Metal-Fatigue Signs Link United 777 Fan Blade Failure With 2018 Incident

U.S. NTSB investigators determined metal fatigue is suspected as the reason a fan blade fractured just before an in-fight engine failure suffered by United Airlines Flight 328 on Feb. 20, board chairman Robert Sumwalt said. 

Updating reporters on a late-evening briefing Feb. 22, Sumwalt said investigators had confirmed the Pratt & Whitney 4077 engine blade that fractured near the root likely had damage propagating steadily over time. 

“A preliminary on-scene exam indicates damage consistent with metal fatigue,” Sumwalt said. “You can actually see these crack-arrest marks. Each time that engine experiences one cycle, there will be one more of those crack-arrest marks. We can come up with a pretty accurate number of when that crack was initiated.” 

The blade was slated to undergo a detailed examination in a Pratt & Whitney facility on Feb. 23, Sumwalt said. 

A second, adjacent blade that fractured mid-span showed damage consistent with being struck by the first blade. 

“Basically, it probably got hit as the other [blade] piece was separating,” he said. 

The incident—which took place as the 1994-build Boeing 777-200 was climbing out of Denver International Airport en route to Honolulu carrying 231 passengers and 10 crew—led to debris from the right, or No. 2, engine being scattered around Broomfield, Colorado, northwest of Denver. The crew declared an emergency and returned to Denver International. No injuries were reported. 

While investigators have not reached any conclusions, the findings linked to the fan blade failure are consistent with a 2018 incident involving another PW4077-powered United 777-200 that experienced an engine failure en route to Honolulu. In that incident, NTSB found the failure started when a fan blade fractured about 4.5 in. from the blade root. Investigators determined low-cycle fatigue fracture as the cause, and cited Pratt’s inadequate inspection procedures as the reason the crack was not found. Inspections identified crack “indications” in 2010 and 2015, but Pratt technicians servicing the fan blades believed the marks were a byproduct of the thermal acoustic imaging process and allowed them to be overhauled and returned to service. 

The NTSB’s final report on the 2018 incident said Pratt reviewed inspection records of all 9,600 PW4000 112-in fan blades—which incorporate a unique, hollow, titanium, shroudless design—to look for similar, missed “indications.” The FAA also mandated Pratt-recommended revisions to inspection intervals in a 2019 airworthiness directive. 

Sumwalt said the NTSB has not reviewed inspection records for the fan blades involved in the most recent incident. The board also has not started comparing findings from the current probe and similar incidents, including the 2018 United failure or the Dec. 4, 2020 incident in which a Japan Airlines 777-200 lost a fan blade shortly after takeoff on a flight from Okinawa, Japan to Tokyo. 

“Certainly we will want to know if there are similarities,” Sumwalt said. 

Aircraft damage was “minor,” Sumwalt said, reiterating an NTSB update issued Feb. 21. The most visible damage besides the engine was along the right-side wing-to-body fairing, where debris left a sizable hole in the composite structure. Despite the airframe damage, Sumwalt said the incident will likely be categorized as a contained engine failure. 

“At this time, it does not appear that anything punctured through the [No. 2 engine] containment ring,” he said. 

But he expressed concern at some of the damage, notably the separation of the front of the engine’s inlet cowling, which ended up in a Broomfield resident’s front yard. 

“We don’t expect a cowling to separate like that,” Sumwalt said. “We want to understand that.” 

He also said that investigators will try to determine whether fire-suppression efforts worked on the failed engine. Videos taken by witnesses appeared to show the No. 2 engine on fire after the crew shut it down and discharged both bottles meant to extinguish any flames. 

“The fuel to the engine was turned off,” Sumwalt said. “We will be looking to see what continued to propagate a fire.” 

Operators of the 70 in-service Pratt-powered 777s pulled their aircraft from service soon after the United incident, while another 60 were already parked, in part because of low demand for long-haul travel. Several regulators banned their operation, ensuring a de facto global grounding while the FAA developed a risk-mitigation plan.  

The FAA late Feb. 23 issued its expected emergency airworthiness directive, ordering inspections of PW4000 112-in. fan blades before further flight to ensure none have latent signs of fatigue cracking.

"Based on the initial results as we receive them, as well as other data gained from the ongoing investigation, the FAA may revise this directive to set a new interval for this inspection or subsequent ones," the agency said.

The interval specified in the 2019 directive is 6,500 cycles.

This story has been updated with new information from the FAA.