Any car seat that has a label stating it is “certified for use in motor vehicles and aircraft” can be used on an airplane. Foreign seats (those purchased outside the US) can be used as long as they are labeled as meeting the standards of a foreign government or the United Nations. This includes infant seats, convertibles, forward-facing seats and combination seats with harnesses (often called “harnessed boosters,” these are allowed if used in 5-point harness mode).
The narrower the better–car seats less than 16 inches wide will fit on most airplanes. It is most important that the car seat is narrow at the area that is between the airplane seats’ arm rests. If the car seat is wider at the shoulder area this shouldn’t be a problem. Check out www.seatguru.com to see the dimensions of many airplane seats.
Lighter car seats are also better for air travel, since you’ll need to carry them down the aisle and lift them up onto the seat.
Aircraft seats and seat belts are different from motor vehicle seats and seat belts, and crash tests have shown that the car seats listed below may not protect a child in an aircraft seat. Therefore, they are not allowed during take-off and landing, even though some have labels saying they are certified for aircraft use.
*Note: Combination car seats, sometimes also called boosters, are allowed if used in 5 point harness mode