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PICTURED - The broken fan blade that caused the Denver air explosion: Engine piece snapped off mid-flight because of 'wear and tear', sparked fire and fell onto soccer field, investigators say

 The mid-air engine fire that caused debris from a Boeing 777 to rain down over Denver on Saturday was caused by a damaged fan blade suffering from 'metal fatigue', officials have announced in preliminary findings.

National Transport Safety Board (NTSB) Chairman Robert Sumwalt said during a virtual press conference Monday there was a 'loud bang' and vibrations coming from the right engine of United Flight 328 about two minutes after take-off from Denver International Airport on Saturday.

With the plane at around 12,000 feet, Sumwalt said two blades of its Pratt & Whitney engine suddenly broke - with one fracturing at the base where it meets the hub, and the second snapping off mid-way. 


The first blade caused 'overload' damage to the second blade, Sumwalt said. One of those blades was later found in a soccer field, and the other remained lodged in the engine.

According to the NTSB, a preliminary on-scene exam of the engine indicates 'damage consistent with metal fatigue.'  

Pictures of the aircraft in a hangar at Denver International Airport yesterday show the damage to the starboard engine up close. 

Damage to the wing and the body fairing of the Boeing 777-200 is also visible, suggesting some of the engine debris struck the plane itself. 

The recovered parts are being flown to a Pratt & Whitney laboratory and will be examined on Tuesday under the supervision of National Transportation Safety Board investigators, who will also probe maintenance records to see whether problems were overlooked during inspections. 

'Our mission is to understand not only what happened but why it happened so that we can keep it from happening again,' Sumwalt said. 

Damaged fan blades can be seen on the starboard engine of United Airlines flight 328 after its emergency landing

Damaged fan blades can be seen on the starboard engine of United Airlines flight 328 after its emergency landing

Damage to the wing and the body fairing of the Boeing 777-200 is also visible

Damage to the wing and the body fairing of the Boeing 777-200 is also visible

The damaged starboard engine of the Boeing 777 involved in the near-disaster on board United Airlines Flight 328 is seen in a hangar at Denver International Airport on Monday

The damaged starboard engine of the Boeing 777 involved in the near-disaster on board United Airlines Flight 328 is seen in a hangar at Denver International Airport on Monday 

Saturday's near-disaster has also led to more than 100 Boeing 777s which use the same engines being grounded around the world, while the FAA has called for ramped-up checks on Pratt & Whitney's fan blades. 

The damaged-blade theory is reminiscent of a 2018 disaster on board a Southwest Airlines flight, when a broken fan blade triggered an explosion that smashed a window, killing a woman who was blown outside. 

Investigators will compare the Denver mishap with the 2018 accident, but Sumwalt said it was not yet clear whether Saturday's failure was consistent with that disaster. 

'What is important that we really truly understand the facts, circumstances and conditions around this particular event before we can compare it to any other event,' he said Monday.  

Sumwalt said the engine involved in Saturday's incident had a 'containment ring' which did manage to hold in some of the parts as they were flying out. 

He also acknowledged the aircraft's underbody was damaged, but clarified that the damage was only cosmetic and not structural. The damaged part is a composite fairing that smooths out the plane to make it more aerodynamic, Sumwalt said.

While the early findings suggest that metal fatigue is a likely explanation for the fan blade failure, full-scale NTSB inquiries can take a year or longer to complete. 

Both the FAA and the NTSB have been so far unable to determine whether the engine that failed Saturday was an original part on the 26-year-old Boeing 777, or whether it was installed later on.  

Video recorded by passengers aboard Flight UA328, which was carrying 231 travelers and 10 crew members, shows the engine on fire

Video recorded by passengers aboard Flight UA328, which was carrying 231 travelers and 10 crew members, shows the engine on fire

Parked: Boeing 777s stand at a production facility in Everett, Washington on Monday as more than 100 of the aircraft were grounded worldwide because of Saturday's near-calamity in the skies over Denver

Parked: Boeing 777s stand at a production facility in Everett, Washington on Monday as more than 100 of the aircraft were grounded worldwide because of Saturday's near-calamity in the skies over Denver 


The fallout from Saturday's incident has led to 69 in-service planes and another 59 in storage being grounded in the US, Japan and South Korea, the only countries with planes using this particular engine. 

United Airlines, the only US carrier with affected planes, said it grounded 24 Boeing 777s and 28 others will remain parked. 

Japanese regulators ordered Japan Airlines and All Nippon Airways to ground 32 planes, and South Korea's Korean Air and Asiana Airlines said Monday they will ground their Boeing 777s.

FAA chief Stephen Dickson said inspectors quickly determined that inspections should be done more frequently for the type of hollow fan blades used in some Pratt & Whitney engines. 

Safety experts were alarmed because debris blew off the disintegrating engine, creating shrapnel that can damage key systems like hydraulic lines or hit the passenger cabin. 


