Just want to throw out another POV on carseats on planes...
Yes, it is more expensive to do this, but PLEASE buy a ticket for every passenger, including babies, and put them in their carseat on the plane! The plane can hit rough turbulence, and there would be no way you could hang on to your baby, and he could be thrown around the cabin. Yes, it has happened! After all, you put your child in a carseat everytime you go in the car, even though it's very unlikely you'll get into an accident! Planes are no different. Many airlines give discounts to babies in carseats, so you won't have to pay the full price, anyhow. Plus, you'll have the peace of mind of knowing your baby will be safe, just as you do in your car...
We flew across the country when my DS was 9 months old. We brought his Britax Marathon and his giant travel-system stroller (Evenflo). When we got to the airport, we just uninstalled the carseat with DS still strapped into it, and sat it on top of the stroller. (Since we also had all our luggage and carryons and junk to carry, it was easiest to do it this way!) It fit fine. People traveling with children get to board before other people, so we gate checked the stroller, and then just carried DS onto the plane while still in the carseat. We strapped the carseat into the plane seat rearfacing with him still in it, and it took all of a minute. Some flight attendants might give you grief about the carseat being rearfacing, but all you have to do is point to the sticker on the side that says, "it must be rearfacing up to a year" and they'll usually leave you alone.
DS stayed in his carseat always during take offs and landings (there's a reason why they tell adults to fasten their seatbelts during those times!) and if we were in the air and DS needed to nurse, we'd take him out of the carseat and I'd nurse him, cuddle him, etc. To relieve his ear pressure during the take offs and landings, I gave him water so he would swallow to "pop" his ears. During our layover, we found a secluded area and let him crawl around and stretch his muscles and whatnot after being in the carseat so long. Then when it was time for the next flight, back in the carseat he went. Since we had gatechecked his stroller, we had it during the layover, so it was no problem maneuveriong the carseat around the airport.
Yes, it's an un-AP way to travel, but for that one day and the return flight, my child's safety took precedence over the sling.
Just my .02 for anyone interested.