| Occurrence Briefs are concise reports that detail the facts surrounding a transport safety occurrence, as received in the initial notification and any follow-up enquiries. They provide an opportunity to share safety messages in the absence of an investigation. |
What happened
On 26 May 2021, a BAE Systems Avro RJ100 (RJ100) aircraft was inbound to Port Augusta, South Australia, conducting a regular public transport service from Adelaide, South Australia, with 37 passengers and 4 crew on board. Also inbound to Port Augusta at about the same time, was an Ayres Corporation S2R-T34 (S2R) agricultural aircraft on a positioning flight from Broken Hill, New South Wales, with the pilot as the sole person on board.
The weather was reported as being generally good and suitable for a visual approach,[1] with a visibility in excess of 10 km, no cloud below 5,000 ft and a southerly wind at approximately 10 kt.
At 1337:06 Central Standard Time,[2] the RJ100 was 30 NM south of Port Augusta Airport at flight level 140,[3] and the crew made their first broadcast on the common traffic advisory frequency (CTAF), stating their intention to join the downwind leg of the runway 15 circuit with an estimated arrival time of 1345.
About 2 minutes later, while 14 NM north-east of the aerodrome and passing 2,500 ft on descent, the pilot of the S2R made their first broadcast on the CTAF advising of the aircraft’s position and estimated arrival time in the circuit area also of 1345 (Figure 1).
Figure 1: Inbound flight paths
Flight path of the Avro RJ100 and Ayres – positional data of Ayres S2R estimated.
Source: Google Earth and Flight Radar 24, annotated by the ATSB
The crew of the RJ100 reported hearing two radio transmissions simultaneously at this time, rendering both transmissions unreadable, but heard the end of the S2R pilot’s transmission stating ‘…Port Augusta’. In response to this, the RJ100 crew reported repeating their inbound radio call but did not receive a response. The pilot of the S2R did not recall hearing that broadcast from the RJ100 crew.
The crew of the RJ100 and the S2R both reported difficulty in being able to make or receive radio calls due to frequency congestion originating from an aircraft conducting circuit operations at Port Pirie aerodrome, which shared the CTAF.
Shortly after repeating their inbound call, and having not received a response, the crew of the RJ100 observed proximate traffic on the aircraft’s traffic alert and collision avoidance system (TCAS) 8 NM south-east of Port Augusta. The S2R was not equipped with a transponder, and as such was not visible to the crew of the RJ100 on their TCAS traffic display. However, the crew of the RJ100 stated that they believed that the TCAS traffic was the same aircraft that had made the previously unreadable radio transmission ending in ‘…Port Augusta’, and assessed that the aircraft did not pose a conflict to their arrival.
At 1343:18, the RJ100 was 5.9 NM south of the aerodrome at 1,500 ft above ground level (AGL) approaching the downwind leg of the circuit, and the crew broadcast on the CTAF that they were joining downwind for runway 15. At this time, the S2R was 9 NM north-east of the aerodrome. The pilot of the S2R reported hearing the transmission but believed the RJ100 was established on the downwind leg of the circuit, and therefore estimated it would be on the base or final leg of the circuit by the time the S2R reached the circuit area.
The pilot of the S2R planned to overfly the circuit at 1,500 ft AGL, descend on the non-active side of the circuit (Figure 2), and then join the circuit at 1,000 ft AGL for a landing on runway 15.
Figure 2: Overfly circuit joining procedure
Source: CASA Visual Flight Guide
A short while later, as the S2R approached the circuit area, the pilot observed an RJ100 aircraft parked on the tarmac and concluded that it was the same aircraft that had previously reported joining the downwind leg and thought it must have landed. This was, however, another (company) aircraft that had operated into Port Augusta that day.
At 1345:03, the RJ100 was in a mid-downwind position for runway 15 at 1,500 ft. The S2R was approaching the circuit from the north-east also at approximately 1,500 ft. The captain of the RJ100 reported seeing the S2R from their window on the left side of the aircraft, as it passed from right to left about 50 ft directly below the aircraft (Figure 3).
Figure 3: Point of closest proximity
Flight path of the Avro RJ100 and Ayres S2R – positional data of Ayres S2R estimated.
Source: Google Earth and Flight Radar 24, annotated by the ATSB
The crew of the RJ100 expressed surprise on sighting the S2R, and broadcast on the CTAF ‘…did you see us?’ to verify if the other pilot had them in sight prior to the aircraft passing below. In response to this, the pilot of the S2R reported that they saw the aircraft pass behind them and had not been aware of the aircraft before then.
Operational factors
Non-towered aerodromes
The majority of aerodromes within Australia operate without the provision of air traffic control services. These aerodromes rely upon pilots broadcasting their positions and intentions on a CTAF and then implementing separation actions that are agreed directly between the pilots.
To guide pilots in interpreting the Civil Aviation Regulations relating to operations within a CTAF area, the Civil Aviation Safety Authority has promulgated guidance in Civil Aviation Advisory Publications (CAAP) 166-01 Operations in the vicinity of non-controlled aerodromes and 166-2(1) Pilot’s responsibility for collision avoidance in the vicinity of non-towered (non-controlled) aerodromes. CAAP 166-01 states that aircraft should fly at a circuit height that is based upon their relative performance category (Figure 4).
Figure 4: Recommended circuit height based on aircraft performance
CAAP 166-01 also states that:
where a pilot is unfamiliar with the aerodrome layout, or when its serviceability, wind direction, wind speed, or circuit direction cannot be ascertained prior to arrival, the overfly procedure should be used.
