Currentley I am able to use all of the default aircrafts and ones that I previously added to the list. However I have downloaded a couple of new ones, and when selecting them to fly it automatically says microsoft flight simulator is not responding and closes the game. I thought this might be the aircraft in particular so I tried a totally different aircraft and the same problem. My aircraft list is very small so I know its not down to too many planes.
This problem is usually caused by an incompatible gauge, sometimes a module. Once installed, it will affect all aircraft.
Solution: Pick out the added gauges one by one and see which one is the culprit. Alternatively you could simply get rid of both aircraft but do make sure you also delete the gauges and modules (if there were any).