In modern military aircraft, complexity of information combined with time stress creates difficulties for the pilot under combat conditions. The first step to mitigate this problem is to measure mental workload. Several methods for doing so have been proposed. In this paper we develop an analysis of mental workload as distinguished from its prior flight mission parameters (risk, challenge) and subsequent effects (mission performance). A model is developed on the basis of extensive mission data, both ground attack and fighter missions, which exhibits a dual coping process as a response to challenge and risk: emotion coping, leading to tension and decreased performance, and problem solving, leading to increased mental energy and improved performance. The model implies a selection of optimal measures for constructing an index of mental workload which is demonstrated to yield a sensitive measure of the pilot's changing mental status over the course of a period of training.