Are you using the GSX editor to edit the file ?
Also, be sure you don't have two copies of it, the one in %APPDATA%\Virtuali\Airplanes takes precedence over the one in the FSX\Simobjects folder so, if you are hand-editing the one in FSX, but have another copy into %APPDATA% you won't see any change. If you use the GSX editor, it work on the one into %APPDATA% by default (if you press "Save" and not "Save As" )