In '81 a Fokker F28 flying over Moerdijk in the Netherlands experienced severe turbulence after entering a thunderstorm, causing one of the wings to separate from the aircraft after it experienced loads of up to +6.8 G and -3,2 G. The F28 is a plane with the engines at the tail. (And quite a tough one, too)
No offence, but I think you're clutching at straws here. Aircraft are designed and built according to certain operational requirements, and those include the limit and ultimate loads ("g") on the airframe. Exceed those and anything can happen.