Mate, I have no idea what you have done, but you have created a black hole.
But in all seriousness: I too had the plane and level disappear when I tried it, I also noticed quite a low framerate when it happened (I ran it at 4% and i had 10FPS).
The very strange thing is that even when the simulation stopped the level and machine did not load back in... odd.
I apologize for not offering a solution, the only thing I can think of is start from scratch (annoying, i know), maybe avoid spawning in extra core blocks and pray it doesn't happen again.
Alright I'm back, with (hopefully) good news!
I loaded your machine back up and started experimenting by deleting certain aspects that I thought would cause issues.
Turns out, for some reason, the braces in the main body of the plane were the issue, once they were deleted the simulation would start as normal, no game breaking issues.
Now unfortunately for you, this quite a structural downfall for the craft, maybe have less braces or redesign the body completely. Hopefully this helped!
Today i was making twin engined plane and i had to restart the game cause of the fps drop bug i saved my machine, so i launched the game again but when i played the simulation when i applied throttle one of the engines was going faster then the other the speeds were set to 2.0 for each but one was colckwise one was counter clockwise so "sometimes" if the wheel is counter clickwise it will run better than clockwise but why they ran at the same speed before but they changed after restarting?
I cant confirm of know but i will confirm it tommorow
(Btw, you may not understand what i said if you wont then tell me so i will try to make it more understandeble ).