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According to a question that I came across while studying for my EASA exam on Aircraft General Knowledge, "icing" is most likely to happen in an OAT between "Between -5°C and +20°C", but how can this be?

I've read in books and online that because of supercooled water droplets, the "freezing" point can go above 0°C, but I didn't came across any writing that goes as high as 20°C.

And here the question from the exam:

enter image description here

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    $\begingroup$ This sounds like carb icing, as opposed to airframe icing? Could you confirm which - carb icing is possible at relative high OAT's due to the venturi and fuel vapour dropping temperatures inside the carb body considerably. Its a very different concept to airframe icing $\endgroup$
    – Dan
    Jan 22 at 11:46
  • $\begingroup$ Please provide a reference that we can crosscheck to validate that EASA actually says this, because this seems like an error. $\endgroup$ Jan 22 at 11:58
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    $\begingroup$ I updated my question with a screenshot from the exam. As you can see it's not mentioning if it refers to the carburator. But, indeed, the questions before this one were about the carburator. $\endgroup$ Jan 22 at 12:00
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    $\begingroup$ As a courtesy, please wait at least 24 hours before choosing an answer so that all the people around the world can get a chance to read and answer your question. $\endgroup$
    – sophit
    Jan 22 at 12:32
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    $\begingroup$ @Fattie - I suspect it is a combination of 2 and 3. Given the OP said that the questions before refer to the carburettor, maybe the question writer 'forgot' to write this one in a stand-alone context. However, expecting a trainee pilot to be very aware of the risks of carb icing is not unfair. When coming in to land, the risks are increased as is the danger - because of the low altitude. Trainee pilots fly a number of solo hours and if all they know is that they 'should' apply carb heat without understanding why, they are in danger of not appreciating the importance of it and forgetting. $\endgroup$ Jan 25 at 7:57

2 Answers 2

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EASA produced a pamphlet for GA Piston Engine icing. It states:

Carb icing is not restricted to cold weather. It will occur on warm days if humidity is high, especially at low power settings. Flight tests have produced serious icing at descent power when the air temperature was above 25°C, even with relative humidity as low as 30%. At cruise power, icing occurred at 20°C when relative humidity was 60% or more. [...]

Icing chart

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    $\begingroup$ Nice find! I extended the quote a bit and included the chart, since it can be quite useful. Hope you don't mind. $\endgroup$
    – Bianfable
    Jan 22 at 12:09
  • $\begingroup$ Dang, that's a nice chart. Someone does science and engineering graphics really well. $\endgroup$ Jan 23 at 20:02
  • $\begingroup$ This answer gives a good authoritative source confirming that it can occur, but doesn’t explain how it can occur at such temperatures. $\endgroup$ Jan 24 at 14:30
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Water cannot exist in solid form at a temperature of 20°C and at pressures in the order of 1 atm. That question is most definitely talking about carburettor icing.

Within a carburettor, temperatures can be much lower than OAT. This is a result of:

  1. Vaporisation cooling: As the fuel evaporates within a carburettor, it needs energy (heat) to do so. It takes this heat from the surrounding air, thereby cooling it down.

  2. Low pressure: Within the carburettor venturi the pressure is reduced, and so is the temperature.

These two factors can easily produce a peak temperature drop of 20°C. So even though the OAT may be relatively high, icing can still occur within the carburettor.

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