Under U.S. law, Delta pilots cannot walk off the job until the National Mediation Board grants them permission.If either side declines, both parties enter a 30-day "cooling off" period, after which pilots and management can engage in self-help - a strike by the union or a lockout by management.The carrier said a "significant progress" has been made in the contract negotiations and only a few issues have been left to resolve."We are confident that the parties will reach an agreement that is fair and equitable, as we always have in past negotiations."Delta pilots have been working without a new contract for nearly three years after their old contract became amendable in December 2019."