If you are going to the expense of purchasing a seat for your child, why not use the travel/car seat he/she is used to? I always brought a car seat, even when I did not purchase a seat, in the event of a turbulent flight and for take offs and landings. If there was space available, in the olden days, I was able to use my child seat. My kids have travelled a lot and now understand that we always buckle up- in planes and in cars. As bulky as it is to travel with the car seat, it IMHO is worth the extra effort.

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