Airframe icing is the accretion and accumulation of ice on the external surfaces of an aircraft during flight. It occurs when supercooled water droplets strike the structure and freeze on impact, gradually building a layer that can cover wings, tail, engine inlets and antennas. This ice may appear as rime, a rough, opaque frost formed by small, rapidly freezing droplets, as clear or glaze ice, a smooth, transparent sheet created when larger droplets flow aft before freezing, or as mixed icing, a blend of both textures.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is airframe icing?
Airframe icing is the formation of water ice on an aircraft, specifically referring to ice accretion and accumulation on external surfaces. This phenomenon is one of the major weather hazards to aviation and seriously impairs aircraft engine performance.
Airframe icing refers to ice accretion on the external surfaces of an aircraft. This type of icing forms on the outside of the aircraft. Supercooled liquid droplets form ice on cold airframe surfaces. Ice accumulates on every exposed frontal surface of the airplane. Ice builds in flight where no heat or deicing boots can reach it.
Conditions conducive to airframe icing mean visible moisture at or below a static air temperature of 5°C (41°F) or a total air temperature of 10°C (50°F). Known icing conditions exist when visible moisture or high relative humidity combines with temperatures near or below freezing. Supercooled liquid is cold enough to pose the risk of ice accumulation when it impacts the airframe.
When does airframe icing occur?
Airframe icing occurs when ambient air temperature is below 0°C (32°F) and visible moisture like supercooled water droplets in clouds or precipitation is present. This type of icing commonly occurs in colder temperatures between -10°C (14°F) and -20°C (-4°F), with half of all reported icing occurring between -8°C (17.6°F) and -12°C (10.4°F).
The effect of airframe icing is that ice increases drag and decreases range, reduces lift by as much as 30 percent, and increases stall speed. Ice on the tailplane causes a roll upset and tail-plane stall occurs after flap deployment when ice is present. Ice on antennas leads to navigation and communication problems or failures, and induction icing blocks airflow in the engine and leads to power loss. Flight instruments rely on external sources like pitot tubes, static ports, and stall warnings and ice causes instrument failures.
Required conditions for structural icing are temperatures close to or below freezing and visible moisture, which includes clouds or water droplets. Icing occurs with either a warm front or a cold front. Icing occurs during any season of the year but is more frequent in winter in temperate climates. Cold soak causes icing when aircraft descends quickly from a high altitude and aircraft skin remains below freezing point even when outside air temperature is above freezing.
Can airframe icing occur in clear air?
Airframe icing occurs in clear, stable air with light winds and the most familiar form is frost that forms near the surface when an aircraft cooled overnight enters warmer, moist air, or when cold-soaked skin - especially near the tanks - remains below freezing after a rapid descent. Hoar frost occurs in clear air when an aircraft passes quickly from sub-zero dry air into warm moist air, and carburetor ice forms at cruise power when skies are clear if relative humidity ranges between 60 and 100 percent and temperatures range between 20°F and 70°F. Sublimation removes accumulated icing in clear air, but the process is generally slow, so clear air is not automatically free of icing hazard. Clear ice is encountered in thunderstorms where large supercooled water droplets cause a rapid build-up of ice on the aircraft, and clear icing is frequent in a cluster of cells.
What must be done in the case of ice accumulation?
If ice accumulates, all ice must be removed prior to takeoff, using heated air or deicing fluid in accordance with manufacturer guidelines. Lift-generating surfaces must be verified completely free of contamination through a tactile hands-on check and engine nacelles, tail, and static ports must also be inspected. Water blown by propellers or splashed by wheels results in serious aircraft icing. Airframe ice protection systems must be closed when on the ground unless conditions are conducive to engine icing while taxiing. If ice pellets are observed on the ground, they indicate Supercooled Large Drops aloft. Pilots must consult Advisory Circular 135-17 Pilot Guide Small Aircraft Ground Deicing for detailed procedures. If icing conditions are anticipated, one must contemplate canceling or delaying the flight.
What are the types of airframe icing?
The types of airframe icing are listed below.
- Clear Ice
- Rime Ice
- Mixed Ice
- Runback Ice
- Frost
Clear ice is heavy, hard, and transparent and it forms when large droplets hit the aircraft and the remaining liquid portion flows out, gradually freezing as a smooth sheet of solid ice. Clear forms in temperatures between 32°F (0°C) and 14°F (-10°C). Rime ice is rough, milky, and opaque and forms when small supercooled droplets freeze rapidly on impact, mostly between -10°C (14°F) and -20°C (-4°F). Rime ice is less dense, brittle, and porous, forms a streamlined extension of the wing, and appears white.