Add us to those who always took a car seat on the plane for our ds when he was little. Our first trip to WDW was when he was 13mos/10adjusted age (also a preemie).
I like the installation tips on
this site:
Installation Tips
1) Use the seat the same way you'd use it in the car (if you use it rear-facing, rear-face on the plane, etc.).
2) Take advantage of pre-boarding. Take that extra time to get the seat installed tightly.
3) Recline the airplane seat back when installing the seat, then bring it upright to get the seat even tighter.
4) For some forward-facing seats, twist the latchplate around once (so that it's "backwards") to make it easier to unbuckle at the end of the trip. Not twisting it may make it nearly impossible to unbuckle the belt. This will also help keep the belt from slipping loose.
5) Put up at least one armrest. This will give the seat more room for installation, and keep it more stable (since resting against an armrest may cause it to tip to one side).
Remember, it's always a good idea to buy your child, no matter what their age, their own seat on the plane so you can use your child safety seat - especially since airplane seats are relatively inexpensive and children tend to be relatively fragile.
Also, if you're using a convertible car seat by then (one that goes rear facing and fwd facing), I always found the best way to get the seat on and off the plane was by carrying it upside down (base up) down the airplane aisle.
Another thing we always did was to board the plane seperately. Since I am sort of the car seat "expert" in our house, I always boarded first with the car seat and carry-on (backpack). DH would wait with ds and board about 5-10mins later. This way, I could get the car seat installed with out our whole family blocking the airplane aisle while that was being done. Using the above tips, I found it to be fairly easy using the airplane lap belt.
We have used several methods to get our stuff from the curb to the airplane and back. With the infant seat "travel system" it should be a snap. We always traveled w/a convertible seat since we had already switched to using that in the car by our first family trip. One trip, I was able to bungie the car seat back to back with the stroller (this was a Cosco Touriva/Scenera seat) by flipping the fwd facing "foot" down and hooking it on top of the stroller basket, and then running a bungie cord thru the fwd facing seatbelt path on the back of the car seat and hooking each end to the sides of the stroller. I then used the bungie cord to secure the stroller closed before boarding the plane. Warning: this may make the stroller tip over backwards. We did not have this problem w/our car seat/stroller combination as the car seat was on the lighter side.
You could also use the baby carrier or sling to carry your LO through the airport. Keep in mind that the baby will probably have to come out of the carrier when you go through airport security. Also, your stroller and car seat will have to go through the scanner as well. Some scanners are too small and if that's the case, they will hand check the stroller and or car seat. I've never had a car seat that didn't fit through the xray machine though (usually, putting it upside down the belt helps if it's a bigger seat).
Really, all this probably sounds a lot more complicated that it really is. If you come prepared, it will make the whole process a lot easier for you.
hth!