* The union representing flight attendants at Northwest Airlines could ask the National Mediation Board this week to declare an impasse in the union's contract talks with the airlines. The flight attendants voted this weekend to strike, which sets the stage for the second work stoppage in a year at the nation's fourth-largest airline. The flight attendants have been in contract talks with the St. Paul, Minn.-based Northwest for almost three years, with pay, pensions and job security among the key issues. They cannot strike until 30 days after the National Mediation Board, which oversees airline labor contracts, declares an impasse. Billie Davenport, president of Teamsters Local 2000 that represents the attendants, said the union could ask this week that an impasse be declared.