Northwest Airlines (search) 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 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 (search) 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 expect 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 (search). The union is running an around-the-clock call-in center to answer pilot concerns about maintenance issues.

Myers said there were reports from Detroit on Friday that some of the tractors used to push airplanes back from the gate had damage to their ignitions, and that keys were broken off in the locks of some jetways. But "we didn't see anything done to aircraft that would pose a safety threat," he said.

It's the first major airline strike since Northwest pilots grounded the airline for 20 days in 1998. But this time, the mechanics are striking alone. Pilots, flight attendants and other ground workers all said they would keep working, and a federal judge barred mechanics at Northwest regional carrier Mesaba Airlines from conducting a sympathy strike.

After talks broke off just before midnight Friday, union negotiator Jim Young said the mechanics would rather see the airline go into bankruptcy than agree to Northwest's terms. The Aircraft Mechanics Fraternal Association represents 4,427 mechanics, cleaners and custodians, about 11 percent of Northwest's 40,000 employees.

The mechanics average about $70,000 a year in pay, and the cleaners and custodians can make around $40,000.

The company wanted to cut their wages by about 25 percent as part of an effort to save $176 million a year. It also wanted to lay off about 2,000 workers, almost halving a workforce already half the size it was in 2001. The cuts would be concentrated among cleaners and custodians.

Northwest has said other airlines get that work done more cheaply with contractors.

"I don't like the strike, but I'm siding with the mechanics," said Bob Gibson, whose daughter was headed off to college in Missouri on a Northwest flight from Detroit. "But why does it have to come down to this? I understand that things are rough, but I think what they're asking them to give up is too much. How can they survive? There should be a compromise."

Mike Tyrna, an aircraft cleaner for Northwest for 16 years, said union members had no choice but strike.

"I know this has devastated a lot of people," he said. "But we can't deal with this. It's impossible in this economy to take a pay cut that extreme. We have families. We have things we have to pay for too.

"When they get all they want, all the corporate types are going to get to walk away with their golden parachutes."

Northwest, based in Eagan, has said it needs $1.1 billion in labor savings from all its workers.

Only pilots have agreed, accepting a 15 percent pay cut worth $300 million when combined with cuts for salaried employees. It is currently negotiating with ground workers and flight attendants, and it has said it can re-open talks with pilots once it gets concessions from the other groups.

Northwest and its regional carriers operate more than 1,500 flights to 750 cities. It has hubs in Detroit, Minneapolis, Memphis, Tokyo and Amsterdam.