The US aviation regulator has started test flights of Boeing’s troubled 737 Max, more than a year after the planes were grounded worldwide after two deadly accidents.

The test flights, which involve pilots and test crew members from Boeing and the US Federal Aviation Administration, are expected to take place over several days.

BBC News is reporting that the FAA said the first flight took off from Seattle on Monday. It was expected to take several hours.

The Max, Boeing’s best-selling aircraft, was grounded in 2019 after crashes in Ethiopia and Indonesia that killed all 346 people on the planes.

The groundings led to Boeing’s worst-ever corporate crisis, which has been compounded in 2020 by the COVID-19 pandemic that has slashed air travel and jet demand.

The grounding of the 737 Max – Boeing’s best-selling plane – in March 2019 after the fatal Lion Air and Ethiopian Airlines crashes triggered hundreds of lawsuits, investigations by US Congress and the US Department of Justice and cut off a key source of the company’s cash.

Investigators blamed faults in the stall-prevention software known as MCAS. Since that revelation, Boeing has spent months overhauling the system to meet new safety demands.

On Monday, a Max plane finally returned to the sky. After a preflight briefing that lasted several hours, crew boarded a 737 Max 7 outfitted with special test equipment at Boeing Field near Seattle.

They were to run methodically scripted mid-air scenarios such as steep-banking turns, progressing to more extreme manoeuvres on a route primarily over Washington state.

The flight plan might 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 of the people involved said.

Pilots will also intentionally trigger the MCAS system, and were likely to perform a full aerodynamic stall, the people said.

The tests are meant to ensure that new protections Boeing added to the flight control system are robust enough to prevent the scenario pilots encountered in both crash flights, when they were unable to counteract the system and grappled with several factors like “stick shaker” column vibrations and other warnings, one of the people said.

Even if these first tests go well, months of further safety checks will be needed before the Max planes can return to general service.

-with AAP