Aircraft icing and how it's reported
The types of structural icing, why they're dangerous, and how icing is forecast and reported.
Structural icing forms when an aircraft flies through visible moisture (cloud or precipitation) at a temperature at or below freezing. Ice on the wings and tail destroys lift, adds weight and drag, and can block pitot/static ports and control surfaces — a serious hazard for any aircraft not certified and equipped for it.
The main types:
- Rime ice — milky, rough ice from small supercooled droplets freezing on contact; follows the airfoil shape, brittle, relatively easy to shed.
- Clear (glaze) ice — from large supercooled droplets that flow back before freezing into a hard, heavy, smooth sheet; the most dangerous, often associated with freezing rain and the worst inside cumuliform clouds.
- Mixed — a combination of the two.
Icing is forecast via AIRMET Zulu and icing SIGMETs, plus graphical icing forecasts (CIP/FIP) and the freezing-level charts; it's reported in real time by PIREPs (the `/IC` field, e.g. "LGT RIME 060-080"). Intensity runs trace, light, moderate, severe.
For a typical light aircraft with no anti-/de-ice, the rule is simple: avoid known or forecast icing. Know your freezing level, don't climb into a cloud layer that's below freezing, and if you pick up ice, change altitude or direction to warmer air or clear of cloud immediately. Carrying even a thin layer of ice can dramatically raise stall speed.
*Reference and training only. Use official briefings for flight planning.*