The story that swept airlines in 2021

This is the story of Boeing 737 Max that started flying in the sky, ending a 20-month grounding since 2019 after two fatal mayhems. That's what about the airlines and Boeing wanted as most people shrugged it. 

Notwithstanding, the aircraft is a crucial piece of hardware that airlines are relying on to help them climb out of COVID-19.

The 737 Max was grounded in March 2019 after fatal crashes in Indonesia and Ethiopia killed 346 people.

The accidents eventually were linked to the airplane's flight control software, and Boeing came under fire for taking shortcuts in developing the plane. After Boeing modified the software,the U.S. 

Federal Aviation Administration (FAA) re-approved the plane in November 2020. Regulators around the world followed suit throughout this year, the largest holdout being China, which has signaled it will re-certify the aircraft in 2022.

The plane had just returned to the skies in the U.S. and a few select countries at the beginning of the year. In fact, there were only 46 Max-operated flights globally on January 1, 2021 - a fraction of 1 percent of global flight activity - according to Cirium schedules. 

And that left airlines with a lot of unknowns: Would travelers return to the jet? And, could carriers bet on aircraft for their planned summer schedules and ambitious climate goals?