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Which set of conditions produces clear (glaze) ice on an aircraft?

  • A

    Large super cooled water droplets at temperature between 0°C and -20°C, which are found in cumulonimbus and nimbostratus

  • B

    Small supercooled water droplets at temperatures below -25°C in cirrus clouds

  • C

    Sublimation of water vapor directly onto the airframe in clear air

  • D

    Rapid evaporation of rain droplets on the leading edge of the wing

Refer to figure.
Aircraft icing type depends on the size of supercooled water droplets and the temperature at which they contact the aircraft surface. 

Glaze ice (clear ice) forms when LARGE supercooled water droplets strike the aircraft surface. Because these droplets are large they do not freeze instantly on impact: instead they spread across the surface as a thin film of water before freezing. This spreading produces a smooth, clear, transparent, dense, hard ice layer that conforms to the aircraft surface shape. 

Glaze ice is the most aerodynamically destructive type of aircraft icing because it is dense, spreads beyond the protected leading edge area, and is difficult to remove. It forms at temperatures typically between 0°C and -20°C where supercooled liquid water can exist in large droplet form. The large droplets are found in cumulonimbus clouds (convective, high liquid water content) and nimbostratus (warm frontal, continuous precipitation with large droplet sizes). 

In contrast, rime ice forms from small droplets that freeze instantly on impact, producing white, opaque, rough ice. 


Large supercooled water droplets at temperatures between 0°C and -20°C in cumulonimbus and nimbostratus → CORRECT. This precisely describes the conditions producing glaze (clear) ice. Large supercooled droplets spread before freezing, producing the smooth clear dense ice layer. The temperature range 0 to -20°C is where large supercooled liquid water droplets exist in significant quantities. Cumulonimbus and nimbostratus are the primary cloud types producing large droplets and thus the most severe glaze icing.

Small supercooled water droplets at temperatures below -25°C in cirrus clouds → INCORRECT. Small droplets at very low temperatures produce RIME ICE, not glaze ice. Small droplets freeze instantly on contact, trapping air and producing white opaque rough rime. At temperatures below -25°C, most water is in ice crystal form (cirrus) rather than supercooled liquid, so significant icing from cirrus is unusual. This description matches rime ice formation conditions, not glaze ice.

Sublimation of water vapour directly onto the airframe in clear air → INCORRECT. This describes HOAR FROST formation, which occurs when an aircraft surface temperature is below 0°C and below the dew point of the surrounding clear air, causing water vapour to deposit directly as ice crystals without passing through the liquid phase. Hoar frost is commonly found on aircraft that have been parked outside overnight in cold clear conditions.

Rapid evaporation of rain droplets on the leading edge → INCORRECT. Evaporation of rain on the leading edge would cool the surface but does not produce glaze ice in the standard aviation icing sense. Glaze ice requires supercooled liquid water that freezes on contact with the surface, not the evaporation of ordinary rain. 

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