What happened

On 20 September 2013, a loss of separation occurred about 17 km west of Adelaide, South Australia, between an Airbus A330 aircraft, registered VH-EBO (EBO) operating a scheduled passenger service from Sydney, New South Wales to Perth, Western Australia, and an Airbus A330 aircraft, registered VH-EBS (EBS), operating a scheduled passenger service from Perth to Sydney.

Both aircraft were within radar surveillance coverage at the time of the occurrence and were equipped with a traffic collision avoidance system (TCAS).

What the ATSB found

The ATSB determined the en route air traffic controller did not adequately assess the traffic for potential conflicts before issuing a climb instruction to the EBO flight crew. The air traffic control computer system subsequently alerted the controller to the conflict and they promptly commenced compromised separation recovery actions. The flight crew of EBS responded to alerts generated by the aircraft’s TCAS. The TCAS in EBO malfunctioned and did not provide the flight crew with traffic information or generate any safety alerts. The reason for the malfunction could not be determined and the equipment manufacturer considered it to be a unique event.

The ATSB identified a safety issue relating to the convergence of many published air routes overhead Adelaide, combined with the convergence point being positioned on the sector boundary of the Augusta and Tailem Bend sectors, which reduced the separation assurance provided by strategically separated one-way air routes and increased the potential requirement for controller intervention to assure separation.

What's been done as a result

Airservices Australia is undertaking a review of air routes and of the Australian airspace design that contributes to risk. There is also consideration to providing more flexible route structures using modern navigation technology to create traffic directional flows and remove convergence points.

In addition Airservices undertook pro-active safety action associated with safety alerting and traffic avoidance advice by air traffic services.

Safety message

This occurrence is a reminder of the potential for skill-based errors to occur when experienced personnel are performing their tasks and highlights the importance of controllers following their normal processes to ensure traffic is assessed prior to providing control instructions.

The occurrence

On 20 September 2013, a loss of separation¹ occurred between an Airbus A330 aircraft, registered VH-EBO (EBO) operating a scheduled passenger service from Sydney, New South Wales to Perth, Western Australia, and an Airbus A330 aircraft, registered VH‑EBS (EBS), operating a scheduled passenger service from Perth to Sydney. Both aircraft were within radar surveillance coverage at the time of the occurrence.

At 1159:56 Eastern Standard Time², following a handover/takeover, an air traffic controller in Airservices Australia’s Melbourne Centre accepted control jurisdiction for the Augusta and Spencer airspace sectors (Figure 1), which were permanently operated in a combined configuration. The Augusta/Spencer controller had previously been monitoring another controller who was conducting a familiarisation shift, following a period of leave, on the air traffic control (ATC) group’s other two sectors (Tailem Bend and Kingscote), which were also permanently combined. The Augusta/Spencer controller was preparing to also take over the Tailem Bend/Kingscote sectors on the same one console, as sector traffic levels and controller workloads were relatively low.

It was normal practice for all of the group’s sectors to be combined at that time of day due to low traffic levels.

Figure 1: Augusta/Spencer and Tailem Bend/Kingscote airspace sectors

Source: Airservices Australia. Image modified by the ATSB.

At the time the controller assumed control of the Augusta and Spencer sectors, EBS was within the Augusta/Spencer airspace at a position 140 NM (259 km) west of Adelaide, South Australia, eastbound on the one-way route Y135 at flight level (FL)³ 390. EBO was within the adjoining eastern airspace (Tailem Bend/Kingscote), 79.6 NM (147 km) to the east of Adelaide and westbound at FL 380.

At 1210:15, EBO’s flight crew contacted the Augusta/Spencer controller as the aircraft approached the airspace boundary. The controller observed that the ATC computer system’s human machine interface prompts displayed on their screen provided a conflicting indication as to whether onwards coordination with the adjoining western sector (Forrest) controller had been completed. To assure that this coordination had been carried out, the Augusta/Spencer controller called the Forrest controller via the internal coordination line at 1211:34 and was advised that coordination had already been completed. In addition, the Forrest controller advised that they had no vertical restrictions for EBO. The Augusta/Spencer controller entered that information into the operational data line of the aircraft’s label in the ATC computer system, and then completed other tasks in preparation for assuming control of the Tailem Bend/Kingscote sectors.

At 1212:57, EBO’s flight crew requested climb from FL 380 to FL 400, which the controller immediately approved. This resulted in a loss of separation assurance⁴ between EBO and EBS as the vertical separation standard of 1,000 ft would not exist when the aircraft passed on their one-way routes at a point where there would be less than the required radar separation standard distance laterally of 5 NM (9.3 km).

