Troubleshooting Add-On Aircraft Installation Issues in FS2004 on Windows 10

I understand you're encountering challenges with add-on aircraft in FS2004 after transitioning to Windows 10. This issue, albeit perplexing, is not uncommon, and I'm here to assist in navigating these turbulent skies.

1. Compatibility Mode: First and foremost, ensure FS2004 is running in compatibility mode for Windows XP (Service Pack 3). Right-click on the FS2004 executable, select 'Properties,' navigate to the 'Compatibility' tab, and select the appropriate compatibility mode.

2. Correct Installation Path: Verify the installation path of your add-ons. FS2004, especially when patched for modern operating systems, can sometimes redirect its primary folder. Ensure the add-ons are being installed in the correct directory.

3. Administrator Privileges: Run FS2004 as an administrator. This grants the simulator the necessary permissions to access all files and directories, which can be a hiccup in newer Windows versions.

4. File Permissions: Check the file permissions of your add-on folders. Windows 10 has more stringent security settings, and sometimes these can interfere with FS2004's ability to access add-on files.

5. Aircraft Configuration Files: Inspect the aircraft.cfg files of your add-ons. In some cases, these files need minor tweaks to function correctly on newer operating systems.

Note: These steps are also applicable to newer platforms like Microsoft Flight Simulator (2020) and X-Plane 12, particularly when dealing with file permissions and compatibility issues.

If the issue persists, do provide more specifics - which aircraft are you trying to install, and are there any error messages? Your detailed input will help in tailoring a more precise solution.

Fly safe and clear skies!