There has been reported a problem in FSX were the incorrect texture is showing even though you have assigned the correct airline texture. Although it does not seem to happen to all aircraft, it does to some, the MD80 springs to mind.
This seems to be a bug within FSX that is causing this. There are two methods of fixing this both are fairly straight forward but do require a bit of work.
METHOD ONE
Find the aircraft that you suspect is not showing up correctly in your FSX\Sim Objects\UT Aircraft\ folder. Make a back-up of the folder outside of FSX and keep it until you are sure you have fixed the problem.
Open up the folder that is called JUST TEXTURE. Then open up the next texture folder. Compare the contents of the two folders, usually in UT AI aircraft there will be TWO files in the TEXTURE folder and ONE file in the next texture folder. COPY the file **_L.BMP which should be the one missing into ALL your texture folders.
Then from ALL your texture folders for that aircraft DELETE the texture.cfg file. An easy way to do this is to do a search of the aircraft folder for texture.cfg and then delete then all from there.
METHOD TWO
If disc space is a problem then this method is better for you.
Find the aircraft that you suspect is not showing up correctly in your FSX\Sim Objects\UT Aircraft\ folder. Make a back-up of the folder outside of FSX and keep it until you are sure you have fixed the problem.
In the aircraft folder create a new folder called TEXTURE.ROLLBACK.
Open your TEXTURE folder and then cut then paste into your new folder the **_L.BMP file.
In your TEXTURE folder open the texture.cfg file and edit the fallback.1 line to read:-
fallback.1=..\texture.rollback
Save the texture.cfg file and then copy it into EVERY texture folder for that aircraft.
Thats it, with luck your aircraft should now all appear.