TAIC confirms two Air New Zealand Boeing 787 inflight occurrences part of known worldwide problem; says regulator actions make urgent recommendations unnecessary.
The New Zealand Transport Accident Investigation Commission (TAIC) says that two Air New Zealand 787 aircraft engine occurrences in December 2017 were caused by a known problem occurring earlier than predicted by the manufacturer’s risk analysis model.
On 3 May 2018, the Commission published an Interim Report (.pdf under Document downloads) for its continuing inquiry into the two events.
“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,” says 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, but obviously it would have been preferable that the model had taken the engines out of service before the December incidents could occur,” Mr Williams said. 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 a draft of today’s interim report and associated draft urgent recommendations, further action by EASA and the United States Federal Aviation Authority meant final recommendations were not needed,” Mr 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. Rolls Royce said it is 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.