An AC file contains information about the appearance of a a 3D model in AC3D format. It stores various appearance information, such as meshes, texture paths, UV-mappings, and material definitions. AC files are typically created by AC3D, an application used to create 3D models for games, flight simulation, and virtual reality.
While AC3D is a popular program foo creating AC files, the files are most commonly used by the X-Plane and FlightGear simulators. AC files can even be used to create X-Plane .OBJ files in AC3D and using the X-Plane plugin to export to the X-Plane OBJ format.
Since AC files are saved in plain text, you can open and modify them using a text editor. To open an AC file with a text editor, you may need to change the .ac file extension to .txt. After viewing or saving changes to the file, change the file extension back to .ac.
NOTE: AC files used by FlightGear are often accompanied by an .XML file, which contains information that animates the object or scenery in the game.