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Under which conditions does rime ice typically form on an aircraft surface?

  • A

    When super cooled water droplets freeze on impact

  • B

    When large supercooled water droplets spread out before freezing

  • C

    Only when flying through heavy rain at temperatures above freezing

  • D

    During a rapid descent from high altitude into warm, moist air

Refer to figures.
Aircraft icing occurs when supercooled water droplets (liquid water at temperatures below 0°C) contact the airframe and freeze. The type of ice that forms depends on the size of the droplets and the rate at which they freeze. 

Rime ice forms when small supercooled water droplets freeze almost instantly upon impact with the aircraft surface. Because each droplet freezes so quickly it traps air bubbles within the ice structure, resulting in a white, opaque, rough, and porous ice formation. Rime ice typically forms in stratiform cloud with small droplet sizes (stratus, stratocumulus, freezing drizzle). 

It has an opaque appearance caused by air being trapped in water droplets as they freeze. Because it freezes instantly, it builds up on the leading edge of airfoils and it does not flow back over the wing.

When supercooled water droplets freeze on impact → CORRECT. Rime ice forms when small supercooled droplets freeze almost instantaneously upon contact with the aircraft surface, trapping air to produce a white opaque rough ice deposit. This is the correct and precise definition of rime ice formation.

When large supercooled water droplets spread out before freezing → INCORRECT. Large droplets spreading out before freezing describes clear ice formation. Clear ice is denser, clearer, and more aerodynamically disruptive than rime ice and is the more dangerous of the two forms.

Only when flying through heavy rain at temperatures above freezing → INCORRECT. Rime ice requires temperatures BELOW 0°C to form. Flying through rain at temperatures above freezing does not produce icing. This option describes conditions that would not cause any ice formation on the aircraft.

During a rapid descent from high altitude into warm moist air → INCORRECT. Descending from cold high altitude into warm moist air would cause water vapour to condense on the relatively cold aircraft surfaces as water or frost, not as rime ice in the supercooled droplet sense. This scenario might produce hoar frost or temporary condensation, not classical rime ice from supercooled droplets.

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