Qantas engineers are inspecting one of its Boeing 747s following the failure of an engine shortly after the plane's departure from Sydney for Los Angeles.

Passengers reported loud noises and flames trailing from flight QF149 on Feb. 3 about 45 minutes after take-off. Observers on the ground reported seeing smoke and flames coming from the plane.

Engine No. 3, on the right-hand side, ceased working, forcing the plane to return to Sydney after 90 minutes in the air, during which time the fuel load was dumped.

A Qantas spokesman said it could take several days to discover what went wrong. He said all safety protocols were followed. Passengers were given snacks and ice-cream in an attempt to keep them calm.

"It is a rigorous and extensive inspection process," spokesman Lloyd Quartermaine said. "The issue with the engine has not been resolved. At this stage, we are still analysing and examining it."

While a few of the more than 270 passengers were placed into vacant seats on a scheduled service to Los Angeles later in the day, Qantas was forced to put on a replacement service, which left at 9.30pm (AEST) that night.

Qantas denied there had been an explosion and described the problem as similar to an engine backfiring. "At this stage, we believe it was an engine surge, which can lead to a flame jetting out the back of it, but it is not an engine explosion," Mr Quartermaine said.