The National Transportation Safety Board said the aviation industry needs improved engine-inspection methods to avoid missing the type of metal flaw that blew apart a jet engine shortly before takeoff.
The aviation industry needs improved engine-inspection methods to avoid missing a type of flaw in metal parts that caused a near-catastrophic breakup of a jet engine on a Boeing 767 in 2016, the National Transportation Safety Board (NTSB) said Tuesday.
The NTSB also pointed to shortfalls in how the American Airlines flight crew handled the emergency after the fire that broke out as the airliner about to take off in Chicago that fall.
And NTSB members criticized the behavior of passengers who tried to haul luggage off the burning plane.
On Oct. 28, 2016, the right engine of the Boeing jet exploded as the plane accelerated down the runway. The pilot aborted his takeoff and brought the plane to a halt within 900 yards, with the engine and wing on fire.
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All 161 passengers and nine crew aboard the plane were evacuated via escape slides to the ground on the left side of the plane.
Twenty people suffered minor injuries, and one passenger, a 77-year-old man, suffered multiple broken bones and a blow to the head when knocked over by the jet blast from the left engine, which had not been shut off.
Explosion on takeoff
The fire began when a heavy metal disk that spins at great speed within the engine core broke into pieces with explosive force. The metal pieces pierced the sides of the GE engine, a highly dangerous outcome known as an “uncontained engine failure.”
The failed disk — in the engine’s high-pressure turbine — was recovered in four pieces. One 57-pound chunk was found more than a half-mile away.
The explosion sent hot metal ripping through the wing, igniting a fire fed by spilling aviation fuel. The blaze eventually destroyed the airplane.
The report highlighted various missteps in the subsequent evacuation.
The pilot shut off the right engine as he brought the plane to a stop. But following a checklist that doesn’t distinguish between engine failure in the air or on the ground, he didn’t immediately stop the engine on the left side.
At that point, flight attendants reported they could not reach the cockpit. Two of them said they couldn’t operate the intercoms to phone the pilots. American had 13 different models of intercoms on its fleet, and flight attendants weren’t trained on each model.
As the fire flared on the right side, flight attendants began evacuating travelers via escape slides on the left side while the left engine was still running.
The NTSB report says alarmed passengers were climbing over seats and rushing for the exits, some carrying their baggage despite instructions to leave it behind.
NTSB Chairman Robert Sumwalt said “one passenger even resisted a flight attendant attempting to take away a carry-on bag … in a burning airplane.”
“Things can be replaced,” Sumwalt said. “People can’t.”
A flawed disk
Investigators determined the engine explosion was caused by a subsurface defect in the turbine disk — a rare but known issue referred to in the industry as a “dirty white spot” — produced during the manufacturing process in 1997.
Such a flaw typically leads to fatigue cracks over time.
Still, it escaped detection during routine in-service engine-maintenance inspections in 2007 and 2011.
The disk was made from an ingot of nickel-based alloy produced in a standard process.
The metal was inspected twice during its production using ultrasonic equipment that would have shown any subsurface cracks or voids in the metal. None were found.
When the engine entered service, American Airlines mechanics inspected the disks twice more during routine maintenance, using equipment designed to detect only surface and near-surface flaws. None were found.
When the in-service inspections were performed the cracks had still not reached the surface, the NTSB concluded.
NTSB engine expert Pierre Scarfo said during the presentation Tuesday that if the in-service inspections had used some subsurface detection method, such as ultrasonic equipment, “the cracks would have been detected before the disk had failed.”
After the accident, GE ultrasonically inspected all engine parts made from the same ingot of metal and found no further defects.
A GE study determined that improvements in the manufacturing process produced fewer subsurface anomalies in ingots of this nickel alloy produced after 2000.
In September, following GE’s lead, the Federal Aviation Administration (FAA) proposed a rule that will mandate ultrasonic inspection of all turbine disks in this type of engine produced before 2000.
The NTSB said the FAA and industry should study whether enhanced ultrasonic methods are required for inspections during manufacture of the nickel alloy.
“Current inspection methods … need a closer look,” Sumwalt said Tuesday.
The safety agency also recommended that airlines provide training to improve communication and coordination between the flight and cabin crews in an emergency situation. American said in an email that it already has done so.