Boeing 767-219ER, ZK-NBC

Approximately six minutes after take-off from Brisbane, Australia on a scheduled passenger service to Auckland, New Zealand, the Boeing 767-219ER aircraft, registered ZK-NBC sustained an uncontained failure of the left (number-1) engine, necessitating a return to Brisbane.

During the return, the flight crew elected to conduct a prepared emergency landing, however communication misunderstandings between the flight crew and the cabin in-flight service director (ISD) resulted in some crew and passengers not being appropriately briefed. The flight crew's subsequent call for the 'brace' position at 500 ft thus came as a surprise to the unaware cabin crew, some of whom adopted the unprepared emergency landing procedures, calling "Emergency - grab your ankles" to the passengers.

Failure of the number-1 engine (a General Electric CF6-80A high-bypass turbofan engine) resulted from the fracture and liberation of a large segment from the first-stage high-pressure turbine disk. The disk failure initiated from a radial fatigue crack at the base of a turbine blade slot, one of three similar cracks that were found during the subsequent investigation. The loss of the disk segment, the resultant imbalance and rapid engine seizure produced extensive damage to the engine casing, accessory components and the engine pylon. The released disk segment impacted the leading edge flap panel immediately above the engine - damaging a 600mm length and resulting in the flight crew electing not to use the leading edge flaps for the return approach and landing at Brisbane. Because the engine pylon and leading edge flap damage sustained during the engine failure was likely to affect the structural strength of the engine pylon and the performance and flight characteristics of the aircraft, the event was classified as an accident, in accordance with the definition published by the International Civil Aviation Organization (ICAO) in Annex 13 to the Convention on International Civil Aviation.

ATSB laboratory examination found that the disk cracking had originated from the rear break-edge corner of the blade fir-tree slots; an area that had sustained heavy surface microstructural damage as a product of manufacturing and/or repair shot peening processes. While subsequent fatigue testing of other blade slots with similar surface damage did not conclusively identify a loss of fatigue life resulting from the peening processes, it is known that overly heavy or abusive shot peening can prove detrimental to fatigue performance.

As a result of the findings of the investigation, the engine manufacturer has implemented several changes to the manufacturing and repair shot peening processes, to avoid the surface damage found on the failed disk. Other safety action taken included revising the inspection requirements for the CF6-80A disks to include the more thorough examination of the slot bottom and rear break-edge areas, as required for the CF6-80C series engines. The US Federal Aviation Administration (FAA) and the Australian Civil Aviation Safety Authority (CASA) have subsequently mandated the revised requirements. The aircraft operator, as part of its own investigation into the occurrence, has developed a series of recommendations aimed at addressing the crew communication deficiencies experienced during the return to Brisbane after the engine failure.

• Severe mechanical and structural damage to the number-one (left) engine and nacelle.

• Associated distortion and structural damage to the engine pylon.

• Surface damage to the number-five leading edge slat above the engine.

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