Originally Posted by Andriyko

BA's T fares use a variety of booking classes on connecting AA flights (up to H) depending on the underlying fare basis. As a suggestion, check if you can book a BA codeshare - many fares require B class if booked as a BA flight even on the cheapest fares on some flights within North America (not sure where you're going but, for example, a MIA-CUN booked as a BA*AA flight would be booked into the full fare B bucket).