On February 20, 2021, United Airlines Flight 328, a scheduled domestic flight from Denver to Honolulu, suffered a contained engine failure shortly after takeoff from Denver International Airport (DEN). The aircraft, a Boeing 777 powered by Pratt & Whitney (P&W) model PW4077 turbofan engines, experienced a fan blade separation due to metal fatigue causing an engine fire and extensive damage to the nacelle. Despite being classified as a contained failure, as the fan blade fragments remained inside the nacelle, large parts of the engine's cowling, inlet and thrust reverser detached, creating a debris field over 1 mile (1.6 km) long across residential areas of Broomfield, Colorado.
The falling debris damaged private property, including the roof of a home and a parked vehicle. Witnesses captured footage of falling debris on smartphones and a dash cam, while passengers recorded video of the damaged engine and posted it to social media. The fuselage sustained minor damage, but the crew was able to shut down the affected engine and return safely to Denver, landing on runway 26 at 1:28 pm MST (06:28 UTC), 24 minutes after departure. No injuries were reported among the 231 passengers and 10 crew, or on the ground.
The U.S. National Transportation Safety Board (NTSB) opened an investigation into the incident. In response, the U.S. Federal Aviation Administration (FAA) issued an Emergency Airworthiness Directive requiring immediate inspection of Pratt & Whitney PW4000-series engine fan blades before further flight. Similar 777-200 aircraft were temporarily grounded by multiple aviation regulators around the world. Japan Airlines, which had experienced a similar engine issue in December 2020, retired its PW4000-powered 777-200s earlier than planned in March 2021. United Airlines, which also had a similar incident in February 2018, grounded its fleet of PW4000-powered 777-200s from early 2021 until July 2022.
Background
Aircraft
The aircraft involved, registered as N772UA, is a Boeing 777-222, the United Airlines-specific variant of the original 777-200 series. It was built in November 1994 (c/n 26930/Line no.5) and delivered to United in September 1995. Originally the aircraft was designated WA005 (a Boeing reference number), one of the original Boeing 777-200s that took part in the flight test certification program prior to its entry into commercial service. Boeing stopped building the 777 with P&W PW4000 series engines in 2013.
The Boeing 777 is a long-range, wide-body, twin-engine aircraft. At the time of the incident it had a relatively low accident fatality rate. The only two 777 crashes with total loss of aircraft (passengers and crew) are Malaysia Airlines Flight 17 that was shot down over Ukraine on July 17, 2014, and Malaysia Airlines Flight 370 that disappeared on March 8, 2014. The other fatal accidents, Emirates Flight 521 and Asiana Airlines Flight 214, were both attributed to pilot error. Two other hull losses with passenger injuries occurred: EgyptAir Flight 667 had a cockpit fire while parked at the gate at Cairo Airport, and British Airways Flight 38 crashed on landing at Heathrow Airport. The latter was attributed to a design defect in its Rolls-Royce Trent 895-17 engines, not the P&W engines on this incident aircraft.
Crew
The captain, Mark Stephenson (60), had been with United Airlines since 1990; the first officer, Michael DeVore (53), joined United in 1999. Both pilots were based in San Francisco. The captain had a total of 28,062 hours of flight time, with 538 hours in the Boeing 777 and the first officer reported a total of 18,612 hours of flight time, with 4,190 in the Boeing 777. During the incident, Stephenson was the pilot flying, and DeVore was the pilot monitoring.
Engine
The original 777-200 was distinctive for its Pratt & Whitney PW4000 engines that are about as wide as a Boeing 737 fuselage. The PW4077 variant used on the United 777-222 nominally produces 77,000 pounds-force (340 kN) of thrust. It is a dual-spool, axial-flow, high bypass turbofan engine, that is a higher bypass version of the PW4000-94 engine originally fitted to the Boeing 747-400. It was redesigned exclusively for the 777 with a larger 112-inch (280 cm) diameter fan section using 22 hollow-core fan blades. The PW4000-112 fan blade is a wide-chord airfoil made of a titanium alloy, about 40.5 inches (103 cm) long and about 22.25 inches (56.5 cm) wide at the blade tip. A PW4000-112 fan blade can weigh a maximum of 34.85 pounds (15.81 kg).
