The US National Transportation Safety Board is co-ordinating with Pratt & Whitney to determine if metal fatigue caused two fan blades of a PW4000 engine to break off during United Airlines flight 328 on 20 February, causing the engine to catch fire. Fan blade fragments that broke off during the flight of the United Boeing 777-220 will be inspected for scratches on the blades that would indicate metal fatigue, NTSB chairman Robert Sumwalt said during a press conference on 22 February. The PW4000 engine will also be removed from the 777 and will be inspected by the NTSB in Washington, DC, where its maintenance history will also be reviewed. The 777 flight-data and cockpit-voice recorders will also be analysed in Washington, DC. "Our mission is to understand what happened and why it happened to keep it from happening again," Sumwalt said. The federal investigation has just begun, so Sumwalt shared with reporters what is known so far about the incident during which nobody was injured, even though engine debris both hit the aircraft and landed in a residential area. Two fan blades broke off from an engine during the United flight from Denver to Honolulu: one broke off at the root and the adjacent blade broke at mid-span. Fragments of one blade became lodged in an engine containment ring. Debris struck the aircraft leaving scratches and causing "minor damage" to the fairing that connects the wing to the fuselage but there was "no structural damage" that endangered the integrity of the aircraft, Sumwalt said. The engine failure caused the PW4000 to catch fire, so the flight crew engaged the emergency fire extinguisher and cut the fuel line to the engine, yet video taken by a passenger shows that it continued to burn. The NTSB is so far uncertain why these steps did not extinguish the fire. While the aircraft landed safely back in Denver after the engine failed, one of the large containment rings meant to catch debris during incidents landed in a soccer field in Broomfield, Colorado, Sumwalt said, adding that nobody was injured by the debris. The NTSB is also so far uncertain why the cowling broke off. Based on initial investigation, the NTSB does not consider this to be an uncontained engine failure according to the traditional definition, "since the ring contained the parts as they were flying out," Sumwalt said. The NTSB will look for similarities with past incidents of PW4000 engines. Two other PW4000 failures in recent years involved a Japan Airlines 777 in December 2020 and a United 777 over the Pacific Ocean in 2018. Investigators traced the 2018 incident to a blade failure. The PW4000 on a Korean Air 777 also caught fire in 2016.

The 20 February failure of a Boeing 777-200’s Pratt & Whitney PW4077 turbofan was apparently an uncontained failure, with images showing damage similar to a 2018 event. Still, details about the 20 February failure, which resulted in no injuries, are scarce and the investigation remains in the earliest phases. John Goglia, an aviation safety expert and former National Transportation Safety Board member, says images and video of the failure powerplant reveal the incident to be an “uncontained failure”. Goglia, who has extensive experience studying turbofan failures, sees evidence suggesting the engine’s containment ring – typically made of impact-resistant composite material – did not stop wayward components from exiting the powerplant. “It cut through… And the front section… came apart,” he says. The incident occurred shortly after the 26-year-old United 777 (registration N772UA) departed Denver as flight 328 to Honolulu. The right-side PW4077 failed a few minutes after the jet departed Denver, according to reports. The pilots made an emergency landing back in Denver 25min after take-off. The failure sent engine debris falling to the ground, with the engine’s nose cowl landing beside a house near Denver. Video of the damaged engine, taken from inside the aircraft, shows the powerplant’s forward cowl section missing, with flames coming from the engine’s mid and aft sections. P&W did not respond to a request for comment. The NTSB is sending local Denver-area staff to the site to begin an investigation, it says. “Extremely rare engine failures like this prove there is no substitute for experience, and that the most-important aircraft safety system is two well-trained, highly skilled, professional pilots at the controls on the flight deck,” says United’s pilot union Air Line Pilots Association, International. In recent years, several jets have suffered uncontained engine failures, which occur when high-energy components fling outward and are not stopped by the containment ring. Investigators, aviation safety regulators and aerospace manufacturers have been working to better understand and prevent such failures. One incident, on 13 February 2018, involved a PW4077 on a United 777-200 (registration N773UA) of nearly the same vintage as the jet involved in today’s incident. Both 777s completed first flights within weeks of each other in autumn 1994, according to Cirium fleets data. The 2018 failure occurred over the Pacific Ocean, as the 777 was inbound to Honolulu. That jet’s PW4077 also lost its cowling, including its forward cowl, during the failure, and engine components punctured the jet’s fuselage. The NTSB attributes the 2018 failure to “a full-length fan blade fracture”. Its report notes that P&W did not uncover blade fractures during a 2015 examination due to lack of formal and recurrent training for inspectors. In December 2020, a Japan Air Lines 777-200 suffered failure of a PW4074, an incident involving loss of a panel and fan-blade damage. That aircraft also landed safely. Other notable uncontained failures include two CFM International CFM56 events involving Southwest Airlines 737-700s – one in 2018 that killed one passenger, the other in 2016. And in 2017, an Air France Airbus A380’s Engine Alliance GP7200 suffered an uncontained failure over Greenland, investigators determined. 