water content.
Refer to figure.
Ice crystal icing (ICI) condition refers to aircraft experiencing icing inflight in high altitude due to high concentration of small ice crystals. At very low temperatures, the water vapour turns directly into solid ice crystals by deposition (often referred to as "sublimation" in meteorology). Engine surfaces and pitot tubes are affected. Several engine power-loss and damage events have occurred in convective weather above the altitudes typically associated with icing conditions. Research has shown that strong convective weather (thunderstorm activity) can lift high concentrations of moisture to high altitudes where it can freeze into very small ice crystal.
- Ice crystals do not adhere to cold airframe surfaces because the ice crystals bounce off. However, the crystals can partially melt and stick to relatively warm engine surfaces. It has also been noted that ice crystals impacting heated windscreens can result in pilots observing “rain” as the crystals rapidly melt on contact with the heated windscreen.
- The main risk of encountering high crystal concentrations appears to be downwind from the tops of large areas of convective cloud, such as Cb - the area where the visible anvil shape is seen when viewed from a distance.
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