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Leon allows to define default 2 alternate airports which will automatically show in a section Schedule, tab FLIGHT.
Click a tab General and type Default ALTN1 airport's name or a code, then do the same for Default ALTN2 (not mandatory).
Once ALTN airports are defined Leon will show them in a section Schedule, in brackets (you need to macr a checkbox 'ALTN' in the left-hand filter, tab 'SHOW' to get columns ALTN & ALTN2 displayed) and also in a tab FLIGHT (see the screenshot on the right).