US Air Shares Sink, Pilots Reject Deal  CHICAGO (Reuters) - Shares of US Airways Group Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=UAIR.O target=/stocks/quickinfo/fullquote"&gt;UAIR.O&lt;/A&gt; slid as much as 23 percent on Tuesday after leaders of  its pilots union rejected the company's latest contract  proposal, dealing a blow to its attempt to avoid a second trip  through bankruptcy.