First came the layoffs. Then the pay cuts. Finally, a slashing of health benefits.

If times weren't tough enough for airline workers, now some employees at the major carriers face the prospect of a poorer retirement.

The alarm sounded last week after a quasi-governmental agency that insures private pension funds said it would not permit the bankrupt carrier US Airways Group Inc. to stretch out payments on its squeezed pilot pension plan. The decision by the Pension Benefit Guaranty Corp. made it more likely that the fund could be terminated. That means pilots retiring in the future could find monthly checks in their mailboxes that are significantly smaller than what they were promised in more prosperous times.

And the US Airways pilots are not alone. Pension troubles are rippling across the nation's major airlines. According to a report issued this week by Fitch Ratings, a New York-based research firm, the larger carriers are staggering under a combined pension-funding shortfall of $18.9 billion. United Airlines, the world's second-largest carrier, has a shortfall of $4.1 billion, Fitch estimated. At US Airways, the gap is $3.1 billion.

Duane E. Woerth, president of the Air Line Pilots Association, said he would not be surprised if pension bombshells hit other airlines, given the carriers' financial straits and their mounting costs for security.

"US Airways happens to be first up and the most serious problem," Woerth said.

The possibility of a pension collapse at US Airways is reminiscent of previous pension crises in the industry. The Pension Benefit Guaranty Corp. took over the pension plans at Eastern Air Lines in 1991, at Pan Am in 1992 and at TWA in 2001. All three airlines ultimately went out of business.

Today, the major airlines are suffering because they must assume the risks of the traditional "defined benefit" pension plans they offer. Under such a plan, a company and a worker pay into the fund. When the employee retires the company pays out a fixed amount each month. The airlines' underfunded obligations have skyrocketed as the stock market has tumbled and interest rates on the funds' fixed investments have fallen.

Low-cost carriers such as Southwest, AirTran Airways, ATA, JetBlue Airways and America West have largely protected themselves from the pension woes of their larger rivals by the use of 401(k), or "defined contribution" plans. Under these plans a company can match an employee's contribution to the fund, but future retirement payouts depend on how well the worker invests his own nest egg. Many 401(k) plans also have been depleted by the market downturn.

To ease its financial burden, Arlington-based US Airways asked the Pension Benefit Guaranty Corp. to allow it to stretch out payments on its pilot pension plan from seven years to 30 years. The guaranty corporation's general counsel, James J. Keightley, rejected the plea. The agency lacks the legal authority to permit US Airways to make its payments more slowly, he said.

The ruling leaves management at US Airways pondering the difficult decision of asking the corporation to terminate the pilot pension plan, which is the most costly of US Airways' four worker pension plans. It would be the first time an airline has taken such a step. In previous terminations, such as those at Eastern, Pan Am and TWA, the government has essentially seized the plans.

David Castelveter, a US Airways spokesman, said the company is seeking congressional intervention that would enable the carrier to stretch out the payments, but he acknowledged that time may not be on the company's side. "The clock is ticking," he said.

United Airlines, which filed for bankruptcy Dec. 9 and is losing $20 million a day, also may be forced to make some tough decisions about its pension funds. The pension agency is one of United's largest creditors and holds a seat on the unsecured creditors committee.

Meanwhile, pilots at US Airways reacted angrily to the possibility that their pension fund could be terminated. Under such a scenario, the pilots would be limited to a government-set pension limit of about $44,000 a year, roughly two-thirds of what pilots had been told they would receive in retirement.

"The current situation is extremely unfair to us," said Roy Freundlich, a spokesman for the pilots union. The airline suffered significantly from the federal government's decision to shut down its hub at Reagan National Airport for several months after the Sept. 11, 2001, attacks, he said. "The people who are getting harmed the most are the pilots," he said. "This is not the way our government should be treating us."

Pilots are required by the Federal Aviation Administration to retire at age 60.

John Hotz, deputy director of the Pension Rights Center, agreed that the pilots would probably be "financially devastated" if their pension plan is terminated, but he said he did not believe the government pension agency had any choice but to refuse US Airways' request to stretch out the payments. "It could create a slippery slope" where other companies facing financial problems could also try to defer making full payments to their pension plans, he said.

Agency spokesman Jeffrey Speicher said the agency has not received a request from the airline's management that it take over the pilots pension plan.

Other airline employees would probably be less affected than the pilots because few of them expect to earn pension benefits exceeding the government limit. Machinists at US Airways, though, also have been warned that their pensions are at stake.

In a Jan. 2 letter, Jerrold A. Glass, senior vice president for employee relations at US Airways, told the machinists that the company would not terminate their pension plan if they ratified their wage-concession contract. Glass urged the union to make sure its members understood "the gravity of the situation."

"As you know, the Company is facing an estimated $3.1 billion in pension funding obligations during the next seven years," Glass wrote. "The Air Transportation Stabilization Board and the Pension Benefit Guaranty Corp. have both expressed doubts about our ability to meet those obligations. . . . In a liquidation, all defined benefit plans would terminate, and many of our mechanic and related employees would experience dramatic losses in their future pension benefit."

One week later, the machinists voted to ratify the cost-cutting agreement.