No one was injured in yesterday's incident, which comes after a Rolls-Royce engine on one of the airline's Airbus A380s superjumbos disintegrated in flight last week.
Qantas spokesman Simon Rushton said the latest incident was not related to the A380 problem and that the landing was strictly a precaution.
He said the plane involved was a Boeing 767 fitted with two engines built by General Electric. It was carrying 234 people.
Rushton said the flight crew noticed vibrations in the left hand engine about 5 minutes into the flight from the western Australian city of Perth for Melbourne in the southeast. They decided to turn back after doing in-flight checks and landed safely in Perth.
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