The midair disintegration of a jet engine over suburban Denver Saturday is the latest in a string of failures that has raised alarm among regulators about debris evading shielding that’s supposed to keep broken parts from hitting aircraft.

The incident aboard United Airlines flight 328, which showered neighborhoods with metal debris, appears to have been the fifth in five years in which a fan blade broke and destroyed the front section of the engine, according to accident reports and safety experts. That portion of the engine isn’t as protected as the core areas around the jet turbines that are built to contain material in a failure.

“It’s getting more attention with each fan-blade-out event, resulting in these dramatic pictures showing the core of an engine hanging from a wing,” said Jeffrey Guzzetti, the former head of accident investigations for the U.S. Federal Aviation Administration.

No one was hurt in Saturday’s incident and the Boeing Co. 777 safely landed. But a similar episode above Pennsylvania in 2018 killed a woman on a Southwest Airlines Co. jet after a metal chunk from the engine struck the window where she was seated and sucked her partially out of the jet.

On Sunday, regulators in the U.S., Japan and elsewhere said they are concerned that a crack on a fan blade in the Pratt & Whitney PW4077 was allowed to grow to the point that caused it to break off, triggering the failure. They ordered emergency inspections on the hollow titanium blades, which effectively grounded several dozen older 777-200s. Pratt is a division of Raytheon Technologies Corp.

The PW4077 front fan section, which is more than 9 feet (2.7 meters) wide, spins at several thousand revolutions per minute, so a broken blade can cause extensive damage to the mechanism and nearby structures.

The failure raises questions about engine designs that are supposed to prevent debris from escaping out the sides of an engine when a fast-spinning fan blade fails. The FAA told the NTSB last year that it planned to order a design change to prevent such incidents, according to a previously unreported letter last year to the NTSB.

“We are working with Boeing to ensure that the corrective action, in the form of a design change, will address the most critical fan blade impact locations,” FAA Administrator Steven Dickson said in a March 9, 2020, letter.

Both the FAA and the European Aviation Safety Agency are conducting reviews of their aircraft certification rules to determine whether the standards for engine and aircraft design need to be updated, they have each told the National Transportation Safety Board. The safety board recommended such a review after its earlier investigation of the Southwest fatality.

Engine manufacturers go to great lengths to demonstrate that a broken fan blade won’t shower a jet with dangerous shrapnel. Modern turbine engines are encased in Kevlar to prevent high-velocity debris from escaping to protect vulnerable fuel tanks, equipment and passenger areas.

But the guiding theory in ensuring safety was that blades and other debris wouldn’t bounce too far forward through the onrushing air. As a result, the curved structure at the front of the engine — known as a nacelle and engine cowl — aren’t built as strongly.

The five recent events show that debris can, in fact, damage those areas. The result is that the existing engine standards have a gap that may not offer adequate protections during violent fan-blade failures, said a person who has participated in accident investigations. The person wasn’t authorized to speak about the issue and asked not to be named.

The issue has been getting quiet attention in recent years, but Saturday’s highly publicized failure over Denver — in which videos on social media showed a burning engine beneath the wing and metal chunks thumping to the ground — means “it’s now reaching a crescendo,” Guzzetti said.

The United plane was headed for Hawaii and was certified to fly long distances on a single engine over the ocean. But heavily damaged engines cause more drag, raising questions about what might have happened if the failure had occurred hours from an airport, he said.

In 2016, a Southwest jet was forced to make an emergency landing in Pensacola, Florida, after suffering a similar failure. Parts of the left engine broke apart, damaging the fuselage, wing and tail, the NTSB found. The plane lost cabin pressure and passengers had to don oxygen masks.

That engine was made by CFM International Inc., a joint venture between General Electric Co. and France’s Safran SA. They power the popular 737 Next Generation family of airliners.

A similar situation occurred on another United 777-200 on Feb. 13, 2018, as it was preparing to descend to Honolulu. Chunks of the front of the engine tore loose after a fan blade failed, the NTSB said. Metal fragments struck the fuselage, wing and tail sections. The jet flew about 120 miles on one engine and landed safely.

On April 17, 2018, another fan blade failed on a Southwest flight, also a 737-700 using CFM56-7B engines. In this case, a latch mechanism near the front of the engine was flung into the side of the jet, causing a window to break open, killing the woman seated next to it. Pilots landed safely and nobody else was seriously hurt.

The NTSB issued five recommendations to the FAA and its European counterpart, EASA, as a result of the 2018 failure. The safety board called on the regulators to require Boeing to redesign the front of the engine to prevent debris from escaping in the future.

Investigators also sought a broader review of how engines and aircraft are designed to ensure future designs aren’t vulnerable.

A Japan Airlines Co. Ltd. 777-200 suffered similar damage on Dec. 4 after a fan blade broke. In this case, a door on the exterior of the engine broke loose and the plane’s fuselage and tail section were hit by debris, according to a preliminary report by Japanese investigators.

All three of the 777s were powered by Pratt & Whitney PW4077 engines.

Pratt said last June that it had taken corrective actions to address the cause of the 2018 failure. After that incident, Pratt re-inspected all 9,600 fan blades and didn’t find any others with potential safety problems, the NTSB said.

The violent failure on the engine above Denver destroyed most of the relatively unprotected areas at the front of the turbine. At least some debris struck the plane itself, causing minor damage, the NTSB said Sunday in a press release.

One fan blade broke off at the root where it was mounted to a spinning rotor, according to a person familiar with the preliminary investigation who wasn’t authorized to speak about it. A second blade broke off, apparently after it was struck by the first, the person said.

While details of the failure haven’t been provided by the investigation, the violent loss of the blades caused the entire structure at the front of the engine to break loose, according to photos of the remains of the engine. Pilots returned to Denver for an emergency landing.

Both Guzzetti and the person who participated in engine investigations cautioned that the planes in each of the incidents were able to land safely, suggesting that other layers of safety had helped protect against a catastrophe.

The inspections of fan blades ordered by FAA and other regulators also will help lower the chances of a similar failure in the future.

GE spokesman Perry Bradley said it was more appropriate for Boeing to answer questions about the design of the front of the engines since it’s the plane-maker’s responsibility under FAA regulations, not the engine manufacturer. Boeing and Pratt & Whitney didn’t immediately respond to a request for comment on the issue.

By contrast, the failure of a Pratt & Whitney engine on a Longtail Aviation 747-400 cargo jet on Saturday in the Netherlands didn’t allow debris to fly out and hit the plane, according to Longtail Aviation Chief Executive Officer Martin Amick.

The failure prompted metal shards to fall to the ground, injuring two people, according to local news reports. The metal debris escaped out the rear of the engine, which is how they are designed to fail.

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