Boeing and the US federal aviation regulator plan to start a three-day certification test campaign for the 737 Max, marking a major milestone in its return to the skies after a 15-month long grounding.

Following a pre-flight briefing, pilots and test crew members from Boeing and the Federal Aviation Administration (FAA) will board a 737 Max 7 outfitted with special test equipment at Boeing Field near Seattle, Reuters reported, citing people familiar with the matter. Boeing and the FAA declined to comment.

Marking a date for certification flights is a critical step in Boeing's long journey to resuming service on its best-selling plane since it was grounded in March 2019 following two fatal crashes in less than five months. Boeing's worst crisis in its corporate history has been compounded by the Covid-19 pandemic that brought the travel industry to a grinding halt and dented aircraft demand.

The Boeing and FAA crew will run methodically through scripted mid-air scenarios, progressing to more extreme maneuvers on a route over mainly Washington state, according to Reuters.

The flight plan may also include touch-and-go landings at the eastern Washington airport in Moses Lake and a path over the Pacific Ocean coastline, adjusting the course as needed for weather conditions and other factors, one source said.

The crew will also intentionally trigger the updated flight control software know as MCAS, which has been implicated in the two fatal crashes that killed 346 people, and will likely perform a full aerodynamic stall.

The flight tests are aimed at ensuring the new updates that Boeing made to the MCAS are sufficient to prevent the scenarios that pilots on the crash flights encountered.

Regulators and Boeing are also hoping to hold an international panel of airline pilots to test a proposed new training course for the 737 Max flight crews, possibly in late July and early August, according to Bloomberg.

Air travel restrictions due to the global pandemic have added uncertainty to how such sessions would occur.

Plans are being discussed to allow the group to perform its work remotely in flight simulators around the world, rather than flying to Boeing’s main training center in Miami, Florida, where the number of Covid-19 cases are rapidly increasing, Bloomberg said.

The test flights will come after more than a year of intense scrutiny of the jet certification process, multiple investigations of the plane and a federal criminal probe.