Marathon on a plane?

Calling all Marathon owners: I'm flying next week with my 6 mo, and think it's time to upgrade to a convertible car seat. I'm pretty sure that I'm going to get the Britax Marathon, but had a question about taking it on the plane. I read in BB that it's too big to fit in an airplane seat, but read on some other posts that people have used it for ailine travel. Anyone out there with experience? TIA!

Sarah