The mechanics strike at Northwest Airlines is in its third day. The carrier is expected to get a true test of its ability to keep planes flying as its flight schedule returns to heavier weekday levels. Northwest has passenger hubs in Minneapolis, Memphis and Detroit. About 4,400 mechanics, cleaners and custodians walked off the job Saturday, when the carrier operates about 1,200 flights. Northwest operates nearly 1,500 flights on weekdays. Northwest prepared by lining up about 1,900 replacement workers, vendors and managers. The union says the airline is already hurting but a Northwest spokesman says there were few cancellations and most flights were on time. No new talks are scheduled. The union is refusing to take pay cuts and layoffs that would reduced their ranks almost by half and wages by about 25 percent.