[asheshouse]

If you have not yet installed any mods then start with the Unified Installer
(The link is inoperative)

Its a long download but the wait is worthwhile.
This will install your MODS folder within which will be the air.ini

Alternatively, if you already have mods installed, there is a default air.ini provided with the Spitfire Mk1.
This was one of the first, if not the first, new slot aircraft.
(The link is inoperative)
Use this with Aircraft Installer v3.5 as linked in previous post.

If using this check that you have all the aircraft listed in the air.ini actually installed, otherwise the game will crash.