ptds wrote on Aug 18^th, 2012 at 11:08pm:

I have found that by selecting another aircraft and then Pause FSX, you can then modify the .xml file. Then un pause FSX and re load the aircraft you have changed. It works for me. pete

It can also be done with FSX running by changing aircraft (I switch to the default C172), without pausing, make the changes, the change back to the aircraft you are working. Only problem with changing aircraft all the time is that eventually FSX can suffer a fatal error (CTD).