It is shown that in the practice of designing Flight Control and Guidance System it is necessary to solve not only single-objective optimization problems related to the field of deterministic processes, but also to optimize random variables and functions. It is to such tasks that the analysis of the criterion of accuracy of the designed Flight Control and Guidance System belongs. In the paper, considering the navigation process as a stationary random process, a probabilistic criterion for the quality of aircraft navigation is formulated in the form of a quantitative assessment of the accuracy of the Flight Control and Guidance System that ensures that the aircraft does not leave the specified corridor for a specified time. Formulas are obtained for calculating the accuracy criterion of Flight Control and Guidance System in across track separation. It is shown that in order to calculate the accuracy criterion of the Flight Control and Guidance System, it is sufficient to know the correlation function of the lateral deviation errors, which can be obtained either by the calculation method, or by modeling, or by a flight experiment.