MINNEAPOLIS – Northwest Airlines got off to a smooth start by keeping its planes flying when mechanics struck. But the real test for the company and its replacement mechanics arrives with a far busier weekday schedule.

The strike began on Saturday, generally the lightest flying day of the week. Northwest averages 1,215 flights on Saturdays – but that increases to 1,381 on Sunday and 1,473 on weekdays, a company spokesman, Kurt Ebenhoch, said yesterday.

The airline will find that maintaining its schedule will be tougher as the work week begins, an airline consultant for Leeham Company in Sammamish, Wash., Scott Hamilton, said.

“Sooner or later, if the replacement mechanics can’t keep on top of it, it’s going to start causing cancellations,” he said.

Northwest’s unionized mechanics, cleaners, and custodians walked off the job Saturday morning after refusing to take pay cuts and layoffs that would have reduced their ranks almost by half. No new talks have been scheduled.

Northwest said there were few cancellations and most flights were on time, though the company declined to provide specifics.

Terry Trippler of cheapseats.com, a ticket fare Web site, said Northwest’s schedule had recovered from a work slowdown just before the strike began Saturday morning. Northwest apologized to passengers for delays then, and said it would work to resolve them.

“This weekend has gone much, much better than I think [the union] thought it was going to, and maybe a little bit worse than Northwest wanted it to,” he said.

Northwest said there were few cancellations and most flights were on time, though the company declined to provide specifics. The nation’s fourth biggest carrier switched to its fall schedule Saturday, a week earlier than planned, lightening the schedule by about 17%.

Between 6 a.m. and noon yesterday, Northwest had 85 delayed flights at Detroit Metropolitan Airport and 68 at Minneapolis-St. Paul International Airport, the president of Local 5 of the Aircraft Mechanics Fraternal Association, Bob Rose, said. The local represents 916 Northwest employees in Detroit. The company had 234 delays at those two airports on Saturday, Mr. Rose said. He did not have data on canceled flights.

“Northwest is hurting. They’re hurting themselves, and they’re hurting the passengers,” Mr. Rose said.

Northwest said it spent 18 months preparing for a strike by the mechanics union, arranging for about 1,900 replacement workers, vendors, and managers.

There was little action at a command center set up by officials at Minneapolis-St. Paul International Airport to deal with picket line incidents and mass travel disruptions, an airport spokesman, Patrick Hogan, said.

There was no sign of other airlines moving to add capacity to make up for Northwest’s cut. United Airlines and American Airlines both said they weren’t adding capacity now, although they left open the possibility. A spokesman for Continental, Martin DeLeon, said the airline was operating as usual.

“We’ve been running load factors that are in the high 80s, and [Sunday] would be no exception,” a spokesman for American, Tim Smith, said. “So there’s relatively few seats out there for folks to chase. As this goes on, if it does, we might look at something else.”

The Aircraft Mechanics Fraternal Association mechanics averaged 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%.

Northwest also sought to lay off about 2,000 workers, almost halving a work force that is already half the size it was in 2001. The cuts would be concentrated among cleaners and custodians; Northwest has said other airlines use contractors to do that work for less.

Eagan-based Northwest has said it needs $1.1 billion in labor savings. Only pilots have agreed, accepting a 15% pay cut worth $300 million when combined with cuts for salaried employees. It is negotiating with ground workers and flight attendants, and it has said it can reopen talks with pilots once it gets concessions from the other groups.