... I created the files myself and pasted the lines in them.
As I said earlier, the game now crashes at 20% on the loading screen.
hi mate, and welcome to the forum - thanks also for your descriptive post, which helps a lot when trying to solve problems - other newcomers would be wise to use your post as an example of what we require.
so, from the bits i quoted above - the most obvious reason for the crash that i can see offhand, is that you can't simply create new files for these....
the file contains info for all planes, so creating a new one for the jetwar planes, means all other planes are non-existent in this new file.
some things have changed in the official updates for the game since jetwar was first released - what used to be called 'planes_ru' is now simply called 'planes' - planes.properties, in other words. this is the file you add the info to - never, ever should you create new files in these folders, they are only to be edited (and backed up, of course - but i see you're well on top of that
second thing - you make no mention of the two SFS files that Jetwar uses?
this is, in essence, most of the actual content for jetwar, without which you'll get nowhere -
these files, sasamt_jetwar_01.sfs and sasamt_jetwar_02.sfs, must go into a folder in the game root directory, called 'SFS_AUTO'
game version is not a problem, it'll work just fine with 4.12.2 version
forget about the 2.6 RC option in the instructions, stick with 2.7
it sounds as if you've done most everything else right, so have a look at what i've mentioned, and then try it out this way, and let us know how it goes from there...