It was America West who purchased US Airways, not the other way around.
America West Holdings Corporation (owners of America West) announced in April 2005 that it was talking with US Airways Group (owners of US Airways) about the former purchasing the latter. In May 2005, a merger announcement was made. America West’s shareholders approved the merger in September 2005. In the same month, the US Bankruptcy court (US Airways was in bankruptcy at the time) approved a Chapter 11 Plan of Reorganization that cleared the way for the merger.
On September 16, 2005, the US Bankruptcy Court approved USAirways Chapter 11 Plan of Reorganization, clearing the way for the merger to be closed as soon as September 27, 2005. For the merger approval announcement see Plan of Reorganization Approval
The Wall Street Journal said about the merger: “[It offered] a huge break to US Airways and potentially [carried] big risks for America West…” (America West was in better financial shape).
Although the new airline would take the US Airways name, management would still be in the hands of America West managers and the headquarters would be in America West’s Tempe (Arizona) headquarters.
The merger required America West Holdings to be merged into Barbell Acquisition Corporation (a subsidiary of US Airways Group Inc.) with America West Holdings being the surviving company. This was done in September, 2007, the same month that the two airlines operating certificates were combined.
Note to nitpickers: this is a high-level overview. It is not meant to be a highly detailed narrative. If you wish more data or wish to try to prove me wrong, I used the terms “surviving company america west us airways” in Google.