In almost all airline carriers, the carseat needs to be installed by the window so that it does not block anyone's access to the aisle in case of an emergency. The radian does not install well rearfacing, so plan on switching it to forward facing mode while waiting for the airplane to arive. Check out the instructions for installing the seat using a lapbelt forward facing and possibly practice doing that in your vehicle prior to your trip (if you have a lap belt seating position in your car). You shouldn't need a seatbelt extender on the plane like you might with the britax seats forward facing.
When installing on the plane, recline the plane's seat all the way while getting the radian fastened in with the seatbelt. Once you think you have a good install, raise the plane's seat back upright to give you an even tighter install.