The “uncontained failure” that last month sent an engine disk ripping through an American Airlines jet with nearly catastrophic results is the first case of that specific part failing. It has spurred a hunt for microscopic cracks in similar disks, but answers may be elusive.

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The nearly catastrophic explosion of an engine that caused a fire on an American Airlines Boeing 767 just short of takeoff in Chicago on Oct. 28 occurred when a specific part that had never before failed broke into pieces.

The breakup of a heavy metal disk that rotates in the engine core reveals a new risk to airline passengers that, however rare, has already spurred a scramble by engine-maker General Electric and government safety agencies to find and fix the vulnerability.

“The risk will be eliminated,” said John Goglia, a former National Transportation Safety Board (NTSB) member with more than 40 years’ experience in the aviation-safety industry. “Will it be eliminated before the next one? That’s the real question. Everything relies upon the timeliness of the system to correct itself.”

American Airlines Flight 383 to Miami was speeding down the runway at O’Hare International Airport in Chicago when the second-stage disk of the high-pressure turbine broke apart inside the right engine with a loud explosion that blew metal shrapnel out through the engine casing.

Hot metal ripped through the wing. A fire ignited fed by spilling aviation fuel.

In an update Friday, the NTSB described how the pilot aborted the takeoff just seconds before reaching a speed at which he would have had to leave the ground. Slamming on the brakes as the jet reached a speed of 154 miles per hour, he brought the plane to a halt within 25 seconds and 900 yards further down the runway.

Moving quickly, the cabin staff evacuated all 161 passengers and nine crew using the escape slides on the undamaged left side of the aircraft. Twenty people suffered minor injuries.

Behind them on the runway, the black smoke billowing skyward and the blaze melting the right wing into a drooping mess testified to how close the passengers had come to a large-scale tragedy.

A flying ax

GE spokesman Rick Kennedy stresses the reliability of the CF6 engine involved. The latest models, introduced in the 1980s, currently power about 4,000 widebody jets, including Boeing 767s, 747s and Airbus A330s, and have flown for more than 220 million hours.

“The CF6 is the workhorse engine of jumbo jets on a level never experienced before,” Kennedy said.

Yet the danger posed by a so-called “uncontained engine failure” cannot be overstated.

The disk that broke apart in Chicago — one of two disks in what’s called the high-pressure turbine that drives the engine’s air compressor — is about 2 feet in diameter and weighs more than 100 pounds. At full power for takeoff, the temperature in the high-pressure turbine reaches more than 2,200 degrees.

The pod around the engine has Kevlar armor around the big fan at the front, designed to contain any blades that fly off. But the pod cannot possibly contain the enormous energy of flying metal ejected from an engine-core breakup.

The NTSB said Friday one piece of the broken disk went through the right wing, then arced high over the airplane and came crashing through the roof of a UPS warehouse almost 1,000 yards away.

“Any time a turbine disk fails, it’s a big deal,” said Goglia.

In 2006, a CF6-80 engine — the same model as in the Chicago incident — on another American Airlines 767 that was powered up during maintenance, exploded on the ground in Los Angeles, with no passengers aboard. A big piece of the high-pressure turbine disk in the left engine pierced the engine casing, sliced through the airplane’s lower fuselage and embedded itself like an ax-head in the casing of the right engine on the other side of the airplane.

That jarring incident came after two other CF6-80 high-pressure turbine-disk breakups.

In 2000, as a US Airways 767 was undergoing engine maintenance on the ground in Philadelphia, a disk rupture split the left engine clean in two and ignited a fire under the wing.

In 2002, it happened in the air. The left engine of an Air New Zealand 767 blew up six minutes after takeoff from Brisbane, Australia, damaging the wing flaps and forcing an emergency landing. Luckily, there was no fire that time, and all 200 people on board were safe.

A new vulnerability

In all three of those CF6-80 incidents, the disk that broke up was the first of two inside the turbine. Although the two disks look alike, the first stage is next to the hottest part of the engine core and must withstand the highest temperatures and air pressure.

