Correction: This article says that American Airlines had bought a $30 million house in London for use by Thomas W. Horton, now the airline’s chief executive. American Airlines said that the property cost less than that when it was purchased in 1992, before Horton’s arrival.

American Airlines has saved $2.1 billion since 2006, thanks to two congressional measures that allowed it to reduce contributions to its pension plans. The company said it would make up any shortfall later.

Later has come, but there’s been a change in that plan.

American Airlines, whose parent company filed for bankruptcy in November, said this week that it wants to terminate its four pension plans for 130,000 workers and retirees and ask the federal government’s Pension Benefit Guarantee Corp. to bail out its unfunded pension obligations to the tune of $9 billion. It would be the largest PBGC bailout ever. Without the congressional relief, the gap would have been smaller, the PBGC said.

The move, which needs approval from a New York judge, has angered the PBGC and labor unions representing American Airlines workers, in part because the airline had $4 billion in cash when it filed for bankruptcy and because it hasn’t yet made any proposal for restructuring its massive debt to financial institutions.

“American and other carriers have repeatedly asked Congress to give them funding relief in the last six years. More than $2 billion was diverted from their pension funds to their bankruptcy war chest,” Josh Gotbaum, director of the PBGC, said Friday. “In effect, the Congress of the United States and the employees of American Airlines provided half of the money to sustain American in bankruptcy.”

One of those congressional measures, the Pension Protection Act of 2006, gave airlines the flexibility to cut pension fund contributions for two years and spread them out in later years. PBGC analysts estimate that American Airlines was able to cut its pension fund contributions by $1.1 billion in 2006 and 2007 thanks to that legislation.

Some airlines that had already gone bankrupt received even greater leeway on contributions, which then-Sen. Barack Obama decried in 2006. He said it “will distort the market in a way that is unnecessary and unfair to the 10,000 American Airlines workers and retirees in Illinois. Both as a matter of retirement policy and aviation policy, this bill should not favor one airline over another.”

American Airlines — and its unions — kept lobbying for additional relief. In 2007, Sen. Kay Bailey Hutchison (R-Tex.) led a successful effort to tack new airline relief onto a defense appropriation for the Iraq war effort. It passed 80 to 14.

The new bill allowed American Airlines, Continental Airlines and a few others to tweak their accounting, raising assumptions for how much money its pension funds would earn from investments to 8.25 percent a year, thus again reducing the amount the companies were required to put aside. As a result, the PBGC has calculated, American Airlines saved an additional $1 billion from 2008 through 2011.

Even in good times, that would be a high rate of return compared with rates insurance companies use in calculating annuity costs. But the economic downturn made investment returns even worse — and added to the shortfall in American’s pension funds.

In an e-mailed statement, Hutchison said that “the purpose of the 2007 legislation was to make the pension funding rules equal for all airlines and establish one standard for the airline industry. . . . The protection of those contribution standards restored a fair, even playing field for all airlines and allowed carriers a better opportunity to meet their pension obligations to workers and retirees.”

Michael MacMurdy, an actuary and American Airlines pilot who is chairman of the Allied Pilots Association’s national pension committee, said that “it’s always a double-edged sword when you’re talking about funding relief.”

“You give them some relief for a few years and get through a tough spot and, in theory, things get better and they make up the contributions and everybody lives happily ever after,” he said. “But if things don’t get better, it’s that much worse for the participants and that much worse for the government. The company is the one getting off the hook in that situation.”

Will Ris, American Airlines senior vice president for government affairs, said that the company has lost $10 billion over the past decade. During that time, it made $3 billion of payments to its pension funds. “If we hadn’t had that relief, we would have been in the bankruptcy realm sooner,” he said. “We have tried to avoid this so diligently for so long, and this is not the direction we wanted to go.”

But not everyone thinks American tried diligently enough. The PBGC’s Gotbaum, a former investment banker, spent two years as bankruptcy trustee for Hawaiian Airlines, ultimately restructuring the company, repaying creditors and preserving defined benefit pensions.

“We know that other airlines have successfully restructured, preserved their jobs and kept their pension plans. We don’t see why American can’t, too,” Gotbaum said Friday. “We hope that before American takes the drastic action of terminating the pension plans covering 130,000 American employees that it tries hard to find an alternative and shows the world that there is no other alternative.”

The PBGC is financed by premiums from private companies; American Airlines paid $260 million over 30 years. The PBGC guarantees private pensions up to $55,840 a year. That covers most airline workers, but the APA, the pilots’ union, said that about 3,800 of its members would be affected by the ceiling. PBGC would cover $9 billion of the American Airlines’ $10 billion of unfunded pensions.

The agency has filed liens on American Airlines assets, largely offices and facilities in Latin America that are not protected by the bankruptcy. American paid only $6.5 million of the $100 million in pension contributions due Jan. 15, the PBGC said.

“This is not a case of runaway labor costs. This is a case of poor management,” Jamie Horwitz, spokesman for the Transit Workers Union, said. He pointed to infighting among top executives and the bankruptcy filing’s revelation that the airline bought a $30 million London home for Tom Horton, who recently became the company’s chief executive.

“I’m a little bitter,” said TWU International President James C. Little, who had just negotiated concessions and was about to bring them to a vote of the union’s 26,000 American workers. “I’m very frustrated at American’s going to go to court and use the bankruptcy process to terminate the plan.”