'Fatigue fracture' caused Qantas blast
Sydney - A Qantas jet carrying 231 passengers and crew suffered an engine failure at 25 000 feet due to the fatigue fracture of a turbine blade, an inquiry found on Monday.
The Australian Transport and Safety Bureau (ATSB) report said the number-4 engine of the Boeing 747-438 failed 15 minutes after take-off from San Francisco on 20 August 2010, causing severe vibrations throughout the plane.
Sparks and flames spurted from the engine's exhaust, punctured its casing and released debris but the flight, which was headed to Sydney, landed safely back in San Francisco after crew shut down the problem turbine.
The ATSB said it agreed with the findings of engine manufacturer Rolls-Royce that the most likely sequence of events was triggered by the fatigue failure of a single turbine blade.
"An investigation conducted by the engine manufacturer found that the engine failure was initiated by the fatigue fracture of a single stage-2 low pressure [LP] turbine blade," the ATSB report said.
"The ensuing rotor imbalance caused the LP turbine bearing to fail, which ultimately resulted in the uncontained release of debris."
Rolls-Royce said a similar incident with that particular turbine blade failing due to a fatigue cracking mechanism had never occurred in some 40 million hours service over 23 years.
More robust turbine
But after its investigation it instructed operators to fit a more robust LP turbine bearing to reduce the chance of it happening again.
The ATSB said the engine's last overhaul was in May 2009 and both the LP turbine bearing and turbine blades were maintained according to the maintenance manual.
"Neither part had a specified maximum service life and were maintained on-condition," it said, adding that number-4 engine data from previous flights showed no abnormal indications.
The report said when the engine failed it ruptured both sides of the turbine case and debris punched through the largest hole and hit the underside of the wing, but caused only superficial scratches.
The August 2010 event came just weeks before Qantas suffered an explosive mid-air engine blast in one of its Airbus A380 planes after take-off from Singapore which saw the carrier ground its A380 fleet for safety checks.