We recently flew with our 23 month old in a Nextfit on a couple cross country flights. Two problems we had, forward facing he kicked the seat in front of him. We held his legs down and he screamed. Rear facing the seat was not reclined and the person in front couldn't recline. Also, forward facing, the huge seat belt buckle was under his back. I gave the person sitting in front of us a heads up so they could choose kicked seat (with my futile attempts to stop it) or no recline. I personally preferred the rear facing install, because it was easier to do stuff for him and the tray table wouldn't go down over the seat anyway, plus I didn't have to hold his feet. He didn't really understand the in-flight entertainment anyway. He only napped rear facing, but fitfully, because the limited pitch between seats (yay economy class!) meant we couldn't recline the seat much at all.

We used a folding Samsonite luggage cart to transport the seat and our small backpack carry on in the airport.

Agree w pp. bring extra pillows!

I agree the Cosco is a crappy seat but its well regarded. I could not install it either. There are other cheaper, lightweight seats. I would not put a young child in a lap belt.

There are Safety 1st seats that aren't much more expensive than the Scenera and cushier, but still lightweight. That's what we had for a travel seat. I'm not checking a car seat.

I agree the big problem is the distance to the seat in front--especially if the person in front reclines, they will be in your kids lap.
I bought the Cosco scenara for travel and hated it. We ended up just leaving it at the airport. My kid cried the whole time she was in it and I could never get the install tight. The padding was very thin and the straps were not comfortable either. Maybe they've improved the model over the years.
After much experimentation, I ended up bringing extra pillows in a bag (including a big pillow pet) to build a nest in the seat. The airlines won't count a pillow or stuffed animal against your carryon allotment. That worked best for us.
Good luck!

But a cheap Cosco Scenara. It's like $40.

I just traveled with the diono radian and my 26 month old. I used a gogo babyz mini to wheel the seat and my daughter through the airport. We had a lightweight cheap seat but she complained a lot that it was hurting her back on the previous trip. I actually found that the radian was better than other seats w/r/t kicking the seat because it doesn't put legs as close as a higher seat. Also, you can use the tray table because the seat is so low. If you are going to bring the seat and gate check it, I'd use it on the plane.

I would bring a seat or the harness but I would buy a light weight travel seat given you need it on the other end. We just flew with our 5 year old, who did great but he was not comfortable in the lap belt as he kept sliding under. I wish I brought a harnessed seat for him (we did but it was not an approved seat).

CARES.

Easier to use.
Easier to carry.
Lets the kid have way more freedom and mobility.
Doesn't put her feet near the seat in front, so she can't kick person in front.
Definitely fits in the seat.

We are flying in a few weeks across country with our 20 month old. We have flown at lot with her but she was either small enough for the lap (3-9 months) and they were short flights. Or I bought her her own seat and put her in a cares harness (1 hour flight, 18 mnths old).
I can't decide if we should put her in a car seat for such a long flight. On one hand, she might actually sleep/nap better. but on the other, there is no way she is going to let me strap her in for five hours. I also can't imagine the car seat fitting. Its a diono radian which we got beacuse its bigger and great for a tall kid which we have. It seems that just from the angle of it, that her feet are going to be in easy kicking distance to the seat in front of her.
Would love to hear some feedback on this. Otherwise I will get the CARES harness again and just gate check the seat.