On Wednesday, Moody's underscored the significance of the single operating certificate, saying it had withdrawn ratings on Northwest liabilities and obligations following the Dec. 31 "legal merger" between the two carriers. "As the surviving entity, Delta has become the legal obligor," Moody's said. Several steps remain for Delta. During the current quarter, all tickets and fares will be consolidated into the Delta reservation system, and the nwa.com Web site will disappear. Reservations system integration posed a major problem in the most recently completed airline merger, which combined US Airways ( LCC) with America West. In the transition month of March 2007, the combined airline's on-time performance sank to 55.5%, the result of a schedule meltdown that included thousands of flight delays and cancellations. Its impact kept US Airways at the bottom of the DOT's performance measure for most of 2007: on-time performance recovered to 78.3% in March 2008. -- Written by Ted Reed in Charlotte, N.C. .