You may have to override settings via your GPU control panel. Do not use "application controlled" as you can't have
finecontrol over these settings within IL2.
Anisotropic filtering is arguably only beneficial at low z-angles. On a tiled textured runway, for example. Trilinear filtering should be just fine. However, if you still think you need Anisotropic filtering, make sure the extension is enabled in conf.ini:
[Render_OpenGL]
TexFlags.TexAnisotropicExt=1
TexFlags.TexCompressARBExt=0
HardwareShaders=1
TexQual=3
3= normal
2= reduced by factor of 2
1= reduced by factor of 4
0 = reduced by factor of 8.
Only need to reduce for older graphics cards of 16mb or less.
TexMipFilter=3
0=None
1=Bilinear (fastest)
2=Trilinear (Improves quality with little sacrifice in speed)
3=anisotrophic (Best quality but slower)
TexCompress=0
0=No compression best quality
1=16bit conversion
2=S3TC compression, best Performance. with little worse texture quality
A very good explanation of what filtering is and what a MIPMAP is can be
located here.
As you can see, IL2 allows the anisotropic extension to be enabled, but the game itself will not perform anisotropic filtering (and personally, I can't see a reason why you'd want to have it on).