So I was looking for two seats on UA from IAD to New Orleans for Mardi Gras, dates 2/18 to 2/21. Availability going out is not a problem, but coming back on Mardi Gras has the usual crowds. What I can't figure out is that United says that there is no Star Alliance availability on 2/21, but if I go to US Air, they say I can ride on their metal for a standard award of 50k round trip. Ideally I'd take United down and return on US Air for the standard 50k reward, all paid for with United miles. Is there any way to do this, or am I just out of luck? Thanks.