What happened
An aircraft conflict occurred involving VH-WXB and VH-ZLV during circuit operations. While approaching the airport, the flight crew of VH-ZLV broadcast that they were on an early downwind leg, when they were actually on an early crosswind for runway 12. At the same time, other aircraft, VH-EQV and VH-YNH, were also present in the circuit.
The pilot of VH-WXB believed they had visual and TCAS contact with VH-ZLV; however, flight data revealed that the pilot had likely misidentified VH-YNH as VH-ZLV. This misidentification, compounded by the incorrect position report from VH-ZLV, led the pilot of VH-WXB to execute a non-standard circuit entry. The pilot of VH-WXB flew opposite to the established traffic direction while descending to circuit height, crossing the flight path of VH-ZLV. This maneuver resulted in the crew of VH-ZLV receiving a TCAS Resolution Advisory (RA).
The investigation
The investigation examined the radio communications and flight data to determine why the aircraft failed to maintain separation. It was found that the crew of VH-ZLV had established an agreement for VH-WXB to follow them as number two in the sequence. However, the crew of VH-ZLV did not effectively monitor subsequent radio transmissions from VH-WXB.
Because the crew of VH-ZLV believed separation had already been organized, they likely discounted VH-WXB as a potential threat, shifting their focus to other traffic and landing preparations. This lack of monitoring, combined with the difficulty of spotting a lower aircraft while performing a right turn, meant the crew became dependent on visual or TCAS detection to identify the conflict.