Understanding forecasts and pilot reports of icing
There are two forecasts for determining the freezing level. The area forecast provides the approximate freezing level. The winds aloft forecast predicts the approximate temperature for selected altitudes as well as the forecast winds aloft. It is important to stress that these are both just forecasts.
Actual temperatures aloft are given through pilot reports, or pireps. Pilot reports are a great source of information since they provide the actual conditions at a given time and place. If there is any icing, they will also provide the location and intensity of the icing.
However, great care must be taken when reading a pirep from an airliner. Jet aircraft travel at speeds much higher than those of general aviation aircraft. The friction of the air passing over the aircraft generates heat, causing the leading edges to be warmer than the outside air. This effect is most pronounced during re-entry of spacecraft. Above 40,000 feet, the air temperature is below minus 60 degrees Celsius-- yet the speed of re-entry creates a temperature equivalent to that of an industrial furnace.
Outside air temperature, or OAT, is the only temperature used in general aviation. This temperature is referred to as SAT or static air temperature in jet aircraft. TAT, or total air temperature, is the measurement of the effective air temperature caused by the effects of friction. One can think of it as leading-edge temperature. The TAT of a jet in cruise is typically 20 degrees Celsius higher, and about 8 degrees higher below 10,000 feet where speed is limited to 250 KIAS or less. The higher the airspeed, the greater the difference between SAT and TAT.
When an aircraft is operating in icing conditions at a speed that results in the TAT being above freezing, the aircraft's speed will melt the accumulating ice. In what would be light icing conditions, the aircraft will not collect any ice, resulting in a pilot report of "negative icing." However, general aviation aircraft operating at slower airspeeds would accumulate ice. If the accumulation is faster than that which the TAT can melt off, airframe icing will occur. What an airliner reports as "light icing" will actually be moderate icing for aircraft operating at a slower airspeeds. Thus, moderate icing reported by a jet aircraft would be a very serious situation for a light aircraft, even if it has deice equipment. Jet aircraft--with their speed, power, and heated wings--can handle icing conditions very easily.
It is illegal for any aircraft to fly into known severe icing conditions. By definition, an aircraft with deice or anti-ice equipment cannot handle the ice accumulation in severe icing. A report of severe icing from an airliner indicates conditions that would bring an airplane without deice equipment out of the sky in a matter of seconds. Even an airplane with deicing equipment would be descending within a minute or two.
One final note on reading and deciphering pilot reports from jet aircraft: Airline pilots frequently report the TAT as the outside air temperature. Thus, reports of temperature and freezing levels that are wildly different (higher) from other reports and forecasts must be disregarded. Unfortunately, I saw this from a military aircraft, as a KC-135 reported the freezing level as 20,000 feet when the actual freezing level was approximately 10,000 feet lower.
It might seem counterintuitive to teach a student to scrutinize or to completely disregard a pilot report from a professional pilot. However, such advice is important because of the differences in heat caused by friction in faster aircraft. We must make students understand the higher reliability of pilot reports from general aviation aircraft when evaluating temperatures aloft and icing conditions.
David Williams is a CFI and regional airline captain based in the Northeast.
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