The National Transportation Safety Board revealed the right engine failed minutes after takeoff from the Denver airport when the aircraft's pilots throttled up the engines "to minimize time in expected turbulence."
"Immediately after the throttles were advanced a loud bang was recorded" on the cockpit voice recorder, CNN reported.
The new details are part of an update from the NTSB in its ongoing investigation, which typically takes a year or more to complete. The seven-page report did not make any conclusions about the cause of the incident nor did it prescribe further steps for the Federal Aviation Administration, aircraft operators, or the engine manufacturer to make.
A warning light indicated a fire in the engine, according to the update. The pilots followed a procedure to fight the fire, and determined they would not dump fuel to make the aircraft lighter before landing. They concluded that "the magnitude of the overweight landing was not significant enough to outweigh other considerations," the NTSB said.
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The report said a valve that cuts fuel flow to the engine in case of fire had properly closed and said "there was no evidence of a fuel-fed fire." The report cited damage to the "fuel, oil, and hydraulic lines."
The plane returned to Denver International Airport safely with no injuries on board or on the ground, where debris from the airplane rained down on a residential area in Broomfield, Colorado. The report said the engine flared up in flames after landing but that "was quickly extinguished" by firefighters.
The NTSB report said the fan blade that failed was inspected using specialized thermal acoustic imaging technology in both 2014 and 2016. The inspection looks for tiny cracks or signs of metal fatigue which may not be visible to the naked eye.
After a 2018 engine incident on a different plane, the 2016 data was analyzed again, the NTSB said.
The report noted that when the fan blade failed last month, it was less than halfway to the point of requiring another inspection -- a detail CNN has previously reported. It had been operated 2,979 cycles, an approximate measure of how many times the engine has been turned on and off. Inspection was required after 6,500 cycles.
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Days after the February incident, engine manufacturer Pratt & Whitney recommended dramatically shrinking the interval for the inspection to just 1,000 cycles, according to a service bulletin obtained by CNN. The FAA issued an emergency directive requiring the fan blades on the engines to be inspected before flying again.
The inspection interval for this engine series has been a concern for federal regulators. CNN previously reported a Federal Aviation Administration review board met just days before the February engine failure to consider requiring more regular inspections.
In the wake of the engine failure, Boeing has recommended suspending the use of 777s that have the Pratt & Whitney 4000 engine, and United Airlines has already pulled its 777s following the incident. Both the FAA and the NTSB are investigating.
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