What happened
During the takeoff phase of a flight, the aircraft experienced a noticeable tendency to roll to the right while the flaps were deployed. This asymmetric performance was caused by a failure in the left outboard aft flap mechanism. While the aircraft handled normally, a noise was reported by the cabin manager during the flight, which may have been linked to the flaps potentially failing to retract completely during the transition to cruise. No parts from the failed component were recovered from the airport.
The investigation
Technical examinations of the Boeing 737 identified that a fatigue crack had spread through the inboard programming roller cartridge located on the left outboard aft flap. This crack eventually led to the total failure of the cartridge. Investigators determined that the failure likely occurred after the aircraft's previous landing at Gold Coast Airport, as no handling issues were noted during that prior flight. It is also believed the aft flap had retracted properly after the previous flight, as any significant skewing would likely have been caught during the two pre-flight walk-around inspections.
Detailed analysis of the fracture surface showed blackening and pitting, suggesting the crack had been developing for some time. However, the specific timing of the crack's formation could not be confirmed. While a general inspection of the left outboard flap had been performed in October 2020, the crack was located on the side of the cartridge abutting the roller track, making it nearly impossible to see during an on-wing inspection.
Findings
- The primary cause of the failure was undetected fatigue cracking within the inboard programming roller cartridge.
- The crack was situated in an area that was not covered by the existing detailed flap actuation system inspection protocols.
- Although the aircraft, VH-YFZ, was not due for a more detailed inspection for another 5,600 cycles, previous similar incidents involved aircraft that had undergone such inspections.
- The current inspection task cards focused on the rollers themselves but did not include the cartridges housing them, leaving the critical area unexamined.