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Dramatic video on social media showed a jet engine rattling away without its protective covering on a United Airlines flight that eventually landed safely in Hawaii on Tuesday.

The pilot of Flight 1175 from San Francisco to Honolulu called for an emergency landing because of a mechanical issue, the airline told NBC News.

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The airline didn't specify what the problem was. But video and 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.

"There was a loud bang ... and then the plane really started shaking," passenger Allison Sudiacal, who was traveling with her husband, her 4-month-old son and her parents-in-law, told NBC affiliate KHNL of Honolulu.

"It was like rattling, and the plane was kind of shaking, like boom, boom, boom," she said, adding that aircraft crew kept everyone informed. Her husband, Tim, told the station that the flight was "absolutely terrifying."

United said that the flight landed safely on schedule at Daniel K. Inouye International Airport in Honolulu and that all passengers disembarked normally.

The Federal Aviation Administration said the pilot "declared an emergency due to a vibration in the right engine." It said it would investigate the incident, as did the National Transportation Safety Board.

The Hawaii Department of Transportation said aircraft rescue and fire crews responded "as a precaution," adding that airport operations and runways were unaffected.

Erik Haddad's view outside the window of United Airlines Flight 1175, which lost the cowling on its right engine on Tuesday. Erik Haddad / via Twitter

NBC News aviation expert John Cox, former executive air safety chairman of the Air Line Pilots Association, said that losing an engine cowling or casing wouldn't normally affect an engine's performance and that the plane and everyone on board wouldn't have been in any danger.