Thank you for your reply @Ffortuna. Unfortunately for me this looks like a different issue.
This is on PC - DX11, and is affecting the Bredok3d 747 bought from the Marketplace. I’ve followed the cold and dark startup twice now and I have exactly the same issue as @zooze74 has described. However advance in the air 5000 feet and the throttle magically works!
I can confirm that I don’t have the Realism mod installed. Infact I don’t have any mods installed apart from scenery and the old Working Title garmin and a few planes from the marketplace. I don’t go for those environment updates or realism stuff. I would rather just buy MSFS 2024 
It’s an odd one, and I get a mod clash could do this, but I really am not sure what could be causing it. Maybe it’s just an issue inherient with Bredok3d 747 on the current version of MSFS (1.37.19.0)?
