On February 20, 2021, a United Airlines Boeing 777-222, which was flying as Flight 328, suffered a major engine failure soon after departing Denver International Airport (DEN). The domestic flight, which was scheduled to go to Honolulu, was compelled to go back to Denver where it landed safely, 24 minutes after takeoff. The accident, which was characterized as a contained engine failure, involved the removal of large parts of the engine cowling, inlet and thrust reverser, resulting in a debris field that was more than a mile long and spanning residential neighborhoods of Broomfield, Colorado.
There were no reported injuries to the 231 passengers and 10 crew members on board or on the ground, even though the falling debris destroyed personal property, such as the roof of a home and a parked car. The falling parts were recorded on smartphones and dash cameras by witnesses, and passengers also filmed the damaged engine inside the plane and posted the videos on social media. The fuselage was slightly damaged, but the flight crew was able to shut down the damaged engine and make a safe return and landing.
The National Transportation Safety Board (NTSB) in the U.S. immediately initiated an inquiry into the accident. That soon resulted in the U.S. Federal Aviation Authority (FAA) declaring an Emergency Airworthiness Directive (EAD) to order all Pratt & Whitney PW4000-series engine fans to be inspected immediately. As a result, the aviation regulators across the globe temporarily grounded similar Boeing 777-200 aircrafts, which indicated a bigger problem with the type of engine.
Background and Crew Information of Aircraft
The aircraft in question is a Boeing 777-222, a model that was specifically built to suit United Airlines in the initial 777-200 line. It was constructed in November 1994, delivered to United in September 1995 with the construction number 26930 and line number 5. This aircraft was an early Boeing 777-200 that was a part of the flight test certification program prior to its commercial release. In 2013, Boeing stopped equipping the 777 with P&W PW4000 series engines.
The Boeing 777 itself is a wide body, long range, twin engine aircraft, which has a relatively low accident fatality rate at the time of the accident. Well-known crashes of total loss like MH17 (shot down) and MH370 (lost) were not similar to mechanical failures of this kind. There were other deadly accidents such as the Emirates Flight 521 and Asiana Airlines Flight 214, which were blamed on pilot error. The Flight 38 loss of the British Airways hull was, as an example, connected to a design flaw on its Rolls-Royce Trent engines, rather than the Pratt & Whitney powerplants that were involved in the Denver incident.
The crew of the plane included Captain Mark Stephenson, 60, who had been with United Airlines since 1990, and First Officer Michael DeVore, 53, who had been with the company since 1999. The two pilots had their bases in San Francisco. Captain Stephenson had a total of 28,062 hours of flight time, 538 of which were in the Boeing 777, and First Officer DeVore had 18,612 hours of flight time, 4,190 of which were in the 777. Stephenson was the pilot in command during the landing, which was an emergency landing, and DeVore was the pilot monitoring.
Engine Specifications and Previous Failures
The original 777-200 aircraft was unique in that the Pratt and Whitney PW4000 engines were installed, which are characterized by the fact that they are very large, being close to the size of a Boeing 737 fuselage. The designed variant, PW4077 itself, used in the United 777-222, is able to generate 77,000 pounds-force (340 kN) of thrust. It is an axial-flow, high bypass, two-spool, turbofan engine, a higher bypass variant of the PW4000-94 engine that was initially installed on the Boeing 747-400. It is only redesigned to fit the 777, having an increased fan section of 112-inch (280 cm) diameter and 22 hollow-core fan blades. The titanium alloy fan blades (PW4000-112) of the PW4000-112 are approximately 40.5 inches (103 cm) in length and 22.25 inches (56.5 cm) in width at the tip and have a maximum weight of 34.85 pounds (15.81 kg).
The hollow core fan blade that malfunctioned in the Denver accident, because of metal fatigue, had only been through 2,979 cycles since it was last taken to the Pratt and Whitney factory to be subjected to nondestructive testing. This was a thermal acoustic imaging (TAI) test that was used to detect internal defects. This inspection period was significantly shorter than 6,500 cycles of the test frequency set in 2019, which was in turn a reaction to a similar engine failure on a prior United Airlines 777-222 flight to Honolulu (UA1175) in 2018. The particular blade was subjected to TAI tests in 2014 and 2016, and the latter data was re-examined following the 2018 UA1175 accident.
