The chairman of the National Mediation Board...

The chairman of the National Mediation Board halted talks between troubled Eastern Airlines and its largest union and returned to Washington. Chairman Walter Wallace could not be reached for comment, but an Eastern executive said Wallace gave as his reason the failure of efforts to keep the talks with the machinists union secret. Wallace arrived Friday for the talks at a hotel in Key Largo, Fla. The sessions were aimed at ending a contract stalemate that has clouded the future of the airline, which is a subsidiary of Texas Air Corp. of Houston.