If you have selected a PMDG 747-400X, then re-enter the aircraft selection menu and select a PMDG 747-400X of the same engine type, you MIGHT trigger this quirky little bug in FSX, and the PMDG 747-400X will fail to load properly and be unflyable within the simulator. If this happens, simply close the sim and start over again.
We identified this failing to Microsoft but there simply wasn’t sufficient time for them to address the issue.
There is smoke pouring from the engines and the model won’t animate.