To make the matters even worse, the transport ministry of Japan had already instructed the frequency of inspection of similar engines to be increased following the failure of a Japan Airlines (JAL) 777-200/PW4000 engine at Naha Airport in Japan on December 4, 2020. U.S. Federal Aviation Administration had been considering more inspections since the JAL incident but had not taken action before the United 328 incident in Denver, which casts doubt on the responsiveness of regulatory measures.
Flight and Emergency Response Sequence
At 10:50 local time, the plane landed in Denver International Airport as flight UA2465. It took off at 13:04 local time as flight UA328 off Runway 25 towards Daniel K. Inouye International Airport (HNL) normally. Data provided by the Flight Data Recorder (FDR) and interviews with the flight crew by the NTSB showed that about four minutes after takeoff, as the plane was climbing through an altitude of about 12,500 feet (3,800 m) and the plane was traveling at a speed of about 280 knots (320 mph; 520 km/h), the flight crew advanced power. This was done so as to spend less time in anticipated turbulence when they are climbing to the designated flight level 230 (approximately 23,000 ft or 7,000 m).
As soon as throttles were pushed, a loud bang was captured on the cockpit voice recorder (CVR). Data on the FDR later indicated an uncommanded engine shutdown and an engine fire alarm. The failure of a fan blade out in the right (#2) engine caused some of the engine cowling to break off and fall to the ground in the area of Broomfield, Colorado. Although there were no injuries on the ground or in the plane, flying debris led to a hole in the wing-to-body fairing, which is a non-critical composite part that helps to decrease the aerodynamic drag.
When the flight crew announced an emergency, the air traffic control was contacted and asked to turn left and go back to the airport. They had filled in checklists, such as the engine fire checklist, pouring both fire extinguisher bottles into the engine. Nonetheless, the engine fire alarm continued until the plane had been on a long downwind landing approach. The crew was still making preparations regarding the emergency landing, including more important checklists and checking the performance of the airplane. They decided not to dump fuel because of safety and time reasons and concluded that the extra landing weight was not so significant to overrule other factors.
Captain Stephenson managed to make a one-engine-inoperative approach and landing on runway 26 without any additional incident. Airport rescue and firefighting (ARFF) crews greeted the airplane on the runway, spraying water and foaming agent on the right engine. There was a short burst of fire at the engine base, which was soon put out. Following ARFF clearance, the aircraft was towed off the runway and passengers stepped out using air stairs and were bussed to the terminal. These passengers were re-booked onto flight UA3025 which was flown by N773UA, another Boeing 777 and a sister ship to N772UA, and immediately behind it on the production line.
Comparable Accidents and History
The aircraft, N773UA, which has undergone a very similar engine failure in 2018, is also noteworthy as United Airlines Flight 1175 between San Francisco and Honolulu, which happened in 2018, has also added an element of recurring worry to the case. The Denver incident was often analyzed in the media and referred to three related disastrous engine failure incidents of Pratt and Whitney PW4000 series turbofan engines. Two earlier accidents involved the same 777-200 series with the PW4000-112 series engines and hollow-core fan blades that had internal cracks, and one more recent accident on an older widebody aircraft design with the original PW4000-94 series engine.
During a press briefing two days following the incident, NTSB Chairman Robert Sumwalt said it was yet to be determined whether the failure was similar to the one in the preceding United Airlines incident in February 2018. He stressed that it was essential to have all the facts, circumstances and conditions around the Denver incident and then compare them but stated that the NTSB was going to explore any similarities.
On February 13, 2018, N773UA, the aircraft carrying passengers on the UA328 flight, experienced a similar engine failure and engine cowling loss as the plane flew over the Pacific Ocean, some 120 miles (190 km) off Honolulu, which is the destination. The plane fell down to 36,000 feet (11,000 m) and landed safely at HNL approximately 40 minutes later without any injuries or loss of life. The plane was later fixed and put back into service. The inlet cowl and fan doors were separated (as opposed to the UA328 incident), and were not recovered and fell into the ocean. The NTSB concluded that one of the fan blades broke because it had a prior fatigue crack of metal that had existed since 2010. The probe blamed Pratt and Whitney because it had not detected the crack during two previous inspections, and this was due to insufficient training in the thermal acoustic imaging (TAI) inspection procedure at the company, which led to a wrong assessment of an indication that led to a blade with a crack returning to service and fracturing. This 2018 incident had been reported to have prompted Boeing to redesign a replacement fan cowl.
