Let’s pick up with the next factor that influences icing: kinetic heating — Page 297, Lesson 283

Let’s pick up with the next factor that influences icing: kinetic heating. Kinetic heating is the temperature rise caused by the friction of air rushing over the aircraft’s surfaces. Now, here’s the key point: if that temperature rise pushes the skin temperature above 0 °C, it can actually prevent ice from accreting — the surface stays warm enough that supercooled droplets don’t freeze on contact. But if the rise only brings the skin temperature to below 0 °C, it can increase the risk and severity of icing. So it’s not automatically helpful — it depends on whether the final skin temperature crosses the freezing point. There’s a simple formula to estimate skin temperature. It’s: Skin Temperature = OAT + (TAS / 100)² OAT is Outside Air Temperature, and TAS is True Airspeed in knots. So if you’re flying at 200 knots TAS, the kinetic heating contribution is (200/100)², which is 2² = 4 °C. Add that to the OAT, and you get the skin temperature. That tells you whether the surface might stay above freezing or remain cold enough to collect ice. Now, let’s move into icing forecasts. Forecasting airframe icing is really about forecasting clouds — both their type and their vertical extent. The degree of airframe icing is classified as light, moderate, or severe. When rain ice is expected — that’s ice forming from freezing rain — it will be mentioned specifically in the forecast. Forecasts of engine icing are not normally provided. Next, the freezing level. That’s simply the height where the ambient temperature is zero degrees Celsius. It’s usually given in forecasts on an area basis, referenced to the height of the Zero Degree Isotherm — that’s the line on a chart connecting points where the temperature is 0 °C. Now, if there’s a temperature inversion — where temperature increases with height instead of decreasing — you can have two freezing levels. One below the inversion and one above it. As a practical example, freezing levels in the south of the United Kingdom average about 11 000 feet in August and drop to around 3000 feet in February. Now, let’s look at reporting of icing. The UK Air Pilot gives a useful description of the degree of icing encountered in flight. All pilots encountering unforecast icing are requested to report: time, location, level, intensity, icing type, and aircraft type to the Air Traffic Services unit they’re in radio contact with. It’s important to understand that the following icing intensity criteria are reporting definitions — they are not necessarily the same as forecasting definitions. Why? Because reporting definitions are related to the aircraft type and the ice protection equipment installed, and they do not involve cloud characteristics. For similar reasons, aircraft icing certification criteria might differ from both reporting and forecasting criteria. Let’s go through each intensity level. Trace icing: Ice becomes perceptible. The rate of accumulation is slightly greater than the rate of sublimation — that’s ice turning directly back into water vapour. It is not hazardous even if de-icing or anti-icing equipment is not used, unless you encounter it for more than one hour. Light icing: The rate of accumulation might create a problem if flight in this environment exceeds one hour. Occasional use of de-icing or anti-icing equipment removes or prevents accumulation. It does not present a problem if that equipment is used. ICAO defines this as “less than moderate icing.” Moderate icing: The rate of accumulation is such that even short encounters become potentially hazardous. Use of de-icing or anti-icing equipment, or diversion, is necessary. ICAO describes this as conditions in which a change of heading and/or altitude may be considered desirable. Severe icing: The rate of accumulation is such that de-icing or anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary. ICAO says these are conditions in which an immediate change of heading and/or altitude is considered essential. So, to summarise: trace is barely noticeable and safe for under an hour; light is manageable with equipment; moderate demands action even in short encounters; and severe means your equipment can’t keep up — you must get out immediately.