our system has a complete picture of what airports exist in the vicinity and can plan a path to get their fairly trivially. ATC communication can be done by the system automatically as a "broadcast only" emergency message with it's intentions and rely on ATC to give the airplane priority (though we'll have obstacle avoidance as well to avoid other traffic that ATC can't get out of the way). There are other systems on board that let ATC know that your airplane is in an emergency and needs to full priority.