A Qantas jet bound for Sydney was forced to turn back to San Francisco after an engine problem punched a hole in the metal shell around it.

Flight 74, carrying 212 passengers and a crew of 18, left San Francisco for Sydney at 11.05pm on Monday local time (4.05pm AEST on Tuesday) and returned at 12.45am (5.45pm AEST), San Francisco airport duty manager Carol Spear said.

Photo: Channel Ten

 

About 15 to 20 minutes into the flight, the crew noticed "excessive vibration" in engine No. 4 of the Boeing 747-400, Qantas Airways spokesman Simon Rushton said in Sydney.

"After some initial troubleshooting, they made the appropriate decision to shut the engine down," Mr Rushton said.

"They also made the appropriate decision to seek priority clearance to return to San Francisco."

The pilot dumped fuel over the Pacific as a precaution, he said.

There were no injuries during the landing, Mr Rushton said, and the plane was able to pull up to the aerobridge at the terminal so passengers could get off the plane.

Engine failure a very rare event: Qantas

Mr Rushton said engine failures were "very rare events" and there was no fire. He could not say whether similar incidents had occurred to other Qantas planes.

"Engineers have determined the engine does need replacement, and they are checking to see what caused the problem," he said.

"We are ... dispatching a replacement engine early this afternoon to San Francisco along with a team of Qantas engineers who will also take the replacement and begin the investigations into what has happened."

But an engine surge can often cause what appear to be flames. The flames created a large hole in the engine cowling, the metal shell around the engine.

An aviation expert, John Nance, told the San Francisco Chronicle the incident was an "uncontained engine failure" that was "an extremely rare event".

He said Boeing 747s were designed to "lose three of four engines and still be able to get back".

Crew looked alarmed: passenger

Passenger Nolan Goldstein told local television station KTVU-TV she "heard a thud, a bump and the plane veered off to the left".

"It was very uncertain for a period of time until the captain announced that we had an engine that blew up. ... It was a real uncomfortable vibration at first and then the plane began to shake a bit."

"It wasn't turbulence," passenger Neil Dufty told KTVU. "It seemed that there was some pretty serious damage. People on the right-hand side of the plane seemed very shocked. Apparently, there were flames coming out of the engine.

Another passenger, Elizabeth Thomas said she heard a "very strange noise" that was "hard to describe".

"I don't travel that much but I knew it was very wrong," she told KTVU. "The staff looked a little alarmed and began to move very quickly."

Mr Dufty said the pilot "assured everybody that the plane was still relatively safe".

"There was serious damage in one of the engines. We jettisoned some fuel and turned around. The captain made a very good landing. We all cheered."

Mr Rushton said passengers were given hotel accommodation and meals.

"We are transferring them to Los Angeles to pick up Qantas flights to Los Angeles tonight to Australia."

The Federal Aviation Administration (FAA) in the US said Qantas will prepare a report into the incident for the FAA and the National Transportation Safety Board.

"Whatever Qantas does is going to have to meet with our approval too," an FAA spokesman said.

"The bottom line is we want to make sure and know that that aircraft is airworthy when they are saying they want to put it back on line."