I'm back. Damn, Wrench is right: the removal of the flightengine.ini does work for me, but only to an extent. In my mod even stock aircraft bounce a lot, especially the Harriers, even though before the removal of that file they crashed as soon as I started the mission on the runway. After hours of research, trial and error, I came to the conclusion that neither the data.ini of the aircraft nor the .inis of the terrains are involved directly: I've tried to remove the modified .inis of both modded stock aircraft and terrains, forcing the game to use the original ones, but nothing has improved. One thing is sure: the patch TK released ruins most mods, but the stock game works as intended.
I apologize if my English isn't all that clear. After all, here in Italy nobody speaks English, apart from teachers, who sometimes don't even know what they're actually trying to say!