Passengers on a United Airlines Plane Have Experienced Real Horror in the Sky

High over the Pacific Ocean on Tuesday, the casing blew off one of the engines on United Airlines Flight 1175.

Passengers heard a loud bang and felt the plane shake violently. Those seated on the right side looked out their windows and saw pieces of metal flying. By the time the plane touched down safely in Honolulu around 40 minutes later, the engine was bare, its innards on full display.

It was not clear as of Tuesday evening what had caused the malfunction. Ian Gregor, a spokesman for the Federal Aviation Administration, said the agency was investigating.

Haley Ebert, who was in a window seat above the right wing, said she had raised her window shade after hearing a "huge bang" that she likened to a gunshot.

"Everyone on our side flung open their windows just to see what it was," Ms. Ebert said in a phone interview. "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."

Michael Nielson, seated in the middle section of the twin-engine Boeing 777, said flight attendants told passengers to return to their seats immediately and fasten their seatbelts. "The plane shook violently for the better part of five minutes," he said in an email. Later, "the shaking subsided to constant, heavy vibration that stuck around for the rest of the flight."

Ms. Ebert said the shaking was more intense than she had ever experienced, even on turbulent flights. "Back and forth, down one side and down to the other side," she said. "The whole thing felt like it was a roller coaster going to go off the tracks."

United said in an emailed statement that the plane, traveling from San Francisco to Honolulu, had made...