$600 is expensive, but you're right, it's far cheaper than dealing with injuries, worse than injuries, the stress from a lost or damaged seat, having to ship things back instead of using her baggage allowance, and the sanity involved with holding a squirming 15 month old for an entire trip.
I would buy her a seat. Let's say she's 25 pounds. A 25 pound child in a 150 mph collision (your landing gear fails, for instance) is equal to 3,750 pounds of force. That's something I can't hold onto. She'd go flying and slam into something at 3,750 pounds. Whoever she hits may be incapacitated and unable to escape the aircraft. That's not something I'd want on my conscience, and I always try NOT to sit next to an unrestrained infant for the safety of my kids and myself.
In a sudden drop her body will stay where it is until the ceiling of the aircraft pushes her down. That can happen at any time, so best if she's restrained.
The airlines think this is important, even though the FAA doesn't require seats for infants. They strap the coffeepot down and the soap. Things that are far less important than our babies. Maybe they're trying to tell us something.
Then on top of that, if you check her carseat you have a piece of trash coming out the other end. It's no longer safe to use. The airlines cannot guarantee that it will function as it should. And you may not see damage. Something inside may have broken. And then you only find out in the middle of a collision when your daughter goes flying by, even though you installed the seat properly and she's riding in it properly.
On top of that, for my family it would be hell on earth to try to contain Laine (nearly 14 months) on a flight. She doesn't understand that she can't walk around, that she has to stay near me. We call her rotisserie baby, since she tends to spin if you hold her and she doesn't want it. I can't imagine doing that for hours at a time.
Then, let's say she's calm and happy and just sitting there. My three cubic inches that I have to eat, read, do games, drink, etc. is taken up by wiggly baby. I can't eat without fingers in my food, or get a drink since she'd pull it open.
It's nice to have the space, as well. If she is on my lap for part of the ride when the seatbelt sign is off, my book can go in her seat. Or her feet can go in her seat while she nurses on my lap. It's far more comfortable traveling with the space next to you.
And then, as I mentioned before, you get her carry on and any checked baggage allowance. When the girls and I flew home from NY in July I was able to consolidate most everything we were given down to our suitcases. But I had one extra bag. No big deal, it went under the seat in front of Laine.
Ok, as for which seat, if you don't buy a ticket I'd buy a Scenera for travel every time you fly. Throw it out when you're done (and ideally not even use it there). If you travel often, a nicer travel seat may be something you want, or a Scenera just for travel may be perfect anyway. It's lightweight, narrow, easy to install, and inexpensive. All convertibles are FAA approved, so you don't need to worry about that, but if you travel enough it may be nice to have a dedicated travel seat that's smaller than your long term rear facing seat in your car. Something cheaper and lightweight and comfortable to sit next to. I have a True Fit Premier in my car for Laine, but while it's FAA approved, I wouldn't want to sit next to it. The seat in my husband's car is our travel seat, a little Coccoro. I use a Traveling Toddler to attach it to my rolling luggage, and that's it. Nice and simple through the airport and on board. If you check luggage you can get a luggage cart and bungee cord to wheel your seat through the airport, or use a set of GoGo Kidz.
If you tell us more about your traveling, and your budget and desires in a seat, and her size, we can probably help you pick out a seat you'll love. Or two seats.
HTH! Have a great trip.
Wendy
