BACKGROUND: Among the variety of different types of technical compatibility (TC), design, technological, operational and thermal types have a high priority. The reason is that these TC types are commonly the most problematic and typical for the majority of modern machines.
 AIMS: Two approaches to development of new prospective projects of autonomous electrotechnical complexes (ETC) of aircrafts, such as particular system design (PSD) and complex system design (CSD) are considered.
 METHODS: Analysis and synthesis of elements of the ETC of aircraft, as well as a systematic approach to design.
 RESULTS: Details and advantages of the CSD are revealed. Problems and conditions, making practical implementation of this approach possible, are given. All notions and considered design approaches find their application in development and modernization of aircrafts (AC) as well as ETCs. Various types of compatibility, such as operational compatibility, design compatibility, thermal compatibility, size compatibility, energetic compatibility and informational compatibility are outlined. Generally, TC research by engineers and scientists gains more relevance of solving the problem of modern machines improvement. Scale and relevance of this problem only at focal points of ensuring some TC types are given in the article, as more detailed consideration is problematic even at a monograph level and impossible at an article level because the number of researched TC types goes beyond 40.
 CONCLUSIONS: For the complete analysis of thermal mode of the whole AC, it is necessary to have the equation system consisting of the equation for the aircraft as a whole and equations for all onboard elements. Such equation system becomes extremely bulky even for small ACs. Therefore, approximate methods of analysis and calculation are commonly used. The most spread approximate method is consideration of only steady-state thermal processes for various AC operational modes. Final target of this approach is to obtain sufficient quantity of thermal modes points, enough to approximate the whole thermal mode of an aircraft.
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