has anyone used the britax regent on an airplane, i need to know if it even fits in the seat...?

The Britax Regent is not approved for airplane use. So even if it fit (which I don't think it would) you wouldn't be allowed to use it.

You can check it with your luggage for free though (save your box).

I have a britax but for get the model. I know its convertable. I know the one i have is bulky but does fit in planes. Try to get the front seat no lap tray but there is extra room at you feet so it makes flying easier. I have flown across the US 4 times twice with one child and twice with two children in car seats. If the child is cordnated enough i would check the carseats at the gate. I did that with my last flight (international). I would take the seat though for when you land and need to get in a car. If it turns out not to fit then the attendents will move it to under the plane (happened in one of those really small commuter planes). Pack lots of liquids for the trip. Feed during take off and landing (helps the babies ears pop). Good Luck. I know how tiring it can be. I have to lug 3 car seats for my next flight.

Pepper