It has been a troubling couple of years for the airline industry. But an executive for United Airlines said yesterday that the company has turned a corner and has begun to acheive stability. The major airlines still face challenges. Nowhere is that more apparent than at Eagan-based Northwest Airlines. Northwest is asking for mediation help in talks with its largest union while at the same time taking a number of measures to save cash. Northwest received permission from the federal government to supplement pension funds with stock in a subsidiary, and the airline has been criticized for backpedaling on a noise abatement agreement with residents who live near the Twin Cities airport. Joel Denney is an airline analyst with the investment firm US Bancorp Piper Jaffray. He says Northwest's situation was always better than United's, but that Northwest still faces serious obstacles.