How do I know whether an STC has been grandfathered?
Any STC approved or validated by any member state before the establishment of EASA is deemed to be 'Grandfathered' under Regulation 1702/2003 Article 2 (3)(a). Unfortunately, there are tens of thousands of these approvals and it has not been possible to put together a database. We normally recommend an enquirer to contact the STC holder (the FAA website has these details) and check with them directly whether they have any EU customers. The STC holder should know who his customers have been because he has obligations to maintain continued airworthiness for his modifications.