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Considering the general rule of thumb defining potential icing conditions: temperature between +10 and -40 degrees C and presence of visible moisture, is the temperature range regarding TAT (total air temperature) or SAT (static air temperature)?

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  • $\begingroup$ I've never heard of those. $\endgroup$ – CrossRoads Jun 30 '18 at 21:33
  • $\begingroup$ You can find a good definition in the answer here: aviation.stackexchange.com/questions/1640/… $\endgroup$ – Musmus Klapa Jun 30 '18 at 21:36
  • $\begingroup$ It would be wise to make it clear your are discussing STRUCTURAL icing and not ENGINE icing. You came close to a -1 when I saw the +10C and -40C :) You may want to reference your source weather.gov/source/zhu/ZHU_Training_Page/icing_stuff/icing/… $\endgroup$ – jwzumwalt Jul 1 '18 at 4:27
  • $\begingroup$ Thanks @jwzumwalt. In your own reference it says threat exists down to minus 40. Some manufacturers, including Cessna (according to my Citation S2 training at Flightsafety) claim the top end to be plus 10 degrees. This can also be seen here: aopa.org/news-and-media/all-news/2016/december/pilot/…. Please elaborate about the difference in conditions between structural and engine potential icing conditions. At the Citation we turn on engine heating at the above conditions, and turn on TKS at the wings+tail when actual icing is encountered. $\endgroup$ – Musmus Klapa Jul 1 '18 at 10:00
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For the purpose of aircraft limitations, it's TAT for the upper limit of +10C. SAT for the lower limit of -40C.

Generally once you are going faster (in the RJ, above 230kts) you only need to have cowls on for all visible moisture conditions, and wings only need to be on if actual icing is happening or indicated. Ice can form on cowls from the temperature drop related to the inlet pressure drop, like carb ice, so the requirements are more conservative for the cowl A/I system. Cowls are turned on and off way more frequently than wings.

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Thinking of this, I believe a pilot should be conservative and take into account both temperatures. Ice can form in places where ram air hits the plane and stops relative to it (there TAT is relevant due to compression), or in places where it keeps on flowing (there its temperature is lower and closer to SAT). Therefore, if SAT or TAT is between +10 and -40 and visible moisture is present, potential icing conditions should be taken into account.

A good explanation of the difference between TAT and SAT: What is the difference between OAT, RAT, TAT, and SAT?

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