Hmmm, I have seen ppl use marathons on flights, but they have never removed anything from it (at the time, my baby was still in his infant carrier or else we would have lugged his marathon along too). I don't know if it would be safe if you removed it (in fact I did not realize you could remove the base... are you sure it was a marathon?). If you are not sure, I think the best thing to do is call britax and ask them, since it is their product and they know best about the safety of their own product. Personally, I do not trust airline personnel, b/c there are sooo many different carseats out there to deal with and from what I have heard, they are often inconsistent with knowing the, "rules" of carseats on planes to begin with, etc.. It's best for you to get the information direct from the manufacturer. GL!