1. What Supercooled Liquid Water Is
Supercooled liquid water (SLW) is liquid water that remains unfrozen at temperatures below 0°C. Water droplets in clouds can stay liquid in these conditions until something disturbs them. When a supercooled droplet strikes a cold surface, such as the leading edge of a wing, it can freeze almost at once, building up ice on the airframe.
This is why icing is most associated with clouds that are sub-zero but still liquid, particularly layered stratiform cloud and some cumulus, rather than with snow or with clouds that are fully frozen.
2. Rime Ice and Clear Ice
The type of ice depends on how the droplets freeze. Small droplets tend to freeze quickly on contact, producing rime ice, which is opaque, rough and brittle. Larger supercooled droplets can spread along the surface before they freeze, forming clear ice, which is dense, smooth and harder to see. Clear ice can be more difficult to remove and can change the shape of the wing more significantly.
Types of airframe icing
| Type of ice | How it forms | Effect on the aircraft |
|---|---|---|
| Rime ice | Small droplets freeze on contact | Rough surface that increases drag; can be brittle and sometimes shed |
| Clear ice | Larger droplets spread before freezing | Dense, smooth ice that changes the wing profile and can be hard to see |
| Mixed ice | A combination of both types | Often the most complex to predict and to remove |
3. Effects on Flight
Ice on the wings disturbs the airflow, reducing lift and increasing drag. The stall can occur at a higher speed than normal, and the aircraft may become harder to control. Ice on tail surfaces and control horns can affect pitch and control response. Ice that builds on propellers, antennas and pitot tubes can affect power, airspeed indication and other systems. The effects can grow faster than the pilot expects, which is why early action matters.
4. Avoidance and Decision Making
The most reliable protection is to avoid icing conditions. Check the forecast freezing level, temperature aloft, cloud layers and pilot reports before flight. If icing is forecast or reported on the route, consider a different altitude, a different route, or a delay. If ice begins to form unexpectedly, leave the icing area as quickly as possible by changing altitude or direction, if it is safe to do so. Where the air is warmer at a lower altitude, descending may take the aircraft out of the icing layer, but this must be weighed against terrain and minimum safe altitudes.
Ice protection equipment, where fitted, is designed for specific conditions and is only as good as the aircraft's certification and the pilot's training. Follow the procedures in the Pilot's Operating Handbook and do not assume that equipment will cope with every type of icing.
Icing forms in clouds that are below freezing but still liquid. Forecasts, pilot reports and the freezing level are the first checks before a flight.
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