The purpose of this thesis is to analyze the approach of the certification of commercial parts for EASA aircraft, the Airbus A320neo aeroplane and the Eurocopter AS350 Ecureuil (Airbus H125) rotorcraft, which results no safety criticals. The analysis throws its roots in the study of the ICAO aeronautical regulation hierarchical structure and, in particular, the Certification Specifications CS25 and CS27, respectively related to large aeroplanes and to small rotorcrafts. The focus has turned on the parts concerning to on-board systems and equipment, in order to derive the requirements necessary for the certification. In parallel, the applicable standards ARP-4754A and ARP-4761 have been analyzed as a guideline for the development of this work, in order to give emphasis on safety aspects and demonstrate compliance with the airworthiness regulations.