It does vary from model to model depending on how the designer designed things originally.
Converting to DDS or 32 bit isn`t really relevant for this problem. They are general tips to improve the performance or quality of the textures.
Most aircraft will have a texture (or part of a general texture) mapped to the glass areas. The problem can be in finding it - not everyone seperates out the glass texture and gives it a recognisable filename. Some will just have a small area on one of the main textures doing the "glass". Tracking it down can be a process of trial and error. One way would be to move all the textures out of the texture folder and see how the aircraft appears. If the windows are now see-through move the textures back in one at a time and test again after each one
There is also some indication that some FS9 imports may work ok in standard FSX but get "broken" when SP2 or Accelleration is installed. Microsoft only guarantee these updates will work with proper FSX aircraft created using the FSX sdk tools so older models just have to take their chances.
There is also the case where they may display correctly on some graphics cards but have problems on others. This means that the fact other people aren`t seeing the same problem isn`t necessarily evidence of a "fix" but might just be due to different graphics card or drivers. Or even just different display settings in FSX.
Thank you martin for your reply. It's most appreciated. I intend to experiment with moving the textures around as per your suggestion but after perhaps trying a newer, better performing video card.
I have heard conflicting ideas reference what is gained by changing video cards but I suppose that it might be a good place to start.
What I've also found confusing is the display issues that seem to crop up with SP2. I have FSX basic with SP1 only. There is one particular aircraft that loads fine on my SP1 FSX yet loads with the 'black glass' on a friend's FSX with SP2.
OTOH, I have three other aircraft that load with the 'black/missing glass' problem on my FSX SP1 yet others have reported that these same files load properly on their FSX SP2.
This seems to indicate that this problem is most certainly particular to the respective individual aircraft files themselves as you've pointed out...
Although this item started up a long time ago I have a comment to add. In FSX PS2 often black (non-transparent)glass textures are caused by white alphalayers in the glass texture. Changing the colorvalue of the alphalayer to something like black (say RGB 0,0,0) or gray (say RGB 70,70,70 )gives you variants from totally transparent to smooky glass. So open up your glass texture p.e. with DXTBmp and rework the alphalayer with a suitable graphics program. Save it back to DXTBmp and save the results in DXTBmp to the original glass texture. Hope this helps a bit.
