I was on a KLM flight recently and they would not allow someones car seat on board as it was not rated for aviation use.
Some carseats are not approved for use on aircraft. Some of the larger Britax models, for example, are too wide at the base to fit on an aircraft seat and are not approved for aircraft. Generally speaking, most car seats manufactured after 1983 are approved. However, as you witnessed, and with the Britax example, not ALL seats are! So if anyone is going to use a carseat, take a quick minute to inspect all the labels attached to the seat. On one of the sticker labels on the seat, probably on the bottom or side of the seat, it should read "approved for use in motor vehicles and aircraft". If you do not see the words "approved....[and] aircraft", it can not be used on an airplane.