The National Transportation Safety Board (NTSB) has blamed the 2021 uncontained failure of Pratt & Whitey PW4077 turbofan partly on engine design and testing failures and on inadequate fan blade inspections.
In a final report released on 8 September, the NTSB also says inspections prior to the event had actually revealed evidence of metal fatigue in the fan blade that ultimately failed. Despite that evidence, the blade had not been removed from service.
The report also notes that since the incident, fleet-wide PW4000 inspections revealed nine more “confirmed cracked fan blades”.
The incident involved a PW4077 on the right wing of a United Airlines Boeing 777-200. Shortly after the jet took off from Denver on 20 February 2021, the turbofan failed dramatically, shedding multiple components and catching fire. The jet returned to Denver and no passengers and crew were injured.
The NTSB’s report says metal fatigue caused “a full-length fan blade separation”, which set off a cascade of events. But inadequate inspections and design issues contributed to the incident, which prompted the Federal Aviation Administration to order airlines to immediately inspect PW4000 turbofans.
The report says damage to the engine was particularly severe because P&W’s design and testing “failed to ensure that the inlet could adequately dissipate the energy of, and therefore limit further damage from, an in-flight fan-blade-out event”.
Notably, it says P&W’s certification campaign for the engine had involved testing of an “aluminium structure inlet”. The engine that failed had composite components.
“Simulation studies indicated that the carbon-fibre reinforced plastic honeycomb structure of the event engine inlet and inlet aft bulkhead was unable to dissipate and redistribute the energy of the loads imposed by the… event in the same manner as the aluminium structure inlet that was used during certification tests,” the report says.
Asked to comment, the FAA says it responded to the incident by issuing three airworthiness directives in March 2022 that required new inspections and modifications.
“The directives require strengthening engine cowlings, enhanced engine fan-blade inspections and inspections of other systems and components and specific corrective actions depending on the inspection results,” the FAA says.
Neither P&W nor United Airlines immediately responded to requests for comments.
The engine’s fan case had successfully contained the separated blade, but a “displacement wave” deflected the fan case, causing it to hit the engine’s nacelle doors and hinges. That caused the components of the engine’s inlet and fan cowl to fall from the aircraft.
Those failures should not occur under FAA certification rules, which the NTSB notes require that fan blade failures not cause inlets and fan cowls to separate.
“Following this event, Boeing developed modifications to the inlet to ensure that inlets and fan cowls remain in place… and modifications to add strength and ductility to the inlet by incorporating additional metallic structure,” the NTSB says.
“Contributing to the fan blade failure was the inadequate inspection of the blades, which failed to identify low-level indications of cracking, and the insufficient frequency of the manufacturer’s inspection intervals, which permitted the low-level crack indications to propagate undetected,” the report adds.
The report says the failed blade had been inspected in 2016; that inspection revealed “multiple low-level indications” of a problem. Those indications should have prompted a second inspection and a team review, but no such steps were taken, the NTSB says. Instead, people involved in the inspection attributed the abnormality to “camera sensor noise or loose contamination”.
“Two of the low-level indications identified during the 2016… inspection were likely associated with the fatigue crack that grew to result in the blade failure,” the report says.
The blade had again been inspected in 2018, and returned to service.
The NTSB says the incident was the fourth in-service PW4000 failure caused by fatigue cracking.
Boeing responded to the event by issuing service bulletins that called for modifications to inlet cowls and thrust reversers on 777s with PW4000s.
The NTSB’s report also concludes that the PW4077’s “K” flange failed, allowing “hot ignition gases to enter the nacelle… which allowed the fire to propagate past the under-cowl area and into the thrust reversers, where it could not be extinguished”.
About 6-9min after the initial blade failure, that fire melted components of the thrust reversers, causing parts to fall from the jet.
This also violated certification standards, which require those materials be capable of withstanding fire for at least 15min, according to the NTSB.
“Pratt & Whitney is evaluating actions to improve the strength of the K flange and expects hardware to be available in 2025,” the report notes.