CHICAGO, May 6 (Reuters) - An arbitrator ruled UAL Corp.'s (UAL) United Airlines must pay flight attendants $8.9 million over flight assignments at its former subsidiary Air Wisconsin Airlines, the workers' union said on Monday. After a long legal battle, the Association of Flight Attendants said an arbitrator ruled United must pay the fine because United's purchase of Air Wisconsin in the early 1990s violated a scope sideletter in the United flight attendant contract. When United Airlines purchased Air Wisconsin in 1992, United chose to operate the two carriers separately and use Air Wisconsin flight attendants. A sideletter in the United flight attendant contract signed in 1986 requires the No. 2 U.S. airline to use flight attendants from a seniority list. Air Wisconsin, a small carrier that feeds traffic to United, is now privately held. Flight attendants said the final phase of the award was issued April 27 after a protracted battle in the courts and arbitration. A spokesman for United, based in Elk Grove Village, had no comment on the ruling. ©2002 Reuters Limited.