Metal fatigue apparently played a role in the engine explosion on a United Airlines flight over Denver on Saturday, Robert Sumwalt, chairman of the National Transportation Safety Board, said in a virtual press conference Monday night. The explosion of Flight 328 caused pieces of the Pratt & Whitney PW4000 engine casing to fall on suburban neighborhoods.
According to CBS Denver, Sumwalt said the engine made a loud bang and started shaking about four minutes after takeoff from Denver International Airport. He said the plane was about 12,000 feet above houses at the time.
Sumwalt said two fan blades in the engine broke – one at the base where it meets the hub, and the second about halfway through. He said the first blade caused “overuse damage” to the second blade.
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One of the knives was found on a football field.
Sumwalt said researchers will try to determine how long the blades suffered from the fatigue.
According to Federal Aviation Administration data, the aircraft in question was nearly 26 years old, CBS Denver says, but FAA and NTSB officials said they could not clarify whether the engine that failed was an original part or was later installed.
Photos taken Saturday after the plane returned to the Denver airport show a gaping hole under the right wing, suggesting some engine debris hit the plane itself.
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Sumwalt acknowledged that the bottom of the plane was damaged, but said the damage was not structural. The damaged part is a composite fairing that smoothes the face to make it more aerodynamic.
Sumwalt said the cockpit fire lever was activated and two fire bottles were discharged into the engine. He said the fuel flow was turned off and investigators will investigate what continued to fuel the fire despite the precautions taken.
The maintenance history is also looked at during the engine investigation.
Sumwalt added that the agency will compare this event to others in the past.
“Our mission is to understand not only what happened, but why it happened so that we can prevent it from happening again,” Sumwalt stressed.
More than 200 passengers were on the flight to Hawaii. No one was injured on the plane or on the ground. The plane made a safe return to the airport approximately 20 minutes after takeoff.
The Denver incident followed an incident in December involving a Japan Airlines 777 with the same engine type and an engine problem during a United flight in February 2018.
“There may be a common theme” between the three incidents, “but until the investigation is completed, we don’t know,” said Scott Hamilton of aviation news site Leeham News.
Boeing said on Monday that all 128 of the 777s with the same Pratt & Whitney engines were grounded worldwide after Saturday’s emergency landing. Sixty-nine were in service and 59 were in storage.
In addition to United, which retired 24 aircraft, the affected airlines were Japan Airlines and All Nippon and South Korean Asiana and Korean Air.
Egyptian state newspaper Al Ahram reported on Monday that the national airline Egyptair grounded four planes with the Pratt & Whitney engine, although they were not in use, a source close to the manufacturer said.
British Transport Secretary Grant Shapps announced a temporary ban on aircraft with Pratt & Whitney 4000-112 series engines from entering British airspace.
Agence France-Presse contributed to this report