The other incident that was similar happened on December 4, 2020, with Japan Airlines Flight 904. A 777-289 (JA8978), which was flying out of Okinawa-Naha Airport (OKA), had a failure of the fan blade out and some loss of the fan cowl six minutes after takeoff at an altitude of 16,000 feet (5,000 m). It landed safely back at OKA, which made the Japan Transport Safety Board declare it a serious incident and initiate an investigation. This investigation subsequently established that there were also two fractured fan blades in the engine, one with a metal fatigue fracture, similar to the United cases. It was using a PW4074 engine type, which has an engine power of 74,000 pounds-force (330 kN) thrust.
On the same day of the United 328 accident, coincidentally, a Boeing 747-400BCF, the Longtail Aviation Flight LGT-5504, suffered an uncontained engine failure soon after leaving the Maastricht Airport in the Netherlands. The debris of this incident also landed in a residential area injuring two people. This airplane used four PW4056 engines, a predecessor of the PW4000-94 engine, highlighting a trend of problems with the PW4000 family.
NTSB Investigation and Findings
The investigation of the Denver incident by the National Transportation Safety Board (NTSB) started at once. Two investigators and an NTSB structures engineer based in Denver spent some days gathering fallen debris together with local law enforcement and safety agencies. The majority of the structure between the inlet cowl and fan cowl doors that broke off the aircraft was located and identified. These recovered parts namely the inlet cowl, fan cowl door structure, and the inlet cowl attach ring were very meticulously spread out in a hangar to be examined further. The NTSB investigators were to map the patterns of damage and cowl failures following the fan blade failure and examine the further course of fire in the thrust reversers. The wing and the body fairing of the United Airlines flight 328 were also observed to be damaged.
The NTSB in its initial inspection found that two fan blades were fractured: one of them was near its root and another one was adjacent to it around the middle, and a part of one of the blades was stuck in the containment ring. The other fan blades were damaged at their ends and at the front edges. The defective blades were flown by the private jet to the laboratory of Pratt and Whitney in Hartford, Connecticut, where they were examined in detail. On February 22, 2021, NTSB chairman Robert Sumwalt declared that the damage to the fan blades was in line with metal fatigue, according to an initial evaluation. According to the strictest definition of an uncontained engine failure as defined by the NTSB, Sumwalt explained that the incident was not an uncontained engine failure since the containment ring held the parts as they were flying out. He also said that NTSB would examine the reasons why the engine cowling was separated and why a fire took place, even though the fuel flow to the engine was indicated to have been turned off.
The NTSB issued an investigative update on March 5, 2021, which gives additional information on the initial investigation of the fire damage on the right engine. They discovered that it was mostly limited to the accessory components of the engine, the thrust reverser skin, and the composite honeycomb structure of the inboard and outboard thrust reversers. The aft cowl seemed to be undamaged in both halves. The spar valve, which was intended to prevent the flow of fuel when the fire switch is pulled in the cockpit, was closed, and there were no signs of a fuel-fed fire. Investigation of the cockpit established that the right engine fire switch was pulled and set to the DISCH 1 position and both fire bottle discharge lights were on. Investigation of engine accessory revealed that there were numerous broken fuel, oil, and hydraulic lines, and a broken gearbox.
Also, NTSB said that during the initial inspection of the right engine fan, the spinner and spiner cap were found intact and seemed undamaged. The fan hub was not broken but it was not rotatable by hand. The fan hub had all the roots of fan blades, and two blades were broken. The fracture in one of the fan blades was transversal to the airfoil, and the fracture surface was in agreement with fatigue. A second fan blade also fractured transversely across the airfoil, but fracture surfaces were shear lips as in an overload failure. The other fan blades were full length, however, all had some form of impact damage to the airfoil.
On September 8, 2023, the NTSB released a final accident report. This report found that the fatigue failure of the right engine was a result of poor inspections and poor frequency of inspections to identify low-level crack indications. These cracks kept on spreading until eventual failure. In addition, the report noted that the carbon-fiber reinforced plastic material used to make the engine inlet rather than the aluminum material used in the certification tests resulted in the inlet not dissipating the energy of the fan-blade out event adequately, which contributed to further damage. The NTSB also established that the intensity of the fire damage was attributable to the failure of the “K” flanges following the blade failure that enabled the spread of hot ignition gases to components that transported flammable fluids causing the fire to spread to undercowl and thrust reverser areas where it could not be suppressed.
