The Canada Industrial Relations Board declared a strike by 10,000 Air Canada flight attendants illegal Monday and ordered them back on the job after they ignored an earlier order to return to work and submit to arbitration. The strike at Canada's largest airline entered its third day on Monday and is affecting about 130,000 travelers per day during the peak summer travel season. Air Canada suspended plans to restart operations Sunday after the union defied a return-to-work order and said workers would not go back to work.