Loss of separation

An infringement of separation standards occurred 70 NM east of Darwin, NT, between a descending Boeing 737-376 (737) and an Embraer EMB-120 (Brasilia) that was maintaining level flight. The event took place during the hours of darkness and in visual meteorological conditions. The crew of the 737 intentionally flew the aircraft through its assigned level in response to a traffic alert and collision avoidance system (TCAS) warning. The Brisbane sector controller also received a short-term conflict alert (STCA) between the two aircraft from the Australian Advanced Air Traffic System (TAAATS). The STCA alerted controllers when the radar trajectories of two aircraft indicated that separation standards might be infringed. The 737 and the Brasilia passed within 1.6 NM horizontally and 600 ft vertically. The required separation standard was either 3 NM or at least 1,000 ft.

TCAS is an airborne device that functions independently from the ground based air traffic control system and provides collision avoidance protection for a broad range of aircraft. The system fitted to the 737, TCAS II version 6.04, provided recommended escape manoeuvres in the vertical dimension, to either increase or maintain the existing vertical separation between the aircraft. The escape solution was communicated directly to the flight crew via a cockpit display and a synthesised voice attention getter. The TCAS alerts in the 737 used a liquid crystal display (LCD) instantaneous vertical speed indicator (IVSI) with red and green markings to indicate the vertical speeds to be avoided (red), and the desired vertical speed to be flown (green). The display was 70 mm square and had a fixed range of 6.5 NM. Aircraft were depicted using geometric symbols, depending on their threat status. A partial aircraft symbol was displayed at the extremity of the screen for aircraft detected beyond the display range. TCAS information could, in principle, have been incorporated in the electronic flight instrument system display (EFIS) of that aircraft but wiring and space available in the electronics racks of the aircraft would have required a major modification programme. TCAS information that is incorporated in the EFIS display allows the pilot to show traffic at longer ranges.

TCAS equipment interrogates the transponders of other aircraft to determine their range, bearing and altitude. Accordingly, the TCAS does not provide protection against aircraft that do not have an operating transponder. The Brasilia was fitted with an operating mode C transponder (altitude encoding) but was not TCAS equipped and the crew was not immediately aware of the infringement of separation standards. The Brasilia, which had 30 passenger seats and had a maximum take-off weight of 11,990 kg, was not required to be TCAS equipped by the Australian legislation current at the time. That legislation mandated the carriage of TCAS for all turbine-powered aircraft with more than 30 passenger seats or a maximum take-off weight greater than 15,000 kg. When both potentially conflicting aircraft are fitted with TCAS, both TCAS units co-ordinate their intentions to provide appropriate co-coordinated avoidance manoeuvres.

TCAS II can issue two types of alerts:

• Traffic Advisory (TA) to assist the pilot in the visual search for the intruder aircraft and to prepare the pilot for a potential RA; and
• Resolution Advisory (RA) to recommend manoeuvres that will either increase or maintain the existing vertical separation from an intruder aircraft.

Once the risk is over, the TCAS issues a synthesised voice `clear of conflict' message.

The 737 crew were on their fourth consecutive day of duty and completing the last sector of a four-sector day when the incident occurred. It took place at the transfer of control point between Brisbane Centre and Darwin Approach. The 737 was en-route from Cairns to Darwin and was on descent to FL220. The Brasilia was enroute to Groote Island, under the control of Brisbane Centre, with instructions to maintain flight level (FL) 210. Those routes placed the two aircraft on almost reciprocal tracks. As the 737 was passing FL235, the crew were instructed by the Brisbane sector controller to contact Darwin approach control for further descent, but were not advised of the opposite direction traffic.

The Manual of Air Traffic Services (MATS) gave guidance to controllers on how best to manage the situation and advised:

`6.6.5.5 Frequency change management in relation to the transit of an aircraft across airspace boundaries shall be arranged in a manner that normally enables pilot communication with the unit responsible for the airspace within which the aircraft is operating. A lateral tolerance of 10 NM either side of the boundary is permitted, except that when entering controlled airspace, the frequency change shall be within the 10 NM prior to the boundary.

6.6.5.10 Aircraft should normally remain on the frequency appropriate to the airspace in which it is operating. However, aircraft may be transferred to another ATS frequency provided that:

1. significant operational advantage will be gained;
2. workload, communications and equipment capabilities will permit the responsible controller to take such action as is necessary to preserve the separation without delay;
3. the actual separation is in excess of the minimum;
4. there is no possibility of separation being reduced to the minimum with the normal operation of the aircraft.'

MATS does not require controllers to provide traffic information to either crew in these circumstances.

In order to comply with MATS 6.6.5.10, the Brisbane sector controller could have either:

• Initially assigned the crew of the 737, FL230 on descent, instead of FL220, then assigned the responsibility for separation to the Darwin approach controller, coordinating the terms of the transfer of the 737 with him, before transferring the aircraft to the Darwin approach frequency; or
• Assumed the separation responsibility, coordinated the terms of the transfer of the 737 with the Darwin approach controller, and transferred the aircraft once the 737 had passed the Brasilia and a horizontal separation standard existed.

