* Singapore Airlines: engines changes are "precautionary"
* Qantas A380s powered by same Rolls-Royce engines, grounded
* Qantas to update on its A380 fleet by Thursday
* Rolls-Royce shares down 7 pct since Qantas engine failure (Adds comments from Singapore Air CEO in para 7, other details)
By Harry Suhartono and Balazs Koranyi
SINGAPORE/SYDNEY, Nov 10 (Reuters) - Singapore Airlines will replace engines on three of its Airbus A380 planes after finding oil stains on them, almost a week after Australian rival Qantas grounded its A380 fleet due to an engine failure.
Qantas's six A380s have been grounded since Thursday, when a Rolls-Royce engine partly disintegrated mid-flight, forcing the fully laden Airbus A380 to make an emergency landing in the biggest incident to date for the world's largest passenger jet.
Investigations into that incident have focused on oil leaks inside the Rolls-Royce Trent 900 engines, the same model used to power Singapore Airlines' and German Lufthansa's A380 fleet. Lufthansa said on Wednesday its A380 flights were on schedule.
But Singapore Airlines stressed the problems on three of its 11 A380s were precautionary and unrelated as the oil stains were different from the oil leakage in the Qantas turbines.
Singapore's findings may deal a fresh blow to Rolls-Royce, which moved on Monday to contain a crisis of confidence, saying it was making progress in understanding what caused last week's blowout on the Qantas A380 flight.
"This is a precautionary move to find out what caused the oil stains." a Singapore Airlines spokeswoman said. "Rolls-Royce recommended a detailed inspection of the engines."
Asked if Singapore Airlines was confident the engine maker had identified the cause of the problem, CEO Chew Choon Seng told reporters: "I'm quite assured at the highest level that Rolls Royce is doing all that it takes to get to that point."
Singapore Airlines said later its other Airbus A380 aircraft were still in service, but could not rule out further engine changes. It said the engines on the affected aircraft would be swapped in two days.
"At this stage there is no indication that more engines on our A380 aircraft will need to have precautionary engine changes carried out, but I would certainly not rule it out," airline spokeswoman Bryony Duncan-Smith told Australian radio.
INVESTIGATION FOCUSES ON DESIGN FAULT
Rolls-Royce shares have already lost more than 7 percent since the Qantas incident, while Airbus and Qantas shares both slipped 2.5 percent each. Singapore Airlines shares closed 1.2 percent lower on Wednesday.
A spokesman for Rolls-Royce declined to comment.
Experts said the Qantas investigation is pointing to a design fault with the engine which may not be difficult to fix but will take time.
"From information provided to date, it would appear to be a design issue and not a power setting issue. Lower power settings are not the solution," said Peter Marosszeky, an aviation expert at the University of New South Wales.
He added similar engine issues are not unknown to the aviation industry and a fix could be relatively easy, but it could take some time.
"Until then, what could happen is Airbus would allow operators to fly the A380 on a limited basis with restrictions on the engines," Marosszeky added.
However, restriction could be a problem for Qantas, as its routes from Los Angeles to Australia are the longest served by the A380, and lower power limits would mean weight restrictions, making flights less economical.
Sources earlier told Reuters Qantas is reviewing the way it operates its A380s engines and whether its higher power settings on take-off contributed to the engine failure.
Singapore Airlines' three affected airplanes, scattered in London, Melbourne and Sydney, are being flown back to Singapore to be equipped with similar Rolls-Royce engines.
Chief Executive Chew told a media and analysts conference that the aircraft should arrive in the city-state on Wednesday evening and there were enough replacement engines available to carry out the swap.
Qantas is expected to give an update on its A380 fleet by Thursday afternoon. The airline said on Monday it would ground its six planes for at least 72 hours.
"We are still continuing with checks," a Qantas spokesman said on Wednesday.
On Tuesday, a Boeing 787 test flight made an emergency landing in Texas with smoke in the cabin, the first incident of its kind, putting additional scrutiny on the already delayed programme. ($1=1.291 Singapore Dollar) (Additional reporting by Dhara Ranasinghe in SINGAPORE and Michael Smith in SYDNEY; Editing by Jean Yoon and Anshuman Daga) ((kevin.lim@thomsonreuters.com; +65 6403 5663; Reuters Messaging: kevin.lim.reuters.com@reuters.net))