Commercial flights with the troubled Boeing 737 MAX jetliners resumed on Wednesday for the first time since they were grounded worldwide since March 2019, following two deadly crashes that killed 346 people.

Brazil’s Gol Airlines was the first in the world to put paying customers on the plane again since the Federal Aviation Administration approved its return to flight in November. A 737 Max 8 ferried passengers from Sao Paulo to Porto Alegre, according to The Associated Press.

Gol Airlines told the publication it plans to use the 737 Max in regular service starting later this month, and passengers who don’t want to fly on the plane will be able to exchange their tickets. 

Other airlines are getting ready to bring the plane back into their fleets, too. American Airlines has said it plans to reintroduce the 737 Max to its fleets by first flying without passengers and then slowly restoring commercial service starting at the end of the year. Southwest Airlines, which made the biggest bet on the 737 Max when it was introduced a few years ago, says it will restart commercial flights no sooner than the second quarter of 2021. 

One thing holding up a wholesale return to the skies for the 737 Max is airlines must put pilots through new FAA-approved training specific to the problem that doomed the flights in 2018 and 2019. Planes that were in storage also need to be “depreserved,” or brought back into flying shape, and updated with new wiring and software.

The two deadly crashes in late 2018 and early 2019 were caused in part because Boeing originally obscured the need for training on a tricky new piece of the 737 Max’s software. While trying to keep pace with rival Airbus’ fuel-efficient A320neo aircraft, Boeing had redesigned the prior 737 to use bigger, more economical engines. But the placement of those engines could cause a stall in certain takeoff situations. So Boeing developed software known as Maneuvering Characteristics Augmentation System (MCAS), which counteracted the stall by automatically pushing the plane’s nose down. 

In an effort to save time and money, though, the company didn’t tell the FAA or customers about MCAS. That meant the pilots of the two fatal crashes were fighting a system they didn’t even know existed.

THE VERGE