Airservices Australia believed that MATS section 6.6.5.10 did not apply to a change of frequency in these circumstances. According to Airservices Australia the action of the Brisbane sector controller, in assigning the crew of the 737 FL220, was consistent with MATS in that the controller applied a 1,000 ft separation standard between the two aircraft and the 737 crew was transferred to the Darwin approach controller within 10 NM of the lateral boundary between the Brisbane sector and Darwin airspace.

Shortly after acknowledging the instruction to change frequency, the 737 crew received an aural `traffic, traffic' warning and a display indication of an aircraft 5 NM ahead. The pilot in command stated that the TA quickly changed to a RA with a `descend, descend, descend' aural alert. As the aircraft was approaching its assigned level he disconnected the autopilot and pitched the aircraft nose down with the intention of following the RA commands. He stated that the required rate of descent shown on the IVSI was 1,200-1,500 ft/min. On passing FL220 the TCAS command abruptly reversed to a climb RA (aural `climb, climb now') which was followed positively. The climb annunciation continued until the aircraft was at FL225. No more commands were issued and there was no TCAS `clear of conflict', which is normally generated once a RA is removed.

Analysis of recorded data indicated that as the 737 descended through FL230, its rate of descent was approximately 2,900 ft/min. At FL227, the automatic flight system commenced a transition manoeuvre to achieve level flight at FL220. At approximately FL225 the autopilot was disengaged and the descent was continued manually at a rate of descent in excess of 3,200 ft/min to FL215. That was followed by a climb to FL225 at 2,900 ft/min as the pilot responded to the RA reversal (`climb now' advisory).

Maintenance files from the TCAS computer were examined and no indication of TCAS failure was found. Technical expertise was requested from the manufacturer of the TCAS equipment. Their evaluation of the event presented two possible scenarios.

`Explanation 1. The reported `descend' advisory was actually a `reduce descent' advisory that was misunderstood by the crew. A `reduce descent' would be consistent with the expected TCAS response per the reported geometry of the aircraft. Because the advisory was misinterpreted, the rate of descent was increased rather than decreased until the aircraft was below the TCAS required 700 feet vertical separation. Thus, the TCAS was required to issue a `climb now' advisory. The lack of a `clear of conflict' annunciation is explained in the following paragraph.

`Explanation 2. There is a possibility that the intruder aircraft's (equipped with mode C transponder) altitude report was not correctly received by the TCAS. There have been instances when a Mode C reply will not contain all the appropriate pulses in the message or it transmits pulses that are too narrow for the TCAS to detect. This could cause differing altitude reports and could result in multiple unstable tracks at different altitudes for the same intruder aircraft. This being the case, the TCAS could have issued a `descend' advisory for the intruder because it appeared (due to erroneous altitude report) that it was actually above its own aircraft. If subsequent replies had the correct altitude, the erroneous track would be dropped by the TCAS and the TCAS would issue a `climb now' advisory on the track with the correct altitude.

`The reason that `clear of conflict' was not annunciated can be attributed to low track firmness of the intruder aircraft. Mode C equipped aircraft are typically only equipped with a single antenna mounted on the lower hull. Since the intruder aircraft was below the 737 aircraft, it is likely that the TCAS was not able to receive regular replies at close proximity. The TCAS computer unit will `coast' the track of a previously established intruder file if it does not receive a valid or reasonable interrogation response. If the track of the intruder that generated the RA is coasted during the time of the associated RA, then the `clear of conflict' is not announced.'

Since the incident, the operator's TCAS software has been updated to Version 7. The objectives of the Version 7 update were to further increase the safety benefits of TCAS, make TCAS more compatible with the procedures used by ATC and to address operational concerns identified by pilots operating the older versions of TCAS. Improvements to the aural annunciations included a change from `reduce descent, reduce descent' to `adjust vertical speed, adjust'.

The Boeing B737-800 (B737) was cleared to Melbourne via the Sydney RWY 34R MARUB THREE standard instrument departure (SID) to 5,000 ft. The Boeing B767-338ER (B767) was inbound to Sydney from Auckland, NZ, and had been cleared to descend to 6,000 ft with a vector to a right downwind leg for RWY 34R. As the aircraft approached each other 12 NM east of Sydney, an infringement of the radar separation standard occurred.

The pilot in command of the B737 was the handling pilot for the sector and was manually flying the aircraft while tracking via the SID. He had recently completed retraining on the aircraft after having not flown the type for 10 years.

