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To prevent ice crystal icing, it is advisable to steer clear of (1) _____ clouds and avoid from flying (2) _____ of these clouds.

  • A

    (1) Nimbostratus (NS); (2) downwind.

  • B

    (1) Cumulunimbus (CB); (2) downwind.

  • C

    (1) Cumulunimbus (CB); (2) upwind.

  • D

    (1) Towering cumulus (TCU); (2) upwind.

Refer to figure.
Ice crystal icing (ICI) condition refers to aircraft experiencing icing inflight in high altitude due to high concentration of very 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, CBs) can lift high concentrations of moisture to high altitudes where it can freeze into very small ice crystal.

The main risk of encountering high crystal concentrations appears to be downwind from the tops of large areas of convective cloud (CBs) - the area where the visible anvil shape is seen when viewed from a distance.

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