AC3D Model .ac
AC3D’s text format. FlightGear scenery and simulator models.
What an .ac file holds
AC3D files are a compact text format listing objects, their vertices, surfaces and materials in a nested hierarchy. AC3D itself was an inexpensive, focused modeller that the flight-simulation community adopted wholesale, and the format became the standard for FlightGear aircraft and scenery, which is why so much simulator content still arrives as .ac.
Also seen as .ac3d, .acc.
Commonly seen in: AC3D, FlightGear, X-Plane tooling, flight-sim scenery.
| Data | Stored in AC3D |
|---|---|
| Geometry | Yes |
| Materials and colour | Yes |
| Texture maps | Yes |
| Animation and rigs | No |
| Scene hierarchy | Yes |
| Read here | Yes |
| Written here | No |
Tested against a real file
The last sweep loaded SphereWithLight.ac from Assimp test models with its 5 sidecar files through the viewer on this site: 17,664 triangles across 1 meshes and 1 materials, in 2.5 seconds. Every format's result is on the test results page, run 2026-09-08.
Convert AC3D to another format
Every pairing has a page saying what survives the trip.
Other things to do with an .ac file
Questions about AC3D
How do I open an .ac file?
Drop it into the viewer on this site and it opens in the browser, with the scene tree, the materials and the mesh statistics. Nothing is uploaded. If you want it in another program instead, convert it to GLB, which almost everything reads.
Is AC3D a good format to archive in?
AC3D stores geometry, materials and colour, texture maps, scene hierarchy. Anything outside that list will not be in the file at all, so archive the original alongside it if the surfacing or the rig matters.
Does converting AC3D lose quality?
Geometry is copied vertex for vertex, so the shape does not degrade. What gets lost is whatever the target format has no field for: convert to STL and the materials go, convert to OBJ and the animation goes. The comparison table on each converter page says exactly which.
Formats that hold the same things
Or go back to the full format reference.