If your child is riding in your car currently in a HARNESSED seat (ie a convertible seat w/ a harness or a combination harnessed booster that harnesses to 40lbs then you use it as a booster w/o the harness over 40lbs) then you can use this seat on the airplane as long as it is FAA approved. Most car seats sold in the US are FAA approved... a handful are not... just look at the stickers on the seat it will say it there.
Most airline will tell you that a car seat is not "necessary" but the FAA recommends all children that do ride in harnessed seats also ride in them on airplanes.
Here are some guidelines for using car seats on airplanes...
The above has a very complete explanation of all the intricacies of using car seats on airplanes along with pertinent links to the FAA, AAP and other sites for supporting info.
Please note... in my last post I was clear about NOT using the booster on the airplane because you can only use HARNESSED seats that are FAA approved. There are currently no booster seats approved for use on airplanes.
The previous poster is correct... if you have a harnessed seat you should use that and have your room steward store it for you w/ your luggage if necessary. We did that on our last cruise and it worked really well. My daughter was 2.5yo at the time and we brought her car seat. Used it to and from the airport on both sides of the trip, as well as to the pier from our pre-cruise hotel.
We did NOT use the car seat on the buses we went on for excursions. We could of used the car seat in the taxi in Grand Cayman but ended up not doing so since we traveled to the beach on a bus and then returned in the taxi. I couldn't think of anywhere we could have stored the car seat while we were walking the beach.
We rode in buses for excursions in Ocho Rios... and for Cozumel. Buses generally don't have seat belts (even here in the US) and have different safety features than cars. You cannot install a harnessed car seat in a vehicle unless there are seat belts and the seating position is forward facing (even for rear facing seats). That means no rear facing or side facing seats.
Oh and about laws.... US laws vary but FL allows children 4 & up to use an adult seat belt. BUT... the laws of physics supercede any state law... so if in a crash a 4yo in an adult belt could be seriously injured as they don't fit the restraint. Most laws are written that the children the law covers be in an "appropriate child restraint" and that they are "following manufacturer's instructions." SO if you have a large 3.5yo (ie really tall and/or over 40lbs) instead of a harnessed seat there is little chance you will receive a ticket for it. Although I always advocate keeping children in a harnessed seat as long as possible.
Hope this clarifies my post some??