Some company makes a combination FAA approved "car" seat which converts into a stroller. The legs drop out of the bottom and the handles come out from the top of the back. One item then does the job of the plane set, car seat, and stroller. Less to lug around.
I will echo the others' posts. Buy a seat. Not only is it safer, but you have a place to put the little one when he/she falls asleep.
I will echo the others' posts. Buy a seat. Not only is it safer, but you have a place to put the little one when he/she falls asleep.