The federal government's pension insurer is putting price tags on the traditional pension plans of newly bankrupt carriers

Delta Air Lines

(DAL) - Get Report

and

Northwest Airlines

( NWAC) -- and the numbers aren't pretty.

That's bad news for airline employees, the insurer -- the Pension Benefit Guaranty Corp. -- and U.S. taxpayers, who might have to pick up the tab if pension defaults overwhelm the PBGC, already operating with a deficit of more than $23 billion.

In the case of Delta, the PBGC's preliminary estimate of a $10.6 billion pension shortfall dwarfs the company's pre-bankruptcy estimate of $5.3 billion. The PBGC puts Northwest's underfunding at $5.7 billion.

The PBGC would be on the hook for $8.4 billion of Delta's obligations, meaning employees would lose $2.2 billion of benefits if Delta were to terminate its plan and hand it over to the government. The agency has guaranteed $2.8 billion of Northwest's obligations, suggesting a termination of that airline's plans would result in $2.9 billion of lost employee benefits.

Both airlines could make the case in bankruptcy court that they can't successfully restructure without jettisoning their plans and handing them over to the PBGC, which receives no tax dollars and funds itself with premiums from companies whose pensions it insures.

That's still not a foregone conclusion, however. Northwest CEO Doug Steenland said late Wednesday his company would do its best to avoid canceling its pensions and would continue to press for congressional reform that would give Northwest and Delta extra time -- a total of 14 years -- to fully fund their plans.

Relief of that nature is included in bills passed by the two Senate committees with jurisdiction over pensions, but an important House committee has so far nixed extra time for airlines. Its stance may change given the Delta and Northwest bankruptcies, however.

The airline industry is turning into a nightmare for the PBGC, which earlier this year agreed to take over the plans at

United Airlines'

parent

UAL

. The PBGC had guaranteed $6.6 billion of United's $9.8 billion shortfall, making the default the largest in the agency's history.

The PBGC also delivered a clear and firm warning to both airlines to continue making pension contributions.

"The financial challenges facing the airline industry are significant, but nothing in the bankruptcy code requires companies to skip their pension funding payments," said Bradley Belt, PBGC's executive director, in a news release. "As long as companies remain in operation with ongoing pension plans, they have a legal obligation to meet their funding requirements."

The warning appears to be falling on deaf ears, however. Delta has already said it plans to miss contributions coming due soon.

"Missing contributions does not mean that our qualified plans stop paying monthly retirement benefits or that we have initiated the process to terminate the plans," said Gerald Grinstein, Delta's CEO, in a press release announcing the airline's bankruptcy.

And in a regulatory filing Tuesday, Northwest suggested it might skip a $65 million pension payment due Thursday. The filing stated that if the carrier failed to make the payment, "a lien would automatically arise against its assets unless the company had previously sought bankruptcy protection."