I've heard some users are struggling with FG crash (before you crash the aircraft yourself :-p) when using CitationX, 777-200ER, or b1900d. This problem happens when Aircraft is adding a menu inside it's script (and this is a Mac specific bug).
It is already fixed on my local end and a new 2.6.0 package will be made soon (Now I'm working on some more issues to be fixed). Meanwhile you can use the quick-fix updater available at: http://macflightgear.sourceforge.net/wp-content/uploads/launcher/2.6.0/GUIUpdater.tar.gz
Here's the brief instruction of updater:
- Choosing "Advanced Features >> Others >> Install Add-on Data" on the GUI launcher.
- Select the file above (or GUIUpdater.tar if Safari automatically unpack gz).
This will get you back in the air.
This updater is available only applicable to a package for 10.6/10.7. If you're using 10.5 package and have similar issue, be patient and try aircraft that doesn't add a menu item to the menu bar.