This work aims to present the complex certification process of a modification made to the Type Design of an aircraft, in order to obtain a Supplemental Type Certificate from the certifying authority (EASA in this case), which is requested by a client to a Design Organization Approval (DOA) organization. Major Change has been chosen as the guiding thread of this thesis, regarding the Avionic Upgrade on AS 332 helicopter models C /C /L /L1. Main attention is given to the certification program and airworthiness compliance documents, given to the agency as first documentation, in order to obtain the approval of the Major Change.