This is from the Britax UK site the above link is for an American site and the rules regarding carseats are different.

1. WHICH CAR SEAT CAN I USE ON AN AIRCRAFT ?

The CAA (Civil Aviation Authority) accept child seats that are approved to the European Standard ECE R44 and can be fitted with a lap belt only.
The following models from our range meet this criteria:
Britax Freeway (Fisher Price forward facing seat)
Britax Eclipse (including Si & Ultra models)
Britax Renaissance (including Si models)
However, as the ultimate responsibility for use is with the airline in question we recommend that you check with them to make sure that the child seat in question will fit dimensionally on their aircraft seat and gain agreement to use it before you intend to travel.

Please be aware that FAA (USA) airlines may have different regulations.


2. I HAVE SEEN A CHILD SEAT ON THE BRITAX WEBSITE IN THE USA, WHY CAN`T I BUY THIS MODEL IN EUROPE ?

The legal requirements of the USA, Australian and European Standards are incompatible so it is not legal for you to use a European approved child seat in the USA/Canada, Australia, or vice versa.

I have posted the telephone number for Britax here in the UK 01264 386034 for anyone who would like to give there customer service a ring if they are not sure about the seat they have.

PLEASE don`t forget when asking about your car seat to let them know you are flying then driving in America not Europe.