'MAYDAY. MAYDAY' United flight 328 departed on 20 February toward the west along Denver’s runway 25, at the airfield’s south-westerly corner, and began climbing to its assigned altitude of 23,000ft, according to air traffic control audio available at LiveATC.net. The first sign something was wrong was the pilot’s announcement to air traffic control just a few minutes later. “We experienced engine failure. We need to turn,” he says. Shortly after, the pilot declares an emergency, saying, ”Mayday. Mayday”. He continues, “Mayday. Mayday, United 328 heavy just experienced an engine failure, we need to turn immediately”. The controller vectors the aircraft to an easterly heading and clears it to descend to 9,000ft. One of the pilots then tells the controller that they “need to run some checklists”. They then ask which runway they can use to land. “Whatever runway you line up for and you want. Let me know and we’ll make it happen,” the controller responds. “You can have any runway you like.” The aircraft approached from the east, landing on Runway 26.

The US Federal Aviation Administration is ordering emergency inspections of some Pratt & Whitney PW4000-powered Boeing 777-200s in response to a 20 February in-flight uncontained engine failure. The event has also prompted United Airlines and Japan to ground PW4000-powered 777s. The National Transportation Safety Board (NTSB) says the 20 February incident involved two fractured fan blades. “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,” FAA administrator Steve Dickson says on 21 February. “This will likely mean that some airplanes will be removed from service.” “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,” the FAA adds. The agency is meeting “into the evening” with Boeing and P&W to finalise the AD and related service bulletins. “Exact details of the inspection will be specified in the emergency order,” the FAA adds. Airlines in only three countries – the USA, Japan and South Korea – operate 777s with affected PW4000 engines, according to the US regulator. Many of the jets have recently been removed from service in response to the industry downturn. Neither P&W nor Boeing immediately responded to a request for comment about the FAA’s move. At least three PW4000s have failure in recent years, including the dramatic failure of a United 777’s right-side PW4077 after take-off from Denver yesterday. That failure caused separation of the turbofan’s cowl, including its nose cowl, leaving the engine mangled and spewing flames. The United pilots landed the aircraft (registration N772UA), which was operating as flight UA328 to Honolulu, back in Denver. Debris from the engine rained down, with some components landing near Denver-area homes. None of the 229 passengers and 10 crew aboard were injured, says the NTSB. “We are voluntarily and temporarily removing 24 Boeing 777 aircraft powered by Pratt & Whitney 4000 series engines from our schedule,” United tweeted on 21 February. “We will continue to work closely with regulators to determine any additional steps and expect only a small number of customers to be inconvenienced.” “Safety remains our highest priority, which is why our crews take part in extensive training to prepare and manage incidents like UA328,” United adds. “Two fan blades were fractured,” says the NTSB. “One fan blade was fractured at the root.” “An adjacent fan blade was fractured about mid-span,” the NTSB adds. “A portion of one blade was imbedded in the containment ring. The remainder of the fan blades exhibited damage to the tips and leading edges.” The agency says “most of the damage was confined to the number two engine. The airplane sustained minor damage”. A Denver-area NTSB investigator began the inquiry shortly after the incident, working with local law officials to recover engine “components that separated from the engine, many of which landed in residential areas”. The engine’s inlet and cowling separated from the powerplant, the NTSB confirms. The agency has sent a lead investigator and a powerplant expert to Denver, and the jet’s flight-data and cockpit-voice recorders were sent to Washington, DC for analysis. Two other PW4000 failures in recent years involved a JAL 777 in December 2020 and a United 777 over the Pacific Ocean in 2018. Investigators traced the 2018 incident to a blade failure. Six airlines globally operate 60 777s with PW4000s. Those include the United aircraft, 20 jets operated by Japanese carriers (11 by ANA and nine by JAL) and 18 operated out of South Korea (seven by Asiana, seven by Korean Air and four by Jin Air).