Incident Overview
A near collision occurred between aircraft XDI and an ATR transport category aircraft during approach sequencing. The pilot of XDI was instructed to track as number two behind the ATR, but was unaware that the ATR (callsign Velocity 1174) was conducting a straight-in approach. The controller intended this sequence to maintain separation.
Traffic Information and Awareness
The controller likely assessed that XDI was not significant traffic for the ATR and did not pass traffic information to the ATR crew. However, the ATR crew knew an aircraft was operating in the area but did not visually locate XDI relative to their approach path until their Traffic Collision Avoidance System (TCAS) issued a Traffic Advisory (TA).
Pilot Actions and Misunderstandings
The ATR was cleared to land before XDI entered the circuit area. The pilot of XDI assumed the ATR was either on short final or already landed, and therefore did not check for aircraft on long final before turning base leg. When the pilot of XDI did not report sighting the ATR as expected, the controller sought corroborative evidence from the pilot on the ATR's position. This was a missed opportunity by both to ensure separation.
Controller Actions and Oversight
Although not required to provide separation, the controller had identified a potential conflict and established a sequencing plan. The controller passed separation responsibility to the pilot of XDI with an instruction to sight and follow the ATR. However, the controller did not require a readback of the instruction, missing the chance to confirm correct understanding. After issuing the instruction, the controller focused attention on a third aircraft, limiting visual scans of the ATR and XDI, which prevented early identification of their proximity.
Conclusion
These factors combined resulted in the near collision not being detected by the controller or pilots until the ATR crew received a TCAS TA and initiated a missed approach.