At the time the climb request was approved, EBO was west of Adelaide and tracking in a westerly direction on the one-way air route designed Q12 (Figure 1). EBS was west of Adelaide, tracking in an easterly direction on one-way route Y135. On receipt of the level change clearance, EBO’s flight crew reported leaving FL 380 and recorded data from the aircraft showed that it commenced climbing at 1213:08.

At about 1213:32, before both the radar and vertical separation standards were infringed, the ATC system’s Short Term Conflict Alert (STCA) activated, alerting the controller to the imminent loss of separation between EBO and EBS. The controller immediately instructed EBO’s flight crew to maintain FL 380, which the crew acknowledged with advice that they were descending back to that level. Recorded data from EBO showed that the aircraft reached a maximum altitude of 38,350 ft at 1213:37.

Recorded data from EBS showed that, at 1213:27, the crew received a traffic advisory (TA)⁵ from their aircraft’s traffic collision avoidance system (TCAS)⁶ At 1213:37 the TA changed to a resolution advisory (RA),⁷ and at 1213:44 the EBS flight crew advised the controller that they were responding to an RA and the aircraft started to climb.

Recorded data from the two aircraft showed that the minimum vertical separation was 650 ft at 1213:37, when the two aircraft were 4.1 NM (8 km) apart laterally. The minimum lateral separation was 1.6 NM (3 km) at 1213:51, when the aircraft were 870 ft apart vertically (Figure 2). At that time both the vertical and lateral separation were increasing as the aircraft were on separate one-way routes. The vertical and radar separation standards were re-established a short time later.

Figure 2: Aircraft positions at 1213:53

Source: Airservices Australia. Image modified by the ATSB.

Note: Data in this figure is provided from the ATC system with a different level of resolution compared to the data provided from the aircraft’s recorders. The figures ‘55’ and ‘39’ refer to the ground speeds of the aircraft (divided by 10).

Air traffic control information

The Augusta/Spencer controller was initially rated as a controller in 2005. Prior to 20 September 2013, they had the previous 3 days off duty. They reported receiving a normal amount of sleep in the previous two nights, and they commenced their shift on 20 September at 0700.

Controllers reported that workload was relatively low at the time of the occurrence and that there were no operational distractions.

TCAS information

The flight crew of EBS reported that, at the time that the EBO flight crew requested clearance to climb to FL 400, they had acquired EBO visually and observed it on their TCAS display. They could see EBO was heading in the opposite direction and that it appeared to be on a diverging route. They also saw EBO climbing and diverging to the south on the TCAS display before receiving the TA and the RA.

Immediately following the occurrence, EBO’s flight crew advised the Augusta/Spencer controller that they did not receive any indications on their TCAS display of the presence of EBS. The flight crew later reported that they did not see EBS on their TCAS display, and they did not receive a TA or RA. They also reported not being able to see other aircraft on their TCAS display during the rest of their flight in situations where the other aircraft’s crews could see them, until reaching the Perth Terminal Area, where traffic returns were evident. They had been able to see other aircraft on departure from Sydney and there was no indication of a TCAS failure prior to the loss of separation event.

Examination of recorded data from EBO showed that no TA or RA was received. After the aircraft landed in Perth, a built-in test equipment (BITE) test was conducted on the TCAS with no faults indicated. A minimum equipment list (MEL)⁸ was applied for the unserviceable TCAS for the return flight to Sydney. A full system test was conducted in Sydney with a failure identified between ATC transponder 2 and the TCAS computer and TCAS antennas. The TCAS computer and ATC transponder 2 were replaced with spare units and a further system test carried out with nil faults detected.

Although all air transport aircraft are required to have a TCAS, on rare occasions the system can fail or lose functionality during a flight. In such situations a flight crew is usually provided with a fault message, and the flight crew are required to advise ATC. In addition, under specific conditions, aircraft are able to be dispatched for short periods of time without a serviceable TCAS.

Additional information

In high reliability systems, there are multiple risk controls in place to reduce the likelihood that safety-critical personnel will make an error. However, on rare occasions an error will still occur, and systems have additional risk controls in place to detect and recover from such errors, or mitigate the consequences of such errors. In this occurrence, one of these detection and recovery controls did not work effectively (that is, the TCAS on EBO). However, other risk controls were functioning effectively. These included the ATC STCA, EBS’s TCAS, and the use of one-way routes.

Further investigation

The investigation is continuing and will include:

• further analysis of the ATC radar and audio data and the recorded data from the two aircraft
• analysis of the context in which the controller’s actions occurred
• examination of the TCAS computer and related components from VH-EBO
• review of the reliability and availability rates of TCAS.

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