You need to have administration rights to alter all kinds of things with notepad.
This issue will not be restricted  FSX  files. 

It is a very, very common problem that can be resolved through a Google search specifying your operating system and how to gain Admin Rights.

Try this quick fix first before you Google:
(Make a backup copy of your Aircraft cfg file and put it somewhere safe before doing anything to it!)

Make a copy your Aircraft cfg file and place on your desktop.
Edit and save changes.
Copy and paste the Aircraft cfg file back to Aircraft folder.
 
If this doesn't work try the search as suggested above. 


 
I don't know of anyone who hasn't solved this issue.