After the 2006 incident, the government safety system kicked into high gear. The Federal Aviation Administration (FAA) issued an Airworthiness Directive ordering urgent inspections of these disks inside older CF6-80 engines.

Subsequently, GE redesigned the first-stage disk to take more stress, adjusting the shape and the composition of the sophisticated nickel alloy.

All planes flying today have been retrofitted with that redesigned disk, said GE’s Kennedy.

“That’s behind us,” he said.

But the disk that exploded in Chicago was the second-stage disk.

This disk is subject to lower temperatures and air flow, so it undergoes relatively less stress than the first.

“There has never been an Airworthiness Directive associated with a second-stage disk,” said Kennedy. “This is new.”

That’s what’s raised alarm bells at the NTSB and the FAA.

The NTSB said that particular disk had been flying for 18 years and completed almost 11,000 flights, short of the limit of 15,000 cycles after which it must be replaced.

Goglia and other industry experts said the sudden breakup almost certainly stemmed from a microscopic crack, caused by a flaw in the metal invisible to the eye, that grew over years of service.

Using electron microscopes, metallurgists at the NTSB Materials Laboratory in Washington, D.C., are now scanning the pieces of the broken stage-two disk, 95 percent of which was recovered.

“They’ll examine the metal down to the tiniest element to determine the cause,” said Goglia.

On Friday the NTSB said the metallurgists have already found “features consistent with fatigue cracking” at a point next to the central bore of the disk. This crack was initiated at a point in the metal that differs from the composition of the material around it, referred to as an “inclusion” or flaw in the metal.

“Ongoing metallurgical examinations of the disk will focus on detailed characterization of the inclusion and the fracture surfaces,” the NTSB said.

Such a flaw could have originated in tiny impurities in the bar of metal from which the parts were made almost two decades ago, or it may have been introduced during the forging or machining processes.

Metals supplied for critical aerospace use are typically melted and resolidified twice to get rid of impurities. The process is meticulously recorded so that, for every part in the engine, GE knows exactly where the metal came from and when it was produced.

Likewise, every person with the smallest role in the manufacturing of any piece of the engine records and signs off on their work.

“The investigators will look at all aspects of making and machining the disks,” said Kennedy. “There’s a whole paper trail.”

Kennedy said Friday that the company is “chasing down a very finite number of disks” made from the same billet of metal as the one that failed and has told airlines in a letter that only one such disk remains in an engine currently in operation.

Company investigators will inspect the disk in that engine for microscopic cracks and will likely widen their inspections to more engines as the hunt for a cause continues.

If any such cracks are found, the FAA will demand replacement of the disks and also review routine engine-maintenance practices to ensure such cracks are detected in future.

A frustrating search

So how closely scrutinized are these engine blowouts, and how challenging will it be to find and fix the cause?

In 2010, GE redesigned and retrofitted a different part inside older CF6-50 series engines, after an NTSB urgent safety recommendation that followed four serious incidents overseas involving failure of that part.

For the CF6-80 engine models like the one in the Oct. 28 explosion, the FAA in 2009 ordered additional inspections of all critical rotating parts, including the two high-pressure turbine disks.

Until the Chicago incident, there had been no uncontained failures of the CF6-80 for a decade. During that time, the NTSB reports, there were seven other uncontained engine failures in the U.S., mostly less serious ruptures, in a variety of different engines.

But one was similar in its outcome to the Chicago incident: The explosion of a GE-90 engine on a British Airways 777 jet taking off in Las Vegas in September 2015 also ignited a fearsome fire after an aborted takeoff.

That was the first-ever uncontained failure of a GE-90 engine, and so likewise spurred a deep investigation.

Yet GE’s Kennedy said that despite intense scrutiny of a widening circle of similar GE-90 engines, investigators have not been able to find a related issue in any other engine. More than a year later, the root cause of the Las Vegas blowup remains a mystery.

Before the NTSB and GE investigators drilling into the Chicago incident can recommend measures to prevent a recurrence, they need to find the origin of the hidden flaw in the metal.

The Las Vegas investigation shows that’s not easy.