Ice Accretion

Meteorology · Chapter 12 · DGCA CPL and ATPL

How supercooled water turns into ice on a wing, why clear ice is the worst, which clouds and temperatures to fear, and how a carburettor can ice up on a warm day.

Remember

Hoar frost: feathery, in clear air, airframe below the frost point (parked on a cold night, or descending fast into moist air). Not serious.
Opaque rime: small supercooled droplets, freezes at once, white, light, porous. Clear ice (glaze): large supercooled drops spread before freezing, heavy, glassy, sticky: the most dangerous.
Severe 0 to −7 °C, moderate −7 to −12, light −12 to −20, very light −20 to −40 (except CB). Most liquid water, so most icing, near −15 °C.
CI, CS, CC: negligible. AS, NS: light to moderate. AC: light to moderate, severe over mountains. TCU and CB: light to severe down to −20 °C.
Icing adds weight and drag, cuts lift and raises the stalling speed. Carburettor icing: +30 to −10 °C, unlikely below 60% RH, even in clear air.

How can liquid water freeze onto a wing?

How ice grows

Five real photographs from NASA's icing research. Swipe or tap Next.

Liquid, but colder than ice Cloud droplets often stay liquid well below 0 °C, down to about −40 °C (−45 °C in CB). They are supercooled, waiting for something to freeze on. An aircraft is that something. Even with de-icing systems, ice is a serious hazard that can lead to loss of control. This aircraft has just flown through supercooled large droplets (SLD), the most dangerous icing condition.
Small drops: white and brittle Small supercooled droplets freeze the instant they touch the leading edge, trapping air between them. The result is opaque rime: white, light and porous, like the frost on a freezer wall. It does not weigh much, but it changes the shape of the wing and spoils the airflow.
Big drops: glassy and heavy Large supercooled drops (and freezing rain) do not freeze at once. They spread back over the wing before freezing, trapping little air: clear, glassy ice. It is heavy, sticks hard, can unbalance propellers and control surfaces, and breaks off in dangerous lumps. Clear ice is the most serious kind.
Frost from clear air On a cold clear night a parked aircraft can be covered in feathery hoar frost: water vapour depositing straight onto a skin colder than the frost point. It also forms when an aircraft descends quickly from cold heights into warm, moist air. Hoar frost is not a serious hazard; it melts or shakes off quickly. But it must be cleared before take-off.
Which ice? Flying through freezing rain under a warm front at −3 °C.

Takeaway: Cloud drops colder than 0 °C can stay liquid (supercooled). The moment they hit the aircraft they freeze. Small drops make rime; big drops make clear ice; ice crystals simply bounce off.

What makes ice white and brittle, or glassy and heavy?

Three kinds of ice

Watch the drops hit the wing, then tap Next.

Hoar frost On a clear night, an aircraft can cool below the frost point (the temperature at which frost forms). Water vapour then turns straight into feathery ice crystals, with no liquid step. It also forms when an aircraft descends fast from cold, high levels into warm, moist air. It soon goes as the aircraft warms up or shakes, so it is not a serious hazard. But frost on the wings must be removed before take-off.
Opaque rime Small supercooled drops (liquid water colder than 0 °C) freeze the instant they touch the leading edge. Air is trapped between them, so the ice is white, rough, light and easy to break, like frost on a freezer wall. It is not heavy, but it spoils the shape of the wing.
Clear ice (glaze) Large supercooled drops hit, spread out and flow back over the wing before they freeze. Little air is trapped, so the ice is glassy, smooth and often lumpy, like ice cubes. It is sticky and heavy. When it breaks off, it comes away in dangerous lumps. It is the most serious kind of icing.
Mixed Clouds often hold drops of many sizes. Between 0 °C and −40 °C, a wide range of drop sizes gives a mixture: rime from the small drops and clear ice from the large ones. So the drop size, more than anything, decides which ice you get.

Memory hook: Small drops: rime. Large drops: clear. Clear is worst.

Takeaway: Hoar frost: feathery ice that forms straight from water vapour in clear air, on a surface colder than the frost point (parked overnight, or after a fast descent into moist air). It is not a big hazard but must be cleared before take-off. Rime: small supercooled drops freeze at once and trap air, so it is white, rough and light. Clear (glaze) ice: large drops spread back before freezing, so it is glassy, heavy, sticky and the most dangerous.