It'll never fly
It'll never fly.
Fig. It will never work!; It will never be approved! (Refers originally to an evaluation of an unlikely-looking aircraft of some type.) l have read your report and studied your proposal. It'll never fly. Your design for a new electric automobile is interesting, but it'll never fly!
It'll never fly.(American)
something that you say when you think an idea will not be successful He sent me a movie script but it'll never fly - it's just too unbelievable.