AIRLINES would schedule the same plane to fly one leg to LHR from the west coast and return to the east coast( or ORD) or vice versa. This way using 2 777 AA or UA can maximize the use of their international fleet. Since AA and UA fly often to the big three European airports( LHR, CDG, and FRA) they can use planes more efficiently then a plane flying a roundtrip from the same hub.