The hollow-core fan blade that experienced metal fatigue failure in this incident had only undergone 2,979 cycles since its last trip to the P&W factory for nondestructive testing using thermal acoustic imaging (TAI) to find hidden internal defects. This interval is less than half of the 6,500 cycle test frequency established in 2019 after a similar engine failure on a previous United Airlines 777-222 flight to Honolulu (UA1175) in 2018. The subject blade underwent TAI inspections in 2014 and 2016. The TAI inspection data collected in 2016 was re-examined in 2018 after the UA1175 incident.
Japan's transport ministry ordered increased inspection frequency after the similar JAL 777-200/PW4000 engine failure incident at Naha Airport (OKA) in Japan on December 4, 2020. The US Federal Aviation Administration was also considering increased inspections as a result of that incident, but had not acted prior to this incident in Denver.
Incident
The aircraft arrived at Denver International Airport (DEN) as flight UA2465 at 10:50 local time. At 13:04 local time it departed normally from Runway 25 en route to Daniel K. Inouye International Airport (HNL) as flight UA328. According to Flight Data Recorder (FDR) data and flight crew interviews by NTSB, about four minutes after takeoff, the airplane was climbing through an altitude of about 12,500 feet (3,800 m) with an airspeed of about 280 knots (320 mph; 520 km/h). The flight crew indicated to the NTSB that they advanced power at that time to minimize time in expected turbulence during their climb up to their assigned altitude of flight level 230 (roughly 23,000 ft or 7,000 m). Immediately after the throttles were advanced, a loud bang was recorded on the cockpit voice recorder (CVR). FDR data indicated the engine made an uncommanded shutdown and the engine fire warning activated shortly after. A fan blade out failure within the right (#2) engine resulted in parts of the engine cowling disintegrating and falling to the ground in Broomfield, Colorado. No one on the ground or in the aircraft was injured, although flying debris resulted in a hole in the wing to body fairing, a non-critical composite part designed to reduce aerodynamic drag.
The flight crew contacted air traffic control to declare an emergency and request a left turn to return to the airport. The flight crew began to complete checklists, including the engine fire checklist. As part of the checklist, the flight crew discharged both fire extinguisher bottles into the engine, but the engine fire warning did not extinguish until the airplane was on an extended downwind for landing. The flight crew continued to prepare for the emergency landing by completing additional critical checklists and verifying airplane performance for landing. They elected not to dump fuel for safety and time reasons and determined that the excess landing weight was not significant enough to outweigh other considerations.
The captain accomplished a one-engine-inoperative approach and landing to runway 26 without further incident. Airport rescue and firefighting (ARFF) met the airplane as soon as it stopped on the runway and applied water and foaming agent to the right engine. The base of the engine experienced a flare up, which was quickly extinguished. Once cleared by ARFF, the airplane was towed off the runway where the passengers disembarked via air stairs and were bussed to the terminal. Passengers were re-booked on flight UA3025—operated by a different Boeing 777, N773UA, a sister ship to N772UA immediately ahead of it on the production line—that took off hours later. N773UA had previously experienced an extremely similar engine failure in 2018 as United Airlines Flight 1175 from San Francisco to Honolulu.
Similar incidents
Media analysis of this incident frequently cited three related catastrophic engine failure incidents involving Pratt & Whitney PW4000 series turbofan engines: two previous incidents on the same 777-200 aircraft family with the PW4000-112 series engines with hollow-core fan blades that developed internal cracks, and one contemporaneous incident on an older widebody aircraft design with the original PW4000-94 series engine. At his press briefing two days after the incident, NTSB Chairman Robert Sumwalt said it remained to be seen whether the failure is consistent with a previous incident in February 2018 on United Airlines. "I think what's important is that we really truly understand the facts, circumstances, and conditions around this particular event before we can compare it to any other events," he noted. "But certainly we will want to know if there's a similarity."
