The head of Eastern Airlines' largest union filed a federal lawsuit yesterday to block the airline from extending an employe wage-concession program, claiming the company "is running better now than it ever has in its history."

But U.S. District Judge Joe Eaton refused to act immediately on the request by the International Association of Machinists, which represents some 12,000 Eastern employes, for a temporary restraining order against the 18 percent wage cuts the company plans to keep in force this week. He scheduled a hearing for Thursday.

The concessions had been set to expire on Jan. 1, but Chairman Frank Borman recently announced he was extending them unilaterally to save $22 million a month. The Machinists' suit charges the airline with violating the federal Railway Labor Act by "unilaterally altering conditions of employment" through the wage-concession extension.

Borman and the company's board maintain the cuts are necessary for the airline to continue on the road to financial recovery. But Charles Bryan, a board member and president of the local Machinists District 100, which represents about 12,000 employes, says Eastern now is in good financial shape, can turn a profit without "taking the employes' wages," and should focus on further increasing productivity and other cost-cutting measures.

Glenn Parsons, Eastern's director of public relations, said "we have no comment" on the suit.

The Air Line Pilots Association, which represents Eastern's pilots, also said it will sue the company to get full salaries reinstated.