The National Transportation Safety Board revealed that the proper engine failed a few minutes after takeoff from the Denver airport, when the aircraft’s pilots kicked the engines “to minimize time in the expected turbulence.”
“Immediately after the throttles were pushed in, a loud bang was recorded” on the cockpit voice recorder, CNN reported.
The new details are part of an update from the NTSB in the ongoing investigation, which typically takes a year or more to complete. The seven-page report did not contain any conclusions as to the cause of the incident, nor did it prescribe further steps to be taken by the Federal Aviation Administration, aircraft operators or the engine manufacturer.
A warning light indicated a fire in the engine, according to the update. The pilots followed a fire-fighting procedure and decided not to dump fuel to lighten the plane before landing. They concluded that “the magnitude of the overweight landing was not large enough to outweigh other considerations,” the NTSB said.
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The report said a valve that cuts off fuel flow to the engine in the event of a fire was properly closed, saying “there was no evidence of a fuel supply.” The report cited damage to the “fuel, oil and hydraulic lines.”
The plane returned safely to Denver International Airport with no injuries on board or to the ground, where debris from the plane rained down on a residential neighborhood in Broomfield, Colorado. The report said the engine went up in flames after landing, but was “quickly put out” by firefighters.
According to the NTSB report, the fan blade that is defective was inspected in both 2014 and 2016 using specialized thermal acoustic imaging technology. The inspection looks for small cracks or signs of fatigue that may not be visible to the naked eye.
After a 2018 engine incident on another aircraft, the 2016 data was re-analyzed, the NTSB said.
The report noted that when the fan blade broke last month, it was less than halfway to the point that it needed another inspection – a detail CNN has previously reported. It has completed 2,979 cycles, an estimate of the number of times the motor has been turned on and off. Inspection was required after 6,500 cycles.
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A few days after the incident in February, engine manufacturer Pratt & Whitney, according to a service bulletin obtained by CNN, advised drastically shortening the inspection interval to just 1,000 cycles. The FAA has issued an emergency directive requiring that the fan blades on the motors be inspected before flying again.
The inspection interval for this engine range is a concern for federal regulators. CNN previously reported that a Federal Aviation Administration review committee met just days before the engine failure in February to consider the need for more regular inspections.
In the aftermath of the engine failure, Boeing recommended discontinuing use of 777s with the Pratt & Whitney 4000 engine, and United Airlines has already hauled its 777s after the incident. Both the FAA and NTSB are investigating.
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