https://discord.com/channels/850161917569662976/865409726368317461/1040107857179312148
by Hangar Studios 713 on FS² discord https://discord.gg/rR6rAY4QdW

All you need is the Albedo, AO, Roughness, Metallic and a Normal map.
Each can be baked separate. Then the 3 goodies need to be composited in the COMP texture.
You can use GIMP to do that efficiently. 
RGB  - R(AO) G(Roughness) and B(Metallic) I remember like this A-R-M = RGB, It's just how my mind works.
Import them as layers into GIMP backwards - so MRA, it's quicker that way trust me.
Then all you have to do is go into the Colors drop down menu - Composite,
then finally compose they should be in the correct order because you imported them backwards MRA,
let GIMP do it's thing and you'll have the COMP.
Next just make the MSFS Parameters for the material as a standard material,
don't mess with any Blender setting(unless you need to) and just put them in their place.
Albedo/COMP/NORM(3 Textures per material).


added by me (EzRyder):
it seems possible to use just the AO & Roughness, AO & Metallic or Roughness and Metallic
you can set the odd channel by selecting 'Mask'
and then editing the number for that channel manually

import each, then Colors -> Components -> Extract -> Extract Component
using the RGB coler for each
 Red (AO)
 Green (Roughness)
 Blue (Metallic)
then Colors -> Components -> Compose