Investigators have closed probes into engine problems on two Air New Zealand Dreamliners which had failed turbine blades and led to nearly two years of disruption for its fleet.
The Transport Accident Investigation Commission (TAIC) says it has resolved its inquiries into two incidents in December 2017 involving Rolls-Royce Trent 1000 engines on Boeing 787s.
Deputy chief commissioner Stephen Davies Howard says the commission resolved its inquiries because it was satisfied that Rolls-Royce had addressed the safety issues identified in an interim report published in May 2018.
"The Commission resolved that further lines of inquiry would be unlikely to identify further circumstances with significant implications for transport safety; or to enable us to make further findings or recommendations for safer transport in future," said Davies Howard.
Soon after takeoff, the flights to Argentina and Tokyo experienced problems in engines which had to be shut down and the aircraft were forced to return to Auckland.
In each incident, a turbine blade in the engine's intermediate-pressure turbine (IPT) fractured and broke off due to corrosion fatigue cracking caused by a combination of blade design or construction, engine operation and contaminants in the air.
On the Tokyo flight, the broken blade significantly damaged the engine and small pieces of ejected metal struck (without significantly damaging) the tail plane, wing and fuselage.
On the Buenos Aires flight, the damage was mainly confined to the engine.
"The Commission's early investigations prompted Rolls-Royce to improve its system for forecasting when the fatigue might happen," said Davies Howard.
By the time of the interim report, Rolls-Royce was already replacing affected blades.
To address the turbine blade "corrosion fatigue" issue, the engine manufacturer published a service bulletin to replace all affected 114 blades within a single turbine disc. The replacement blades were of a different design, using an alloy that had been used extensively before in other engine types. The new blades were also fully encapsulated by a protective coating to help prevent corrosion.
Like the two New Zealand incidents, six other failures had all involved the same type of IPT blade, and all had occurred soon after take-off or in the climb phase of flight.
Rolls-Royce has now replaced the affected turbine blades in 99 per cent of the global flying fleet.
The disruption caused to Air New Zealand by sending engines to Singapore was severe and the airline had to re-jig its international schedule with available planes from its 14-strong Dreamliner fleet.
"These actions by the engine manufacturer were sufficient to address the safety issues,"
said Davies Howard.