If your child is a lap child (under 2, thus free but no seat), make your baby carseat as a gate-check item, i.e. bring it all the way to the gate and tell the gate agent to gate-check this. The agent would tag the seat and you leave it at the gangway just before you board the plane. Most baby-strollers are gate-checked this way too.
If your child is older than 2 and needs a ticket (thus has a seat), the car seat can serve as a safety seat for the child on the plane - it is safer for little child travelling this way on the plane then just sit in the regular plane seat.
If your child is older than 2 and needs a ticket (thus has a seat), the car seat can serve as a safety seat for the child on the plane - it is safer for little child travelling this way on the plane then just sit in the regular plane seat.