You might ask what’s a “Statutory Instrument”? Well, in the United Kingdom they are a key form of delegated or secondary legislation. That’s distinct from Primary legislation, which in the case of civil aviation is the Civil Aviation Act 1982[1]. Part III of the Civil Aviation Act 1982 calls for an Air Navigation Order (ANO).
The latest UK Air Navigation Order (ANO) 2016[2] is a Statutory Instrument. The UK ANO forms the legal basis for many areas of civil aviation that are regulated at a national level. In effect, it gives the UK Civil Aviation Authority (CAA) is powers and its responsibilities.
The UK ANO is amended from time to time to provide the links between European legislation and national legislation. This Statutory Instrument needs to reflect the general details of the civil aviation regulatory framework that’s expected to work.
It’s well worth remembering that, unlike the USA, the European civil aviation system is not a federal system. To work effectively, it needs the common European part and the national part to work together harmoniously. That has been achieved with remarkable results. It has been an achievement that has enabled great efficiency, a fertile environment for international commercial success and safety improvement.
Taking one example in the filed of aircraft airworthiness. Today, “certification” is defined as meaning: any form of recognition that a product, part or appliance, organisation or person complies with the applicable requirements including the provisions of this Order (ANO) or the Basic EASA Regulation and its implementing rules, as well as the issue of any certificate attesting to such compliance.
If Brexit happens many choices are open to the legislator. That’s the Government and Parliament in the UK. One would be to maintain links to the Basic EASA Regulation. Another would be to delete all references to the Basic EASA Regulation and reference an alternative new text. Naturally, there are numerous combinations and permutations that can be imagined. Whatever happens the resulting new Statutory Instrument(s) will need to comply with existing international obligation namely; the Chicago Convention.
Here I’ve described a great deal of detailed work and no doubt its sitting on someone’s desk. Above I referred to “certification” but that’s just one aspect of a much wider range of aviation activities. Clearly, its one where errors and omissions are likely to be costly to industry and ultimately the traveling public. Normally, consultation and deliberations on significant legislative changes take place over many years. Thus, it’s reasonable to be concerned about what might happen over the coming year. Simple this is not.
[1] https://www.legislation.gov.uk/ukpga/1982/16/contents
[2] http://www.legislation.gov.uk/uksi/2016/765/contents/made
John Vincent 
I think a point that keeps getting missed in all this is that even if the UK government severs all links with the EU and EASA, the UK aviation industry and aviation community dosen’t have to. As I understand it the requirement for the Principal Place of Business and majority ownership to be within the EU applies to airlines (AOC Holders) only. All other approved organisations can be in third countries, regulated directly by EASA. Pilots and Engineers with Part-FCL, Part-66 licences (respectively) can reside outside the EU as well. So any outcome of Brexit negotiations will not be binding on UK industry and UK licensed personnel. The companies can go to EASA, the pilots and engineers can be re-licensed by any EU Member State (e.g. Ireland) and current UK EASA aircraft can be re-registered in another Member State and have their CsofA reissued there. For the background see my article on my Linkedin page that was published by Flight Training News in January.
I agree. For example, a Production Organisation Approval (POA) can be granted to an organisation in a Third Country (non-EU Country).
Inside the EASA system, the issue of POA related certificates and their continued surveillance is a matter for the National Aviation Authority, namely UK CAA. Outside the EASA system, the issue of POA related certificates and their continued surveillance is a matter for EASA.
So, ironically if the UK leaves the EASA system, and if an organisation wishes to retain its approval the day-to-day regulatory activities would shift from the UK CAA to EASA. Industry has the choice to hold multiple approvals, but this always comes with a cost. Efforts over the last couple of decades have been to reduce that cost by agreement and harmonisation. It seems a shame to throw that out for no tangible benefits.