The Boeing 737 MAX returns to commercial service on Wednesday

The nearly two-year grounding of the Boeing 737 MAX is set to come to an end on Wednesday.

GOL, the largest domestic carrier in Brazil, is slated to resume commercial service with the troubled jet, with the first flights on routes to and from the company's hub in Sao Paulo.

The airline plans to have all seven of its MAXes back in the sky by the end of December.

In announcing the restart of commercial MAX operations, Celso Ferrer, vice president of operations at GOL said "our first priority is always the safety of our customers."

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Prior to returning the MAX to service, GOL, along with all other operators, must implement a software update designed to fix the MCAS system, which was partially blamed in two crashes of the jet that took 346 lives. In addition, pilots must undergo a recertification process, which includes a three-hour simulator-based training, running through a combination of emergency scenarios.

According to GOL, all 140 of its pilots have already undergone the required training. Plus, the company has operated some test flights with pilots and other employees before relaunching commercial service.

The countdown to the first commercial MAX flight has been ticking since Nov. 18 when the Federal Aviation Administration announced that it would lift the 20-month grounding order that kept the MAX parked while regulators worked on a fix for the plane's issues.

Immediately following the un-grounding order, every major U.S. carrier announced their intentions to restart MAX flights within months.

Of the three U.S. airlines with MAXes in their fleet, American Airlines has been the most ambitious with its plans to restart MAX service by the end of the year. The Fort Worth-based carrier is slated to re-inaugurate MAX service on Dec. 29 with a flight from Miami (MIA) to New York LaGuardia (LGA).

In fact, on Dec. 2, American flew a media delegation on the MAX on a roundtrip flight from Dallas/Fort Worth (DFW) to Tulsa (TUL) as the first public flight since the plane's grounding. Members of the media heard from the carrier's top brass of pilots and mechanics about how the carrier is bringing the plane back to the skies.

In the weeks leading up to Dec. 29, American is operating several employee-only charter flights with the MAX to "ensure our own team members are comfortable" flying the jet.

Related: A pilot and mechanic explain what’s needed to bring the 737 MAX back

Interestingly, both American and GOL, are citing the MAX's efficiency as one of the top reasons to bring it back so fast, especially during the demand downturn due to the pandemic.

In a recent Skift forum, American's president, Robert Isom said that the MAX "can really do things in an efficient way. From a sustainability perspective, the single best thing an airline can do is buy new planes — and we can't wait to get these MAXes in our fleet." According to the Brazilian airline, "the 737 MAX is critical to GOL's expansion plans due to the greater fuel efficiency and reductions in carbon emissions."

Other U.S.-based carriers aren't acting as fast as AA or GOL. Southwest plans to have its MAXes back in the sky in the second quarter of 2021, while United is gearing up to restart flights from its Denver and Houston hubs in the first quarter of 2021.

Alaska Airlines, which just announced that it's leasing 13 more MAXes in exchange for 10 A320s, plans its first MAX flight in March.