WASHINGTON, Jan 11 (Reuters) - A federal judge on Friday refused to block a presidential order preventing United Airlines mechanics from going on strike. U.S. District Judge James Robertson rejected a motion for a preliminary injunction by the International Association of Machinists union, saying the timing and merits of the group's case were largely impractical. The union wanted the court to set aside the executive order issued in December that created a presidential emergency board to resolve a long-running contract dispute between the machinists and the No. 2 U.S. airline. The action by President George W. Bush effectively prevented the union from carrying out its threat to strike until late February at the earliest. Robertson ruled quickly to give the union time to appeal. Lawyers for the machinists said no decision on that option had been made. At issue were union claims that federal mediators acted arbitrarily when they determined that a strike would hurt the economy and recommended that Bush step in to prevent a walkout. The union said that when the National Mediation Board issued its recommendation, data was readily available showing there was excess capacity in the airline industry. Thus, the union claimed, the situation did not meet the threshold under the law for presidential action, and the recommendation to intervene interfered with the collective bargaining process by blocking the union's right to strike. Robertson questioned whether federal courts had the authority to even review such action, and whether the life-span of a presidential emergency board, up to 60 days, made a legal challenge in this instance practical. The union represents 15,000 mechanics and related workers at UAL Corp.'s (UAL) United Airlines and has been in talks with the company for two years.