The electronic sets that are being used in aviation industry are commonly summarized as “avionic = aviation electronic equipment”. Nearly seventy years ago the first avionics devices used on aircraft were communication and navigation systems based on old gauge instruments and analog systems. Since then, the industry has evolved a lot and today the avionics systems require for new and smarter functionalities thus driving the overall aviation research to an exponential rate towards high grade avionics systems and architectures. In this research project, a complete investigation has been performed regarding to the maturity of avionics systems from different phases of the development. The report describes in detail the methodologies and constraints related to the avionics modification from electromechanical to digital avionics bus. In addition, the problems regarding to the maintenance and modification of legacy aircraft has been presented in detail. Using the knowledge acquired in the different phases of avionics modernization, two different top level avionics system design architectures have been developed for a medium jet civilian aircraft with digital avionics systems involving military standard 1553B (MIL-STD-1553B) bus and Integrated Modular Avionics (IMA) e.g. ARINC 653 and ARINC 664 protocols. The top level design presents the complete avionics architecture of the aircraft, the avionics systems used, the data communication between different avionics systems and the complete hardware and software layout architecture. During the research, some military aircraft such as C-130 Hercules (American) and IL-78 Ilyushin (Russian) were taken as the main study and reference avionics architectures for the modification from electromechanical to digital systems. Apart from these two models, several other aircraft such as F-22 Raptor, Boeing B777 and Airbus A380 were taken as the study cases for the Integrated Modular Avionics (IMA) technology