People bring car seats on planes?? I've never seen anyone do that. Sorry, not judging, just saying I've never seen it. I would call up the airline you booked on regarding their policy and ask what accommodations they have regarding small children before you lug something large and most likely inconvenient on the plane. They might provide a booster seat for you or have a special seat belt type thing. If they don't, I'd echo another poster and say bring a booster seat. It might just be a little less bulky and inconvenient for you (plus, depending on how big it is, you might not be able to get through security that easily too...if it's too big for the scanners you are probably looking at special fun probing from your lovely TCA officer)
EDIT: Ok. I did a little research for you. Just in case you do decide to, you need to make sure your car seat is certified for automobiles as well as planes:
The label should read, "This restraint is certified for use in motor vehicles and aircraft." Flight attendants are instructed to look for the label, and you may run into problems if it's missing.
Be sure to measure the car seat you're planning to travel with, too. To fit into a typical coach seat, the car seat should be no wider than 16 inches (though you can lift the airplane seat's armrest to accommodate slightly wider car seats).
Secondly, do you know if you have a window seat booked? It would appear they often require this because otherwise said car seat would cause a blockage in case of an emergency:
You'll want to book a window seat. That's where you'll need to put the car seat, to make sure it won't block the escape path in an emergency. You may not put a car seat in an aisle seat or exit row.
Here's what the FAA recommends for children riding on airplanes:
- Less than 20 pounds? Ride in a rear-facing car seat.
- 20 to 40 pounds? Ride in a forward-facing car seat.
- Over 40 pounds? Use the airplane seat belt.
It seems like your daughter is just on the edge of what the FAA recommends for when to start just riding with the seatbelt so the choice is up to you. I think you'd honestly be ok without it, but again, choice is up to you.
EDIT #2: Sorry, also find out if they will charge you. Airlines charge for everything these days and it wouldn't surprise me in the least if they charged that as an extra carry on or something.