Passengers on board a United Airlines flight from San Francisco to Honolulu on Tuesday (13 February) had a terrifying experience after the casing around one of the engines ripped off mid-flight over the Pacific Ocean. The pilots of United flight 1175 declared an emergency due to vibration in the right engine.

The passenger jet, with more than 370 people including crew members on board , made an emergency landing at Honolulu International Airport, even as fire fighters and aircraft rescue personnel were ready and waiting, Hawaii News Now reports.

united airlines
The plane with engine problems is seen taxiing on the tarmac in Honolulu, Hawaii Twitter/Peter Lemme/via REUTERS

Passengers said the malfunction took place about 40 minutes before the plane was scheduled to land when they heard a loud bang.

"Everyone on our side flung open their windows just to see what it was," Haley Ebert, one of the passengers, told the New York Times. "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."

It is not clear what caused the piece that covers the plane's engine, known as the cowling, to blow off during the flight. Ian Gregor, a spokesman for the Federal Aviation Administration, said the agency is investigating the incident, Reuters reports.

"Our pilots followed all necessary protocols to safely land the aircraft," United said in a statement. "The aircraft taxied to the gate and passengers deplaned normally."

Several passengers on board the Boeing 777-200 have taken to social media to share photos and videos captured from the flight.

Google engineer Erik Haddad, who was on the flight, threw in a few puns and jokes when tweeting out a few photos of the incident.

"That looks bad, plane and simple," Haddad wrote.

In another tweet, he posted a photo of the plane's safety manual with the exposed engine in the background saying: "I don't see anything about this in the manual."

"Scariest flight of my life," marketing consultant Maria Falaschi tweeted along with a photo of the damage. "It was about 45 minutes until we landed. Heard a big bang and the plane started to shake violently. The pilots and crew did a great job at keeping us updated."

This isn't the first time an aeroplane engine has broken apart in mid-air.

Last year, an Airbus A380 superjumbo jetliner from Paris to Los Angeles suffered a mid-flight engine explosion forcing the Air France flight to make an emergency landing in Goose Bay.

In November 2010, a Qantas A380 from Singapore to Sydney made an emergency landing after a Rolls-Royce engine exploded shortly after take-off - an incident that grounded all six A380s in Qantas' fleet for nearly a month.