Depends what you wish to do. The MDL files used on the less complex models in earlier versions of FS can be decompiled & manually edited but this is very time-consuming & you need to know exactly what you're doing. Small changes like renaming the texture files used on repaints can be made using a hex editor but again you would need to know exactly what you're doing as the MDL file looks like a load of gibberish when opened in a hex editor. This is no longer necessary in FS9 anyway.
PS.
.MDOEL
Correct spelling is most important if you intend doing any manual editing.
