WASHINGTON — The airline industry's ongoing push to reduce costs by outsourcing jobs — a key issue for striking Northwest Airlines mechanics — has intensified in recent years, and now more than half of all major maintenance on U.S. airlines is done by contractors rather than airline employees.

The union at Northwest has said the trend raises safety and security issues, but the company contends the outsourcing is necessary for the airline to remain competitive.

Northwest demanded pay cuts and sought to slash its payroll, preferring to send the work to lower-priced U.S. contractors and overseas.

About 4,400 unionized Northwest mechanics, cleaners and custodians walked off the job Saturday, and no new talks have been scheduled.

Among U.S. airlines, 53 percent of maintenance work is being done by contractors rather than in-house mechanics, 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 soon will 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 contracting out the major repair work. Northwest has trimmed its work force of mechanics and other maintenance workers from 10,000 in 2001 to 4,400 today, by cutting jobs and outsourcing, according to the union.

"Northwest directly competes in the global marketplace, with airlines based on three continents and across two oceans," said Kenneth Hylander, the airline's vice president of safety and engineering. "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."

Despite the strike, Northwest Airlines kept its planes flying again yesterday. But experts say the real test arrives with a far-busier weekday schedule.

Saturday, when the strike began, generally is the lightest flying day of the week.

It might be tougher for the airline to meet its schedule as the workweek begins, said Scott Hamilton, an airline consultant for Leeham in Sammamish. "Sooner or later if the replacement mechanics can't keep on top of it, it's going to start causing cancellations."

Northwest's unionized mechanics average about $70,000 a year in pay, and cleaners and custodians can make around $40,000. The company wants to cut their wages by about 25 percent.

Northwest has said it needs $1.1 billion in labor savings.