I believe that's only true for planes flying over/through other countries, and it is not universal. In order for a plane to be sold in EASA governed countries it has to be certified by the EASA. So in effect, the 737 is getting certified twice. In general aviation there's this common misconception that the Cirrus SR line can't recover from a stall, which is because the FAA drops spin recovery testing if you ship with a BRS (parachute). Reality is, when the SR line was certified by the EASA it was required to demonstrate spin recovery. Another discrepancy is that the EASA does not recognize "experimental" has a category of aircraft, so an FAA classified home built experimental that flies to an EASA country would not be allowed to take off after unless the national government has rules that allow it (also applies to ultralights).
xenadu02|2 years ago
FWIW I'm very glad in the US we still allow experimental and home-built aircraft. That's how we got aircraft in the first place and many experimental planes (most?) are built from kits engineered to similar specs as certified aircraft, they just don't go through certification.
However EASA does not actually make Boeing go through a full from-scratch certification again. For the most part EASA reviews the FAA's certification and does not actually put the 737 through a from-scratch certification. This is part of the various "Bilateral Aviation Safety Agreements".
For certain hard requirements (as you noted) there may be disagreement on the rules and the manufacturer may be required to do additional certification with FAA or EASA for the differences but again it is not usually a from-scratch process - it only covers the differences. Technically other countries have similar processes and their own agreements but FAA (being the origin country of powered flight and home to the most aircraft manufacturers) and EASA (home to Airbus and also highly involved since the early days of powered flight... the Wright brothers toured France with their flier to much fanfare, disproving a lot of skeptics in the process) are the leaders and everyone else mostly just follows them rather than inventing their own rules and certifications.