Passengers on United Airlines Flight 1175 were shaken but landed safely yesterday afternoon after the plane lost engine parts while en route to Honolulu from San Francisco.

The Boeing 777 departed San Francisco at about 9 a.m. for an approximately 5-1/2-hour flight to Honolulu and arrived at the gate at 1:02 p.m., 22 minutes late, according to flight tracker flightaware.com.

“United Flight 1175 traveling to Honolulu from San Francisco landed safely after the pilots called for an emergency landing because of a loss of the engine cowling (the covering of the engine),” United said in a statement. “Our pilots followed all necessary protocols to safely land the aircraft. The aircraft taxied to the gate and passengers deplaned normally.”

United said it would fully investigate and cooperate with the Federal Aviation Administration and National Transportation Safety Board, but could not comment on what the cause was.

“We are currently reaching out to all of our customers on this flight, and plan to fully refund every ticket,” United said in an email.

Several passengers posted photos and videos of the engine on social media while in flight.

Passenger Maria Falaschi (@mfalaschi), a marketing consultant from the San Francisco Bay Area, tweeted several photos of the plane’s engine and wrote, “Scariest flight of my life #ua1175.”

Erik Haddad (@erikhaddad), an engineer at Google who lives in San Francisco, posted a video on Twitter of the damaged engine while in the air, saying, “I don’t see anything about this in the manual #ua1175,” adding later that he had landed safely. He also posted a clear photo of the damaged engine after landing in a follow-up tweet, saying, “For those fellow flight enthusiasts wanting to see more detail #ua1175.”

Another passenger, Haley Ebert (@haleylora), posted video of the damaged engine and violently shaking wing, saying, “So glad we are all safe after our emergency landing.”

State transportation spokesman Tim Sakahara said in a statement, “The plane landed safely with Aircraft Rescue and Fire Fighting personnel standing by as a precaution. The plane has been taken to the hangar. There is no impact on airport operations or runways.”

Aviation expert Peter Forman of Honolulu said he has heard of an engine cowling coming off an aircraft before but that it is a rare occurrence. In cases in which the cowling was missing, as well as other cases in which an entire engine has failed, Forman said the airplane is still flyable and able to land safely.

“I would call it a potentially serious matter,” Forman said. “Although the airplane will fly perfectly without the cowling, the lack of the cowling can create secondary problems; for example, it could have damaged other parts of the plane when it blew off.”

The pilots would need to monitor systems to make sure there was no secondary damage and to ensure there were no fuel leaks.

Still, he said, “This type of failure is not normally taught in recurrent training for pilots, so pilots have to do some troubleshooting on the way down and assess what else could have been damaged.”

Though he was not a witness, Forman said that from what he could observe, the “pilots handled the situation right.”

Another aviation expert, Peter Lemme, had just arrived in Honolulu from Seattle for a business trip when he heard of the United flight on Twitter. He went to the gate where the plane was and, observing it from there, said it appeared the first-stage fan was still attached, but he could not tell whether there was any blade damage.

The plane, he said, taxied away on its own power, without being towed.

A similar event happened on a Southwest Airlines 737 a few years ago, Lemme said, but with a completely different engine.

“Usually an engine imbalance causes excessive vibration, which could have caused the inlet to break off,” he wrote in an email. “The imbalance is usually from a blade breaking off, which by design should be contained. Inlet cowls should not be breaking off.”

FAA spokesman Ian Gregor said in an email that the plane declared an emergency due to a vibration in the right engine and that it will investigate.