With my Brompton in a hard case I had to argue quite assertively that it was "bike parts", not a whole bike, even though it was in airline size-compliant casing, coming back from Boston. Detroit - Boston was asked "is there anything in there I should know about". Again, I said parts for a bike. I don't like the dishonesty (ish), but nor should I have to pay extra because it is "a bike" unlike unfolding bikes.

Otherwise I agree with JimBeams83. It is asking for trouble to carry it on, not least because scanners seem so variable in size between airports.