A mechanics strike is causing mounting delays for Northwest Airlines flghts, according to surveys of the No 4 airlines' on-time performance.

NEW YORK (CNN/Money) - More than six out of 10 Northwest Airlines flights were late Monday, the airline's first weekday since it was struck by its mechanics union, according to one survey of the struck airline's operations.

Other surveys are showing roughly half the Northwest flights are flying late, although only a small percentage of flights are being cancelled.

Shares of Northwest (down $0.13 to $5.53, Research), which were higher Monday on reports that the airline had weathered the first weekend of the strike, were slightly lower in early trading Tuesday.

While the airline says that its use of replacement workers, outside contractors and management employees has allowed it to maintain close to normal operations, the Aircraft Mechanics Fraternal Association charges its three-day strike is causing significant operational problems for the nation's No. 4 airline.

One survey, conducted by business travel site Joe Sent Me.com, only 37.5 percent of 99 flights surveyed arrived on time. The survey also found that the average delay of the flights that were late was a bit less than an hour, or a slight improvement from the hour-plus average delays found in identical surveys Saturday and Sunday.

The worst delay found in the business travel Web site's survey was a 6-hour, 29-minute delay on Northwest Flight 411 from Flint, Mich., to Minneapolis St. Paul, on what was scheduled to be a 41 minute flight.

"A lot of talking heads opined that Monday, Day Three of the mechanics strike, would be Northwest's first 'real test.' In that case, Northwest failed, and failed spectacularly," wrote Joe Brancatelli, editor and publisher of Joe Sent Me.com.

A Northwest spokesman claimed that the survey of 99 domestic flights by Brancatelli was not scientific. The survey has looked at Northwest's own Web site for information on all flights whose flight number ended in "1" until it had 99 flights to consider.

Brancatelli said that he thought there was a good chance that Northwest's on-time performance might be worse than the survey.

"If they had numbers that said their on-time is better, they would have said that," he said. "They're not even releasing their on-time performance in internal company postings."

A Northwest spokesman would not comment on why the company is not releasing on-time performance data, or say when it might be released. The government's on-time performance data is not available for more than a 30 days after the end of a month.

Northwest has been able to get the overwhelming majority of its flights in the air and to their destinations during the strike.

It had only four of 99 flights cancelled Monday, up from two of 99 grounded Saturday and three of 99 on Sunday. But that is above the company's average of only 1.2 percent of scheduled flights cancelled during the first six months of the year, according to Transportation Department data. That data also shows Northwest had 21 percent of its flights delayed during the same period.

In other surveys of Northwest's Monday flights, USA Today reported that a sample of 35 non-stop flights from its Minneapolis or Detroit hubs on Monday showed 43 percent were late departing, while the Detroit News found slightly more than 50 percent of flights out of Detroit Metro on Monday to Northwest's most popular destinations were delayed by 15 minutes or more.

The strike by 4,400 members of AMFA is not being supported by the unions representing pilots, flight attendants or other ground employees at Northwest. The union says it would lose more than half its jobs at Northwest if it agreed to the management's demands. The company says it needs $176 million in savings from AMFA if it is to save the $1.1 billion it says it needs to avoid filing for bankruptcy court protections.

For a look at the broader implication of the Northwest strike on labor-management relations, click here.