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Please see the official X‑Plane Support page for help.
I am not from Laminar Research; just a flight simmer from down under.
You haven't indicated what version of X-Plane you have. If you have accidently downloaded the XP11.40 Beta version it is a known fact by Laminar that problems exist with some older aircraft.
If you have downloaded the beta version then you need to view the following link found at https://forums.x-plane.org/index.php?/forums/topic/160879-thinking-of-downloading-a-x-plane-beta-version/
You haven't indicated what platform you have been running the PMDG MD-11. If you have been using XP10 then you may have a good chance of converting by loading the plane into planemaker (supplied with X-Plane) and then saving the file as a XP11 aircraft. If on another platform you may have to dump the software in the bin and start again
You should also have a good viewing of the following link found at https://www.youtube.com/watch?v=V4RjIxrhkXs
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