What happened
During a period of high traffic volume, a surface movement controller experienced a potential conflict between two aircraft at an intersection. The incident occurred while the controller was managing the morning peak period, which included ongoing taxiway works and a high density of traffic.
While transitioning control from a trainee to himself, the controller issued a clearance to VH-VWQ to taxi through intersection A/T. However, the controller incorrectly believed he had instructed the crew to hold short of the intersection. Shortly after, the controller cleared VH-VZB to taxi through the same intersection. This sequence of instructions created a potential conflict between the two aircraft. Although the conflict was eventually identified, radio communications delayed the instruction for VH-VWQ to give way to VH-VZB until after avoiding action had already been taken.
The investigation
The investigation focused on the factors contributing to the controller's increased task demands. It was noted that the controller had recently taken over from a trainee, a process that inherently increases workload and distraction. The investigation examined the timing of the handover, noting that while there had been a brief 30-second lull in radio communications, the controller began the handover process immediately following five minutes of near-continuous communication.
Investigators found that the controller's workload and the distractions associated with the handover process likely contributed to a delay in detecting the conflict. Furthermore, the controller's reliance on memory without a prompt to record taxiing instructions meant that the initial clearance limits were forgotten.
Findings
- The controller's workload was increased by high traffic volumes, the transition from a trainee, and ongoing taxiway construction.
- The handover process itself increased the risk of error through heightened distraction.
- The controller forgot the specific taxiing instruction issued to VH-VWQ due to high workload and reduced memory performance.
- The timing of the handover, occurring shortly after a period of intense radio activity, may have contributed to the error.