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The US aviation regulator has started formal test flights for Boeing's troubled 737 Max.
The test flights, which will run through a series of potential emergency scenarios, are expected to take place over several days.
Boeing's best-selling aircraft was grounded last year after two crashes killed all 346 people on the flights.
The tests are a milestone for Boeing, but even if they go well, months of further safety checks will be needed.
Aviation regulators grounded the 737 Max about 15 months ago following two crashes - a Lion Air flight and an Ethiopian Airlines flight - within five months of each other.
The ruling triggered a financial crisis at the 103-year-old company, sparked lawsuits from victims' families, and raised questions about how Boeing and the US regulator, the Federal Aviation Administration (FAA), conducted their safety approval process.
Investigators blamed faults in the flight control system, which Boeing has been overhauling for months in order to meet new safety demands.
Carried out by FAA pilots, the tests will evaluate key systems, including a revised version of the flight control software which was implicated in the two accidents.
The original system is believed to have overridden the pilots' commands and forced both aircraft into catastrophic dives.
Image copyright Getty Images Image caption Ethiopian Airlines' flight ET302 crashed shortly after take off in March 2019The FAA told BBC News that the first certification flight took off from Seattle on Monday. The flight is expected to take several hours.
Test flights had been planned for last year, but investigations uncovered an array of new safety issues that have delayed a return to service.
It could take weeks to analyse data from the test flights. But even if this process is successful, further flying, training of pilots, and clearance from European and Canadian regulators will be needed.
The European Aviation Safety Agency has maintained that clearance by the FAA will not automatically mean a clearance to fly in Europe.
Norwegian Air, TUI, and Icelandair are among airlines using the 737 Max in Europe, while other carriers have the aircraft on order.
Boeing declined to comment.
