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Once you have landed your jets back into your airbase (with full health) at your Capital City, you will notice the jets in the airbase have taken damage. Take note that your city did not run out of fuel, nor did the jets run out of fuel. (The HP of the jets is shown to be lower while in the airbase than when being dispatched)
This is because when you send your jets back to the airbase, the boost (Jet General that is applied) will be gone, as the boost only works when the jets are outside of the airbase.