Creating a promising aircraft is an important area of state policy. Making the right decision to create a promising aircraft technology contributes to the economic assessment of the development efficiency. The subject of economic assessment is the updating of resources allocated for the development of the aircraft. An important component of ensuring high reliability of evaluating the effectiveness of the development of a promising aircraft is the methodology of its implementation. The methodology of economic assessment is aimed at determining economic indicators. The authors present the procedure for using the probability of economic success indicator in the economic assessment of the effectiveness of developing a promising aircraft when determining the feasibility of adopting a draft tactical and technical task for the development of an aircraft. To improve the accuracy of the economic assessment, it is proposed to use the standard error of alternative methods for calculating the cost of performing activities at the stages of the aircraft life cycle. The authors tested the proposed method on the example of a promising auxiliary power plant of a military transport aircraft. It is advisable to evaluate the cost of developing a promising auxiliary installation of a military transport aircraft based on the complexity of the development, determined depending on its basic complexity and cost. The authors tested the proposed method on the example of a promising auxiliary power plant of a military transport aircraft. The proposed method is applicable for calculating costs for other types of equipment at all stages of its life cycle.