Passengers on a United Airlines flight to Hawaii have described hearing a "bang" before the plane started "really started shaking", with one calling it the "scariest flight of my life".

Flight 1175, which was travelling to Honolulu from San Francisco, landed safely on Tuesday in Hawaii after an engine cover came off during its flight from California, the airline said.

"Our pilots followed all necessary protocols to safely land the aircraft," United said in a statement, adding that all passengers departed the Boeing 777 normally at the gate.

The airliner landed as emergency responders waited nearby, said Hawaii Department of Transportation spokesman Tim Sakahara.

Images posted on social media show an engine with the exterior cover missing as the plane approaches Honolulu. Video shows the engine shaking back and forth with pieces of the cover flapping in the wind.