Swan Song

Delta Air Lines Inc. will begin to sing a long, slow farewell to its Song discount affiliate starting May 1, but officials are promising they won't be dropping any Song routes out of Boston's Logan International Airport or other cities.

As Delta begins a six-month process of folding Song back into the mainline carrier, conventional Delta jets will begin to displace Song's Boeing 757s that feature leather seats and seat-back televisions on Boston-Florida routes. By autumn, Delta will reallocate the 48 Song jets to cross-country routes and refit the now one-class Song planes with a separate first-class cabin, Song chief marketing officer Tim Mapes said in an interview.

Although Delta officials insist they will find ways to incorporate ''the best of Song" in improving conventional Delta service, the demise of Song represents to many industry analysts the latest failure of a legacy carrier to successfully launch a discount unit.

Song started in April 2003 as Delta's second effort at launching a low-fare affiliate after Delta Express, which lasted from only 1996 to 2001. In the wake of other failed ventures like Continental Lite in the 1990s and US Airways' MetroJet, United Airlines' Ted unit will by next month be the only surviving example of an airline set up by an old-line carrier to compete with discounters like Southwest Airlines Co. and JetBlue Airways Corp. (US Airways wound up merging with discounter America West to emerge from bankruptcy protection last September, and the combined company has tried to portray itself as a new low-cost carrier, even adopting ''LCC" as its stock market ticker symbol.)

''The legacy carriers have been singularly unsuccessful" in trying to create true low-cost airlines, said Allen Michel, a Boston University School of Management professor who has studied and written about the aviation industry. ''It's very difficult to make two organizations operate independently and successfully within the same umbrella."

United has bristled at widespread industry skepticism of the Ted operation, which flies from Chicago and Dulles International Airport, outside Washington, D.C., to Florida and the Caribbean, and operates several southwestern routes. In a recent speech United chief executive Glenn F. Tilton called Ted ''a success story" and a moneymaker that is being expanded by 20 percent to 56 airplanes this year.

But Hugo Burge, head of US operations for travel website cheapflights.com, said, ''Certainly if Ted were a big success, it would be growing like a rash" to other cities including Boston.

''History keeps repeating itself, with there being something really seductive for the major carriers about launching a low-cost airline," Burge said. ''The demise of Song is a victory for the skeptics."

Song's Mapes said from his perspective, the single biggest problem Delta faces is a belief Song is shutting down, rather than just being folded back into Delta.

''There are a lot of people under the misimpression that Song is going away. Everything that we serve today will continue to be served," Mapes said.

Song aircraft will be gradually redeployed to serve chiefly flights of 1,700 miles or longer. From Boston, that would mean people who have become used to the Song experience for flights to Las Vegas and Los Angeles will often continue to ride on Song-style 757s. Delta is blending the 48 Song planes with another roughly 70 long-haul jets, including Boeing 767s and 737-800s, to create by 2008 a revamped fleet of two-class, long-haul planes, Mapes said.

Between now and November, Boston passengers flying to Song's Florida destinations -- Fort Lauderdale, Fort Myers, Orlando, Tampa, and West Palm Beach -- may still from time to time ride on souped-up green-and-white planes, not Delta jets.

''It's reasonable to think they could be there over the next six months, but I don't think it's there for much more than that," Mapes said.

Unclear now is the fate of a Song plane with special significance for Boston: the one renamed in honor of Red Sox slugger David Ortiz as ''Big Papi" and christened by Ortiz himself in a ceremony at Logan last October.

Delta spokeswoman Chris Kelly said the airline had envisioned the jet being named Big Papi for a defined period of time, but officials are still reviewing whether the name might survive when the plane is repainted in Delta colors in the coming years.

Another local innovation that will fade away is a Song menu of in-flight meals and snacks designed by Peter Davis, chef at Henrietta's Table and the Charles Hotel in Cambridge.

But in a recent Globe interview, Delta chief operating officer James M. Whitehurst said Delta remains eager to find ways to infuse more Song creations into the parent airline.

''Song is very stylish, and out there a little bit. Delta's more of a traditional business service kind of brand," Whitehurst said. ''We want to marry the two and make it a little more contemporary-classic. We're trying to add a little more style to Delta."

Peter J. Howe can be reached at howe@globe.com.