All FAA approved seats that sit rear facing for infants will fit in the belts on US airlines. Infants under 1 (or 20 lbs on some) are required to be rear facing and the airline is required to allow for the seat to be installed correctly according to manufacturer directions for air travel. I have used a Cosco Scenera Next rear facing with no issues at all, and know many people who have had infants needing rear facing seats that had no trouble. Infants necks are not strong enough to support their head and need additional recline to protect their airway that the forward facing position does not provide, making it unsafe for them.