It is valid, by why would you choose a surface sector LHR//DXB instead of flying LHR-DXB-LHR? It is a nice flight in A, and you are within the allowed segments. The ANC service is operated mid-June to mid-September (roughly); you can't use an AA codeshares on AS. Rumor has it that the ORD-ANC flight from past years will not be resumed in 2005 but might be an AS codeshare, hopefully DFW-ANC will be on AA metal but that remains to be seen. At the moment it is in the AA schedule and bookable so your ticket can be issued and AA will reroute you if the flight schedule is changed.