Ice on an aircraft is not just extra weight; it changes the shape of the wing, and a wing that has lost its shape has lost lift it may badly need.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
What airframe icing is
Structural, or airframe, icing forms when an aircraft flies through supercooled liquid water: water droplets that are still liquid even though their temperature is below freezing. When the aircraft strikes them, they freeze onto the leading edges, the windscreen, the antennas, and any other exposed surface. The two ingredients are simple and must occur together: visible moisture (cloud, rain, or drizzle) and an airframe at or below 0 degrees C. The FAA's pilot guide to flight in icing conditions (AC 91-74B) sets out these conditions, and the FAA Aviation Weather Handbook (FAA-H-8083-28) covers the meteorology behind them.
Ice harms the aircraft in several ways at once: it adds weight, it spoils the airflow over the wing and tail so they stall at a lower angle of attack, it increases drag, and it can jam control surfaces or block the pitot and static system that feeds your instruments. The shape change matters more than the weight.
The three types of ice
The kind of ice you get depends mainly on droplet size and temperature, and the FAA and NASA's icing research describe three families:
- Rime ice comes from small droplets that freeze almost the instant they hit the surface. The trapped air gives a rough, brittle, opaque, milky deposit, usually building forward into the airflow. It tends to form in colder cloud.
- Clear ice (also called glaze) comes from larger droplets that do not freeze on contact but flow back across the surface before freezing. The result is a smooth, hard, glassy, and much heavier layer that is difficult to see and harder to shed. It tends to form in warmer cloud and in freezing rain.
- Mixed ice is a combination of the two, with the roughness of rime and the density of clear ice.
As a rule, colder air and smaller droplets favour rime, while warmer air and larger droplets favour clear ice; the two shade into one another rather than switching at a single temperature.
Where ice lives, and supercooled large droplets
Supercooled water exists across a wide band. Most airframe icing occurs roughly between 0 and minus 20 degrees C, although supercooled droplets can persist to around minus 40 degrees C, as the FAA Aviation Weather Handbook describes. Below that, cloud is usually made of ice crystals, which generally do not stick to a cold airframe.
The worst case is supercooled large droplets (SLD), the size of freezing drizzle or freezing rain. These are large enough to run back past the leading edges that a de-icing or anti-icing system protects and then freeze, building ridges of ice on unprotected surfaces behind the boots. Many aircraft are certified only for the smaller-droplet icing envelope and not for SLD, which is why the FAA treats freezing rain and freezing drizzle as a distinct and serious hazard, and why certification rules define separate icing envelopes for them.
The forecast, and what each authority expects
Icing is forecast and warned about in the products you already brief. A SIGMET uses SEV ICE for severe icing, an AIRMET uses MOD ICE for moderate icing, and TAFs, area forecasts, and significant-weather charts carry the icing picture, all under the meteorological framework of ICAO Annex 3.
The rules on flying into it differ by authority and by aircraft, so this is where to be precise. In the United States the FAA's AC 91-74B frames flight in icing around whether the aircraft is approved for flight into known icing and around avoiding conditions beyond its certification. In the United Kingdom the UK CAA's Safety Sense leaflets give equivalent guidance for general aviation, including the handling of airframe icing and the limits of light aircraft. The common thread is that an aircraft without ice-protection equipment, or one outside its approved envelope, must avoid icing conditions rather than fly through them.
Common pitfalls
- Clear ice is hard to see, especially at night or on a wet windscreen, yet it is the heavy, fast-building kind. Do not wait until you can see it clearly.
- Tailplane icing can bite first. The tailplane is often a smaller, sharper aerofoil that collects ice faster than the wing, and tailplane stall behaves differently from a wing stall.
- Freezing rain and freezing drizzle are a trap, because the droplets are large and may exceed what the aircraft is certified to handle.
- Carburettor and induction icing are separate problems that can occur even in air well above freezing on a humid day, so they are not covered by an airframe-icing forecast alone.
In Pilot EFB
Pilot EFB pulls the weather for your route and shows the decoded report alongside the raw text, so the SIGMETs, AIRMETs, and forecasts that warn of icing are in front of you in plain language with the original code preserved. It does not predict whether your particular aircraft will pick up ice, and it is not a certified Electronic Flight Bag. Treat it as a study and planning aid: judge icing against your aircraft's approved limitations, its ice-protection equipment, and your official meteorological source. Saved weather stays readable offline; pulling a fresh forecast needs a connection.