NTSB details what part failed on Boeing 787 Dreamliner
The National Transportation Safety Board on Wednesday revealed what part of the General Electric engine failed during a Boeing 787 Dreamliner pre-flight taxi test in North Charleston last month — but not why.
How a jet engine works — and broke
Air enters through the front of the engine, passing through the big fan blades. It is then compressed, injected with fuel and combusted. That energy turns the turbines.
The high-pressure turbine drives the compressor and the low-pressure turbine, at the very back of the engine, drives the fan in the front.
The incident last month involved the fracture of the shaft that runs through the middle of the engine, connecting the low-pressure turbine to the fan.
Source: TECOP International Inc.
The investigative agency determined that one of the GEnx-1B engine's central drive shafts “fractured” toward the front of the engine between its booster and compressor.
“As a result of the investigative work to date, the NTSB has determined that a fan mid-shaft on the failed GEnx engine fractured at the forward end of the shaft, rear of the threads where the retaining nut is installed,” the release stated.
That failure led to debris ejecting from the back of the engine, igniting a small fire near the runway and closing Charleston International Airport for more than an hour on the afternoon of July 28.
What caused the break and what it means for General Electric, Boeing and all the planes either flying the engine type or with GE-powered 787s on order is not yet clear.
Analysts said while the update forecloses some possibilities, it's still too early to say how major a problem GE and Boeing have on their hands. All Dreamliners being assembled in North Charleston use the GE engine.
Hans Weber, a California aviation consultant who has advised the Federal Aviation Administration on jet engine inspection procedures, said a failure at the threaded area where the two engine shafts are screwed together could have been caused by a one-time machining error that can easily be corrected or by something more problematic.
“But we have to wait for the result for what really caused the failure,” Weber, president of TECOP International Inc., cautioned. “We're not out of the woods yet with this one. It's not negative news. It's news that points in a positive direction, but until we know what really happened, we're not out of the woods. Because you can also argue that there's a design flaw that the dimensions of the threaded zone are inadequate ... you could hypothesize that, too.”
Scott Hamilton, of Issaquah, Wash.-based aviation consulting firm Leeham Co., agreed it's still too early to draw any definitive conclusions.
“While they have determined what happened, there's a lot of work they have to do to determine why it happened,” Hamilton said, “to narrow down whether it's a one-off situation or a systemic situation, and if it's a systemic situation, how widespread is it, how many engines are affected ... and so on. How's this corrected? This raises many more questions than have been answered so far.”
NTSB investigators in Cincinnati are still examining the engine and a metallurgical analysis of its components, and their counterparts in Washington, D.C., are analyzing the affected plane's cockpit voice recorder and flight data recorder. They have also begun to look at the engine's manufacturing and assembly records.
“This investigation is ongoing,” Wednesday's update stated. “The information released today is factual in nature and does not include any analysis. Additional factual information may be released as it is developed.”
GE Aviation spokesman Rick Kennedy said once the shaft broke, the engine “did exactly what it's supposed to do” by spewing the pieces of the low-pressure turbine straight out the back of the engine in what's known as a contained engine failure.
Weber said that's what happened in this instance but said a shaft breakage can cause the attached turbine to “overspeed,” which can lead to a catastrophic failure in which engine pieces pierce the engine casing and either puncture the plane's fuel tanks or fuselage.
Shaft fractures are “exceedingly rare,” Kennedy said. This was the sixth such incident in a GE engine in a decade, or 600 million flight hours, he said.
Kennedy said the shaft came from a Japanese supplier, IHI Corp., which is part of the ongoing investigation into why such a new part would fail. “They're a long-time supplier with GE and they're doing a fabulous job,” he said.
He said the NTSB update would have no effect on the four Japan Airlines 787s with the GEnx-1B engines now in service or the 747-8 jets that are powered by GEnx-2B engines.
“No, the planes are still flying today and there haven't been any emergency inspections at this juncture,” Kennedy said. “We're not aware of any issue with the engines that would hazard the safe flight of those aircraft.”
Boeing spokesman Doug Alder said the company can't comment on the investigation. But, he said, “I can tell you that we're working very closely with the investigative team and GE.”
While questions remain, Wednesday's update means that the problem likely originated in General Electric's supply chain, not at Boeing South Carolina, where the engine was installed on the second S.C.-assembled Dreamliner earlier this year, according to Weber, the engine expert.
“Based on what I know, I don't see how it's Boeing's fault at all,” he said. “Boeing, I'm sure, is sitting there very impatiently for GE to do its work and find out what went wrong.”
Reach Brendan Kearney at 937-5906 and follow him on Twitter at @kearney_ brendan.