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An immolated and heavily damaged Pratt & Whitney PW4000 hanging precariously from the wing of a United Airlines Boeing 777 is an unambiguously evocative sight. It instantly grabs the attention and amplifies a natural emotional response, one built from our animal instinct for survival and also the memory of experience.

What manifests in the case of United flight 328 and its dramatic turn back over suburban Denver on Saturday is first, demonstrable evidence of what happens when good systems are in place when bad things happen. When the 777 adopted fly-by-wire technology and new expectations of reliability and redundancy to operate flights hours away from suitable diversion airports, it made possible a multi-layer safety net for United 328 and its crew to have both time and options to return safely to the ground.

Related: FedEx and Sikorsky quietly begin single-pilot tests for cargo airliners

Yet, this safety crisis, the first since the 20-month long grounding of another Boeing airplane, cannot be viewed in isolation. The legacy of the 737 Max remains a persistent shadow over aviation safety and global regulatory relations. “In a post-737 Max world, I was surprised at how quickly the [FAA] Administrator [Steve Dickson] grounded the airplanes,” said Jeff Guzzetti, former head of accident investigation for the FAA and now an independent safety consultant. “The old FAA would not have been that quick on the draw.”

It’s been just over three months since the 737 Max was cleared by the FAA to return to flying passengers after a 20-month grounding by global regulators, following two fatal crashes in 2018 and 2019. The aircraft is gradually returning to service around the world as individual regulators evaluate the changes made to the aircraft and crew training, but the heavy impact of aviation’s most acute contemporary crisis is only just beginning to be felt on industrial relationships.

The FAA’s emergency order on the 777 came three days after the initial incident. Released late Tuesday, the regulator spelled out an immediate inspection requirement before further flight for every PW4000 fan blade for 777s. The pace of those examinations is expected to be slow, paced by the limited global capacity of thermal acoustic imaging testing. “The fleet will be grounded for months,” according to an industry official familiar with the regulator’s plans that will see airplanes coming back into service gradually as inspections are completed.

Of the 1,600 777s delivered since 1995, the PW4000 represents a tiny portion. Only about half of the 128 777-200 and 777-200ERs powered by Pratt’s PW4000 are in operation as twin-aisle flying has been decimated by the COVID-19 pandemic. The last Pratt-powered 777 was delivered to Asiana Airlines in the summer of 2013 and the engine maker no longer has active manufacturing operations for the enormous 112-inch model of the engine or its fan blades.

Related: 737 Max grounding threatens to unravel the aviation certification world order

Because the blades have a hollow titanium design, traditional borescope inspection of these fan blades is not an option. Further complicating matters, the inspections require the use of global capacity of thermal acoustic imaging (TAI), the availability of which is extremely limited. Fan blades will be removed from each engine and sent to Pratt & Whitney in Connecticut for evaluation, according to the engine maker.

The inspections take roughly eight hours per 40.5-inch long blade, according to Dickson. A person familiar with the inspections said that the company currently can process about 12 engines worth of blades each month. Each engine has 22 blades.

A Pratt & Whitney PW4000 engine for the Boeing 777 sits at the National Air & Space Museum's Udvar-Hazy Center in Virginia.

The last Boeing 777 to be powered by Pratt & Whitney engines was delivered to Asiana Airlines in the summer of 2013.

The last Boeing 777 to be powered by Pratt & Whitney engines was delivered to Asiana Airlines in the summer of 2013.

Throughout Tuesday, the inspection interval under consideration was a moving target, according to two people familiar with the regulatory deliberations. FAA had been considering allowing the aircraft to continue to fly with inspections on the blades within 1,000 cycles, but later Tuesday the FAA eventually settled on the most conservative path requiring the inspections of all fan blades before they would be allowed to resume flying.

“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,” said the U.S. aviation regulator.

While the grounding of the Pratt-powered 777’s draws Boeing back into another safety crisis, responsibility for the engine ultimately lies with its manufacturer. NTSB chairman Robert Sumwalt said in a Monday evening briefing with the media that the initial inspection of the accident engine is “consistent with metal fatigue” on the blade that initiated the failure. The remaining fragments that the NTSB had recovered from the engine were being sent to Pratt facilities in Connecticut.

However, Boeing is not expected to escape scrutiny by the NTSB’s chair, who said the disintegrated structure around the engine is an area of inquiry. “We don’t expect a cowling to separate like that. We want to understand that.” said Sumwalt.

The Air Current has confirmed that a new nacelle design had been planned by Boeing and shared with the FAA for PW4000-powered 777s, but not yet implemented nor formally mandated by the FAA. The Wall Street Journal reported earlier Thursday that Boeing had been planning a modification as a result of structural and moisture concerns.

“We are in constant communication with our customers and the FAA, and engaged in ongoing efforts to introduce safety and performance improvements across the fleet,” the plane maker said in a statement. “We will continue to follow the guidance of the FAA on this issue and all matters related to safety and compliance, and we continue to provide updates to our customers.”

