Schemes and organization of the working process of turbojet engines with variable working process (PIA), made on the basis of double-circuit engines with mixing of flows, peculiarities of characteristics of the main adjustable elements and conditions of their joint operation in the engine system are considered. General methodological issues of the working process, research and analysis of power plants with PIA are described. Methods are described and the results of calculation of the characteristics of power plants with optimal control of the main adjustable elements are given. The laws of control of DIP elements are considered. For engineers of aviation industry and civil aviation.