MINNEAPOLIS - Northwest Airlines jets roared into the sky over the heads of striking mechanics Saturday as the nation's fourth-largest carrier turned over its maintenance to replacement workers on Day 1 of the industry's first major walkout in seven years.

The union mechanics walked out rather than take substantial pay cuts and layoffs that would reduce their ranks almost by half. They said they don't believe the replacements will be able to maintain the fleet, the oldest among domestic airlines.

Saturday afternoon, Northwest was already facing at least two maintenance jobs in Detroit, one of its hubs.

One Northwest plane blew out four tires as it landed on a runway, and another made an emergency landing after flight attendants reported smoke in the cabin. No injuries were reported in either incident. The airline said the cause of the blow-out was likely "an anti-skid braking issue," the vapor appeared to be an air conditioning system problem, and neither had anything to do with the strike.

Earlier Saturday, Northwest executives said their contingency plan was working flawlessly, with few cancellations and flights predominantly on time. The airline switched to its fall schedule Saturday, a few weeks early, lightening the schedule by about 17 percent.

No new negotiations were scheduled, said Bob Rose, president of AMFA Local 5 in Detroit. "We'll either live or die on the picket line out here," he said.

Steve MacFarlane, assistant national director for the mechanic's union, said the union never expected the walkout to have an instantaneous effect.

"As airplanes break through the normal flight day, these airplane need to get fixed. And if these guys can't fix them they get set off to the side," MacFarlane said. "We're confident that over a period of time it begins to snowball, and they're going to have a real problem maintaining their schedule."

Northwest's pilots said the airline appeared to be running smoothly, said Hal Myers, spokesman for the Air Line Pilots Association. The union is running an around-the-clock call-in center to answer pilot concerns about maintenance issues.