"They let us know that we had to brace for impact in case there was a rough landing", said Ms Sudiacal, who was travelling with her four-month-old son, her husband Tim and his parents. She added that the flight UA1175 crew "did a great job" in guiding passengers through the "scary" situation.
The plane managed to land safely in Honolulu 22 minutes late at 1.02pm (10.02am AEDT) while Aircraft Rescue and Firefighting arrived to inspect the cause of the incident.
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The airline didn't specify what the problem was. but images posted to social media, apparently by passengers on the flight, showed what appeared to be the right engine of a Boeing 777 without its front cowling.
United said it was "fully cooperating" with investigations by the Federal Aviation Administration and the National Transportation Safety Board.
A passenger onboard a flight from San Francisco to Honolulu has captured the moment when bits of the plane's engine began falling off.
The Boeing 777 is powered by Pratt & Whitney's PW4000-112 inch engines, the engine's manufacturer said.
The plane was able to land safely and there were no reports of injuries. "The flight landed safely and all passengers deplaned normally at the gate.?" The FAA will investigate the incident.