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An airport that you can turn off.
Now there are a few known problems here, with the first being that every time it has to do a calculation you can see it sitting and thinking about it. This will be my aim of the next version, to have another thread continuously getting foreseeable circumstances results.
The other small problem is that if changing from an airport to not having an airport and if there is a plane landing it doesn't have a long flight path, it may not be able to find a viable solution. This is small fixable but not a show stopper so am release this version.