A day after the dangerous explosion of an engine on United Airlines flight 328 shortly after takeoff from Denver, the older Boeing 777-200 models involved were effectively grounded worldwide.

The Federal Aviation Administration (FAA) ordered immediate stepped-up inspections of Boeing 777-200 airplanes equipped with certain Pratt & Whitney engines on Sunday, as airlines operating such jets in the U.S. and Japan suspended flights.

United Airlines grounded 24 similar 777s, while the Japanese aviation regulator ordered all planes equipped with this type of engine to cease flying in Japan until further notice.

Boeing followed late Sunday with a statement that “recommended suspending operations of the 69 in-service and 59 in-storage 777s powered by Pratt & Whitney 4000-112 engines until the FAA identifies the appropriate inspection protocol.”

The speed and decisiveness of the grounding of the jets is spurred by the fact that there have been two previous engine blowouts on similar 777s with the same Pratt & Whitney engine.

Another United flight suffered a similar engine failure three years ago, and it happened again on a Japan Airlines 777-200 in December.

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These are older 777 airplanes. The last time Boeing used an engine made by Pratt & Whitney in a 777 was in 2013.

FAA Administrator Steve Dickson in a statement said that he directed his team “to issue an Emergency Airworthiness Directive that would require immediate or stepped-up inspections of Boeing 777 airplanes equipped with certain Pratt & Whitney PW4000 engines.”

“This will likely mean that some airplanes will be removed from service,” he added.

Dickson said his agency’s experts “concluded that the inspection interval should be stepped up for the hollow fan blades that are unique to this model of engine, used solely on Boeing 777 airplanes.”

He added that “the FAA’s aviation safety experts are meeting into the evening with Pratt & Whitney and Boeing to finalize the details” of the order detailing the inspections that will be required in his emergency order.

United, the only U.S. operator with this type of engine in its fleet, in a statement said that “starting immediately and out of an abundance of caution, we are voluntarily and temporarily removing 24 Boeing 777-200 aircraft powered by Pratt & Whitney 4000 series engines from our schedule.”

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“As we swap out aircraft, we expect only a small number of customers to be inconvenienced,” United said.

Meanwhile, the Japan Civil Aviation Bureau directed operators of airplanes equipped with this type of engine to cease flying in Japan until further notice.

According to the most recent registry data, airlines in only three countries operate airplanes with the affected engines — the U.S., Japan and South Korea.

In Saturday’s incident, the front rim of the engine pod — known as the inlet — was ripped off and fell close to houses on the ground along with other large metal pieces of the cowling that encases the engine.

Luckily, no one was injured on the ground and the United crew shut off the engine, turned back and landed the jet safely with no injuries to those aboard.

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Safety experts from Boeing, Pratt & Whitney and the National Transportation Board (NTSB) will investigate the cause of the accident.

The NTSB reported Sunday that initial examination of the engine revealed the inlet and cowling separated from the engine and that two of the 112-inch titanium fan blades at the front of the engine broke. One fan blade was broken off at the root and an adjacent fan blade broke off at midspan.

This suggests the destruction of the engine was initiated by the fan blade breaking off, with the scimitar-like metal colliding at high speed with the casing around the fan — which is strengthened with Kevlar to contain any metal shrapnel from a broken blade.

The NTSB said a portion of one blade was embedded in the Kevlar containment ring. The other unbroken fan blades exhibited damage to the tips and leading edges.

While the engine was destroyed, the NTSB said the rest of the airplane sustained only minor damage.

Pattern of similar failures

Though engine failures are dangerous, an aircraft typically can land safely as long as the casing contains the broken metal and protects the fuselage from penetration.

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However, a pattern of similar accidents with this same engine on the same model 777 has emerged that explains the decision to ground the jets for inspections.

A very similar incident occurred on Feb. 13, 2018, during a flight from San Francisco to Honolulu, when an engine fan blade broke off on another Boeing 777-200 also operated by United, with the same Pratt & Whitney engine.

And in December, two fan blades broke off in flight on a Japan Airlines 777-200 with the same engine on a flight from Naha to Tokyo.

In both these prior accidents, the planes also landed safely without injuries.

In the case of the United flight to Hawaii, UAL Flight 1175, the inlet duct and fan cowls separated from the engine, just as in Saturday’s incident in Denver.

Investigation of UAL 1175 revealed a metal fatigue fracture that began beneath the surface on the interior of the hollow core fan blade.

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Alarmingly, indications of a nascent fracture on that broken fan blade had been noted during two previous engine overhauls.

In 2010, inspectors using thermal acoustic imaging noted a small indication at the location of the origin of the crack. Then again in 2015, records show that there was a larger indication in the same area — the precise location where the 2018 crack originated and turned into a fracture.

On both those occasions, the inspectors attributed the indication to a defect in the paint that was used during the inspection process and allowed the blade to return to service.

The training provided to the inspectors was judged inadequate. Pratt & Whitney said it was working to correct that, and the FAA mandated repetitive inspections on similar fan blades.

In the case of the Japan Airlines accident in December, Flight JA8978, one blade broke off near the root and the other halfway along, just as in the Denver accident.

That accident is too new to have a definitive indication of the cause. Still, clearly the similarity with what happened in Denver spurred the Japanese regulator to act quickly Sunday by grounding all similar 777-200s with the same engines.

This article has been updated to reflect that Boeing 777 jets continued to be built with Pratt & Whitney engines until 2013.