Boeing’s 737 MAX has been grounded since March. It almost feels like the new normal. But sooner or later the MAX will get airborne again. The big US airlines seem tentatively optimistic this maybe around the end of the year. But the US and their regulatory agency, the FAA, are just one part of the larger equation. Boeing’s 737 MAX flies into countries and jurisdictions all over the world. Each will need their regulatory agency to tick off the MAX and there’s no guarantee they will simply follow the FAA’s lead.
In excess of forty carriers around the world have a combined total of 376 737 MAXs sitting idle. The vast majority of the airlines that have 737 MAXs are based outside the United States. Each has a local regulatory equivalent of the FAA. In addition, an airline may plan to fly the MAX into a country which may not be the home base for any 737 MAXs of its own, but that country will not allow the 737 MAX into its airspace until it is confident Boeing has sorted out problems with the aircraft.
Collected and summarized from the source below by Quynh Hoa https://simpleflying.com/boeing-737-max-simultaneous-return/