An arbitrator certified union representation for 4,100 workers at Litton Industries Inc.'s Avondale shipyard in New Orleans. labor sources said. Avondale workers had been locked in a bitter fight with management over union representation since 1993, but when Litton bought the shipyard in August, the company agreed to remain neutral and let workers sign cards indicating they favored a union. The Metal Trades Department of the AFL-CIO, a federation of U.S. labor organizations, said confirmation of the successful signature drive had been received. "We will be asking Litton to plan for full-scale negotiations as early as possible in the new year," said a statement by John Meese, president of the national Metal Trades Department.