Certification of Aircraft, Parts and Appliances
Part-21 and approvals
Regulation (EU) No 748/2012 (Part-21) governs design and production. EASA issues the type certificate (TC) when the product meets its certification basis; a restricted TC covers limited-use aircraft such as historic types. Major design changes lead to a supplemental type certificate (STC). Changes and repairs are classified as major or minor: major ones need approval by EASA or by a Design Organisation Approval (DOA) holding the privilege; minor ones may be approved by the design organisation.
Parts and appliances are approved through an ETSO authorisation or as part of a TC/STC installation; production is controlled by a Production Organisation Approval (POA). Parts released under Part-21 carry an EASA Form 1. Standard parts (AN, MS, etc.) do not need a Form 1. The certificate of airworthiness is issued by the State of registry on the basis of the TC, alongside the registration certificate.
Traps: assuming an ETSO authorisation permits installation - it approves the article, not its installation. Assuming a Form 1 for a part is an airworthiness certificate of the aircraft - it is not. Calling an FAA PMA an EASA approval. Classifying a change by its size rather than by its effect on airworthiness, weight, performance or noise.
Worked example: fitting a new avionics unit requires a valid Form 1 or ETSO article plus approval of the installation; that installation change is assessed as major or minor, approved under Part-21 if major, and released by a Part-145 organisation.