Northwest Air Getting Control Of Continental
Northwest Airlines agrees to buy controlling stake in Continental Airlines for $519 million to create third-largest domestic and international airline network behind United Airlines and American Airlines; Northwest and Continental will merge flight schedules and frequent-flier programs but maintain separate identities and managements; consumers can expect larger choice of flights from new alliance, to virtually any spot from Latin America to Asia, but probably not lower prices, since two carriers do not now compete on many routes; merger is subject to approval by Departments of Justice and Transportation, and by Northwest's pilots; Northwest's proposal of an alliance, which would create jobs, was deciding factor for Continental's board; board rejected merger offer from Delta which would have resulted in widespread layoffs; map; chart (M)