For Boeing Chief Executive Officer Dave Calhoun, the Alaska Air episode is another blow to his efforts to stabilize the company after half a decade of upheaval, coming just a few days into a new year he had heralded as crucial to a turnaround. Boeing is still feeling the reverberations of two deadly 737 Max crashes almost five years ago that shook confidence in the company. Now Boeings fraught relationship with its biggest supplier — Spirit AeroSystems Holdings Inc. — stands to face fresh scrutiny.