'That was a substantial hit,' said John Goglia, a former NTSB member. 'If that had hit the wing, things might have been different because the wing is full of fuel' and the broken engine was still on fire. 

Investigators will also look at why the cowling, which covers the front of the engine, broke off, and also why there fire a fire despite indications fuel to the engine had been turned off, in keeping with safety protocol.

Sumwalt says the fire handle in the cockpit was activated and two fire bottles in the engine were discharged. He said investigators will seek to determined what continued to fuel the fire, despite precautions being taken.  

Todd Curtins, a former Boeing engineers, said the engine fire could indicate a fuel leak.  

Mid-air panic: United Airlines Flight 328 on its way back to Denver International Airport on Saturday with its starboard engine on fire following the explosion shortly after takeoff

Mid-air panic: United Airlines Flight 328 on its way back to Denver International Airport on Saturday with its starboard engine on fire following the explosion shortly after takeoff 

Grounded: Aircraft belonging to All Nippon Airways, a Japanese carrier which uses the same Pratt & Whitney engines on board its Boeing 777s, are seen in a file photo at Tokyo's Haneda Airport

Grounded: Aircraft belonging to All Nippon Airways, a Japanese carrier which uses the same Pratt & Whitney engines on board its Boeing 777s, are seen in a file photo at Tokyo's Haneda Airport 

A map showing the Denver suburb of Broomfield in relation to the airport where the United Airlines flight had taken off and subsequently made an emergency landing

A map showing the Denver suburb of Broomfield in relation to the airport where the United Airlines flight had taken off and subsequently made an emergency landing 


United Flight 328 was heading to Honolulu on Saturday from Denver International Airport when the plane's right engine blew shortly after takeoff, causing it to erupt into flames.

The captain had been giving an announcement over the intercom when a large explosion rocked the cabin, accompanied by a bright flash. Passengers recalled their horror as they looked out the window to see engine casing and chunks of fiberglass falling from the plane, and thick black smoke emanating from the wing.

The incident forced the pilot to pull off an emergency landing back in Denver just 20 minutes after take-off, at around 1.30pm local time.

Video recorded aboard Flight UA328 captured the moment it touched back down on the runway safely, prompting the cabin to erupt in applause and cheers of relief.

The NTSB said the cockpit voice recorder and flight data recorder were transported to its lab in Washington for the data to be downloaded and analyzed. 

NTSB investigations can take up to a year or longer, although in major cases the agency generally releases some investigative material midway through the process. 

Former National Transportation Safety Board Chairman Jim Hall called the incident another example of 'cracks in our culture in aviation safety [that] need to be addressed'.

Hall, who was on the board from 1994 to 2001, has criticized the FAA over the past decade as 'drifting toward letting the manufacturers provide the aviation oversight that the public was paying for'.  

Investigators will compare Saturday's incident, in which the 231 passengers and 10 crew were unhurt, with previous mishaps including the 2018 disaster in which a woman was killed. 

The same year, another United Airlines Boeing 777 suffered an engine failure that caused parts of the housing to break off and fall into the Pacific Ocean as the plane flew from San Francisco to Honolulu.

In a report last year on the incident, the NTSB said Pratt & Whitney missed signs of cracking in previous inspections of the fan blade that broke, and faulted the company's training. The company told the NTSB it was fixing the shortcomings. 

In December last year, a Japan Airlines Boeing 777 with the same Pratt & Whitney engines suffered fan blade damage and lost a large panel. 

The plane returned safely to Naha with none of the 178 passengers and 11 crew injured, but officials said at the time that the left engine experienced a malfunction at approximately 16,000-17,000 feet. 

And on Saturday, hours before the Denver flight, a Boeing 747 cargo plane in the Netherlands suffered an engine failure that resulted in engine parts falling to the ground. 

Although the plane has Pratt & Whitney engines, they are different from those on Boeing 777s, and nothing yet shows a link to the problem on the United plane, said a spokeswoman for the European Aviation Safety Agency.  


The announcement came after the Federal Aviation Administration (FAA) said it would require stepped-up inspections of 777 aircraft with Pratt & Whitney PW4000 series engines after the right engine failure on United Flight 328

The announcement came after the Federal Aviation Administration (FAA) said it would require stepped-up inspections of 777 aircraft with Pratt & Whitney PW4000 series engines after the right engine failure on United Flight 328 


Pratt & Whitney, a subsidiary of Raytheon, is one of the major players in the aircraft-engine market along with Boston-based General Electric and Britain's Rolls-Royce. 

Cai von Rumohr, an aviation analyst with Cowen, said events around Saturday's flight would be a bigger issue for Raytheon than for Boeing, whose reputation was battered by two deadly crashes in 2018 and 2019.   

Boeing has said the aircraft should be taken out of service until federal regulators had determined an inspection procedure.   

'While the NTSB investigation is ongoing, we recommended suspending operations of the 69 in-service and 59 in-storage 777 aircraft powered by Pratt & Whitney 4000-112 engines,' the company said. 

