Thanks @ItsAMysteriousOfficial !!!
One model had 83.000 vertices instead of 10.000. I had to remodel quite a bit and it doesn't look so good but it works :-) .
BTW do you now if its possible to have multiple textures for one model like exterior.png and interior.png?
The mod is called "MCHeli Mod"and its basically the Flans mod without bugs (no planes stuck on the roof or in the floor :-D ) and way better.
Seriously try it!!! It's one of the best mods but pretty unknown (from a Japanese guy).
I'm only making a Content Pack for it.