With a strike by thousands of mechanics in its third day, Northwest Airlines Corp. (NWAC) on Monday faced its biggest test yet to keep operating, but its stock rallied as investors bet the carrier would prevail.

Northwest shares rose 24 cents at $5.62 on Nasdaq, after earlier touching $5.86.

The No. 4 U.S. airline kept flying over the weekend without major disruptions as it called in 1,500 replacement workers in an attempt to weather the first U.S. airline strike since 2001. Monday — the first strike day of the workweek — promised to be the busiest flying day since the labor action began.

The strike has pushed Northwest, with hubs in Minneapolis and Detroit, to run a fall flight schedule, which usually begins after Labor Day, rather than a summer plan featuring more flights to vacation destinations.

Informal surveys showed Northwest's on-time performance over the weekend at 50 percent to 58 percent, significantly worse than its June average of 72.5 percent.

Rating agency Standard & Poor's (search) on Monday said it may cut Northwest's debt ratings to "junk" status, citing the labor troubles, which are compounding pressure from high oil prices and pension liabilities. The ailing airline has warned it may face bankruptcy if it can't contain costs.

Members of the Aircraft Mechanics Fraternal Association (search) picketed major airports again on Monday, and a union representative said the walkout by 4,400 mechanics and maintenance workers was taking a toll.

But Northwest shares jumped as much as 9 percent, as analysts said the carrier has survived the strike well.

"Northwest's extensive contingency planning appears to be paying off," Morgan Stanley analyst Douglas Runte said in a research note.

Traffic at Detroit Metropolitan Airport looked very thin for an unusually slow Monday morning. Of 115 outgoing Northwest flights, only three cancellations were listed, with 25 flights delayed.

Keith Parks, a mechanic picketing the airport on Monday, said Northwest has reduced its schedule there by about half. He said he saw about 15 flights go out on Monday morning, compared with a usual schedule of 30 flights.

Parks, wearing a black-and-white AMFA T-shirt with the logo "United we stand, divided we fall," said replacement workers took four hours to change a tire on Saturday — a task that he said typically takes Northwest mechanics 30 minutes.

Bob Rose, president of the local AMFA chapter in Detroit, said he expects Northwest's DC-9 fleet, estimated to be more than 30 years old, to be grounded within a week to 10 days.

Flight 1412 bound for Pittsburgh from Detroit was returned to Detroit after reports of smoke in the cabin. The airline cited a problem with the air conditioner.

Also on Saturday, Flight 210 from Seattle had four tires blow out when it landed at Detroit. The airline attributed the problem to the braking system. In Guam on Friday, a Northwest 747's front landing gear collapsed, causing the aircraft's nose to hit the runway.

No one was hurt in any of the incidents.

"Our evidence is that Northwest is having a great deal of trouble keeping their operation running," said Steve MacFarlane, AMFA's assistant national director, adding that the union would not return to the bargaining table unless Northwest substantially sweetened recent proposals.

Northwest would not return calls seeking comment.

Analysts have said Northwest's operating expenses are the highest among the so-called legacy carriers.

The airline is seeking $176 million in annual wage and benefit concessions from AMFA members, part of which would come from firing about 50 percent of them. Those concessions are part of a broader $1.1 billion in wage and benefit cuts from unions Northwest says it needs to stay competitive.

"We are clearly prepared and ready to last months if that's what it takes," said the union's MacFarlane.

Michelle Williams, on a flight from San Jose, Calif., to Hershey, Pa., on Sunday, missed her connection in Minneapolis because of a two-hour flight delay. Williams and her children were put up at a hotel by Northwest and "basically abandoned," she said.

"I will not fly Northwest again," she said. "They gave me some coupons, but I don't care, I won't fly Northwest again at all."

But Bill Houlihan, who flies between Newark, New Jersey, and Minneapolis every week, said his plane took off and arrived on time. "The pilots and flight attendants seemed to be in good spirits. I wasn't concerned about safety," he said.