Singapore Airlines has grounded three Airbus A380 superjumbo jets after tests revealed oil stains in the plane's Rolls-Royce engines. The move comes just two days after Qantas announced troubling oil leaks in its A380s.

The engine of a Singapore Airlines Airbus A380 is checked at the airport in Zurich, Switzerland. Tests have uncovered oil stains in three Rolls-Royce engines on Singapore Airlines' A380 superjumbos. (Steffen Schmidt/Keystone/Associated Press)The oil on the Qantas and Singapore planes was discovered during tests prompted by the explosion of a Rolls-Royce engine on a Qantas A380 during a flight from Singapore to Sydney last week. The plane made a safe emergency landing in Singapore, but the Australian airline immediately grounded its entire fleet of A380s while it investigated the cause.

Singapore Airlines said it does not know whether the oil stains found in its engines have any connection to the engine oil leaks found on Qantas but was temporarily pulling the planes from service as a precaution. The planes, in Melbourne, Sydney and London, will be flown without passengers to Singapore, where they'll be fitted with new engines.

'We apologize to our customers for flight disruptions that may result.'— Nicholas Ionides, Singapore Airlines

"We apologize to our customers for flight disruptions that may result and we seek their understanding," airline spokesman Nicholas Ionides said in a statement.

Twenty planes operated by Qantas, Germany's Lufthansa and Singapore Airlines use the Trent 900 engines. With the decision by Singapore, nine aircraft with the engines have been grounded.

On Monday, Qantas CEO Alan Joyce said tests had uncovered oil leaks in the turbine area of three engines on three different A380s. The leaks were abnormal and should not be occurring on new engines, he said. All six of the Australian airline's A380s remained grounded Wednesday.

Rolls-Royce advises review of Trent 900 engine

London-based Rolls-Royce, an aerospace, power systems and defence company that is separate from the manufacturer of Rolls-Royce cars, had recommended a series of checks for the Trent 900 engines used in the A380s operated by Qantas, Singapore Airlines and Germany's Lufthansa.

An engine blowout on this Qantas A380 has led to closer examination of the Rolls-Royce Trent 900 engine. (Vivek Prakash/Reuters)Singapore Airlines grounded its entire fleet of 11 A380s following last Thursday's engine explosion on Qantas, but after initial checks, returned them to service Friday. On Wednesday, however, based on fresh analysis of the tests, Singapore took three of its A380s out of service again because of oil stain results.

Singapore's eight other A380s, also flying with Trent 900 engines, remain in service.

Lufthansa spokesman Thomas Jachnow said the airline was aware of the Singapore problem, but its maintenance crews had not found any oil leaks in their Trent 900 engines.

However, the German airline did say it had replaced the Rolls-Royce engine on one of its A380s after detecting a problem it said was not connected to the oil leaks that grounded the Qantas and Singapore superjumbos.

The most likely cause of an oil leak or stain is a tiny crack in the oil supply pipe that lubricates the engine's bearings, said John Page, an aircraft designer and senior lecturer in aerospace engineering at the University of New South Wales in Sydney.

Oil leaks could pose serious danger

Oil leaks in an engine's turbine area can spark fires, which could then put too much stress on the rapidly spinning engine parts and lead to a breakdown. There are many possible culprits behind a crack, such as poor design or chafing from the pipe rubbing against another engine part.

The Australian Transport Safety Bureau is leading an international investigation of the incident. The agency has said it is trying to find a missing piece of a turbine disc that could help explain what happened.

The Qantas and Singapore incidents are not the first problems Rolls-Royce has faced with its engines. In September 2009, a Singapore Airlines A380 was forced to return to Paris mid-flight after an engine malfunction. Last August, a Lufthansa crew shut down one of its engines as a precaution before landing in Frankfurt after receiving confusing information on a cockpit indicator.