More than a dozen union locals representing American Airlines workers have been asked to schedule strike authorization votes this month.  The Transport Workers Union of America says it’s at an impasse with Fort Worth-based American.  However, an airline spokeswoman stresses that only the National Mediation Board can officially declare an impasse.  Under federal law, airline unions may not go on strike until official mediators do, in fact, declare an impasse and then there has to be a 30-day cooling off period.  In March, the union asked the National Mediation Board for an impasse declaration but mediators ordered talks to continue.