Incident Overview
On December 8, 2002, a Boeing 767-219ER aircraft, registered ZK-NBC, sustained a failure of the number-1 (left) engine during a scheduled regular passenger transport flight from Brisbane to Auckland. The flight crew reported a loud 'bang' and felt the aircraft move to the right, followed by a progressive yaw to the left. The crew shut down the left engine, a GE CF6-80A, and completed checklist items before returning the aircraft to Brisbane.
Damage Assessment
An initial engineering evaluation of the left engine revealed the loss of the outboard fan cowl section and a large rupture of the outboard core cowl, with associated damage to the engine strut and wing leading edge flap above that area. The engine core showed the complete circumferential disruption of the high-pressure turbine casing and the loss of a large segment of the first-stage high-pressure turbine disk. The low speed (N1) spool shaft had fractured between the first and second-stage turbine disks.
Cause of Failure
The failure of the number-1 (left) engine from ZK-NBC occurred as a direct result of the rupture of the first-stage high-pressure turbine disk, liberating a large disk section that was uncontained by the engine casing or nacelle. Disk fracture had initiated from prior fatigue cracking at the base of a turbine blade fir tree slot. Fatigue cracking had initiated from the rear corner of that blade slot and propagated radially inward and axially forward, before turning to intersect an adjacent rim-bolt hole. At that stage, the crack was below critical size and incapable of directly causing the disk rupture. Subsequently, the growth of a secondary radial-axial crack from the base of the primary crack enlarged the total defect to a depth of 39 mm, where it reached critical size and initiated the overload failure of the remaining section, with the release of the disk fragment.
Investigation
The investigation is under the jurisdiction of the government of Australia. Further information pertaining to this accident may be obtained from the Australian Transport Safety Bureau (ATSB).