Japan has requested airlines avoid using Boeing 777 planes with Pratt & Whitney 4000 engines for take-offs, landings and overflights in its territory until further notice, authorities said. 

In Britain, transport secretary Grant Shapps announced on Monday morning that all Boeing 777s with PW4000 engines will be banned from flying over British airspace until further notice.  

Pratt & Whitney meanwhile says it is 'actively co-ordinating' with planemakers and federal regulators, adding that it had dispatched a team to work with investigators looking at what went wrong on Saturday's flight.    

Flames could be seen coming from the engine of the plane after it exploded at 15,000 feet

Flames could be seen coming from the engine of the plane after it exploded at 15,000 feet 

Terrified United Airlines passengers clapped in relief as their flight touched down safely in Denver on Saturday after suffering catastrophic engine failure
Remarkably, there were no injuries reported either on board the flight or on the ground

Terrified United Airlines passengers clapped in relief as their flight touched down safely in Denver on Saturday after suffering catastrophic engine failure


Greg Feith, a former NTSB air safety investigator, expressed his concern over not only how the engine came to be damaged in the first place, but that it caught fire and continued to burn until it landed

Greg Feith, a former NTSB air safety investigator, expressed his concern over not only how the engine came to be damaged in the first place, but that it caught fire and continued to burn until it landed

The Broomfield Police Department posted photos on Twitter showing large, circular pieces of debris leaning against a house in the suburb about 25 miles north of Denver

The Broomfield Police Department posted photos on Twitter showing large, circular pieces of debris leaning against a house in the suburb about 25 miles north of Denver 

Cops in Broomfield responded to reports of objects falling from the sky on Saturday afternoon and saw huge metal objects in front lawns

Cops in Broomfield responded to reports of objects falling from the sky on Saturday afternoon and saw huge metal objects in front lawns

On Sunday, the FAA had said it would require stepped-up inspections of all the 777s with Pratt & Whitney PW4000 series engines. 

FAA administrator Steve Dickson said: 'After consulting with my team of aviation safety experts about yesterday's engine failure aboard a Boeing 777 airplane in Denver, I have directed them to issue an Emergency Airworthiness Directive that would require immediate or stepped-up inspections of Boeing 777 airplanes equipped with certain Pratt & Whitney PW4000 engines.

'This will likely mean that some airplanes will be removed from service,' he added. 

Dickson said that his team has 'reviewed all available safety data following [the] incident,' and 'based on the initial information, we concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes'. 

According to Dickson, the FAA 'is working closely with other civil aviation authorities to make this information available to affected operators in their jurisdictions'. 

He said his team will be meeting with Pratt & Whitney and Boeing 'to finalize the details of the Airworthiness Directive and any accompanying service bulletins to ensure that the appropriate airplanes are included in the order'. 

This image provided by KCNC-TV in Denver shows the damage done when debris fell through the roof and into the kitchen of a home in Broomfield, Colorado on Saturday

This image provided by KCNC-TV in Denver shows the damage done when debris fell through the roof and into the kitchen of a home in Broomfield, Colorado on Saturday

Pieces of the aircraft landed on a football field as seen in the above image posted to Twitter by a local resident in Broomfield

Pieces of the aircraft landed on a football field as seen in the above image posted to Twitter by a local resident in Broomfield

Prior to Flight UA328 landing safely on Saturday, large chunks of debris had fallen from the plane on the Denver suburbs below, narrowly missing homes and other buildings. 

Denver resident Kirby Klements was inside his home with his wife when they heard a huge booming sound.

A few seconds later, the couple saw a massive piece of debris fly past their window and into the bed of Klements' truck, crushing the cab and pushing the vehicle into the dirt. 

Another local resident, Kieran Cain, told CNN he was with his children at a nearby elementary school when the aircraft flew over. Seconds later, they heard a loud explosion.

'We saw it go over, we heard the big explosion, we looked up, there was black smoke in the sky,' Cain toldsaid.

'Debris started raining down, which you know, sort of looked like it was floating down and not very heavy, but actually now looking at it, It's giant metal pieces all over the place.

'I was surprised that the plane sort of continued on uninterrupted, without really altering its trajectory or doing anything,' he said.

'It just kind of kept going the way it was going as if nothing happened.'

Flight 328 passenger David Delucia recalled for the Denver Post how he grabbed his wife's hand after hearing the explosion, telling her: 'We're done for.'

'The plane started shaking violently, and we lost altitude and we started going down,' Delucia, who sat directly across the aisle from the side with the failed engine, said. 

'When it initially happened, I thought we were done. I thought we were going down. I thought we were going to die at one point,' he said, adding that he and his wife took their wallets containing their driver's licences and put them in their pockets so that 'in case we did go down, we could be ID'd'.  

Police in Broomfield released this photo showing debris from the United Airlines aircraft strewn across a football field

Police in Broomfield released this photo showing debris from the United Airlines aircraft strewn across a football field

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