I recently downloaded a B747-400F to use instead of the default B747-400 the reason was the new aircraft had decent strobe lights that the default did not have. And you can not change the strobe lights in the default because they are hard coded into the MODEL file
After downloading I took it out for a trial flight to see how it handled. Like the default aircraft it did not taxi worth a flip very slow making turns but manageable I took off with no problems and flew around the air port to land making sure to go far enough out so I could make a proper approach . Once I disengaged the auto throttle the plane fell out of the sky like a lead anvil. I could not control the decent. Normally I would just delete the aircraft but I decided to see if I could fix it.
The first thing I did was to compare the new plane's aircraft.cfg file with the default B747-400 but found only minor differences between the two so changing it was not going to help. I than decided to try something I have never done before I knew that the only other thing that effects how a aircraft handles in the air is the .AIR file so I wondered what would happen if I used the default .AIR file I had nothing to loose so I did
I could not believe it! it actually worked and now the plane handles good
All I had to do was to copy the file into the new plane's folder and than change the name in the Aircraft.cfg file