For US domestic flights, the size weight recommendation is not set by the airlines, it is set by the FAA. Their recommendation is that a child who weighs under 40 lbs. should use an approved CRS (Child Restraint Device, aka carseat) when on a commercial airliner. It is not *required* by gov't regulation, but it is suggested for the sake of safety, especially in turbulence conditions.
US-based carriers DO NOT provide carseat equivalents for use aboard aircraft; their liability insurance does not allow it. If you want a CRS for your child to use on the aircraft, you must provide it yourself, and it must be a seat with an integral upper-body harness. Seats that lack an upper-body harness will not be approved for aircraft use.
FYI: I have heard reports lately that some US carriers are now mandating that *if* a separate seat is purchased for a child under age 2, that a carseat must be used in that seat. Those airlines that do have this policy are apparently not invoking this rule for children over age 2. The imposition of the rule appears to be a bit spotty, but from what I've heard, Delta is the carrier most likely to insist.
There has been some confusion here lately about the "FAA sticker" that has to be on a US-sold carseat. While the sticker's presence is mandated by the FAA, the text on the sticker does not actually say "FAA approved". However, the wording has been mandated by law since 1985. There must be two labels, actually, one that says "(1) This child restraint system conforms to all applicable Federal motor vehicle safety standards and another that says "(2) THIS RESTRAINT IS CERTIFIED FOR USE IN MOTOR VEHICLES AND AIRCRAFT in red lettering."
If the seat was purchased outside the US, it will be acceptable if it "bear either a label showing approval of a foreign government or a label showing that the seat was manufactured under the standards of the United Nations."
You can read the US standards for yourself by going to the webpage for the Code of Federal Regulations. The section of the law that cover the use of carseats on aircraft is: 14 CFR 121 (
http://ecfr.gpoaccess.gov/cgi/t/tex...view=text&node=14:2.0.1.4.19.11.10.9&idno=14.
NOTE that that regulation says that a booster seat may not be used on board an aircraft. It is *very* important to note that what the FAA defines as a booster seat is NOT necessarily what the mfr. of the seat might define as a booster seat. The exact description of what the FAA considers to be a booster seat may be found at 49 CFR 571.213. If a seat is convertible for use with and without an upper-body harness, there will be a caveat on the "certified for use" sticker, that caveat will specify that the certification is only valid when the upper-body harness is properly installed in the seat.
