US investigators have concluded that a manufacturing defect of a high-pressure turbine stage two disc caused the uncontained failure of a GE Aviation turbofan engine on an American Airlines Boeing 767-300 in 2016.
The US National Transportation Safety Board, which released its finding on 30 January, says new inspection methods could likely have detected the subsurface defect on the CF6-80C2B6 before it failed.
"Through extensive examination of the disk fragments at the NTSB lab in Washington, investigators determined there was a subsurface defect in the disk at the time of manufacture," says the NTSB. "Because of the nature of the defect and the limits of inspection methods, the NTSB concluded the defect was likely undetectable when the disk was produced in 1997."
The failure occurred as the American 767, registration N345AN, accelerated for takeoff from Chicago O'Hare airport, bound for Miami, on 28 October 2016.
Fragments of the right engine's disk ruptured a fuel feed line and punctured the fuel tank, causing a fire. The pilots aborted takeoff and stopped the aircraft on the runway, and 161 passengers and nine crew evacuated, with one passenger suffering serious injuries.
"The passenger who was seriously injured sustained those injuries as a result of evacuating the airplane, as directed by a flight attendant, and encountering jet blast from the engine that was still running," says the NTSB.
The NTSB determined that ultrasonic inspections of the disk performed during production were likely unable to detect defects. A company called ATI Specialty Materials manufactured the nickel-alloy disk in 1997, it says.
American Airlines also in January 2011 performed "eddy current" and "fluorescent penetrant" inspections of the disk, but those tests are unable to detect subsurface defects, says the NTSB.
However, the NTSB says newer "enhanced ultrasonic inspection techniques, such as multi-zone and phased-array inspections, could better detect internal defects".
"If a subsurface ultrasonic inspection had been required at the time of the disk’s last inspection, the cracks that developed… would most likely have been detectable," it says.
The FAA issued a proposed airworthiness directive in September 2017 that would mandate ultrasonic inspections of CF6-80 disks.
GE in 2017 also issued a service bulletin calling on airlines to perform ultrasonic inspections at regular shop visits of all CF6-80C2 first and second stage high-pressure turbine disks produced before 2000.
The company says some 1,400 pre-2000 CF6-80C2 disks remain in service and that about 50 of those disks have been inspected. But it notes that the older aircraft are being continuously retired and that the "AA incident was a very rare occurence".
"Over the past 17 years, the melt processes for advanced alloys used in these components have improved significantly," says GE.
Contacted for comment, American Airlines cited a 30 January media release noting that the "NTSB determined that the engine failure was caused by a manufacturing defect in the GE engine that could not be detected by the manufacturer’s Federal Aviation Administration-approved inspection requirements".