Without going into the whole history of the last 50 years, there has been considerable success at harmonisation of aircraft certification rules in Europe. Not only at a level of the requirements and specifications to be applied by the processes and procedures used too.
Now to consider a “no deal” outcome of the negotiations between the EU and UK. This is the case where there are no working arrangements or informal agreements of any kind on the day after the assigned leaving date of leaving the European Union (EU).
In accordance with the Chicago Convention there are a series of certificates that are mandatory in international civil aviation. Those concerning aircraft airworthiness are in the articles of the convention and its Annexes, most particularly Annex 8 and its associated manual.
The complete framework of European aviation regulation is compliant with these standards. So, a European aircraft Type Certificate (TC) or a Certificate of Airworthiness (CofA) or an Airworthiness Release Tag (EASA Form 1) is recognised across the globe. To build this European system, decades of cooperative working and confidence building were invested. It’s is not only that the rules and procedures are agreed and correct, it’s also that their implementation delivers the desired results.
In a “no deal” outcome of the negotiations between the EU and UK the EU Regulations responsible for this regulatory system no longer apply in the UK. Now, I have assumed that the technical content of these regulations will be adopted lock-stock and barrel into UK law. Thus, the situation may be that the UK will not change any aspect of the rules and procedures for the issuance of a certificate on the day after Brexit.
The significant difference that arises is that the certificate issued will be a National certificate and not a European one. Thus, it will not be automatically mutually accepted by EU Member States. For existing certificates, the UK may choose to adopt all those that exist at the time of change. Nevertheless, each one of these would need to be accepted as a UK National certificate.
The European system does validate certificates from “third countries” but this is usually done under the framework of a working arrangement or a Bilateral Aviation Safety Agreement (BASA). If no such arrangement or agreement exists, then either there’s no recognition or an ad-hoc grandfathering of privileges might prevail. Such ad-hoc measure might be time limited and contingent upon the conduct of an audit.
In any event the movement of aircraft and related products, parts and appliances between the UK and EU Member States will not be as free as it is now. In the extreme worst case “no deal” there will be no movement. This would be commercially catastrophic for all sides.
Speculating about ad-hoc or emergency measure is difficult. One analogy that could shed light is that of the position of Turkey. In 2009, the Joint Aviation Authorities (JAA) was disbanded. Non-EU members of the JAA like; Norway, Iceland, Switzerland and Liechtenstein joined the European system. Turkey did not. There is a working arrangement between the EASA and Turkish DGAC but it is limited. Relationships like these are subject to continuing negotiations which are not without frustration.