What happened
On 21 October 2021, at 0743 local time, a Beech Aircraft B200 (registered VH-WXB) departed Roma Airport on a passenger charter flight to Brisbane West Wellcamp Airport (Wellcamp), Queensland. At 0806, a Saab 340B (registered VH-ZLV) departed Brisbane Airport on a scheduled passenger service flight to Wellcamp. Both aircraft operated under instrument flight rules and estimated their arrival at 0827.
Before leaving controlled airspace, the Brisbane Centre air traffic controller provided traffic information to both aircraft in two separate broadcasts with updated arrival times. The pilots made several calls on the common traffic advisory frequency (CTAF) to coordinate separation at the non-controlled aerodrome. However, at about 0828, VH-WXB conducted a 180° left turn on the active side of the circuit, crossing in front of VH-ZLV. This reduced separation to 300 ft vertically and 1,000 m horizontally.
As VH-WXB began the left turn, VH-ZLV's traffic alert and collision avoidance system (TCAS) issued a traffic advisory (TA), followed by a resolution advisory (RA). The pilot flying immediately disconnected the autopilot, followed the RA instructions, and climbed until clear of conflict. Around the same time, the pilot of VH-WXB received a TCAS TA and began a visual lookout.
The flight crew of VH-ZLV advised VH-WXB they had received a TCAS RA. Further communication occurred between both pilots to confirm and visually identify each other's position in the circuit to ensure separation. Both aircraft landed safely at Wellcamp Airport.
ATSB findings
The Australian Transport Safety Bureau (ATSB) determined that the pilots of both aircraft had an incorrect mental model of each other's position, and neither had positively sighted the other before the conflict. The flight crew of VH-ZLV broadcast an incorrect position when approaching the circuit, which likely caused the pilot of VH-WXB to misidentify another aircraft on their TCAS as VH-ZLV.
This misunderstanding affected the pilot of VH-WXB's decision to fly opposite the downwind circuit direction while descending, then conduct a 180° left turn in front of VH-ZLV, believing that aircraft was further ahead on the downwind leg. Additionally, the flight crew of VH-ZLV did not effectively monitor the radio, resulting in an incorrect mental model of VH-WXB's position and failure to identify it as a threat.
As neither crew had positively sighted the other aircraft, alerted see-and-avoid was limited. The last line of defense was the TCAS, which prevented a potential collision.
Actions taken
The operator of VH-WXB advised that they would:
- Increase buffer between regular public transport (RPT) flights and their aircraft by orbiting at a waypoint further out to allow the RPT flight to be on final approach when joining the circuit.
- Brief pilots on the event and emphasize adherence to procedures in CAAP 166 - Operations at Non-Towered Aerodromes.
- Encourage pilots to consider orbits for greater spacing when traffic congestion is anticipated.
- Discuss congestion with the flight training school at Wellcamp to limit training aircraft during scheduled RPT arrivals.
The operator of VH-ZLV advised that they would:
- Include operations at non-towered aerodromes as a focus item in periodic aircrew check cycles.
- Use this occurrence internally as a human factors case study for operations at CTAF airports.
Safety message
The ATSB's SafetyWatch highlights safety concerns from investigations and industry data. One priority is safety around non-controlled aerodromes. Insufficient communication between pilots is the most common cause of safety incidents near non-controlled aerodromes. Pilots should ensure they understand the location and intentions of surrounding traffic and clearly communicate their own intentions while maintaining visual lookout.
Safe operation at any aerodrome requires sound judgment and adherence to standard procedures and CASA guidance. Using standard procedures at non-towered aerodromes helps maintain situational awareness and separation.
Developing and maintaining situational awareness is essential for safe flight, particularly at non-towered aerodromes. In addition to radio communication, systems such as ADS-B and TCAS provide valuable information to assist pilot decision-making.