UPDATE 5-Oil leak blamed for new Qantas A380 mishap-sources

* Airbus A380 plane was bound for London from Singapore

* Engine problem traced to external oil leak-sources

* Rolls was aware of weakness and seeking fix-sources

* One year since blowout grounded Qantas A380 fleet

By Narayanan Somasundaram and Mahmoud Habboush

SYDNEY/DUBAI, Nov 4 (Reuters) - A London-bound Qantas A380 was forced to divert to Dubai after en engine problem on Friday -- a year to the day since a mid-air engine blowout prompted the Australian airline to ground its entire fleet of Airbus superjumbos for nearly a month.

British writer and comedian Stephen Fry was on board flight 31 which landed safely on three engines. He immediately began tweeting, turning the event into a social media hit but capping a nightmare week for Qantas.

"Bugger. Forced to land in Dubai. An engine has decided not to play," he told his 3.3 million followers on Twitter.

"Not a great week for Qantas," he added.

Qantas has just emerged from the grounding of its entire fleet over the weekend following a long-running labour dispute.

The weekend shutdown stranded almost 70,000 passengers, forcing the government and the nation's labour tribunal to intervene and ban all further strikes at Qantas.

Friday's engine problem was traced to a leak in one of the external oil pipes surrounding the engine, industry sources familiar with a preliminary investigation told Reuters.

Engine manufacturer Rolls-Royce had already identified the potential weakness and was talking to customers about providing a solution before Friday's incident, they said.

The sources spoke on condition of anonymity because the preliminary results of investigations have not been annnounced.

A Rolls Royce spokesman said the company was working closely with Qantas to provide appropriate support and technical assistance and declined to comment on the cause.

Friday's flight, with 258 people on board, had an "oil quantity defect" in one engine which was switched off according to standard procedure, a Qantas spokeswoman in Sydney said.

The location of any leak was seen as important because of fears of a repeat of another leak which caused a fire and explosion on a Rolls-powered Qantas A380 exactly one year ago.

Technicians discovered however that the latest leak was in a cooler and safer part of the engine, the sources said, even though it caused disruption that Airbus, Rolls and airlines will be anxious to avoid repeating as they seek to lift the superjumbo's image.

Qantas said the two incidents were not related.

The airline said in a letter distributed to passengers and posted on Twitter by Fry that engineers would take a number of hours to conduct "mandatory inspections."

It added it was rebooking the passengers on alternative carriers.

"Qantas... have said they shut down the engine to check oil and that there was no explosion," an aviation industry source familiar with events said.

A second source with knowledge of events said Qantas identified the glitch about 90 minutes after take off.

"If it had been really serious they would have turned back and not flown on to Dubai," the source said. "Engine systems are so sophisticated now they can flag any discrepancy so airlines can act."

ONE YEAR ON

A Qantas Airbus A380 aircraft suffered an engine explosion on Nov. 4 last year shortly after leaving Singapore for Sydney. It returned to Singapore and landed safely.

"The two issues are completely unrelated. This is a one-off and we will look to get the aircraft back in the skies as soon as possible," Qantas spokeswoman Olivia Wirth said on Friday.

Each Qantas A380 is powered by four Rolls Royce engines. The carrier has 10 A380s in service and is due to take delivery of two more by the year-end. It also has two more on order and deferred the delivery schedule for six others.

REBOOKED

Fry later tweeted that he was on a flight to London with Emirates, which operates the world's biggest fleet of A380s but powered by engines supplied by a joint venture between General Electric and Pratt & Whitney .

An analyst who wished not to be named said Fry's Twitter feeds inflated the incident.

"This doesn't seem like a big deal," said the analyst. "It seems like a precautionary measure. But the fact that it is Qantas again and on the same date as last year with Stephen Fry tweeting about it has made it a big deal."

In last year's engine blowout, a turbine disc disintegrated and sent supersonic shrapnel through the aircraft's wing, severing systems and narrowly missing the cabin.

Investigations pinpointed a manufacturing fault in an oil pipe which could lead to oil leaks and ordered safety checks.

Rolls-Royce replaced or upgraded dozens of engines.

Rolls Royce engines power the A380 fleet of Qantas, Singapore Airlines , and Lufthansa and China Southern .

Airbus has sold 236 A380s. By the end of September this year it had delivered 57. The four-engined double-decker airplanes sell for $375 million each at list prices.

Qantas shares closed up 2.5 percent at A$1.62, in line with the broader market.

Shares in Rolls-Royce fell 0.2 percent to 712 pence. Airbus parent EADS fell 1.5 percent to 21.33 euros.