Here are the guidelines from American Airlines - it does address seats mfg outside of the US. I would check with your carrier to see if they have a similar page - then you can check the seat. fwiw, I can't imagine a new seat, especially a Britax not meeting requirements to be used on the aircraft.
again c&p from aa.com:
FAA Approved Infant Seat or Child Restraint Devices
Most restraints that are used in automobiles are acceptable for use in aircraft by an infant or small child. Acceptable restraints manufactured in the United States will bear one or both of the following labels:
"This child restraint system conforms to all applicable federal motor vehicle safety standards"
Additionally, the restraint may carry a second label with red lettering which states the following:
"This restraint is certified for use in motor vehicles and aircraft."
Restraints manufactured outside the United States may be accepted provided that they bear either a label showing approval of a non-U.S. government OR a label showing that the seat was manufactured under the standards of the United Nations (U.N.).
U.N. approval is designated by a label with a circle surrounding the letter "E" followed by the distinguishing number of the country which has granted approval, plus an indication of the category and mass group of the child restraint which will be affixed.
I believe the Britax Eclipse si is an European carseat, therefore it would not have any specific details on it meeting American safety standards (FMVSS 213.X and FAA). If you are a resident of the US, please be aware that it 'illegal' to use a seat not US-certified. If you are visiting the US, it should be alright to use on board a US aircraft, if it's approved for such use in Europe.
thankyou all for your replies, i have just phoned britax uk and they informed me none of their uk car seats are officially recommended for aircraft but the britax freeway can be used on an airplane ( not the one i have, it is greatly inferior), not any of the others because of the width and size!! also is is illegal to use a Uk carseat in the states and US one in the UK.
following this i phoned the airline (TCD charter) to find if they had an infant seat i could prebook - they don't but i am welcome to bring my car seat from home to use as long as it is forward facing!!! i then asked if it had to be a particular make, model ect to which she replied as long as it is forward facing they will allow it.