The mechanics' strike at Northwest Airlines Corp. has refocused attention on the airline industry's ongoing push to reduce costs by outsourcing maintenance jobs.

A key issue for the mechanics union was Northwest's insistence on reducing its in-house maintenance work. In addition to demanding pay cuts, Northwest sought to slash the number of mechanics on its payroll, preferring to send the work to lower-priced contractors in the United States and overseas.

Outsourcing of maintenance tasks has surged in recent years, raising concerns about the experience and background of workers performing the upkeep on the nation's aircraft. Fifty-three percent of all major maintenance on U.S. airlines is done by contractors rather than airline employees, an increase from 47 percent in 2003, according to the Transportation Department's inspector general. In 1990, only a third of the carriers' maintenance went outside the company.

JetBlue Airways and America West Airlines have their planes serviced in El Salvador. Northwest and Continental Airlines use repair stations in Hong Kong and Singapore. Delta Air Lines' planes will soon be serviced by a division of Air Canada. Many other carriers send maintenance work to third parties in the United States, many located in the South, where labor rates are lower.

Like other unionized mechanics, the Aircraft Mechanics Fraternal Association at Northwest has questioned whether airlines are compromising safety and security by moving the delicate task of major aircraft repairs thousands of miles from their headquarters. Northwest has trimmed its workforce of mechanics and other maintenance workers from 10,000 in 2001 to 4,400 today, through a combination of job cuts and outsourcing, according to the union.

"Northwest directly competes in the global marketplace, with airlines based on three continents and across two oceans," Kenneth J. Hylander, the airline's vice president of safety and engineering, said in a statement. "Other airlines and aircraft operators are using the full range of competent maintenance opportunities available to efficiently maintain and safely fly their aircraft. This is the world we find ourselves in and in which we must compete."

The trend poses new challenges for the airlines and the federal government in keeping on top of safety and security. In January 2003, a US Airways Express flight operated by Air Midwest crashed shortly after takeoff from Charlotte, killing all 21 aboard, partly because an inexperienced mechanic at a third-party maintenance contractor performed improper work on the aircraft. In March, the Immigration and Customs Enforcement agency arrested 27 illegal immigrants who were working at one of the country's largest contract aircraft maintenance facilities, in Greensboro, N.C. None of the immigrants, from Central and South America, Sudan and the Philippines, had terrorist ties, but the incident raised questions about the hiring standards of contractors.

Sarah MacLeod, executive director of the Aeronautical Repair Station Association, said she rejects the notion that recent events indicate contractors' standards are any less safe than those of the airlines. "I think a lot of emotion has been injected in this because the government is finally enforcing its regulations and people are getting caught," MacLeod said. "It does not make an entire aviation maintenance industry unsafe."

MacLeod said half of her 732 member companies based in the United States and overseas conduct their own criminal background checks on employees. She said the contract maintenance industry is similar to many other industries that are growing as companies look for ways to reduce costs.

Union officials said workers at contract repair stations are not required to meet stringent criminal background checks and pass drug tests, as technicians who work directly for airlines are. Congress mandated that the Transportation Security Administration develop security protocols and procedures for maintenance workers and facilities overseas by August 2004, but the agency still has not issued any plans. A TSA spokesman said the agency plans to file a public notice of its proposed rules soon.

"In a post-9/11 environment, the in-house guys undergo tremendous surveillance and criminal background checks, and they are subject to screening in secure areas of an airport," said Lawrence I. Willis, general counsel for the transportation trades department of the AFL-CIO. "It's clearly not the case for third-party repair stations."

The Department of Transportation's Office of Inspector General has found that the Federal Aviation Administration has also been slow to keep up with the changing industry. The FAA's workforce of safety inspectors continue to be focused mostly on maintenance at airlines' in-house hangars rather than contractors in the United States and overseas, according to a status report sent last month to Rep. James L. Oberstar (D-Minn.), who asked the office to conduct an audit of the FAA's inspection program.

"FAA initially planned to implement all of our recommendations by August 2005," stated the inspector general's letter to Oberstar. "However, progress has been slow: FAA has completed only one of nine promised actions."

The FAA said that its progress has been hampered by laws and regulations overseas that are far different from those in the United States, such as those pertaining to routine drug tests. The agency plans to work out a bilateral agreement with the safety agency in Europe to ensure that countries on both sides of the Atlantic can ensure safe inspections of each other's maintenance facilities.

"Clearly, we need to refine our oversight of repair stations -- specifically as they become more popular," said Greg Martin, an FAA spokesman. "The inspector general's concern is perhaps maybe those actions aren't as quick as they should be, but we are moving forward with due speed."