What happened
During a scheduled flight from Horn Island to Cairns, the flight crew experienced unexpected aircraft performance during the takeoff roll and initial climb. While the aircraft successfully became airborne, the pilot flying noticed that the aircraft's performance did not match expectations. Upon investigation, the first officer discovered that the flaps had been set to 5° instead of the required 15°. To compensate for the reduced performance, the pilot flying applied continued back pressure on the controls, eventually reducing pitch to increase airspeed and maintain a positive climb. The flight proceeded to its destination without further incident.
The investigation
The investigation focused on why the incorrect configuration was not detected during the pre-flight phase. It was determined that the first officer inadvertently selected the 5° setting, a mistake likely driven by habitual behavior. Because a 5° setting had been used for the previous two flight sectors and is a common setting across the operator's network, the error was identified as a 'slip'—an unintended action during a routine task.
Although the crew followed standard pre-departure procedures, which included four separate opportunities to identify the error, the error went undetected. The investigation found that the crew was managing a high workload due to additional calculations required for weight, weather, and performance limitations. This high workload likely led to a high degree of automaticity during the after-start checklist, meaning the crew performed the checks without consciously verifying the flap position against the required performance cards.