Even the Latitude calculations can get a bit screwy on things like trans-Pacific flights because the actual flight path will usually follow the "Great Circle Path" which is hardly ever parallel to lines of Latitude. For those, you'd need to calculate the Lat/Lon Limits using the half-distance thing for every point. That'll still be lightning quick compared to trying to compare to all the airports of the world.
--Jeff Moden
Change is inevitable... Change for the better is not.