New York ? American Airlines took a huge step toward preventing bankruptcy Monday by reaching tentative cost-cutting agreements with key unions representing pilots, flight attendants and mechanics.

While the company appeared to secure the $1.8 billion in concessions needed to avert Chapter 11, the agreements still must be voted on by union members. The ratification process will begin today, union officials said.

The news lifted shares of American by 33 percent as investors considered a bankruptcy filing much less likely.

Company chairman Don Carty praised union leaders, saying they “have enabled us to avoid an immediate filing with the bankruptcy court.”

Despite the tentative deals, the carrier is not guaranteed a smooth ride. The industry is still battling its worst downturn ever, precipitated by fears of terrorism, an ailing economy and now the war in Iraq.

“If the war lasts for many months or if there’s another act of terrorism, then even these cost savings could prove insufficient,” said Philip Baggaley, Standard & Poor’s airline analyst.

Even if employees ratify the agreements, analysts said it will still be critical for parent company AMR Corp. to secure additional financing as well as better deals from suppliers and lessors.

Experts said the talks were probably affected by last week’s agreement between United Airlines and its pilots’ union on $1.1 billion in cuts.

“I think that has helped focus their minds,” said Peter Cappelli, a professor of management at the University of Pennsylvania.

United, which is restructuring under Chapter 11 protection, has said deep cuts were needed to avoid liquidation. American, meanwhile, had indicated it would file for bankruptcy protection soon unless it reached tentative agreements with its major labor groups Monday.

Other major airlines also are pushing employees to accept wage and benefit cuts as the industry suffers huge losses, and analysts said the momentum at United and American could spread.