How did a plane’s engine fall apart minutes into a flight from Denver?

A United flight bound for Hawaii had 231 passengers on it when some saw an engine breaking apart outside their window. The NTSB says there was evidence of metal fatigue.
8:35 | 02/27/21

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Transcript for How did a plane’s engine fall apart minutes into a flight from Denver?
Here's ABC's gio Benitez. You know, the boom, it was just a big jar. Like your body shook with it. Reporter: A loud bang. Chaos. And a prayer. That's what Brenda remembers. I did scream. People were gasping. Why me, why us? Reporter: The North Dakota mom, her husband, and three kids were on a last-minute vacation to Hawaii last Saturday when one of the engines on united flight 328 broke apart. It's got a blown engine! Oh no! Reporter: Massive pieces of metal raining down onto neighborhoods below. Mayday, mayday, united 328, heavy mayday, mayday, aircraft just experienced engine failure, need to turn immediately. Reporter: The incident raising alarms about what went wrong and leaving Americans wondering whether it's safe to fly. Raining down parts and seeing pictures like that, it's just absolutely amazing. This was kind of a last-minute trip. Last-minute idea. Reporter: The trip was supposed to be a relaxing break for the Dohn family. Brenda, husband Eric, and their kids, 15-year-old daughter Cameron, sons 12-year-old Eli and 10-year-old Jed. I'm a bit of a nervous flier. So once we landed in Denver, I thought, here we go. Reporter: Flight 328 took off from Denver at 12:49 mountain time, bound for Honolulu. 231 passengers on board. We took off. Everything seemed smooth. It seemed like a regular flight. Reporter: Barely 10 minutes later, the plane, still ascending, a terrifying sight. We were up near the clouds. That's when that explosion happened. And that's the only way I can describe it. It was just a huge boom. Reporter: Passengers on board filming the terror right outside their window. We started dropping down, like you could feel going down in an elevator, it's like a quick drop. Plane started shaking. All I kept thinking is -- this couldn't be my time. I thought, if this blows up, I want to be remembered. Reporter: They were in different rows. I bent down and grabbed my rosary, put it in my hand, and I grabbed my daughter's hand. And we held on to that rosary and I started praying. Reporter: She says it was motherhood and faith that kept her calm. I just saw debris flying at our window. And I decided it would be a good time to shut that window. Because I just didn't want her to be more terrified than she already was. My husband had just said, you know, we saw sparks of fire, and almost what looked to be fireballs flying at us. I'm glad I didn't see it. Reporter: Witnesses on the ground say they heard a loud boom then saw wrackage falling from the sky. Oh my god. Hang on, we're getting blown up with 911 calls. I'm sure you are. Debris falling everywhere -- Reporter: Others running to avoid being hit by plane parts. This giant piece part of the engine casing falling on Kirby Clemens' lawn. His house was spared but his truck was crushed. My wife and I were sitting in the living room, looked at each other, what was that? Looked out the front window and this great big piece just rolled past the front window, laying up against the tree. Reporter: The debris field at least a mile wide. Those enormous pieces now evidence turned over to the NTSB. The whole sky was just covered with that -- falling down like ash from a fire or Reporter: The pilots turned the plane around. Feel like there's a little black cloud of smoke -- Reporter: An agonizing 20 minutes before making an emergency landing back at the conference airport. Everyone on the plane cheered and clapped. The landing was rougher than normal, but no different than if you had been in bad wind or whatnot. Landed very good. I was lucky enough to talk to the pilot. While we were waiting to get off. I just had asked him, have you ever experienced anything like this before? He right away said no. Reporter: Now the first clues into what may have gone wrong. The NTSB saying it saw evidence of metal fatigue in one of the fan blades. Here you can clearly see a broken blade. You're a former NTSB investigator. What are the key questions that need to be answered here about this incident? The question is, was the fatigue crack just due to pure time in service? Or was it a fatigue crack due to some flaw that was there, either during the manufacturing or somehow damage that occurred later in its service history? When NTSB talks about fatigue cracks, it's just that, cracks that develop from too much use. These engines should last longer, but they seem to be breaking in ways that weren't anticipated when they were first built. Reporter: The engine, manufactured by Pratt and Whitney, was an older model not used in newer planes. But dozens of 777 planes still use the engine, and they've now been grounded. FAA ordering inspection of these fan blades before the planes can get back in the air. Fatigue crack that is occur in older engines are not easy to detect. You can't see them with the naked eye. It requires very sophisticated testing techniques. Reporter: The extent of the damage to the plane seen in these images from NTSB, a gaping hole in the fuselage created by debris from the engine. Debris that fortunately missed the fuel tank. The danger of an uncontained failure is when it throws hot, heavy metal outside of that engine casing and punctures the side of the airplane, which we saw here, which we saw in the southwest mishap. Reporter: That incident in 2018 killed a mom from new Mexico. The number one engine failure. Reporter: Southwest flight 1380 took off out of New York's Laguardia airport headed for about 25 minutes into the flight, with the plane at 32,000 feet over Pennsylvania, the left engine exploded. Everybody breathe, relax! Reporter: Sitting in the 14th row, right next to the window, Jennifer ryerton, on her way home from a business trip. She's -- had a very warm smile. I wish I'd talked with her more. Paula Mackay was in the aisle seat. A broken fan blade caused that explosion. Debris shattered the window and air pressure sucked her partly out the window. I put my hand on Jennifer's back, so if she was conscious or could feel anything, she would at least know we were there. If I ever had a conversation with her family, I would want to be able to tell them that she wasn't alone. Reporter: Pilots Tammy Jo Schultz and Darren ellisore credited with managing the emergency landing in Philadelphia. We did what we were trained for. People in the back of the plane were heroic. They paid for a seat in the back to relax. And they were up and at it to help. It's always stirring when you see heroics in other people. Reporter: Within days the FAA ordered emergency inspections of the fan blades on those engines made by another company, cfm. I don't have to tell you, people already have fears about flying during the pandemic. This doesn't help, does it? An incident like this, it makes it even more nervousing. Well, maybe she drive. However, I can tell you it's a lot safer to fly than to drive. Reporter: The last deadl commercial airplane crash in the U.S. Was in 2009. We got off the plane and we had to walk down some stairs. We did take a picture. It was just something we'll never go through again in our lives. Reporter: After the emergency landing back in Denver, the Dohn family made the choice to continue on with their vacation, hopping on another hawaii-bound flight just three hours later. So it was kind of like, well -- you get bucked off the horse, you've got to get back Reporter: The shock only wearing off once they landed. I had a really rough time sleeping that first night. It seemed like every time I shut my eyes, I kind of could feel that plane accelerating. I could kind of feel those jumps. I was glad to be in Hawaii, because I really think it would have been very difficult to get on the plane the next day. Reporter: Brenda, her husband, and the kids say they refuse to be scared. We've got a wild group of they're always willing for the next adrenaline rush. We're already talking about coming back next year. We're not going to stop from flying.

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

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