Question

What program can open MDL and AIR files to edit? 

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What do you wish to edit in the MDL file?
I want compare 2 MDL file from different aircrafts and Edit one of them to make virtual cockpit and another to make the doors open the right way.
You would need the original source file & the same 3D modelling program used to create the aircraft.
This is the SimV "Orphan" files page. http://www.simviation.com/adoptfsc.htm
You would need either FSDS or Gmax to work on them.
Can u give me link to Gmax or FSDS to download.