Transition Layer,
What
Buster said earlier.
Thanks
Buster,
I think that when the 6 ERs were ordered back in 2000 or early-2001, the idea was to use them for direct DFWs, the idea being to link in with AA's main hub and provide better connections to AA's US network than that available through LAX. I
think (though clearly not
sure) that there was also some noise about trying direct ORDs for similar reasons.