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Message: <blockquote data-quote="Umbran" data-source="post: 2522263" data-attributes="member: 177">Note that everything but the last sentence above is mostly irrelevant.  To be more clear about it - Yes, the engine takes in air, that that air contains water.  That air and water get heated, and shot out the back of the engine.  So far, the overall amount of water in the air has not increased, and the mixture is warmer when it leaves than when it entered.  So far, we should end up with a lower relative humidity in the air leaving the engine - same amount of water, but higher temperature.  That should discourage condensation.  Even if it cools, it would only have the same water content as the other air nearby, and nothing interesting should happen.But, aviation fuel is made of hydrocarbons.  When it burns, many of the hydrogen atoms in the fuel combine with oxygen inthe air to create water.  So, a whole lot of new water comes out of the engine than went into it.  So, when the exhaust cools, the air can end up supersaturated with water, and condensation might occur.</blockquote>