Structural icing
Structural icing is ice building on the airframe in flight. It needs two things at once: visible moisture (cloud, rain, drizzle) and a temperature at or below 0 °C (TC AIM AIR 2.12.3.1). Take either away and no ice forms. The water is supercooled: still liquid below 0 °C until it touches the aircraft and freezes.
The size of the drops decides the type:
| Clear (glaze) ice | Rime ice | |
|---|---|---|
| Drops | large, supercooled | small, supercooled |
| Freezing | slow, spreads back over the surface | instant, on contact (traps air) |
| Looks | hard, clear, smooth, heavy | opaque, white, rough, brittle, light |
| Found in | freezing rain, cumuliform cloud | stratiform cloud, light drizzle |
| Hazard | worst: hard to remove, heaviest | lighter, easier to shed |
- Mixed ice is both at once, from cloud that holds a range of drop sizes.
- Freezing rain means there is a warm layer aloft (an inversion): snow melts as it falls through above-freezing air higher up, then drops into a cold layer below and freezes on your aircraft. It is a classic warm-front hazard.
- Hoar frost is a different thing: water vapour turning straight to ice (sublimation) onto a cold ground surface on a clear night. It is not in-flight icing, but it must be removed before flight (the clean-aircraft concept), because even a thin, rough layer spoils lift.
Intuition: a big drop holds a lot of unfrozen water, so it freezes slowly and runs back over the surface before it sets, glazing a clear sheet. A small drop freezes the instant it lands, trapping air between the grains, so rime comes out white and crumbly.
Trap: clear versus rime is set by drop size, not by temperature alone. And do not read freezing rain as colder air above: it means warmer air aloft.
References
- TC AIM AIR 2.12.3 (in-flight airframe icing)
- TC AIM MET 2.4 (Table 2.4, Icing Types)
- TP 10643 (When in Doubt... aircraft critical surface contamination)