It will now likely be more than three months before the National Mediation Board decides whether to certify a vote by members of a Northwest Airlines union to join another union . The workers voted to leave the International Association of Machinsts for the Aircraft Mechanics Fraternal Association. The IAM complained Northwest and AMFA interfered with last fall's election. After an intital review, the National Mediation Board has decided to conduct a more thorough investigation into the allegations.