We flew to Colorado last July with a 2 1/2 year old and 13 month old. I think on Frontier, so I'm not sure about other airlines. I would call. I also took the print out from the airlines website with carseat regulations with me, just in case we had issues with the airline, we didn't. We took both our Britax Marathon seats with us. We ended up putting my 2 1/2 year olds seat in cargo as we boarded by our choice (he had his own seat, so we just belted him.) It was just so cumbersome lugging two and it was good we did anyhow. My daughter sat in her car seat. They will only put them in a window seat and the only problem we had was that her feet touched the seat in front of her. It was a tight fit, but worked out. I just did blocking and tackling with her feet so she wouldn't kick the seat in front until she fell asleep. There was a pregnant, very understanding woman in front of her. Nonetheless, we didn't want her ride to be unpleasant! No way could we have used it for my son, his little legs would have been crammed up against the seat. I would take all the formula you think you'll need even in case of delays. They won't fight you on that. They will make you empty any sippy cups as you go through security. So just don't have any on that side until you get through in a bottle or cup. I'd label it clearly too. All airports are different, but good luck! Also ask to be pre-boarded so you can get situated. Some airlines are great about that, some ARE NOT & don't help at all when traveling with little ones. Others are wonderful. So, at least ask.