Version 2.5X of blender omits an AC3D file format importer, this is a format commonly used in FOSS games (eg, flightgear).

I have set myself the task of updating an existing import/export utility to work in 2.5.9 of blender.

So far I have updated a V2.48 AC3D import script to work in the new 2.5 framework (borrowed some code from the existing python scripts for import/export) to work as an importer for .ac files. Some testing with simple and complex models has been carried out (opening an example flightgear model as well as a simple 4 point rectangular plane with no textures)

Known issues:
- Poor formatting of the code
- Not picking up textures in the path of the opened .ac file
- doesn't check for alpha/transparency
- probably an issue with use of PolyLine to triangulate ngons with vertexes>4
- most of code largely unchecked
- export not yet done

create folder (either in ~/.blender or in blender installation folder/shared script folder):
blender/2.59/scripts/addons/io_scene_ac3d

Download files into newly created folder, startup blender 2.5, enable the addon in the user preferences (ctrl-alt-U or file->user preferences) "addons" tab