An oil leak in a Rolls-Royce Trent 900 engine has been identified as the cause behind a partial power loss on a Qantas Airways Airbus A380 this February.
The leak, which led to a gradual decrease in oil quantity in the number four engine, prompted the aircraft's flight crew to reduce the powerplant to idle thrust during a Singapore-London flight on 15 February. The aircraft, registration VH-OQC, did not experience further oil level variation during the rest of the flight.
Inspections after the incident showed that there was an oil leak from an external high pressure / intermediate pressure (HP/IP) oil tube at its connection to the engine case, said the Australian Transport Safety Bureau (ATSB).
The ATSB add that it is now conducting a "holistic investigation" into the oil leak and other reported cases of oil leaks in Trent 900 engines.
Checks of the tube involved in the February incident showed that the attaching nut had less than the required torque, said the ATSB. "A torque check of the tube revealed the attaching B-nut was at 80 pound force/inch. The correct torque value for the B nut was 240 pound force/inch," it added.
It is not certain how the attaching nut had less than the required torque. An uncontained engine failure last November involving another Qantas Trent 900 engine had led to inspections of all of the airline's Trent 900s. These inspections involved the removal of the HP/IP oil tube to allow for an internal borescope inspection of the engine, said the ATSB.
"Specific torque procedures for the re-installation of the tube were required on completion of the borescope. A review of the operator's maintenance documentation confirmed the correct installation procedure had been carried out," it added.
That inspection on the engine involved in the partial power loss was conducted 20 days before the leak developed, said the bureau. During this time, the engine completed 20 cycles and about 241 flight hours. There was no other maintenance carried out on the oil tube during the 20-day period.
During the incident, the engine's oil quantity began decreasing three hours into the flight while the aircraft was cruising near New Delhi.
The quantity continued decreasing gradually to about 3.5 quarts at about 7.5 hours into the cruise, said the ATSB. The oil pressure had dropped to 75 psi from 100 psi. The flight crew was then advised by the airline to reduce the engine to idle thrust for the rest of the flight, which stabilised the oil pressure at 45 psi.
Upon arrival at London, only 0.7 quarts of oil remained, said the ATSB. The oil tube in the engine was subsequently replaced.
The incident was not an isolated one as Qantas and another A380 operator have reported a total of seven HP/IP oil tube leaks due to "low torque of the attachment fitting", said the ATSB.
"All of these incidents were following removal / installation of the oil tube to facilitate a borescope installation," it added.
Shortly after the 15 February incident, another incident involving decreasing oil quantity on another Qantas A380 was reported on 24 February. The engine in that incident, which also took place on a Singapore-London flight, was also reduced to idle thrust.
"As a result of the multiple events, the ATSB initiated an investigation... to conduct a holistic investigation into the factors surrounding these failures," said the bureau.
The ATSB is still investigating last November's uncontained Trent 900 engine failure, which has been traced to a manufacturing defect in an oil feed pipe in the IP turbine module.