Legal Fight Menaces Aircraft Debt Ratings  NEW YORK (Reuters) - Standard   Poor's on Friday said  ratings on about \$30 billion of aircraft-backed debt could be  threatened if creditors lost a legal dispute with bankrupt UAL  Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=UALAQ.OB target=/stocks/quickinfo/fullquote"&gt;UALAQ.OB&lt;/A&gt; unit United Airlines over the right to  repossess aircraft.