. this is "nightline." tonight, cleared to do fly, two years after a pair of deadly crashes. boeing 737 max returning to the skies. now the year-long investigation, how a flawed flight control system led to disaster. >> it was given the power to try to kill them essentially until it finally succeeded. >> and what a flight simulation may reveal about the final minutes. >> pull up. pull up. >> plus live science, from creating the largest nerf gun to showing how germs spread, why he is trending in the pandemic. and the holiday home coming that is worth the six-month wait.

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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 fire storm. now, nearly two years after being grounded, boeing 737 max with significant changes 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 on the market. saving them a lot of extra time and money. >> one of the big customers early on, was a young budget carrier in indonesia, called lion air. it was a huge deal. lion air had asked boeing about simulator training for 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, idiots. >> well, they were mocking them, because iflion air required simulator training, other airlines will 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 and it is rattling in the captain's hands. >> just ten minutes later, a tug boat captain was out on the job at sea, just east of 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 to 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 characteristics augmentation system. this system under certain circumstances can trigger in such a way that it will cause the ear plaairplane to nose dow. >> 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 no idea why this plane is going down against their will. so, they try to fide it back. >> 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, carrying 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 we see? >> in this airplane, while it's not a max, the displays are similar. what is unique about the max in this case is the mcas. >> shortly the takeoff, faulty data from the plane's angle tac sensor, triggers it. in the middle of the distractions, mcas suddenly activa 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 electronic power to the stabilizer in the plane's tail. >> now the problem is that the nose is so far down they are having trouble holding it back, yeah. >> and a last ditch effort pilots decided to turn the electronic stabilizer system back on. >> the mcas pirates and this fire goes and goes, i'm pulling 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 the 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 that's 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 the justice 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 recertification process. they say they take the findings seriously 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 simulator 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 team. coming up. how this former nasa engineer is taking science during the pandemic on a rocket ride to youtube stardom. just between us, cleaning with a mop and bucket is such a hassle. well i switched to swiffer wet jet and it's awesome.

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. with millions students stuck on virtual learning in the pandemic, one scientist is hoping to make science he has been going viral and breaking world records. >> this is the world largest nerf gun and this is the world's biggest kid. his name is mark. take one part literal rocket scientist, mix two parts youtube sensation and add in a load of messing around and he will have you feeling like a kid again, only smarter.

it's larger than life experiments like these, that have transformed mark in to a social media phenomenon, with 15 million subscribers on youtube, racking up 1.5 billion about views. >> growing up i had bill nye the science guy. and today, kids have mark rober, who is that? >> mark rober is excited about getting people stoked about science and engineering that is my m.o.i will suck people in with something big, world's largest nerf gun or largest super soaker. but at the end of the day, it's about telling them about the science of what is going on there. >> chemistry, math, my nightmare growing up. for mark, formulas and a accumen for innovation came to him. how did you first get curious about science? >> i just always loved mathity and science, physics and so, a lot of my videos come down to physics principals.

i'm trying to explain to people, this world is magical, magical in a way that if you understand how it works you can predict the future and make cooler thing-s. >> but mark is not your every day diy science guy. he has the resume to back up the experiments of astronomical proportions. i mean, long before, this was not your job. you were not a youtuber. you were a rocket scientist? >> yeah, so, i spent nine years at nasa, at the jet propu had lsion laboratories. seven of those were working on the curiosity recovery, something that i designed and tested, it's roving on the dot in the skien 90 million miles away. it's a cool feeling. >> here a relatively simple halloween costume. while he was sending aircrafts to space. he posted the science experiment to youtube. it got him nearly 10 million views. >> and that was a cool feeling. and i'm like, i should, i want to do this more. i should do it again, iright?

so i have done one video a month since that point. >> before long, he left behind, and i cannot stress it enough, a job at nasa. to make youtube videos full time. >> yeah, there was a survey that went out recently that was like, more kids want to be professional youtubers than astronauts. and everyone like, even i'm like, man. that's so lame, but then i was like, holdup, i quit my dream job at nasa to make youtube videos so i'm not one to judge. >> sounds crazy, right? the gamble paid off literally. now mark is making a living off of experiments that are often sponsored by major brands. here at his work shop, we got to mess around with gadgets. >> there you go. no, that is perfect. >> yeah! i'm having the time of my life and mark said it's all about education. he advocates for important

causes. trying to save the planet with super celebrities like bill gates. >> wow, that's pretty good. >> as a youtuber, as a scientist, as a dad, why is it important for you to be kind of advocating for the causes? >> it's weird to think that like i can tap a microphone and like 25 million people will listen. right? and so, i can't just sit by and not do something knowing that i could. right? and i have the power to do something about it. this is the world's largest nerf gun, i have kind of like opened it up here so you can see. >> but it's hard not to admit that being a big kid is in mark's dna. so mark loaded up the world's largest nerf gun, our producer john is sitting in the trampoline, he is going to get the nerf shot straight at him, are you ready, john? we pray we don't take you out, but if we do, sorry. are you ready? >> yeah, i'm ready. >> all right, say hello to my

little friend. >> oh! yeah! >> that was awesome. >> shot gun, mike. >> took the shot gun light off the camera. can we get a shot of that? >> the mic got taken straight off the camera. how are you feeling? >> fine. >> that is 40 miles an hour of pure toilet plunger. >> no producers were hurt in the making of the piece. but i do think we need some more space. and a drone. we definitely need a drone. >> oh! if 8-year-old mark rober could see you now, what would he think? >> he would be really stoked. >> for "nightline" we are in silicone valley. yee-haw! >> our thanks. up next, what a surprise, home for the holidays. so, what should we do today?

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>> she can't believe her eyes, the florida officer with the st. petersburg police department, face to face with her son, private second class davion oliver. the two have not seen each other since july when davion left for basic training, now reunited with the help of secret santas in blue. good for them. home is where the heart is. that is "nightline" for this evening. watch our full episodes on hulu, see you back here same time tomorrow, thank