I had no problems with Iceland Air (Zurich - Seattle and back). It went through both times as hand luggage (without determining the weight).

I made it as far as Spain with it, but the depth in particular is borderline for hand luggage and the fact that you can't remove the thick hip belt. You're on the safe side with the 400, also because the 500 easily exceeds 12kg when fully loaded. Then hopefully no hand luggage check.