Response by the Regulators and Industry
The direct impact experienced a swift social and state reaction. The Broomfield, Colorado Police Department made a post at 13:41 on the day of the incident that confirmed that there were reports of engine trouble and debris, but that there were no reported injuries and asked the public not to touch debris. Public Information Officer Rachel Welte, in an impromptu press conference, was relieved, saying that it was quite remarkable, considering the number of people that are at this park at this time of the day, that no one was hurt.
The regulatory agencies responded promptly. The grounding of 32 Boeing 777s of Japan Airlines and All Nippon Airways was ordered by the Japanese Ministry of Land, Infrastructure, Transport and Tourism on the day after the UA328 incident on February 22. The Federal Aviation Authority of the U.S. mandated more inspections of Boeing 777s with PW4000 engines, and United Airlines voluntarily withdrew 52 of these airliners (24 in service, 28 in reserve) out of service. Boeing issued a recommendation to ground all 128 Boeing 777-200 series aircrafts of its fleet that were powered by Pratt and Whitney PW4000-112 engines worldwide. The British Civil Aviation Authority prohibited the use of Boeing 777s powered by PW4000-112 engines on UK airspace, and Transport Canada thought of similar measures in Canadian airspace, although none of their airlines used the type.
On February 23, Pratt & Whitney released a statement that it was cooperating with federal investigators and coordinating with operators and regulators to facilitate the establishment of a new inspection period of PW4000 engines. On February 24, the FAA became the first of other countries to issue an Emergency Airworthiness Directive (EAD), which mandated that U.S. operators of planes with some Pratt and Whitney PW4000 engines conduct thermal acoustic-imaging inspections of their fan blades before any further flight. This practically put the whole U.S. fleet of affected planes, which were operated by United only, on hold. The unusually rapid issue of the EAD, three days after the incident, was observed by industry observers, and speculated that the urgency was informed by the experience of the Boeing 737 MAX crisis. A source in the industry on February 25 reported that these fan blade inspections required about 8 hours per blade and Pratt and Whitney was only able to do 10 sets of 22 blades per engine per month. This implied that the concerned aircrafts would take a very long time to resume service unless the FAA chose to soften the TAI inspection standards after reviewing initial findings.
The implications remained a challenge to airline operators and regulators. Japan Airlines declared on April 5 that it would retire all its P&W powered Boeing 777s by March 2021, one year sooner than it had originally planned. As early as April 21, United Airlines announced its plans to resume service with its Boeing 777-200s powered by PW4000 engines soon. One week after, the British Civil Aviation Authority lifted the ban and these Boeing 777s were allowed to fly back into the UK airspace. At an FAA Safety Oversight Hearing on May 12, Administrator Stephen Dickson announced that the agency was asking the manufacturers to repair the cowling to strengthen on the affected aircraft. Boeing admitted that, it is a demanding and time consuming work.
But the service was not resumed as promptly as it had been hoped. By June 28, the grounded 777-200s of United were still not flying, and the airline declared it was reworking its schedule on Hawaii flights, although it still hoped they would be back soon. On August 30, the Wall Street Journal reported that United 777-200s that had been grounded were not expected to resume service until 2022 because of further safety measures. Finally, on May 17, 2022, the FAA approved the 52 Boeing 777s of United to resume service, and Jin Air followed suit, putting its first 777 back into service on June 12, 2022.
International Effect and Security Heritage
The accident in Denver caused major groundings around the world affecting many airlines. In less than a week, United Airlines grounded its 52 planes (24 in service, 28 in storage) (All Nippon, 2000).
Airways 24 (10 in service, 14 in storage), Japan Airlines 20 (7 in service, 13 in storage), Korean Air 17 (7 in service, 10 in storage), Asiana Airlines 7 (6 in service, 1 in storage), and Jin Air 4 (1 in service, 3 in storage), a total of 124 Boeing 777-200 series aircraft with Pratt & Whitney PW4000 engines.
This accident, which was a part of a worrying sequence of engine failures with Pratt and Whitney PW4000 series engines, was a wake-up call to the fact that aviation safety must be maintained at all times. The comprehensive investigation by NTSB, which resulted in the conclusions of poor inspections and design defects of the engine inlet and K-flanges highlighted major areas in which to improve. The worldwide regulatory reaction manifested in the rapid groundings and mandatory inspections showed a concerted effort to avoid the future events. The long-term campaigns by manufacturers, airlines and oversight bodies though time consuming proved how the industry was committed to learning through such events and strengthening the safety measures that have formed the foundation of air travel across the globe so that the skies are safe to everyone.
