Preliminary findings from an investigation into Air New Zealand Dreamliner engine incidents late last year have found they relate to global problems known to Roll-Royce but that occurred earlier than predicted.
The New Zealand Transport Accident Investigation Commission report finds that besides damage to the engine on a flight to Tokyo on December 5, the aircraft's wing and fuselage suffered some damage when spayed with parts from the engine although it was minor.
A flight to Argentina the next day also suffered an engine anomaly.
"The Commission's continuing inquiries into these incidents suggest that the failures were consistent with a known problem with unmodified Trent 1000 engines. Rolls-Royce has been replacing engine components with a new design, and managing the safe operating life of unmodified engines using a risk analysis model," said the commission's manager of Air Investigations, Peter Williams.
After the December incidents Rolls-Royce reduced the number of flights Air New Zealand could make under its risk analysis model, he said.
"But obviously it would have been preferable that the model had taken the engines out of service before the December incidents could occur."
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After the December incidents Air New Zealand had also voluntarily reduced the maximum time to a diversion airport for which it would fly its aircraft with unmodified engines.
Since the Air New Zealand events, the European Aviation Safety Authority (EASA), which certifies the engines, also directed that aircraft should not be powered by two unmodified engines which had flown similar numbers of flights.
Since the commission began consulting on the interim report and associated draft urgent recommendations, further action by EASA and the United States Federal Aviation Authority meant final recommendations were not needed, Williams said.
According to Rolls-Royce there had been six in-flight intermediate pressure turbine blade separations in Trent 1000 engines worldwide before the Air New Zealand incidents. All eight incidents have occurred during the take-off or climb phases of flight when engines are subjected to the highest stress.
According to Rolls-Royce, the blade separations have followed cracking in the blade shank that has been initiated by corrosion, the commission said.
Rolls-Royce said it was "likely that a combination of environmental and operational factors are involved and that these may be operator specific".
Rolls-Royce has been replacing blades in the Trent 1000 single-stage intermediate pressure turbine modules with redesigned blades made from a different alloy and with an improved corrosion protective coating. It has been using the risk analysis model to determine how many flights unmodified engines may make safely.
Problems with another part of some Rolls-Royce Trent 1000s have also been identified and have disrupted flights for Air New Zealand and other airlines.