It is necessary to stay on course. They erected lighthouses along the Mediterranean Sea, something we can hardly think of when flying above clouds nowadays. X-Plane uses the standard aeronautical navaids, like non-directional beacons (NDB's), very high frequency omni-range beacons (VOR's), instrument landing systems (ILS's).
X-Plane comes with over 40,000 NAVAIDs, but if you want to add a few more you can do it in World-Maker in NAVAID editing mode in the "edit" menu. You can use World-Maker to edit and add these navaids, or simply open the "nav.dat" file with a word processor and edit it manually.
