American Airlines filed for bankruptcy protection on Tuesday to cut labor costs in the face of high fuel prices and dampened travel demand, capping a prolonged descent for what was once the largest U.S. carrier.

AMR Corp, the parent of American Airlines, also filed for bankruptcy and replaced its chief executive.

The company, which employs about 88,000, has been mired for years in fruitless union negotiations, complaining that it shoulders higher labor costs than rival domestic and foreign carriers that have already restructured in bankruptcy.

United Continental Holdings Inc's United Airlines and Delta Air Lines Inc, both of which used Chapter 11 to cut costs and later found merger partners, are now the largest U.S. carriers. American ranks third.

"The world changed around us," incoming Chief Executive Tom Horton told reporters on a conference call. "It became increasingly clear that the cost gap between us and our competitors was untenable."

AMR named Horton as chairman and chief executive, replacing Gerard Arpey, who retired.

American plans to operate normally while in bankruptcy, but the Chapter 11 filing could punch a hole in the pensions of roughly 130,000 workers and retirees.

AMR pension plans are $10 billion short of what the carrier owes, and any default could be the largest in U.S. history, government pension insurers estimated.

Ray Neidl, aerospace analyst at Maxim Group, said a lack of progress in contract talks with pilots tipped the carrier into Chapter 11, though it has enough cash to operate. The carrier's passenger planes average 3,000 daily U.S. departures.

"They were proactive," Neidl said. "They should have adequate cash reserves to get through this."

PROBLEMS TO ADDRESS

Bankruptcy gives AMR a chance to pare less profitable operations, and could result in the sale of flight routes. The process also gives AMR more flexibility, according to Jack Williams, a professor of law at Georgia State University.

"There are considerable tax benefits that they will be able to use in a bankruptcy case, and they will be able to more aggressively manage their liabilities," Williams said.

But analysts question whether the bankruptcy will address operational shortcomings that have eroded revenue.

"Bankruptcy is not necessarily the be-all, end-all," said Helane Becker, an analyst with Dahlman Rose & Co. "They've got more problems to address in addition to the cost problem."

Shares of AMR closed Tuesday down $1.36, or 84 percent, at 26 cents, down from a 52-week high of $8.89 on January 7. Stock typically is wiped out in bankruptcy.

Shares of rival airlines rallied on expectations that reduced competition could boost fares. AMR had kept a lid on industrywide fares in its effort to keep its airplanes full.

United Continental shares closed up 6.3 percent at $17.63, Delta rose 5 percent to $7.80 and US Airways Group Inc climbed 4.4 percent to $4.46.

AMR shares were halted 28 times on the NYSE on Tuesday for triggering a circuit breaker rule, activated when a stock moves up or down at least 10 percent within five minutes.

SLIMMED-DOWN AMR

In its bankruptcy petition filed in Manhattan, AMR reported assets of $24.72 billion and liabilities of $29.55 billion. The company has $4.1 billion in cash.

One bankruptcy rule is "don't wait too long," Harvey Miller, a partner at Weil, Gotshal & Manges representing AMR, said at a court hearing. "Don't wait until the course is irreversible. That is what American Airlines is doing today."

AMR's bankruptcy filing showed few details about how the company would proceed, said Stephen Selbst, a bankruptcy attorney with Herrick Feinstein in New York.

"It's possible they are still in negotiations and don't want to put something on paper that might prejudice those negotiations," he said.

Experts believe AMR stands to save billions by restructuring its obligations in bankruptcy.

"AMR will no longer have its defined benefit pension plan, helping absorb nearly $7 billion in debt," Morningstar equity analyst Basili Alukos said.

"I imagine the company can save between $1.2 billion to $1.5 billion in labor costs, in addition to savings on repair and maintenance and better fuel burn," he said.

MERGER IN THE OFFING?

AMR said the bankruptcy has no direct legal impact on non-U.S. operations. It also said it was not considering debtor-in-possession financing.

But it could susceptible to unsolicited takeover bids from rival carriers. AMR has long said it could thrive on its own.

Robert Herbst, an analyst with AirlineFinancials.com and a former American pilot, said there was a "95 percent" chance American would join up with another carrier within two years.

"US Airways is probably toward the top of the list but it wouldn't be the only (potential merger partner)," he said.

A US Airways representative did not immediately return a phone call seeking comment.

Most large U.S. carriers are the products of mergers.

United Continental combined the former United Airlines and Continental Airlines, while Delta bought the former Northwest Airlines. US Airways was formed from a 2005 merger with America West Airlines.

US Airways and United Airlines filed for bankruptcy protection in 2002, and Delta and Northwest in 2005. US Airways had tried to buy Delta out of bankruptcy.

Japan Airlines Co, one of American Airlines' alliance partners, filed for bankruptcy last year.

American Airlines said it would remain an active member of the oneworld global airline alliance.