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What I did in the end was make a lookup table in excel with the coordinates of the airports and then write a little function that computes the distance between airports assuming a pythagorean triangle using the cosine of the average latitude to determine the east-west part of the triangle (the north-south part is a big circle). Good enough to determine if something was bigger than 50 NM or not.
A bit like drawing a circle on a Chart, I suppose....
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