Blessed by a resurgence in airline traffic, higher fares and lower fuel prices, the employee-owners of UAL Corp. have been flying high. A 15-month-old buyout by the company's unions and salaried employees has been a model of labor-management relations. After six years and $3.1 billion in losses, the Chicago-based parent of United Airlines is back in the black. Just last week, UAL posted record third-quarter earnings of $243 million. So, why ruin a good thing?