Fly any plane:
Go to the mission builder, and create a mission. Click on "Add plane" if
there is not already one on the runway. Then, right click on the plane icon.
Change the aircraft type from the normal F-15E to any plane in the game
(Su-27, MiG-29, 707, etc.) Note: The plane that is selected will still look
like the F-15E on the inside. Press [F8] to switch to the outside view to
see the aircraft. Additionally, the plane will still perform like the F-15E,
allowing you to do barrel rolls and loops in a 707,
Go to make your own mission. Your aircraft should be listed. Delete what it
says is you. From here, click "Add Aircraft". Select what you want to fly
from the list. Select any other information and then click the "Computer
Controlled" box to change it to "Human Controlled" if it already is not
listed as such. Add what you want , save it, and fly it in the missions menu.