Northwest Air CEO Quits for UnitedHealth  NEW YORK (Reuters) - Northwest Airlines Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=NWAC.O target=/stocks/quickinfo/fullquote"&gt;NWAC.O&lt;/A&gt; on  Friday said Chief Executive Richard Anderson is leaving the  company to join a health care group, as the airline struggles  with cutting costs, low fares and tough competition.