Passengers on a United Airlines flight to Hawaii have described hearing a "bang" before the plane "really started shaking", with one calling it the "scariest flight of my life".
Flight 1175, which was on a scheduled five-and-a-half-hour flight to Honolulu from San Francisco, landed safely on Tuesday in Hawaii after an engine cover had come off during the flight from California, the airline said.
"Our pilots followed all necessary protocols to safely land the aircraft," United said in a statement.
It added that all the passengers had departed the Boeing 777 normally at the gate.
The airliner landed as emergency responders were waiting nearby, said Hawaii Department of Transportation spokesman Tim Sakahara.
Images were posted on social media showing an engine with the exterior cover missing as the plane was approaching Honolulu.
Video shows the engine shaking back and forth, with pieces of the cover flapping in the wind.
"There was a loud bang ... and then the plane really started shaking," one of the passengers, Allison Sudiacal, told Hawaii News Now.
"There was a loud boom and then it was like rattling, and the plane was kind of shaking, like boom, boom, boom," she told the news outlet.
Ms Sudiacal was travelling with her four-month-old son, her husband, Tim, and his parents.
"They let us know that we had to brace for impact in case there was a rough landing," she said. "It was scary. But they did a really good job," Ms Sudiacal added.
Erik Haddad, a Google engineer who was on board the flight, posted a picture of the exposed engine on Twitter.
He jokingly tweeted: "I don't see anything about this in the manual."
Another of the passengers, Maria Falaschi, said that the crew did a "great job" but it was the "scariest flight of my life".
"You could tell everyone was scared. But the crew and pilots did a great job," she added.
Passenger Haley Ebert told the 'New York Times' that she heard a "huge bam" as part of the casing came away from the engine.
"Everyone on our side flung open their windows, just to see what it was," Ms Ebert said. "The casing to the engine had sort of flown off. There were pieces flying into the ocean, nuts and bolts flying out a little bit. A bolt hit the wing, and it just made this huge bam."
An emergency was declared due to a vibration in the right engine, said Federal Aviation Administration spokesman Ian Gregor.
In an email, he added that the agency will investigate the incident.