This paper presents an approach for identifying the optimal configuration of avionic systems of the unmanned aerial vehicle (UAV) based on an additive multi-attribute utility function. The function arguments are technical and economic indicators of avionics design quality that are specified in accordance with the UAV requirements. The method is developed on an improved decision model that has a high degree of adequacy primarily by the increasing number of the utility attributes and using advanced reliability models of avionic systems. These reliability models consider in addition to the reliability parameters of main and standby elements of fault-tolerant units the effectiveness of non-perfect detection and switching devices. The proposed method enables the increasing certainty of design analysis results. It allows determining the optimal configurations of the avionic systems and rational maintenance regime ensuring needed effectiveness and reliability, minimizing the expenditure of resources.