After a British Airways 777’s jet engine broke up explosively on a runway last month, investigators have pinned down the precise location of the initial fracture.

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A month after a British Airways 777’s jet engine broke up explosively on a Las Vegas runway, investigators have pinned down the precise location of the initial fracture in the innards of the engine.

A person with knowledge of the investigation said inspections of engines on some older 777s are expected to be ordered in the next couple of weeks, as soon as investigators decide which of several hundred engines of this type are the most urgent priority.

A National Transportation Safety Board (NTSB) update Tuesday confirmed that the incident was a rare and dangerous “uncontained engine failure” that began with a break in the eighth of 10 spinning disks that form the high-pressure compressor inside the GE-90 engine.

These rotating disks compress the air sucked in by the large fan at the front of the engine, before the air is mixed with injected fuel and ignited.

GE spokesman Rick Kennedy said the engine involved was an early model GE-90 that powers older 777s.

He said about 400 engines of this early GE-90 type are now in service on 167 airplanes.

Scrambling to understand what happened, investigators from the Federal Aviation Administration (FAA) and GE are trying to narrow down the possible causes and the number of engines to be inspected.

Disk breakup

As the 777 jet began its takeoff run on Sept. 8 with 157 passengers and 13 crew, the engine exploded, severing fuel lines and catching fire.

The outcome could have been catastrophic if the pilots had not reacted well or if the engine had broken up while in the air.

Capt. Chris Henkey, 63, a 42-year veteran, aborted the takeoff, slammed on the brakes to bring the 777 to a stop, issued a mayday call for firefighting support and ordered passengers to evacuate.

As fire engulfed the left side of the airplane, all on board were able to exit on emergency-escape slides from the right side, with only a few minor injuries.

The most dangerous aspect of the incident is that hot metal shrapnel from the broken parts pierced the protective pod around the engine.

That protective pod, called a nacelle, is engineered to withstand and contain a blade coming off the fan at the front.

However, the inner parts in the core of the engine move with such speed and energy that if they break up, no nacelle is strong enough to hold them, said retired Boeing propulsion engineer Barry Latter.

GE-90 uncontained engine failure

“When an engine is certified, it’s certified for blade containment, not disk containment,” Latter said. “If one of those disks lets go, there’s so much energy in it, you just can’t stop it. It’ll go right through anything.”

Such an uncontained failure is unusual but dramatic.

In 2006, as an American Airlines 767 maintenance crew conducted a maximum-power ground test with no passengers on board at Los Angeles airport, a GE engine on the left wing exploded when a disk in the high-pressure turbine broke into four pieces.

One piece of the disk was found deeply embedded in the right engine, on the other side of the airplane. Another piece landed on a vacant lot nearly 900 yards away.

Urgent inspections

The Las Vegas incident was the first uncontained engine failure in the 20-year history of the GE-90 engine.

GE’s Kennedy said the engine involved entered service in 1997, but the compressor disk that failed was built in 1995, the first year the GE-90 was produced.

Kennedy said that in addition to examining the parts from the British Airways engine at a facility in Evendale, Ohio, GE investigators are separately inspecting a set of 38 compressors of similar vintage and similar usage that had previously been removed from 777s and warehoused for refurbishment.

“They are looking for any anomalies, any wear, any usage characteristic that could help us, give us a clue to determine which ones to inspect in the field,” Kennedy said.

In 2000, a different GE engine on a 767 suffered an uncontained engine failure as a Varig Brasil Airlines flight took off in São Paulo, Brazil.

Just two months later, the NTSB recommended that the FAA issue an airworthiness directive “to require the expeditious removal from service” and inspection of engines of the same type that were deemed “most at risk of rupturing.”

Once investigators have determined their priorities in last month’s Las Vegas incident, they’ll order field inspections around the world, Kennedy said.