for weds m.e. When Northwest Airlines tried to win back customers after a walkout by its pilots union, the airline offered double and triple frequent flyer miles for travel. That strategy was testimony to the popularity of frequent flyer miles, which have been called the most powerful marketing programs in history. But with their popularity is coming criticism that frequent flyer plans promise more than they deliver. Minnesota Public Radio's Bill Catlin reports. Frequent Flyer plans were designed to promote airline customer loyalty. They were so successful, companies in other industries