United Airlines Flight 1175
On February 13, 2018, the same replacement aircraft used to accommodate the passengers from this incident, N773UA, originating from San Francisco as United Airlines Flight 1175 (UA1175), had a similar engine failure and loss of the engine cowling. This incident occurred over the Pacific Ocean approximately 120 miles (190 km) from Honolulu (HNL). The flight descended continuously from 36,000 feet (11,000 m) and landed at HNL approximately 40 minutes later with no reported injuries or loss of life. The aircraft was eventually repaired and returned to service.
The separated inlet cowl and fan doors fell in the ocean; unlike the UA328 incident, they were not recovered. The NTSB determined that a fan blade fractured from a pre-existing metal fatigue crack that had been slowly propagating since 2010, leading to the failure. The investigation faulted Pratt & Whitney for not identifying the crack in two previous inspections due to a lack of training in Pratt & Whitney's thermal acoustic image (TAI) inspection process, resulting in "an incorrect evaluation of an indication that resulted in a blade with a crack being returned to service where it eventually fractured." Boeing had been working on a redesign for a replacement fan cowl as a result of that incident, according to documents reviewed by the Wall Street Journal.
Japan Airlines Flight 904
On December 4, 2020, a 777-289 (JA8978, ex-Japan Air System) operated as JL904 from Okinawa-Naha Airport (OKA), also experienced a similar fan blade out failure and partial loss of the fan cowl six minutes after takeoff at an altitude of 16,000 feet (5,000 m). It returned to OKA and landed safely, but the Japan Transport Safety Board considered it a "serious incident" and launched an investigation. They later confirmed that the engine also had two broken fan blades, one with a metal fatigue fracture, similar to both United incidents. The Ex-JAS 777-289 version is powered by a PW4074 engine variant rated for 74,000 pounds-force (330 kN) of thrust.
Longtail Aviation Flight 5504
Coincidentally, on the same day as the United 328 incident, a Boeing 747-400BCF operating as Longtail Aviation flight LGT-5504, experienced an uncontained engine failure shortly after departing Maastricht Airport in the Netherlands, and two people were injured by debris that also fell in a residential area. That aircraft was powered by four PW4056 engines, a version of the earlier PW4000-94 engine.
Investigation
The National Transportation Safety Board (NTSB) investigatied the incident. An NTSB structures engineer and two investigators from the NTSB's Denver office collected fallen debris with local law enforcement and safety agencies over several days immediately after the incident. Most of the structure from the inlet cowl and fan cowl doors that separated from the aircraft was recovered and identified. Recovered portions of the inlet cowl, fan cowl door structure, and inlet cowl attach ring were laid out in a hangar (pictured). The inlet cowl, fan cowl doors, and thrust reversers will be examined further by NTSB investigators to map damage and cowl failure patterns after the fan blade failure, and to examine the subsequent progression of fire in the thrust reversers.
The NTSB noted upon initial inspection two fan blades had fractured, one near its root and an adjacent one about mid-span; a portion of one blade was embedded in the containment ring. The remainder of the fan blades exhibited damage to the tips and leading edges. The failed blades were removed and flown by private jet to Pratt & Whitney's laboratory in Hartford, Connecticut for further examination.
On February 22, 2021, National Transportation Safety Board Chairman Robert Sumwalt announced that the damage to the fan blade is consistent with metal fatigue, according to a preliminary assessment. Sumwalt also said that, "by our strictest definition," the NTSB did not consider the incident an uncontained engine failure because, "the containment ring contained the parts as they were flying out." He said the NTSB will look into why the engine cowling separated from the aircraft and why there was a fire, despite indications that the fuel supply to the engine had been turned off.