We take the Britax Roundabout on the airplane all the time. It must say its FAA approved and we've always had to place on the window (if sitting in the ABC or DEF configuration). But might be okay in the center section of the airplane if the seating is 2-3-2 in the CEG configuration.

We also have the Britax Super Elite (now called Husky) and that is the granddaddy of car seats but would never bring that on an airplane as its too big and heavy!