A crumbling nacelle isn’t unique to the 777 and Pratt & Whitney. Southwest Airlines suffered two separate incidents in 2016 and 2018 with CFM56-7B-powered 737s. In both cases fan blades failed, causing major damage to the engine and surrounding structure. And in the case of the second accident aboard Southwest 1380 when the nacelle tearing away punctured the fuselage, it killed one passenger. It was the first mainline U.S. airline fatality since 2001.

The NTSB's final report on Southwest 1380 detailed how the CFM56's engine fan blades and surrounding nacelle came apart.

The NTSB’s final report on Southwest 1380 detailed how the CFM56’s engine fan blades and surrounding nacelle came apart.

That prompted the FAA and CFM to call for inspections of fan blades with more than 30,000 cycles within 20 days of its order, and within six months for blades with more than 20,000 cycles and then all others after they reached 20,000 cycles and repeated every 3,000 cycles. That was later updated to every 1,600 cycles.

The NTSB recommended a redesign of the fan cowl structure on the CFM56-7B engines. That has not been adopted by Boeing or the FAA.

“In a post-Max world that accident would not have been treated the same,” said Guzzetti, who was at the FAA during both Southwest incidents. “There would’ve been more eyes and attention paid to it.”

Just two months before Southwest 1380, another United 777 — flight 1175 — suffered a similar engine failure as United 328. The aircraft was about 120 miles from Honolulu when one of its PW4000 engine blades failed. The aircraft was 328’s sistership, just in front of it on the 777 production line — the fourth and fifth built by Boeing.

Related: NTSB examining collateral damage of failed engine structure on Southwest 1380

In March 2019, the FAA ordered stepped up inspections on PW4000 fan blades that had accumulated more than 6,500 cycles and then required repeating the examination every 6,500. Operators had 500 cycles or 180 days to comply with the directive, which was issued in February 2020. It is not yet clear if or when the blades on the engine involved in United 328 had been examined under the new requirement. Reuters reported Thursday that the aircraft had flown nearly 3,000 cycles since its last inspection.

Last summer, following NTSB recommendations after United 1175, Pratt had shifted its inspection process for examining PW4000 fan blades and adding an additional quality review to ensure that cracks aren’t missed or erroneously identified as a false positive.

Just three months ago on December 4, 2020, another Pratt-powered 777 suffered an engine failure, an apparent broken fan blade during a Japan Airlines flight between Okinawa and Tokyo-Haneda Airport. The similar situation to United 1175 prompted Japan’s Ministry of Transport to call it a serious incident, but the occurrence didn’t set off the same alarm bells as were seen after United 328. Guzzetti attributed the limited U.S. response to the JAL incident to a combination of the nearing Christmas holiday break, the distraction of the COVID-19 pandemic and the impending change of Presidential administrations.

Japan Airlines uses older Boeing 777-200s in high-cycle domestic operations to compete with high speed trains.

Japan Airlines uses older Boeing 777-200s in high-cycle domestic operations to compete with high speed trains.

With this first major test of the NTSB and FAA after the 737 Max’s return, one industry official said the FAA and Pratt were focused on instilling trust and confidence in the U.S. regulator’s emergency airworthiness directive for international regulators. The Japan Civil Aviation Bureau, the country’s aviation regulator, on Sunday said that air carriers flying PW4000-powered 777s were “requested to avoid” taking-off, landing or flying within the territory of Japan “until further notice.”

Related: Lessons of DC-10 grounding pave way for 737 Max return

The industry official suggested that with the emergency directive now settled, the work now turns to Pratt working with FAA to build confidence in the prescribed inspection regime with JCAB so carriers can resume operations in Japan, including allowing United to resume flights to the country. The official said the next steps reflect the “tenderness” of the relationship between the FAA and other regulators like the JCAB after the U.S.’s botched certification and grounding of the 737 Max.

The dissection of the FAA has continued since the Max was cleared to return to flying in the U.S. in November. The Department of Transportation’s Inspector General released a report (PDF) on Feb. 23 detailing the deficiencies in how the regulator oversees Boeing and other delegated certification efforts. “To its credit, FAA is taking significant action to correct identified weaknesses,” the report concluded, adding that “Much work remains to address weaknesses in FAA’s certification guidance,” suggesting 14 recommendations that it believes “will be vital to restore confidence in FAA’s certification process and ensure the highest level of safety in future certification efforts of major passenger aircraft.”

Write to Jon Ostrower at jon@theaircurrent.com

Jon Ostrower is Editor-in-chief of The Air Current. Prior to launching TAC in June 2018, Ostrower served as Aviation Editor for CNN Worldwide, guiding the network's global coverage of the business and operations of flying. Ostrower joined CNN in 2016 following four and half years at the Wall Street Journal. Based first in Chicago and then in Washington, D.C. he covered Boeing, aviation safety and the business of global aerospace. Before that, Ostrower was editor of the award-winning FlightBlogger for Flightglobal and Flight International Magazine covering the development of the Boeing 787 Dreamliner and other new aircraft programs from 2007 to 2012. Ostrower, a Boston native, graduated from The George Washington University's School of Media and Public Affairs with a bachelor's degree in Political Communication. He is based in Seattle.

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