As United Airlines prepares to ask workers for a new round of cutbacks, its pension plans look increasingly vulnerable. The airline has four big plans, and shedding any one could lop off more than $1 billion in debt.

Such a drastic step could nudge other airlines to trim their pension plans as well, to keep their labor costs competitive. The long-term prospect could be a series of failed pension plans and lost benefits reminiscent of those in the steel industry, a costly outcome for the government.

Which workers' pensions at United are most at risk? Those with the biggest pensions - the pilots - might not, in fact, be first in the cross hairs.

Because the pilots' fund had good returns during the stock market boom, it built a big reserve of credits for funding purposes. That cushion has allowed United, a unit of the UAL Corporation, to contribute less cash to that plan than to the others since entering bankruptcy, even though the pilots have been promised by far the most benefits.

The most recent data suggest that the pension plan for United's mechanics has been consuming the most cash in the last two years. United's plan for administrative workers and managers appears to have required the second-largest amount of cash, followed by the plan for flight attendants.

As long as this pattern continues, United could conserve more cash in the short term - and make itself more attractive to lenders - by chopping one or more of its skimpier pension plans. It could either freeze the benefits at their current level, or terminate one or more plans outright - a far more drastic step that would require approval by the bankruptcy court.

A termination would save the airline more money but also cause an uproar in the workplace. To minimize the backlash, United might start with the plan that has promised the smallest benefits - the flight attendants' plan - because government insurance would cover more of those promises. The flight attendants have already agreed to pension reductions, and they are bitter about a new plan to cut retiree health insurance. United might ease the pain by giving them other retirement benefits, like an enriched 401(k) plan.

United declined to discuss any aspect of its pension plans, and officials of the unions that represent its employees said the airline had not yet contacted them for discussions. Just a few weeks ago, United said in a bankruptcy court filing that it viewed its pension plans "as untouchable unless there was no other choice." But that was before the government denied loan guarantees to United. O. V. Delle-Femine, national director of the Aircraft Mechanics Fraternal Association, said he now feared the worst.

"You've got to gut the pension plans," he said. "I don't see any other way."

Whatever United does will be closely watched by the other major airlines and their employees, who have substantial pensions of their own to worry about. If United ultimately revives itself by terminating one or more of its pension plans, other airlines may also try to shed pension debt, to remain competitive.

This would not happen overnight. Pension terminations are difficult and costly. But over time, the industry could find itself in a long, slow race to the bottom - a succession of bankruptcies and pension defaults similar to those in the steel industry over the last quarter of a century. Steel maker after steel maker went bankrupt, and the only ones to bounce back were those that scuttled their pension plans.

In the process, the government had to take over $9.4 billion worth of pension obligations. Because pension insurance has limits, many steel workers had their benefits reduced.

A replay of those grim events in the aviation sector would be painful for airline employees, and ominous to workers in other mature industries, like automaking, where the pension obligations are also large and growing faster than revenues. And it would probably swamp the government's insurance program.

In May, the Pension Benefit Guaranty Corporation disclosed that it was beginning to stabilize after two years of losses, but it warned that it had just classified $23.4 billion worth of airline pensions as "reasonably possible" to default.

The agency did not specify how much of that amount was owed to United employees. But last year it calculated that if United terminated all four pension funds immediately, they would be $7.5 billion short.

In earlier rounds of cost-cutting, United scaled back some of its pension plans. The flight attendants, for instance, are building their pensions at a slower rate than before the bankruptcy filing, saving United a reported $43 million a year.

Mr. Delle-Femine of the mechanics said that the unions might be able to form a coalition to negotiate pension cuts. But the employee groups' interests diverge, and Mr. Delle-Femine said that if full-blown terminations were coming, as he expected, the unions would be unable to stick together.

"Everybody's crying that they're not going to take it out of my hide," he said.

United has disclosed that it has contributed $127 million to its four pension funds this year. It must still contribute $598 million, some by July 15 and the rest by Oct. 15. A United spokeswoman declined to specify what portion applied to each plan, or to provide up-to-date information about the plans' financial strength.

Despite the sums at stake, the airline is not required to disclose current, detailed information about its pension funds. No company is. Some pension data must be disclosed in financial reports, but companies with several funds, like United, usually provide aggregate figures.

More detail on individual plans can be found in the annual reports filed with the Labor Department. But the filing deadline is seven months after the close of the year, and companies are given ample extensions, so the information is out of date - particularly in rapidly changing situations.

United's most recent pension reports are dated Dec. 31, 2002. Since then, the stock market has turned up, some plans have been reduced, and Congress has relaxed the pension-funding rules. But the numbers offer a rough sense of how the termination of any one pension plan might affect United's finances and workers' benefits.

Not only has the pilots' plan promised the most benefits, but it has promised them to the smallest number of people, meaning very rich individual benefits are at stake. As of December 2002, about 500 active pilots had worked enough years to earn pensions of more than $100,000 a year. Sixty-four of them had earned pensions of $150,000 a year.

If United defaulted on that plan, the pilots at these levels could experience big losses. Government insurance generally covers a maximum of $44,386 a year. People who have already retired when a plan fails can sometimes get more, depending on the amount in the plan. But those too young to retire are often out of luck. Those who have tried to recover their losses in court, like the pilots of Pan Am, have found that the pension agency is a tireless litigator.

The average United pilot was about 44 years old in 2002 and had built up a pension of just $26,000, the records show. Such pilots would probably have full insurance coverage if the plan failed. But they would miss the opportunity to accumulate the six-figure pensions that were standard in the past, and would probably not back down without a fight.

When US Airways found it had to terminate its pilots' pension plan to emerge from bankruptcy, the pilots threatened to force the airline into liquidation. US Airways countered with a very rich retirement package that fell short of a guaranteed pension plan. That satisfied US Airways' lenders, but the cost of the new benefits canceled out much of the savings from ending the old ones.

All that makes the pilots' pension plan an unappealing starting point for United to look for savings.

The mechanics' plan has some similar issues. Its benefits are much smaller than the pilots', but a significant share of them were granted in 2002. The government does not fully cover benefits granted within five years of a pension termination. So part of the mechanics' most recent raise would disappear if the plan went to the government.

"They'd lose big time," Mr. Delle-Femine said, adding that if United gave notice of a termination, the union would try to block it in court. "If they said, 'Let's sit down and negotiate this stuff,' I'd say, 'Let's see what the bankruptcy judge says first.' "

The other two plans, for flight attendants and administrative workers, might be the most vulnerable. The administrative workers, managers and ticket agents are not represented by a union. Terminating their plan would wipe some $1.7 billion in unfunded pension debt off the books. These workers tend to earn pensions safely within the government's insurance limits. The 34 longest-serving employees in this category have earned pensions averaging about $32,000 a year, the records show.

Most flight attendants would also be fully covered by the government's insurance. The most senior had salaries of a little more than $40,000 a year in 2002. Forty-two had 39 years of service, entitling them to pensions of about $36,000 a year.