A United Airlines plane was able to land safely in Hawaii Tuesday afternoon after one of is engine casings blew off over the Pacific Ocean during a flight from San Francisco to Honolulu.

According to Federal Aviation Administration spokesperson Ian Gregor, Flight 1175, a Boeing 777 out of San Francisco International Airport, declared an emergency due to a vibration in its right engine. The plane landed on the runway at Honolulu International Airport 12:40 p.m. local time Tuesday afternoon without incident, he said.

AP

An FAA official told NBC News that it appears the engine cowling or casing came off during the flight. The engine does not appear to have come apart and appears to still be operating normally, the official said.

A San Francisco couple said flight attendants told them to brace for impact.

"I saw a bunch of other people crying around me," Tim Sudiacal said. "I think we were bracing for the worst, like, what if this actually lands on the water."

His wife, Delia, added:

"My husband and I were just praying because we had never gone through this experience. I really thought we were going to die."

The FAA is investigating the incident, and the NTSB has sent a team of two to look into how the cover might have come off. In a follow-up statement, the agency said the flight landed safely after the pilots called for an emergency landing because of a loss of the covering of the engine. "Our pilots followed all necessary protocols to safely land the aircraft. The aircraft taxied to the gate and passengers deplaned normally," a spokesperson told NBC News.

NBC News aviation expert John Cox said that losing an engine cowling should not impact the performance of the engine. The plane and everyone on board should not have been in danger, Cox said.

Max Trescott, a certified flight instructor agreed.

"This sounds like the cowling had not been properly secured the last time maintenance had occurred, and so it's great that it was something like that as opposed to an engine shutting down, which would be a bigger issue," he said.

Google engineer Erik Haddad, who was on the United flight, tweeted out photos and videos.[[473983853, C]]

"Aloha," Haddad tweeted at 2:45 p.m., along with a photo of parts of the engine coming apart.

"That looks bad, plane and simple," he tweeted next, with a video of the engine casing flapping in mid-air. That tweet was followed by another photo of Haddad holding an emergency manual in front of the window, with the malfunctioning engine part in the foreground. "I don't see anything about this in the manual," the tweet said.

Haddad's tweet was retweeted more than 400 times within the hour. He also responded to various media outlets, letting everyone know he was safe.

Haley Ebert, who was also on the same flight with her sister, tweeted out a video of the cowling coming off, along with a photo of the exposed engine after the plane landed on the ground.

"When the engine blows off during her flight," she captioned her video.

Later she tweeted: "#ua1175 so glad we are all safe after our emergency landing."

Another passenger, Maria Falaschi, tweeted out more pictures of the casing coming off, captioning it "scariest flight of my life." Falaschi told reporters passengers heard a big bang and the plane started to shake violently. "The pilots and crew did a great job at keeping us updated," she said.

According to Falaschi, the incident took place about 45 minutes before the flight landed at Honolulu airport.

"Where did that engine's cowling go?" a Twitter user asked her.

"It's floating somewhere in the Pacific Ocean...," she responded.

NBC Bay Area's Ian Cull and NBC News contributed to this report.

