Mission efficiency of an air wing is shown in the air base view window, that is shown if you click the airfield in which the air wing is stationed. Mission efficiency in a strategic region is shown in the strategic air view window, that is shown if you click a strategic region in the strategic air map mode. Note this is different from mission-specific modifiers like Air Superiority Mission Efficiency described below. The mission efficiency of a wing influences what percentage of its planes can actually carry out the assigned missions.
Air base view showing red mission efficiency of the first air wing and yellow mission efficiency of the second air wing.
