The Boeing 737 MAX was launched as a test flight by US regulators on Monday, a pivotal step in recertifying the jet that has grounded for more than a year following two fatal crashes.

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The plane took off from Boeing Field in Seattle at 1655 GMT, a Federal Aviation Administration spokesperson said.

The initial flight will last “several hours” and will be followed by additional trips expected to take about three days, the spokesperson said in a statement.

But though the flight is a critical step, the regulator stressed that the process for returning the once top-selling aircraft to the skies is not over.

“While the certification flights are an important milestone, a number of key tasks remain,” the FAA spokesperson said, adding that the agency will be “deliberate” and “thoroughly review Boeing’s work” before allowing the plane to return to service.

“We will lift the grounding order only after we are satisfied that the aircraft meets certification standards.”

The MAX has been grounded globally since March 13, 2019, following an Ethiopian Airlines crash that killed 157 people. That crash came just a few months after a Lion Air MAX crash that killed 189 people.

The similarities between the two accidents, both of which occurred shortly after takeoff, along with the pilots’ inability to regain control of the plane, led global aviation authorities to ground the model indefinitely.

For months, the US aerospace giant has been struggling to get the medium-haul aircraft back into service.

The MAX’s anti-stall flight system, the MCAS, was partially to blame for both crashes. But other technical malfunctions, including one involving electrical wiring, were subsequently detected during the aircraft’s modification process, slowing down its recertification.

Shares of Boeing opened sharply higher Monday and rose further following news the flight had gone off. Near 1630 GMT, Boeing shares were up 10.2 per cent to $187.26.

Civil aviation authorities won’t approve the modified model until they have examined how it performs in the air and looked at the thousands of data points collected during the test flights.

All MAX aircraft manufactured since the crashes also will need to be personally inspected by FAA staff, the agency said.

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“The FAA has not made a decision on return to service,” the agency said.

A few months ago, Boeing anticipated the MAX would be back in the skies in mid-2020. But the coronavirus pandemic, which resulted in travel restrictions and lockdown measures to try to slow its spread, upset the schedule.

Boeing needs to get the 737 MAX back in the air in order to pull itself out of a historic crisis.

The aircraft accounts for more than two-thirds of the company’s order book and is therefore crucial to the survival of the manufacturer — which, like the entire aviation industry, is suffering from the effects of the coronavirus crisis.

In March, credit rating agency S&P downgraded its debt grade for Boeing to BBB from A-, moving it into a speculative category.

Additional modifications on the MAX expected to be required by foreign aviation authorities also could add substantial costs to the program.