BUSH ISSUES ORDER PREVENTING STRIKE BY AIRLINE UNION
Pres Bush isues executive order barring potential strike by mechanics at Northwest Airlines and says he will take 'necessary steps' to block walkouts at any major carrier this year; Bush's pledge comes as United Airlines, American Airlines and Delta Air Lines are nearing end to contract negotiations, some of which have lasted for years; mechanics at Northwest, who are represented by Aircraft Mechanics Fraternal Assn, have been seeking wage increase and new contract for more than four years; executive order requires that mechanics stay on job for 60 days while three-member emergency board is convened to make settlement recommendation (M)