I can fit my B35 in Clemence under a normal seat, however, if it were a less slouchy bag the handles would be at risk.I flew with k32 and b30 in Econ or premium Econ and they fit under the seat in front of me. I put them inside a Rume bag (recycle shopping bag) though. When I fly business, that is when 10 out of 10 times the FA would get me to put the bag up the compartment. But then in business I get the whole compartment to myself so I have no issues with it neither. However I do not believe the 35 will fit under the seat though.
In general I love traveling with a Birkin in particular (for the flight) and find a Kelly amazing for almost any city walking...but I've just bought a Bolide and think it may be a better plane bag. I have accepted some scuffs and dirt for the B when it goes on a plane but I'd rather not have to
