Investigations into an uncontained engine failure on a Qantas Airways Boeing 747-400 in August 2010 have found that it was caused by the fatigue fracture of a low-pressure turbine blade.
The ensuing rotor imbalance had caused the low-pressure turbine bearing to fail and the debris ejected ruptured the turbine case and fairings, producing a large perforation in the right side of the engine nacelle, says the Australian Transport Safety Bureau (ATSB).
Debris ejected through the hole also scratched the aircraft's wing skin and bent its leading-edge flaps.
The aircraft, registration VH-OJP, had departed from San Francisco International Airport for Sydney in Australia on 30 August 2010 when its number four engine failed.
The engine was shut down and the aircraft returned without incident to San Francisco.
The aircraft had four Rolls Royce RB211-524G2-T engines. The number four engine had accumulated 5,059 hours and 518 flight cycles since its last overhaul in May 2009.
The Australian investigators found that the high-service time, low-pressure turbine blades had reduced fatigue endurance because of vibratory stresses sustained during operations at high speeds.
"It was likely that as a result of the reduced fatigue endurance limit, a single stage two low-pressure turbine blade fractured and separated from the low-pressure disc during engine operation," says ATSB.
Vibratory stresses from the low-pressure turbine blade loss then resulted in the turbine support bearing to collapse, allowing contact between the low-pressure and intermediate-pressure turbine shafts. This led to the severing of the intermediate-pressure turbine shaft, resulting in the uncontained engine failure.
Rolls Royce has since instructed operators of the RB211-524 engine variants to fit a more robust low-pressure turbine bearing during the next maintenance check to reduce the likelihood of an engine failure resulting from rotor imbalance.
The engine manufacturer is also recommending that operators replace high-service time, stage two, low-pressure turbine blades to increase their fatigue endurance.
ATSB also found that the cockpit voice recording of the event was not available as it was on a continuous loop and recording of the failure event had been overwritten.
The Australian Civil Aviation Safety Authority has since amended the civil aviation order to require air operator's certificate holders to preserve cockpit voice recorder or flight data recorder data for 30 days in case of reportable events.