The plane has been taken to a hangar, the department said, and the incident didn't spur any delays at the airport. "A bolt hit the wing, and it just made this huge bam", Haley Ebert, a passenger on the flight, said in a phone interview to NY Times.
"Scariest flight of my life", she tweeted, along with photos of the engine and its missing cover.
The FAA and National Transportation Safety Board are investigating the incident.
The aircraft landed safely on one engine and passengers evacuated with no reported injuries sustained.
In a separate photo, the laid-back passenger added: "I don't see anything about this in the manual".
Passenger Allison Sudiacal told the station: "There was a loud bang...and then the plane really started shaking".
"The pilots and crew did a great job at keeping us updated", Ms Falaschi added. By the time the plane landed safely in Honolulu around 40 minutes later, the engine was bare as its insides were on full display.
United Airlines confirmed that the flight had been cleared for an emergency landing at Honolulu airport due to a "mechanical fault". After the worrying noise, pilots immediately sent out a distress call before making the emergency landing.
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