Solving this problem
Okay, a couple of things here. First, the aircraft fleet was never intended as a "stand-alone" add-on, but rather as an extension to the shuttle fleet and the expansion packages. However, I never really explicitly stated this, so that's my fault.
Now, if you only want to use the aircraft fleet and not install the shuttle fleet to your Orbiter installation, then download and install this file:
[ame="http://www.orbithangar.com/searchid.php?ID=3777"]STS Expansion Pack V4.1.8[/ame]
That will give you the minimum you need to use the Aircraft Fleet. The preferred installation, including the shuttle fleet consists of the following add-ons, installed in the following order (with the Aircraft fleet installed last...)
1.) [ame="http://www.orbithangar.com/searchid.php?ID=3730"]Shuttle Fleet V4.1.5 FULL Package[/ame]
2.) [ame="http://www.orbithangar.com/searchid.php?ID=3744"]Shuttle Fleet V4.1.9 (SP2)[/ame]
3.) [ame="http://www.orbithangar.com/searchid.php?ID=3777"]STS Expansion Pack V4.1.8[/ame]
4.) [ame="http://www.orbithangar.com/searchid.php?ID=3220"]NASA Aircraft Fleet V4.0.1[/ame]
This will provide you with a FULL shuttle fleet, expansion package, and Aircraft fleet. Then the Enterprise, STA, etc. should all work properly.
If after installing these files (either method) you are still having problems, then post your Orbiter.log file and the scenario that you are having problems with. I do ask that you install ONLY these add-ons into a "Clean" Orbiter installation (with only Orbitersound V3.0 or V3.5 added) and NO other add-ons. I have created a similar Orbiter installation to aid in troubleshooting problems, and I will not attempt to troubleshoot other peoples add-ons.
Dave