What happened
On 19 May 2001, a Boeing 767-319, registration ZK-NCH, was performing a scheduled flight from Auckland to Sydney. During the approach to Sydney, passengers noticed that the left outboard spoiler and flap appeared misaligned and were not sitting flush with the wing. They also observed a dark, flexible object protruding from the wing into the airflow. Upon landing in Sydney, the crew was alerted to these observations by the passengers, and ground engineers conducted an inspection. Finding no visible defects, the aircraft was cleared to return to Auckland.
During the return flight's final approach to Auckland International Aeront Aerodrome, a 3 kg section of the left outboard flap deflection control track separated from the aircraft. The component fell to the ground, striking the roof of an occupied warehouse in Manukau, approximately 2 km from the runway centerline. The impact created a 15 cm hole in the warehouse roof but caused no injuries to the personnel inside. The flight crew, unaware of the separation due to the lack of impact on aircraft performance, completed the landing safely.
The investigation
Investigators examined the fractured component and the aircraft's maintenance history. The investigation established that the aircraft had been maintained according to all required schedules. Analysis of the metal fracture revealed that the failure was caused by a fatigue crack.
Testing indicated that the structural integrity of the track had been compromised by a manufacturing defect—specifically, the random inclusion of slag within the metal. This defect acted as a stress concentrator. Furthermore, the investigation found that the component had been subjected to extreme cold-soaking at temperatures around -55°C during routine flight operations. This exposure to very low temperatures made the material more brittle and susceptible to fatigue damage during the high-load period of the landing approach.
Findings
- The primary cause of the failure was a fatigue crack in the flap control track.
- The progression of the crack was accelerated by the presence of slag within the part and the brittle nature of the metal following exposure to extreme cold.
- The loss of the single track did not compromise the immediate flight safety or control of the aircraft.
- The incident had the potential to cause significant injury or property damage to the public.
Safety action
Following the incident, the aircraft operator notified Boeing and other 767 operators. The manufacturer subsequently developed a new inspection procedure, to be released via a service bulletin, establishing specific intervals for checking flap control tracks. The manufacturer also worked toward a redesign of the track assembly to eliminate the need for such frequent inspections, while the operator began replacing older track models with the updated series used in the 767-400ER.