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Okay, this confuses me. As it gets colder doesn't it mean more possibility of icing? And is the -40 degrees SAT or TAT?

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  • $\begingroup$ I'm curious: where do you get the -40°C limitation from? The anti-ice systems description for the §25 aircraft that I am most familiar with states that anti ice should be used between OAT of +10°C and -30°C. However, the aircraft limitations require that all anti-ice systems must be on when operating in visible moisture at an indicated RAT of +10°C or lower. $\endgroup$ – J Walters Jan 24 '17 at 19:46
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    $\begingroup$ @JonathanWalters it's not specifically mentioned in my company 737 limitations. I get that information from 737 limitations of the other company which operate in areas known for icing condition $\endgroup$ – Azizi Akbar Jan 25 '17 at 1:19
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That -40 is SAT and in degrees Celsius.

The reason that anti-ice is generally no longer needed below that temperature is because at that point, the air is so dry that there isn't enough moisture left to form frost or ice that will stick to the airframe or engine parts.

Normally, it's engine anti-ice, rather than wing anti-ice, that is still on when you get to the point in the climb that the temperature drops below -40. At least in the 737, the engine anti-ice is kept on continuously while the temperature is below +10 TAT and above -40 SAT and you're in IMC or other visible moisture (i.e. rain, snow, etc), while wing anti-ice is only used "as needed" to remove ice buildups from the wings.

Also of note, the "below -40 is too cold" logic only exists in climb and cruise, not in a descent. The rationale there being, you don't want to miss the transition from the air is no longer "too cold for icing" as you descend, and find out that your motors ARE now accumulating ice, when they start to act sick... that makes your day entirely too interesting!

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    $\begingroup$ Note that the -40 is also in degrees Fahrenheit :) $\endgroup$ – IanF1 Jan 24 '17 at 7:47
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    $\begingroup$ There is some serious confusion around. If you cool down an air parcel then the relative humidity rises! You can easily have 100% relative humidity and -40°C. The absolute humidity will be probably small, because the after the saturation the superficial water will condense and freeze and form snow. But just saying that cold air = dry air is really incomplete. $\endgroup$ – Vladimir F Jan 24 '17 at 11:47
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    $\begingroup$ @AziziAkbar -40 F and -40 C are very close, so saying "below -40 C" is nearly equivalent to saying "below -40 F". Warmer than that, the F number is larger (ie at the freezing point of water, the F number is 32 while the C value is 0), while colder than that, the reverse is true. SAT and TAT are always reported in Celsius, though. $\endgroup$ – Ralph J Jan 25 '17 at 2:46
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    $\begingroup$ @RalphJ whoa, that's where I missed. I tricked by thinking that there is no convergence in numerical value between celcius and fahrenheit when stating the temperature. Thank you good sir! $\endgroup$ – Azizi Akbar Jan 25 '17 at 3:40
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    $\begingroup$ I think thi important fact is the absence of supercooled water which sticks really fast. Snow does not, it is blown away. The water vapour content is small indeed and the icing from that can be neglected. Even at sever icing conditions it is mainly the supercooled water that sticks and freezes so quickly. $\endgroup$ – Vladimir F Jan 25 '17 at 6:38

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