What happened
In early December 2017, two separate incidents involving Air New Zealand Boeing 787-9 aircraft occurred during the climb phase of flight from Auckland. On 5 December, the aircraft registered ZK-NZE experienced a reduction in speed in the right engine. The flight crew performed necessary checks, shut down the engine, and successfully returned to Auckland International Airport.
Less than 18 hours later, on 6 December, another Boeing 787-9, registered ZK-NZF, departed Auckland for Buenos Aires. During the climb, the crew received over-temperature alerts for the right engine. Following standard procedures, the crew reduced thrust on the affected engine and returned to Auckland without further incident.
The investigation
The investigation examined the Rolls-Royce Trent 1000 engines involved in both events. Physical inspections of the engines revealed that a turbine blade in the intermediate-pressure turbine (IPT) module had fractured and separated from the disc. In the first incident, the released blade caused significant damage to the engine modules and struck the aircraft's horizontal stabilizer and fuselage.
Investigators analyzed the engine manufacturer's Corrosion Fatigue Lifing (CFL) model, which was designed to predict when engines should be removed for modification to prevent such failures. The investigation found that both engines experienced the failure significantly earlier than the model had predicted. Specifically, the first engine was only five cycles away from its scheduled removal notice, while the second engine still had 192 cycles remaining before its scheduled maintenance.
Findings
Detailed examinations of the fractured blades identified corrosion fatigue cracking in the blade shanks. The manufacturer noted that this mechanism likely resulted from a combination of environmental contaminants, engine operational factors, and blade design.
Crucially, the investigation determined that the CFL model failed to provide an adequate safety margin, as the cracks reached critical depths much sooner than the mathematical model anticipated. This discrepancy meant that engines were remaining in service with a much higher risk of failure than the regulatory framework suggested.
Safety action
Following the incidents, the engine manufacturer implemented a 400-cycle reduction in the removal limit for the operator's fleet to increase the safety buffer. Additionally, the European Aviation Safety Agency (EASA) issued an emergency airworthiness directive requiring the 'de-pairing' of high-cycle engines to prevent the risk of a simultaneous dual-engine shutdown. The FAA also issued a directive limiting single-engine diversion times to 140 minutes to mitigate risks during long-range operations.