You can add custom airports to Endless ATC. Many ready-to-play, user-created custom airport txt files are available to download thanks to the external communities on GitHub and Discord:
All the files that are available to download via the links above are created and shared by users of Endless ATC, not by the developer of Endless ATC. Use the files at your own risk.
To create your own airport file, read the instructions about custom airports at itch.io.
