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If one can't get a clearance, it would mean it is unsafe for more aircraft to operate inside it. (ie. too much traffic)
If only that was the reason. I have sat next to a controller with zero traffic within 40nm and asked how one would ask for a clearance to cross his class "D" airspace. His answer? I dont let anyone in to my airspace, neither with nor without an engine or transponder. The sky was empty.....That is the problem.
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