Did you check your suspension? I often had this issue and thought it was a bug, but upon further examination, it was just the CoG sitting above underpowered landing gear, which meant it's completely compressed, which results in a backwards tilt or even tipping over. Try adding better ones or try more and glitch them into each other so they look like one unit.

edit:
Just some pics of what I mean, from an older post. The CoG is clearly forward and the nose LG is clearly shorter, but you can see the result in pic 1 It's due to the suspension loads being very much the polar opposites for front and main LG. In pic 2 it stands correctly with a noticably forward tilt (as designed), there was nothing wrong with CoG calculations.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3518143659

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3518144409

Thanks
Can understand what you mean
Gear compresses -> tail goes down.
The compression is too soft is the reason ?

Okay , then if the calculation is correct.
It looks like the Turbo fan engine weight is too light so that can't make a COG forward.
Anyway thanks , seems I'm not to be here in this game.

It's not too soft, it's overloaded. Too much weight for a singe set of gears. Adding more or a better type is the solution. Then it won't compress as much.

Thanks but already uninstalled this game.