Structural Icing Explained
Why ice forms on an airframe, the types of icing, how it degrades performance, and how icing is forecast and reported.
Structural icing forms when an aircraft flies through visible moisture (cloud or rain) at a temperature at or below freezing. Supercooled water droplets — still liquid below 0 °C — freeze on impact with the airframe, and even a thin layer can be dangerous.
Types of ice
- Rime ice — milky and brittle, from small droplets freezing fast; roughens the surface but follows the airfoil shape and is relatively easy to shed.
- Clear (glaze) ice — glossy and hard, from large droplets that flow back before freezing into a heavy, smooth sheet; the most dangerous, and often associated with freezing rain and the worst cumuliform cloud.
- Mixed ice — a combination of the two, common in practice.
Why it's dangerous
Ice changes the shape of the wing, which:
- Reduces lift and raises the stall speed — a wing can stall at a much higher airspeed than normal.
- Increases drag and weight, degrading climb and range.
- Can jam controls and ice the propeller, antennas and the pitot/static system.
How icing is forecast and reported
Icing is forecast by AIRMET Zulu and icing SIGMETs, by graphical Current and Forecast Icing Products (CIP/FIP), and by freezing-level charts. It is reported in real time by PIREPs — the /IC field, e.g. "LGT RIME 060-080". Intensity runs trace, light, moderate, severe; a "severe" PIREP means the anti-ice cannot keep up and is a signal to leave immediately.
Avoidance
Most light aircraft are not certified for flight into known icing (FIKI), so the plan is avoidance: know the freezing level, avoid visible moisture near and below it, and always have an escape — a climb to colder/drier air, a descent to above-freezing air, or a 180° turn. Supercooled large droplet (SLD) conditions and freezing rain are especially hazardous and demand an immediate exit. See also aviation cloud types and SIGMETs and AIRMETs explained.
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*Reference and training only — not for navigation or as legal advice. Always consult the current 14 CFR, the AIM, your aircraft’s POH/AFM and a certificated flight instructor.*