NEW YORK -- Delta Air Lines announced a long-speculated deal to acquire Northwest Airlines for about $3.1 billion Monday, a combination that will create the world's largest airline and could lead to a series of other deals to reshape the U.S. airline industry.

The new carrier will operate under the Delta name, and be based in Atlanta.

Delta said the carrier will maintain the nine hubs of both airlines in the United States, Europe and Asia, serving more than 390 destinations in 67 countries. The combined carrier will have $35 billion in annual revenue, more than 800 airplanes and 75,000 employees, according to Delta.

But many of the employee unions at Northwest were quick to voice opposition to the deal, even though Delta said it is not looking to cut non-office staff.

A review of maintenance records showed a test of the fire- suppression system on its 777s hadn't been performed, the Chicago-based airline said in a statement today. The equipment is now being scrutinized and the U.S. Federal Aviation Administration has been informed.

``United will not operate these aircraft until the tests are complete,'' the company said. The airline said it's working to provide additional planes or accommodate passengers with other carriers. Customers should check the status of their flights before leaving for the airport, it said.

United is one of at least four major U.S. carriers whose maintenance regimes are under scrutiny after it said this week that crossed landing-gear wires may have caused two Airbus SAS A320 jets to skid off runways. Regulators last month proposed a $10.2 million fine for Southwest Airlines Co. over missed inspections and American Airlines and Delta Air Lines Inc. last week grounded jets while complying with a government directive.

Mission accomplished at the American Airlines maintenance site in Kansas City.

American has achieved its previously stated goal of generating $150 million in additional value at its local overhaul base, part of AMR Corp.’s program to make its maintenance operations a profitable part of the company.

American management, union officials and rank-and-file employees will discuss how the overhaul base generated $150 million new revenue and cost-savings at a news conference today.

After several years of financial and employment losses in the years after the Sept. 11, 2001, terrorist attacks, American has posted two consecutive years of profits.

The airline also has been recalling employees who were laid off during the tumultuous period earlier this decade, including workers at the Kansas City facility. About 30 mechanics recently returned to work, raising the number of hourly workers there to nearly 850.

Jake Brace, chief financial officer of UAL Corp., told investors in New York this morning that a number of potential buyers of the airline's maintenance unit have "gone through a very thorough due diligence process and we expect in the very near future will have multiple bids in hand and will take that to the next step."

Once the airline has offers in hand, it plans take the proposals to its unions.

The unions, primarily the Aircraft Mechanics Fraternal Association, "have the ability to block something like this," Brace told a Calyon Securities Inc. conference.

The 204,000-square-foot facility, used in recent years by Delta Air Lines and before that by now-defunct Eastern, will be home to more than 400 ASA employees and about 100 mechanics.

Moving to the North Hangar at 990 Toffie Terrace will quadruple ASA's current space and give the airline its first hangar operation at Hartsfield-Jackson.

ASA, one of Delta's regional contract carriers, already is using the 1960s-era hangar's bays for overnight maintenance. Maintenance support staff will begin moving in next month. Headquarters employees will begin moving into the office space sometime after that, and the move should be complete by April, said Kate Mondolo, ASA's director of corporate communications.

American Airlines will recall or hire 200 to 250 maintenance employees to meet increased workloads in 2008, the Fort Worth-based company said Monday.

The recall and hiring will start in December and will continue through the first quarter of 2008.

Employees will be added at American's overhaul bases in Fort Worth, Tulsa, and Kansas City, Mo., as well as at other maintenance facilities across the airline's system.

"In reviewing our needs for 2008 and beyond, it became very clear that increasing our maintenance work force would ensure that we continue on the path of becoming 'Best in Class,'" said Carmine Romano, senior vice president of maintenance and engineering. "While this will be an ongoing process for the next few months, we are very pleased to welcome more people returning to work or starting a career with American."