The accumulation of airframe ice. Types of icing are:

1. Rime Ice – Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.
2. Clear Ice – A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.
3. Mixed – A mixture of clear ice and rime ice. Intensity of icing: Trace – Ice becomes noticeable; accretion rate less than 1/4 inch per hour on the outer wing. Light – Requires occasional cycling of manual deicing systems; 1/4 inch to 1 inch per hour on the unprotected part of the outer wing. Moderate – Requires frequent cycling of manual deicing systems; 1 to 3 inches per hour. Severe – Ice protection systems fail to remove the accumulation and ice accumulates in locations not normally prone to icing; more than 3 inches per hour. By regulation, immediate exit is required.

Source FAA Pilot/Controller Glossary↗ · checked 2026-08-25