US regulators have launched a test flight of Boeing's 737 MAX plane, a key step in recertifying an aircraft grounded for 14 months following two fatal crashes.

A MAX plane took off from Boeing Field in Seattle at 1655 GMT (5.55pm Irish time), a Federal Aviation Administration (FAA) spokesperson said.

The initial flight will take "several hours" and will be followed by additional trips expected to take about three days, the spokesperson said.

Boeing Field is the manufacturer's birthplace in Washington state.

The MAX has been grounded globally since 13 March following an Ethiopian Airlines crash that killed 157 people. That catastrophe 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 aviation giant has been struggling to get its medium-haul aircraft - whose sales were its main source of revenue before the grounding - back into service.

The model'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.

For weeks, Boeing has been awaiting the green light from authorities to conduct test flights to prove the modifications provide maximum safety.

Civil aviation authorities cannot approve the modified model until they have examined how it performs in flight. They will also look at the thousands of data points collected during the flights.

The New York Times said an FAA pilot will be at the controls to test out the modifications conducted on the plane, and a Boeing test pilot will also be on board.

In general, test flights are meticulously prepared for.

A few months ago, Boeing anticipated the MAX would return to service in mid-2020, around June. 

But the coronavirus pandemic, which resulted in travel restrictions and lockdown measures to try and slow its spread, has upset the schedule.

According to The Seattle Times, European and Canadian aviation authorities have demanded substantial new changes to the plane's flight control system.

The two regulators, along with the FAA "have agreed Boeing will be required to make these additional design changes only after the MAX returns to service," the newspaper reported.

When asked for further details, a Boeing spokesperson said Friday that safety is the company's top priority.

The spokesperson also said that Boeing is committed to answering regulator questions and meeting all certification and regulatory requirements.

Boeing urgently 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 mid-term survival of the manufacturer - which, like the entire aviation industry, is suffering from the effects of the coronavirus crisis.

At the end of April, Boeing released details of a downsizing plan to cut total headcount by 10%, or roughly 16,000 employees in all.

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

The additional modifications required by foreign aviation authorities could add substantial costs to the MAX programme.

They could also slow the increase in deliveries that Boeing would need to rebuild its cash flow.