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

Delta’s filing late Wednesday afternoon 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 U.S. history, according to bankruptcy tracker New Generation Research Inc. The ranking did not change following Delta’s recent $425 million sale of feeder carrier Atlantic Southeast Airlines to SkyWest Inc.

Northwest, which said it had nearly $17.92 billion in debt, said it would continue to fly its normal schedule while reorganizing. Its filing, made in U.S. Bankruptcy Court in the Southern District of New York, listed $14.35 billion in assets.

The airlines say that passengers should not expect to see any immediate effects from the filings. 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,” Gerald Grinstein, chief executive of Delta, said.

Chapter 11 protection will allow Delta to pursue wage cuts for its more than 65,000 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.

A Delta Air Lines jet enters the airline's main hub at Hartsfield Jackson Atlanta International Airport in Atlanta. Delta Air Lines Inc. and Northwest Airlines Corp. filed for bankruptcy protection from creditors Wednesday.

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 during 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. The airline has since scaled back its operations in Dallas.

Delta follows into bankruptcy UAL Corp., the Elk Grove Village, Ill.-based parent of United Airlines, and Arlington, Va.-based US Airways Group, Inc., which is undergoing reorganization for the second time in three years.

Fort Worth, Texas-based AMR Corp., 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 Inc.

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.

But even the stronger carriers are finding business harder with fuel prices soaring, carrying crude oil futures past $70 a barrel earlier this month.

Some smaller carriers, including Honolulu-based Hawaiian Airlines and Indianapolis-based ATA Airlines Inc., also have filed for bankruptcy in recent years. Hawaiian emerged from bankruptcy in June.