Bilateral arrangements in aviation safety rely on the assumption that the rules and practices across different jurisdictions are largely interchangeable.
That is to say, if Transport Canada or the US Federal Aviation Administration (FAA) approves an aircraft, then Europe's regulator will have few qualms about following suit.
But the aftermath of the Ethiopian Airlines Boeing 737 Max crash on 10 March threatens to, if not fracture, at least strain that convention.
Safety regulators in Canada and Europe have stressed that until they are independently satisfied with the safety of the re-engined narrowbody, they will not allow airlines to fly the jet again.
Patrick Ky, executive director of the European Union Aviation Safety Agency (EASA), went so far as to promise the bloc's elected representatives that without satisfactory answers, its grounding would remain in place no matter "what the FAA does".
Perhaps that represents a belated admission from EASA that it could or should have probed more deeply into the operation of Boeing's anti-stall system – under scrutiny since the Lion Air Max crash of October 2018 – during the original certification process.
Boeing says there was nothing unusual in the validation of the Max, or the Maneuvering Characteristics Augmentation System (MCAS). But the manufacturer's insistence that the 737 Max is safe is unlikely to distract attention from the painful reality of two fatal crashes, and 346 deaths, just five months apart.
As Ky points out, though, one of EASA's reasons for grounding the narrowbody was that, despite Boeing's issuance of instructions on how to deal with a malfunctioning MCAS, the actions required are incredibly difficult to replicate or train for.
Human factors mean that while one pilot may respond with icy, robotic precision, another – confronted with a cockpit full of competing alarms – may freeze. This represents a huge challenge for Boeing. Even if it considers the system safe, how does it ensure flightcrews respond correctly to maintain that security?
Recent coverage in the UK media included an article asking: "After two deadly disasters in five months, can Boeing survive?"
Of course it will. But its response, and how it goes about convincing operators, crews and passengers of the Max's safety, will determine whether its reputation similarly endures.