To mitigate the risk of a potential conflict between two aircraft of differing performance categories, and therefore circuit heights, where one aircraft is planning to overfly the aerodrome, the CAAP recommends that:
At aerodromes with high performance traffic in the circuit, the overfly height should be no lower than 2,000 ft above aerodrome elevation.
In this occurrence, the relative performance of each aircraft would mean that the RJ100 should fly a circuit at 1,500 ft AGL, and the S2R should fly a circuit at 1,000 ft AGL (Figure 5). In which case, the pilot of the S2R should conduct the overfly procedure at 2,000 ft AGL to remain safely above the RJ100’s circuit altitude.
Figure 5: Relative circuit heights of aircraft of different performance categories
Source: CASA Visual Flight Guide
The pilot of the S2R reported being unfamiliar with Port Augusta Airport, and as such elected to overfly the circuit prior to joining – in line with the requirements of CAAP 166-01. Unfortunately, the pilot of the S2R was unfamiliar with the operation of high-performance aircraft and the differing circuit heights stipulated in CAAP 166-01, and erroneously believed that the RJ100 would be conducting their circuit at 1,000 ft AGL. This resulted in the S2R conducting the overfly procedure at 1,500 ft, which was the same height at which the RJ100 was conducting its downwind leg.
Communications
Most non-towered aerodromes use a standard frequency for the CTAF. Some aerodromes that experience higher volumes of traffic, or are located close to other aerodromes, are assigned a discrete frequency. At the time of the occurrence, Port Augusta shared a frequency with the nearby aerodrome of Port Pirie. The crews of the RJ100 and S2R reported that the pilot of an aircraft operating at Port Pirie Aerodrome broadcast their position at each leg of the circuit being flown. This does not conform to the recommended broadcasts contained in CAAP 166-01 or align with its recommendation that the fundamental principle of operating in the vicinity of a non-controlled aerodrome is to only make the broadcasts necessary to ensure other aircraft are aware of your operation. The excessive and unnecessary transmissions contributed to the crew of the RJ100 being unaware of the S2R’s position and limited their opportunity to implement satisfactory separation.
Traffic alert and collision avoidance system
The TCAS enhances a pilot’s situation awareness by displaying traffic information regarding the position and altitude of other aircraft operating nearby. For aircraft equipped with a TCAS unit, the system will alert the pilot to aircraft in close proximity through a traffic advisory, and then issue an avoiding action to prevent a collision through a resolution advisory if required. The TCAS system gathers position and altitude data through an aircraft’s transponder output to display and generate traffic information to the pilot of a TCAS-equipped aircraft. In this occurrence, the RJ100 was fitted with an integrated TCAS unit, however the S2R was not fitted with a transponder. This resulted in the crew of the RJ100 not receiving any traffic information or resolution advisories regarding the S2R from the TCAS unit throughout the occurrence.
Safety action
In response to this occurrence, the operator of the RJ100 advised the ATSB that further advice had been disseminated to the company’s pilots regarding operations at non-towered aerodromes. Specifically, pilots had been requested to ensure that positional broadcasts are as accurate as possible and include the provision of ‘early, mid or late’ to best describe the aircraft’s position when broadcasting joining the downwind leg of the circuit. Pilots have also been encouraged to verify their intended circuit altitude in circumstances where any doubt exists as to the awareness of this among other aircraft.
The operator of the S2R advised the ATSB that they have reviewed the relevant CAAP regarding operations at non-towered aerodromes and ensured that all company pilots are familiar with the possibility of aircraft operating at differing circuit heights depending on their performance category.
Prior to this occurrence, safety concerns around frequency congestion and broadcast interference in the Port Augusta area had been reported to the ATSB through REPCON, the aviation confidential reporting scheme. This report and the ATSB’s comments are available as REPCON AR2020-0066.
Safety message
This incident highlights one hazard associated with operations at non-controlled aerodromes and reinforces the importance of pilots being thoroughly familiar with the recommended procedures and the likely traffic mix operating at the aerodrome. It is also a reminder to pilots to make clear and concise radio calls and eliminate unnecessary broadcasts, particularly within the CTAF environment.
Further, this incident serves as a reminder of the risk of confirmation bias during operational decision making. Confirmation bias is defined as the tendency to interpret new information as confirmation of existing hypotheses, and as such is a threat to situational awareness and sound decision making in the aviation environment.
In this occurrence, a company RJ100 on the ground at Port Augusta led the pilot of the S2R to erroneously conclude that the aircraft they heard joining the circuit had landed. While the conclusion drawn by the pilot was not unreasonable, it likely reduced the vigilance of the S2R pilot who reported then turning their attention to other tasks of navigation and communication.
The ATSB SafetyWatch highlights the broad safety concerns that come out of our investigation findings and from the occurrence data reported to us by industry. One of the safety concerns is Communication and self-separation in non-controlled airspace.
About this report
Decisions regarding whether to conduct an investigation, and the scope of an investigation, are based on many factors, including the level of safety benefit likely to be obtained from an investigation. For this occurrence, no investigation has been conducted and the ATSB did not verify the accuracy of the information. A brief description has been written using information supplied in the notification and any follow-up information in order to produce a short summary report, and allow for greater industry awareness of potential safety issues and possible safety actions.
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- Visual approach: A visual approach is an approach when either part or all of an instrument approach procedure is not completed and the approach is executed with visual reference to the terrain.
- Central Standard Time (CST): Coordinated Universal Time (UTC) + 9.5 hours.
- Flight Level: at altitudes above 10,000 ft in Australia, an aircraft’s height above mean sea level is referred to as a flight level (FL). FL 140 equates to 14,000 ft.