What happened
Two aircraft, VH-SYT and VH-HPA, were operating in Class C airspace when a convergence event occurred. The pilot of VH-SYT was flying at a higher speed and trailing behind VH-HPA. Because VH-SYT was positioned below the leading aircraft during the convergence, the pilot of VH-HPA had a limited opportunity to identify the potential conflict. As the faster aircraft closed the gap, the wing structure of VH-SYT obscured the view of VH-HPA, causing the pilot of VH-SYT to lose visual contact. At the moment sight was lost, the aircraft were approximately 500 m apart horizontally and 800 ft apart vertically. Although the pilot of VH-SYT notified Air Traffic Control (ATC) of the loss of visual contact, no maneuvering was performed to maintain separation, and no alternative clearance was requested.
The investigation
The investigation focused on the effectiveness of the see-and-avoid principle and the accuracy of ATC surveillance. While the controller provided traffic information and offered alternative tracking, no specific avoidance instructions were issued because neither pilot requested assistance. The investigation found that a safety alert was only triggered when the vertical separation on the situation display dropped to approximately 500 ft. However, the actual separation was significantly less than indicated. This discrepancy was caused by radar accuracy and resolution limitations, combined with the fact that the altitude of VH-SYT was outside the required 200 ft tolerance on the display. As a result, the safety alert was issued only after the aircraft had already passed in close proximity.
Findings
- The controller did not verify the initial altitude of VH-SYT, which would have revealed the discrepancy between the reported and displayed altitude.
- The pilot of VH-SYT failed to maneuver to maintain sight of VH-HPA or request a different clearance after losing visual contact.
- Structural obstructions, such as an aircraft's own wing, significantly impact a pilot's ability to maintain the see-and-avoid principle.
- The lack of an Airborne Collision Avoidance System (ACAS) on the aircraft contributed to the difficulty in managing the proximity event.