A federal judge in Miami ruled yesterday that pilots who struck Eastern Air Lines and later offered to return to work are entitled to replace strike-breaking trainees who were hired instead. The returning pilots also are entitled to back pay.

The decision, which an attorney for the Air Line Pilots Association estimated could expose Eastern to as much as $100 million in damages, puts further financial pressure on the already reeling airline.

Eastern said it will appeal the ruling and file for a stay of the order. U.S. District Judge Edward Davis ruled yesterday that Eastern acted improperly when it opted to hire trainees instead of striking pilots who offered to return to work. The case grew out of an announcement by Eastern on Aug. 11, 1989, that jobs were no longer available to striking pilots who wished to return to work. All but about 200 of Eastern's pilots walked out of the airline on March 4, 1989, in sympathy with a strike by the International Association of Machinists. Less than a week later Eastern filed for protection from its creditors under Chapter 11 of the federal bankruptcy code. As the strike dragged on, individual pilots returned to work, and on Nov. 22, 1989, ALPA announced that it was ending its strike.

Although exact numbers are not available, the union has estimated that more than 1,000 pilots are interested in returning to work.

The back pay award could be particularly troubling to Eastern. ALPA attorney James L. Linsey said that it would take precedence over claims of Eastern's unsecured creditors.