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 1(EU) No 748/2012 Article 4. Unfortunately, a central repository of such approvals does not exist .Please contact the STC holder directly or review the websites of our Member States National Aviation Authorities. Please also refer to our FAQ “Who do I contact to request a copy of a grandfathered STC? | EASA (europa.eu)