Quoting The Tsukuno family:" Question how do u get a carseat on a plan because on us airways it free of charge to being with but does it go under the plane or what? Also does it need to be in one of those carseat bags?"
I prefer to use the car seat on the plane. Mainly so I can strap him in and not have him crawling all over me. If you don't want to use it but you need to take it, you can gate check it right before you get on the plane (although some don't reccommend it because the workers just throw everything into the plane and aren't careful). They have put mine in a bag before when I gate checked it, but like I said, I always use it. The only time I didn't use it was before he turned 2 and he was a lap baby. If there was an extra seat on the plane, I could use it for free but if the plane was full, then I gate checked it.