Introduction
On 5 December 2017, an Air New Zealand Boeing 787-9, registration ZK-NZE, departed Auckland on a scheduled passenger flight to Narita, Japan. During the climb phase, the right engine began to spool down. The flight crew completed the necessary checks, shut down the engine, and returned to Auckland International Airport without further incident. The Transport Accident Investigation Commission (Commission) opened inquiry AO-2017-009.
On 6 December 2017, another Air New Zealand Boeing 787-9, registration ZK-NZF, departed Auckland on a scheduled passenger flight to Buenos Aires, Brazil. During climb, the flight crew received engine over temperature alerts for the right engine. The crew reduced the thrust lever for the right engine to idle and returned to Auckland International Airport without further incident. The Commission opened inquiry AO-2017-010.
Investigation
The Commission was made aware of six previous Trent 1000 intermediate pressure turbine (IPT) blade separations worldwide, all involving the same blade type and occurring soon after take-off or during climb. According to the engine manufacturer, the blade separations followed cracking in the blade shanks initiated by a 'corrosion fatigue mechanism'. The manufacturer believed the mechanism involved a combination of blade design or construction, engine operational factors, and environmental air contaminants.
Both engines were Rolls-Royce Trent 1000-J2 (Package C) engines. Engine number 10231 from the first incident and engine number 10227 from the second incident were sent for teardown examination at Singapore Aero Engine Services Private Limited. The affected turbine blade sets were later examined at the Rolls-Royce laboratory in Derby, England. Borescope inspections found that a turbine blade in the IPT module had fractured and separated from the IPT disc, with features indicating corrosion fatigue cracking.
Damage
In the first occurrence, the released blade caused significant damage to the IPT and low-pressure turbine modules. Small pieces of turbine and stator blades were ejected through the exhaust nozzle and struck the leading edge of the right horizontal stabiliser. Some pieces also chipped the underside of the wing and the side of the fuselage towards the rear, but these did not cause significant damage. In the second occurrence, damage was mainly confined to the IPT module.
Safety Actions
The Commission issued a draft interim report with recommendations addressing early safety issues. The European Aviation Safety Agency (EASA), Federal Aviation Administration (FAA), and New Zealand Civil Aviation Authority (CAA) took safety actions that addressed the issues, so no final recommendations were issued. The engine manufacturer published a service bulletin to replace all IPT blades with redesigned blades made from a different alloy and with an improved corrosion-protective coating. A Corrosion Fatigue Lifing (CFL) model was developed to predict crack propagation and engine removal schedules. This action was mandated by EASA airworthiness directive 2017-0056.