ATLANTA – Delta Air Lines, and Northwest Airlines, hobbled by high fuel costs and heavy debt and pension obligations, filed for bankruptcy protection from creditors yesterday, becoming the third and fourth major carriers to enter Chapter 11 since the 2001 terrorist attacks.

Delta’s late afternoon filing included its low-fare carrier Song and was followed shortly after by Northwest’s.

Delta’s total debt is roughly $28.3 billion, and it listed $21.6 billion in assets, according to the filing. The asset figure would make Delta’s bankruptcy the ninth-largest in American history, according to bankruptcy tracker New Generation Research. The ranking did not change following Delta’s recent $425 million sale of feeder carrier Atlantic Southeast Airlines to SkyWest.

Delta and Northwest said passengers were not expected to see any immediate effects from the filing.

Delta also promised to honor all tickets and sent a letter to frequent-flier customers seeking to reassure them.

“We are operating our full schedule of flights, honoring tickets and reservations as usual, and making normal refunds and exchanges,” the chief executive of Delta, Gerald Grinstein, said in the letter.

[The bankruptcy filings by Delta and Northwest may add as much as $10 billion to the record deficit at the agency that insures the pensions of 44 million American workers, Bloomberg News reported. Recent airline and steel bankruptcies have left the quasi-government agency that insures the plans, the Pension Benefit Guaranty Corporation, with a deficit of $23.3 billion.

“It looks like the claim on the PBGC from Delta would be in the ball park of $6 billion and from Northwest in the ballpark of $4 billion,” the president of the Center on Federal Financial Institutions, a Washington-based research organization, Douglas Elliott, said.

While the PBGC has enough cash to continue covering pensions for about 15 years, any additional airline claims underscore a growing difficulty at the pension agency, Mr. Elliott said.]

Chapter 11 protection will allow Delta to pursue wage cuts for its 65,000-plus full-time employees, as well as pension and health benefits for workers and retirees, that would have been more difficult or impossible without protected status.

Delta was expected to continue its normal schedule. However, as the company makes its way through bankruptcy court, some changes to Delta’s operations could occur, analysts say.

Atlanta-based Delta, the nation’s third-largest carrier, has lost nearly $10 billion over the last four years despite announcing it would cut up to 24,000 jobs. In September 2004, it also said it would shed its Dallas hub as part of a sweeping turnaround plan aimed at saving the airline. It has since scaled back its operations in Dallas.

Delta follows into bankruptcy UAL, the Elk Grove Village, Ill.-based parent of United Airlines, and Arlington, Va.-based US Airways Group, which is undergoing reorganization for the second time in three years. Fort Worth, Texas-based AMR, the parent of American Airlines, the nation’s biggest carrier, teetered on the verge of bankruptcy before winning deep concessions from its employees. The other so-called legacy carriers, those with a large presence in multiple regions prior to deregulation in 1978, are Eagan, Minn.-based Northwest and Houston based Continental Airlines.

Continental and American are in no immediate danger of bankruptcy. Continental had a big cost advantage over other traditional airlines after it slashed expenses during two bankruptcy reorganizations in the 1990s. American may be the strongest financially of the traditional airlines, thanks to $1.8 billion in annual labor concessions it won in 2003. Its parent company actually turned a profit in the second quarter.