Development of the Occurrence
The incident involved two aircraft, designated SYT and HPA, operating under visual flight rules in Class C airspace. The pilots were required to see and avoid each other. SYT was trailing but faster than HPA, a relative positioning that reduced the opportunity for the pilot of HPA to identify the developing proximity event, as SYT was below HPA as the two aircraft converged.
As SYT closed on HPA, the pilot of SYT lost visual contact because the wing structure visually obscured HPA. Although the pilot of SYT identified that his aircraft was about to overtake HPA, he did not manoeuvre to keep HPA in sight or request an alternative clearance. The pilot of SYT lost sight of HPA while the aircraft were about 500 meters and 800 feet apart.
Despite losing visual contact, the pilot of SYT advised air traffic control (ATC) that he had lost sight of HPA. When requested, ATC can provide VFR flights in Class C airspace with a suggested course of action to avoid other VFR flights, but the pilot is still required to see and avoid other aircraft. Neither pilot requested ATC to provide avoiding action, so the controller did not issue alternative segregation instructions at that time, and the two aircraft continued to converge. The controller did, however, provide traffic information to both pilots and offered alternative tracking.
The controller issued a safety alert and instructions when the vertical separation depicted on the situation display between the aircraft reduced to about 500 feet. However, due to a combination of radar accuracy/resolution and the altitude of SYT on the situation display not being within the required 200-foot tolerance, the aircraft were much closer than the indicated 500 feet. Consequently, the safety alert was issued after the near collision, and neither pilot had an appreciation of just how close the aircraft were until they re‑sighted each other as they passed in close proximity.
Had the controller verified the initial altitude of SYT on first contact, they would have identified the discrepancy between the reported and displayed altitudes. The controller would then have had an accurate indication of the vertical distance between the two aircraft.
See-and-Avoid Limitations
Obstruction by the aircraft’s wing is one of many factors identified by ATSB research that affect a pilot’s ability to sight another aircraft. This occurrence therefore highlights the difficulties of the see-and-avoid principle, even when the pilot is given information about (or alerted to) the other aircraft’s position. Airborne collision avoidance systems (ACAS) provide valuable information to alert pilots of other aircraft in their proximity and can direct the pilot to take avoiding action, thereby reducing the risk of collision.