Incident Overview
At Mildura, aircraft ZPJ and XGA came within 0.125 nautical miles and 200 feet vertically during a near collision. The incident involved two aircraft operating under visual flight rules at a non-controlled aerodrome. Both pilots complied with the minimum radio broadcasts required by the Aeronautical Information Publication (AIP).
Radio Communications
It is likely that the pilot of XGA was not monitoring the Mildura Common Traffic Advisory Frequency (CTAF) when the pilot monitoring (PM) on board ZPJ broadcast that they were joining the circuit. When the pilot of XGA switched to the CTAF, they were initially occupied with communicating with another public transport aircraft inbound from the north, considering that aircraft their only potential conflict. The reason why the flight crew of ZPJ did not comprehend the presence of a potential conflict from this radio traffic could not be determined. The flight crew of ZPJ had previously dismissed both XGA and another aircraft, XGY, as potential conflicts for their arrival.
After ZPJ turned onto the base leg for runway 09, the PM made a base radio broadcast. It could not be determined why the pilot of XGA did not comprehend the presence of a potential conflict from this broadcast. The investigation could not rule out that other aircraft operating at Mildura or Wentworth might have made broadcasts that interfered with one or several of the broadcasts from ZPJ or XGA.
Visual Detection
Shortly after the base leg broadcast from ZPJ, the flight crew received a Traffic Collision Avoidance System (TCAS) Traffic Advisory (TA) aural alert. At that time, ZPJ was below 900 feet, so TCAS Resolution Advisory was inhibited. However, the TCAS visual display showing the relative position of XGA cued the crew's visual search and facilitated quick identification. XGA appeared stationary against a background of sky to ZPJ's crew. Conversely, to the pilot of XGA, ZPJ was probably against a more complex background with no relative motion, contributing to the difficulty in detecting ZPJ before being alerted by a radio call.
Conflict Resolution
The PM on board ZPJ directed a radio broadcast to the 'traffic inbound to Mildura from the west', which alerted the pilot of XGA to other traffic and prompted a search. The PM then directed the pilot of XGA to 'turn immediately away to the south' to avoid a potential collision. After receiving an acknowledgement, ZPJ's crew focused on executing a go-around, as their turn onto the final leg had been delayed due to monitoring XGA.
Outcome
During ZPJ's go-around to join the upwind circuit leg on the south side of runway 09, XGA continued to converge, reaching a closest point of 0.125 NM behind and about 200 feet above ZPJ. XGA then made an abrupt turn to the south. This resulted from the pilot flying XGA intending to join the upwind leg to the south before realizing ZPJ was also conducting a go-around from its approach.