Three of the nation's largest airlines could be operating under bankruptcy protection in coming weeks, analysts say, the latest sign of the industry's upheaval as it lurches through a historic transformation.

United Airlines already is in bankruptcy. US Airways has warned for months that its second bankruptcy filing in two years could come as soon as this month. And Delta Air Lines, which is struggling to lower costs, looks increasingly like a candidate for Chapter 11.

All three airlines are locked in contentious talks with employees in a bid to lower costs by reducing pay and benefits or eliminating jobs outright. The failure to win concessions from workers not only could hurl the airlines into bankruptcy, it also could precipitate their demise, industry observers say. The chairman of US Airways, David G. Bronner, has warned that a trip into bankruptcy may be a dead end resulting in the company's liquidation.

Simultaneous bankruptcies, which would cover 42 percent of the nation's flights, could force the industry to accelerate its overhaul, leading to drastic cutbacks for workers and a sharp reduction in the number of seats available to travelers.

"Today, you have an entire industry that has been subjected to a paradigm shift by the virtue of new [low-cost] carriers," said Robert L. Crandall, former chief executive of American Airlines. "The outcome in the end will be that every carrier, in one way or another, is going to have to get their costs down to the point where they are essentially the same as low-cost carriers. That means [get] rid of fixed-benefit pension plans, completely and profoundly change their labor contracts, eliminate restrictive work rules, and reduce wage rates."

A triple bankruptcy hasn't hit the industry since 1992, when Continental Airlines, Trans World Airlines and America West Airlines were all in Chapter 11. At the time, the industry was stronger than it is now, and the bankrupt carriers affected only 19 percent of the country's airline operations, according to J.P. Morgan Chase & Co. analyst Jamie Baker. Continental, America West and TWA all emerged, though TWA eventually folded in 2001.

This time, the stakes are larger. The industry is at a turning point. The growth of low-cost carriers and high fuel prices threaten the survival of the traditional airlines. Even in a strong economy, the major airlines would be unable to raise prices much because the low-fare carriers and the Internet have changed the fare game.

The industry's restructuring began after the Sept. 11, 2001, terrorist attacks, when travel fell off and the airlines were forced to cut about 100,000 jobs and seek billions of dollars in aid from the government. But the carriers failed to take drastic enough steps to forestall continued turmoil even as passengers returned.

The struggle to emerge from bankruptcy, or avoid a slide into it, preoccupies United, US Airways and Delta. United's reorganization under bankruptcy protection has dragged on for nearly two years. Its executives are expected to release details of its restructuring later this month. It is likely to include job cuts and a squeeze on pay and benefits to trim costs enough to attract the $2 billion in financing needed to emerge from bankruptcy.

United spokeswoman Jean Medina said yesterday that the airline may have to reduce its workforce as part of its overall plan to rein in costs.

"It is clear that achieving cost competitiveness will require additional job reductions over time," she said. "The details of how we achieve the required savings and the potential impact on United's workforce, however, are still under discussion, and it is premature to comment on specifics."

In a recorded message to employees this week, Peter D. McDonald, United's chief financial officer, said that despite the cuts the airline has already made, "it may be more difficult as we go forward," he said.

Four of United's employee pension plans also may be targeted for elimination. Last month the airline said it was considering eliminating the four plans, a move that would wipe out about $2 billion in future retirement benefits for many of its 120,000 retirees and workers.

Leaders of the union representing United's flight attendants this week said they had lost confidence in management and wanted the senior executives removed.

US Airways, based in Arlington, is trying to extract about $800 million in annual cost savings from its employees by the end of the month. The airline said that if it fails to get those cuts, which are part of its $1.5 billion restructuring plan, it will have to file for bankruptcy.

After nearly a week of round-the-clock negotiations at the Key Bridge Marriott in Rosslyn, US Airways executives and leaders of its pilots union continued to negotiate on concessions yesterday. The pilots' leaders plan to meet today to discuss whether to send the airline's final proposal to the rank and file for a vote. The carrier is seeking $295 million a year in concessions from its pilots.

Also pressuring the airline is a $110 million pension payment due Sept. 15. Later this month, the carrier must be able to show it has reduced its costs enough to meet requirements of a $900 million loan guarantee granted by the Air Transportation Stabilization Board.

Earlier this week, US Airways executives dismissed a proposal from its machinist union suggesting operational changes that the group said could save the airline $115 million. Robert Roach Jr., general vice president of the International Association of Machinists and Aerospace Workers, which represents US Airways' mechanics, said slashing employee pay and benefits is a bandage that doesn't fix the airline's long-term operational problems.

"You can't expect employees to make sacrifices when they see those sacrifices being wasted," Roach said. "You can't restructure this industry on the backs of its employees."

US Airways spokesman David Castelveter said that there was nothing "new, unique or innovative" about the mechanics' proposal and that the mechanics' suggestions would have added expenses rather than reduced them. The airline said the only way to reduce its costs is through cuts in pay and benefits across its labor groups.

Delta Air Lines continues to negotiate with its pilots in the hope of securing $1 billion in annual pay and benefit cuts from the union. As early as next week, chief executive Gerald Grinstein is expected to unveil a new strategic plan, which some close to the carrier said may include the elimination of jobs and a reduction in flights as well as a possible bankruptcy filing. Yesterday, Delta executives renewed their request with bondholders to change their lending agreements, part of the airline's effort to restructure about $20 billion in debt.

As the airlines struggle to find their way, the ripples will spread. "The one who is going to take this the worst, besides the bankers and the Wall Street investors, is the airline employees," said Darryl Jenkins, visiting professor at Embry-Riddle Aeronautical University in Daytona Beach, Fla. "They're the ones who are going to be the walking wounded for a long time to come."

United Airlines has been in bankruptcy protection for nearly two years. The carrier is planning to cut more jobs and is examining the possibility of eliminating four of its employee pension plans.