How the Boeing 737 Max returned to the skies after 2 deadly crashes kill hundreds

The Boeing 737 Max was approved to fly again after the company modified its flawed flight control system, which pilots initially knew nothing about and had caused some aircraft to nosedive.
10:53 | 12/29/20

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Transcript for How the Boeing 737 Max returned to the skies after 2 deadly crashes kill hundreds
Upgrade to Febreze Fade Defy Plug. Good evening and thank you for being with us, grounded no more, the 737 max set to take off. The crash fresh in the lives of loved ones, demanding accountability. It was a brand new model called the Boeing 737 max-8. They didn't understand the danger that they were walking in to. What started out as routine flights. Pull up. Quickly became a pilots worst nightmare. The plane had flown in to the sea at full speed. With two airplanes down and 346 people killed by 2019, Boeing becoming the focus of a global storm. Now, nearly two years after being grounded, Boeing 737 max with significant Chang is set to carry U.S. Passengers once again. The max was born out of Boeing's need to keep up with the closest competitor. Air bus. Instead of developing a new plane, they decide to revamp a model the market. Saving them a lot of extra time and money. One of the big customers early on, was a young budget carrier Indonesia, called lion air. It was a huge deal. Lion air had asked Boeing about simulator training their pilots and Boeing dissueded them from this. Now, according to internal Boeing documents, one of their employees ridiculed the request in a message to a colleague saying now, lion air might need a sim to fly the max and maybe because of their own stupidity I'm scrambling trying to figure out how to unscrew this now, Well, they were mocking them, because iflion air required simulator training, other airlines say, if they are requiring it, maybe we need it and then the whole house of cards would start to fall apart. On the early morning of October 29th, 2018 at the airport in Jakarta, the capitol of Indonesia, lion air flight 610 was getting ready to take off, the destination was the city of pinang. Shortly the flight takes off, you know, the pilots are struggling with the plane. The stick shaker pops off. It's loud T is rattling in the captain's hands. Just ten minutes later, a tug boat captain was out on the job at sea, just east Jakarta. When news of the crash broke, I immediately flew in from beijing, where I was based at the time. So the sun has just gone down and you see the police officers on the dock. The crews have been pulling up and carrying out debris piece by piece. There was no way anyone could have survived. And we thought that it was pitching up and down, down and up, and down and up it's up and down for about 24 times. It was revealed that pilots were rifling through the pilot Manuel, trying figure out what to do. Turns out that Boeing had installed a flight control system on the 737 max that had the power to take control of the plane from the pilots without them even knowing about it. The program was called mcas. Maneuvering character augmentation system. This system under certain circumstances can trigger in such a way that it will cause the airplane to nose down. They put new engines on an old air frame. And that changed the way the airplane flew. And the plane starts to want to pitch up all by itself. The idea was to provide something that would offset that pitching moment or causing the nose to come up. The pilots have Noa why this plane is going down against their will. So, they try to fideback. Yes, and the way mcas was designed, it was given the control authority to on do it as many times as necessary. In other words, it was given the power to try to kill them essentially until it finally succeeded. Just a few days later, Boeing the tells operators for the first time that there's a new and previously undisclosed system on the aircraft. And that is mcas. That was a shock. It's not in our manuels at all, except in one section, our abbreviation section. It meant nothing to it. Pilots were furious. Pilot unions said, why didn't you tell us that you put something on the airplane that is going to interfere with my job? The bottom line is the 737 max is safe. And the safety is a core value for us at Boeing. But then, four months later, another fatal crash. This time in ethiopian airlines flight,rying 157 passengers. First, Indonesia, and now Ethiopia. The big question here is could the same thing happen? The faulty angle of semsor triggered mcas right after takeoff and that is almost exactly what happened on the lion air aircraft. Despite similarities between the two crashes, this time there was a huge difference. Not only that the pilots knew about mcas, they followed Boeing's instruction how to disable it. To better understand why those pilots could not recover control of the plane, we asked two experienced American airlines pilots and members of the allied pilots association to take us through what might have occurred in the ethiopian airlines cockpit, based on information from the black boxes. In here, what do W see? In this airplane, while it's not a max, the displays are similar. What is unique about the max in ase is the mcas. Shortly thetakeoff, faulty data from the plane's angle of tac sensor, triggers it. In the middle of the distractions, mcas suddenly activates. Mcas is firing. Starts to push the nose down. I'm pulling back. It was at this point that the ethiopian airlines pilots followed Boeing's instructions, and cut off stabilizer in the plane's Now the problem is that the nose is so far down they are having trouble holding itck, And a last ditch effort, the pilots decided to turn the electronic stabilizer system back on. The mcas pirates and this fire goes and goes, I'm pul back, pulling back, a bit of trip. And now, the nose is pointing down and it's over. Stop the sim. All of us at Boeing are deeply sorry tore the loss of life in ethiopian's airlines flight 302 and lion air flight 610 accidents. He is sorry that we lost our family members doesn't take accountability for is having caused their deaths. Their 25-year-old daughter was on ethiopian airlines flight. She and her husband quickly turned their anguish in to action. We flied to Chicago and filed the first lawsuit against Boeing by an American family. I want her death to not be in vein, I don't want anybody else to die. Natty and Michael and more than 100 other families of victims around the world allege that max had a defective design and Boeing issued inadequate warnings about the plane. Something that Boeing denies. Lawmakers held congressional hearings in to what went wrong with the 737 max. The committee uncovered e-mails and communications showing there was lower level employees who raised questions several times about mcas. These are questions of life and death. But not how they treated them in the company. Some Boeing employees openly questioned the safety culture of the company can. During development of the 737 max. In one e-mail, a worker said, frankly, right now, all my internal warning bells are going off and for the first time in my life, I'm sorry to say that I'm hesitatent about putting my family on a Boeing airplane. The safety was evicerated under pressure. They have since fired Dennis mule enburg their CEO and they currently face a criminal investigation led by theustice department. The investigation is focused on whether Boeing withheld information from regulators. With the max set to return to U.S. Commercial service tomorrow, clouds of suspicion over Boeing remain. Earlier this month, a report released by senate investigators found that Boeing officials in appropriately coached FAA test pilots during the max's recertification process. They say they take findings riously and will review the report in full. However, the company has made a number of changes to the max plane. Mcas will rely on two sensors and will only activate once. And pilots will have to complete flight simulaor training and Boeing said they have made a series of meaningful changes to strengthen the safety practices and culture. I don't want other mothers and families to go through this had. I don't want them to be trying to hear their daughter's voice and you know, wonder what you would say to them. People should not innocently buy an airplane ticket and then be in a flying coffin. Our thanks to Bob and the

This transcript has been automatically generated and may not be 100% accurate.

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