REX SF34 at Wellcamp on Oct 21st 2021, wrong position report, loss of separation in traffic circuit
Last Update: September 29, 2023 / 06:40:56 GMT/Zulu time
An Air Charter Coordinators Beech 200 registration VH-WXB with 8 passengers and 1 crew, also operating under instrument flight rules, was descending towards West Well Camp Airport, when Brisbane center released both aircraft to enter the unmonitored airspace at the destination airport, announcing to ZLV that WXB was estimated to land at 08:30L and to WXB that ZLV was estimated to land at 08:29L.
At the time 5 other aircraft were in the traffic circuit for West Wellcamp Aiport.
ZLV incorrectly announced on CTAF that they were on an early downwind and were reducing speed and track second to WXB (while they were on the cross wind).
WXB however had developed the mental model ZLV was ahead of them and they would track second to ZLV. In order to increase the distance to ZLV the pilot decided to enter the circuit initially flying opposite to the circuit direction. While doing that both aircraft received TCAS TAs and ZLV subsequently a TCAS RA, which ZLV followed until clear of conflict.
The separation between the aircraft reduced to 300 feet vertically and 0.5nm horizontally.
Only at that point they identified each other visually and became fully aware of their positions. Both aircraft continued for safe landings.
The Australian TSB released their final report concluding the probable causes of the incident were:
- The flight crew of VH-ZLV broadcast an incorrect position of their aircraft when approaching the circuit. This probably resulted in the pilot of VH-WXB misidentifying it for another aircraft in the circuit and influenced their decision to conduct a non-standard circuit entry contrary to the traffic flow.
- The flight crew of VH-ZLV did not effectively monitor the radio, resulting in them having an incorrect mental model of VH-WXB’s position and thus not perceiving VH-WXB as a threat.
- The pilot of VH-WXB manoeuvred their aircraft opposite to circuit traffic direction while descending into the active side of the circuit in the vicinity of the airport resulting in a conflict with VH-ZLV.
- Neither flight crew identified the other aircraft visually or on their TCAS, leading to VH-WXB turning in front of VH-ZLV and resulting in the crew of VH-ZLV receiving a TCAS RA.
The ATSB analysed:
Incorrect mental models
Mental models are a form of cognitive structure that enables an individual to effectively interact with their environment by organising knowledge into meaningful patterns (Reynolds & Blickensdefer, 2009). An individual, when performing a task will develop a mental model of what they think will occur during the task being completed. Their mental model is based upon the information available to them at the time.
The flight crew of ZLV advised the pilot of WXB that they were on early downwind, when they were actually on early crosswind for runway 12. At the time of this broadcast, there were 2 other aircraft in the circuit: VH-EQV on mid-downwind and VH-YNH on mid-crosswind.
The pilot of WXB recalled seeing ZLV to the left of their position on their TCAS screen at about the same time as when they broadcast that they were continuing on an easterly heading. However, a review of recorded flight data identified that ZLV was not to the left of WXB until after the 2 aircraft had crossed paths and it was VH-YNH to the left of WXB at this time. The pilot of WXB also stated that they had ZLV visual most of the time and the only period that ZLV was not visual to them was when the pilot was conducting the left turn to position behind ZLV on downwind.
After WXB crossed ZLV’s track and was in communication with the flight crew of ZLV, the pilot asked the crew to confirm ZLV was on base. At the time of this broadcast, VH-YNH was on base and ZLV was to the left of WXB on downwind.
The pilot of WXB's description of when they first became aware of ZLV on their TCAS screen and their common traffic advisory frequency (CTAF) broadcasts after crossing ZLVs flight path, suggested the pilot of WXB had sighted VH-YNH and not ZLV visually or on the TCAS screen.
The advice from the crew of ZLV that their aircraft was on early downwind when they were on crosswind, would have likely also confirmed the pilot’s assumption that VH-YNH was ZLV and the pilot possibly assumed VH-EQV was VH-YNH.
The decision by the pilot of WXB to fly opposite to the traffic direction on downwind while descending to circuit height, before turning left, across ZLV’s flight path indicates that it is likely that the pilot had not identified ZLV either visually or on the TCAS.
Both flight crew members of ZLV recalled WXB’s field estimate was 0830, which was initially given to them by the air traffic controller. However, after they transferred over to the CTAF, the pilot of WXB broadcast their new arrival time of 0827 on 2 separate occasions, including a broadcast directly to ZLV. This was the same estimated arrival time as ZLV. It is evident that the flight crew of ZLV were aware of the potential arrival time conflict as a discussion occurred between the pilot monitoring and the pilot of WXB, for the flight crew to slow down ZLV to go in number 2 to WXB.
Due to the earlier incorrect positioning call from the flight crew of ZLV, leading the pilot of WXB to believe that ZLV was already established in the circuit on downwind, the pilot of WXB advised the flight crew that they would track number 2 to ZLV and join the circuit behind them on downwind. The pilot monitoring of ZLV acknowledged the broadcast.
The pilot monitoring did not recall hearing anymore broadcasts from the pilot of WXB, after they had organised that WXB would track behind them, until after the TCAS resolution advisory was received. The pilot of WXB had made one other broadcast prior to this, just before they conducted the 180° turn onto downwind, that included intentions to descend and to shortly join the circuit via a left turn. If the flight crew of ZLV had of been effectively monitoring the CTAF, this transmission should have been a trigger for them to look for WXB and respond to confirm their mental model. At this time, ZLV was 1,300 ft higher than WXB (4,800 ft vs 3,500 ft) and in the process of conducting a right turn onto early downwind, making sighting of a lower aircraft more difficult.
It is possible that after organising separation with WXB and agreeing that WXB would go number 2 behind them, the crew thought that WXB was aware of their position and therefore discounted WXB as a threat. Believing adequate separation had been organised, focus switched to VH-YNH and configuring the aircraft for landing.
The flight crew’s ineffective monitoring of WXB’s broadcasts and their incorrect mental model meant that they were now dependent on either visually acquiring WXB or the TCAS detecting them.
Neither crew had positively sighted the other aircraft
The pilot of WXB recalled having ZLV in sight both visually and on the TCAS screen prior to the left turn to join the circuit. If the pilot of WXB had accurately identified ZLV’s location, it is very unlikely that they would have assessed that it was safe to turn left in front of ZLV and cross their flight path. Therefore, the pilot of WXB probably did not identify ZLV visually or on the TCAS until the completion of the left turn when both aircraft were on downwind.
The flight crew of ZLV, reported that while they were on early downwind, they did not hear any broadcasts from WXB about joining the circuit. The first indicator they had that WXB was in the vicinity of their aircraft was when they received a TCAS traffic alert followed shortly after by an RA. They recalled, during their initial communications with WXB, being unsure where WXB was planning to join the circuit, which was why they were initially happy for WXB to go first. After organising separation with WXB, they reported being under the impression that WXB would slow down and join the circuit behind them either on downwind or base. They did not recall seeing WXB on the TCAS prior to the RA. The pilot flying recalled seeing WXB visually for the first time when the aircraft crossed their flight track from left to right.
Separation in a CTAF is dependent on pilots organising their own separation through radio communication, as well as conducting standard circuit procedures. Neither crew positively identified the other aircraft’s location while in, and prior to joining, the circuit, so the potential conflict was not recognised.
Conducting standard circuit procedures provides the best opportunity and risk control for aircraft to maintain separation. Finally, if available, it is also important to follow TCAS RA information. In this instance, it prevented a potential collision.
This article is published under license from Avherald.com. © of text by Avherald.com.
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