S.H.
I put the car seat right into the sitting area of the stroller and put my toddler in the car seat. I always traveled with a combi stroller and a cosco scenera car seat ( the best car seat for the airplane....lightweight narrow safe and only $40). The edge of the bottom of this car seat fit snuggle into stroller. It was so easy this way.
But I always had my car seat on the plane....with an active 2 year old why wouldn't you. I have flown so many times over the years with my 4 and learned the hard way to bring a car seat on at that age...always!!!