After take-off, the B737 entered cloud and encountered turbulence as it climbed through 3,500 ft. The pilot in command was monitoring the aircraft's weather radar and stated that he became distracted while assessing the meteorological conditions. Although the co-pilot gave the 1,000 ft to assigned altitude call at 4,000 ft, he was also observing the weather situation and did not monitor the flight instruments as the aircraft approached the assigned altitude. The B737 continued to climb above 5,000 ft and reached 5,700 ft before the pilot in command descended the aircraft back to the assigned level. During the descent the aircraft's traffic alert and collision avoidance system issued a Traffic Alert.

The departure controller issued a turn instruction to the crew of the B737 for avoidance action and an evasive turn instruction to the crew of the B767, in addition to providing traffic information on the B737. Recorded radar data indicated that lateral separation between the aircraft reduced to 2.8 NM with a vertical separation of 900 ft. The required radar separation standard was 3 NM laterally or 1,000 ft vertically.

A Boeing 747-48HE registered VH-OEB (OEB) was en-route from Los Angeles, USA to Auckland, New Zealand was maintaining flight level (FL) 330 as assigned by Tahiti air traffic control (ATC). A Boeing 747-4H6 registered VH-OED (OED) was en route from Auckland to Los Angeles and was also maintaining FL330. The crew of OEB reported that they observed, on their traffic alert and collision avoidance system (TCAS), another aircraft that was on a reciprocal track at the same level (OED). The crew of OEB turned their aircraft right 15 degrees and descended to FL325. The crew of OED later reported that they observed, on their TCAS, another aircraft that was on a reciprocal track at the same level (OEB), and climbed their aircraft to FL333. A third aircraft, a Boeing 747, was en-route from Los Angeles to Auckland at FL340. The crew of OED also observed an indication of that aircraft on their TCAS.

The vertical separation standard was 1,000 ft. The vertical distance between OED and OEB reduced to 800 ft, and to 700 ft between OED and the third aircraft. There was an infringement of separation standards.

The crews of OEB and OED were communicating with Tahiti ATC via both Controller-Pilot Data Link Communications (CPDLC) and high frequency (HF) radio. CPDLC was a 'means of communications between a controller and pilot using data link for [Air Traffic Control] communication' (ICAO Doc 4444 ATM/501 14.1.1). Messages were compiled and initiated either by the crew of the aircraft or by ATC and were, in this case, pre-formatted. The use of pre-formatted messages was 'intended to reduce the possibility of misinterpretation and ambiguity' (ICAO Doc 4444 ATM/501 14.3.4).

The crew of OEB had requested climb from FL320 to FL340 but that request was denied. About ten minutes later the crew of OED requested climb from FL330 to FL350. Tahiti ATC asked the crew of OED, via CPDLC, when they could reach FL350 and then denied the request for climb. The French Bureau d'Enquetes et d' Analyses pour la Securite de l'Aviation Civile (BEA) produced a report on the occurrence. The BEA reported that the crew of OED then contacted Tahiti ATC via HF radio and advised that they could reach FL350 by time 1140 universal coordinated time. The controller responded via HF radio and instructed the crew of OED to maintain FL330. The crew of OEB then requested, via CPDLC, climb to FL330. The CPDLC response provided to the crew of OEB was 'climb to and maintain FL330 due to traffic' even though FL330 was not available. The message was selected by the controller from the menu of pre-formatted messages available in the system.

The South Pacific Operations Manual (SPOM Part 5.5) stated that 'when a clearance request is denied, the controller shall use the element "UNABLE" in the uplink message'. The SPOM detailed the procedures and requirements applicable in the South Pacific flight information regions (FIRs) for CPDLC equipped aircraft and applied within the Tahiti FIR. The SPOM (Part 5.1) also stated that `generally, when a CPDLC aircraft is operating within a CPDLC FIR, CPDLC will be the primary means of communication'.

Subsequent to the occurrence OEB returned to FL320 and OED returned to FL330. The crews then reported to Tahiti ATC at those respective levels. The controller had not intended to assign FL330 to the crew of OEB and did not realise that they had been assigned FL330, or that they had climbed to FL330 and subsequently returned to FL320. When the crews reported at FL330 and FL320 respectively, after the occurrence, that information was consistent with the information the controller had recorded on the flight progress strips. The controller was not aware that there had been an infringement of separation standards.

The BEA reported that the controller believed there were possibly two reasons why a climb instruction had unintentionally been assigned to the crew of OEB:

1. In reply to the request by OEB for climb to FL330, the controller pre-selected the wrong pre-formatted CPDLC message and sent the message without checking it, or

2. The controller confused the two aircraft because of their similar callsigns.

The BEA reported that four controllers had been rostered for the period between 1900 hours and 0700 hours (Papeete local time) and were rostered to cover the aerodrome control position, the approach control position and the area control position. The event occurred at 0050 Papeete local time. The controller involved in the occurrence was alone in the tower at the time of the occurrence and was performing all three functions. That controller considered that the workload was high due to poor quality HF radio, increased coordination with other centres in relation to aircraft using 'flexible routes' and difficulty validating CPDLC messages with OED.

The controllers at Tahiti had been trained in France but that training had not included the use of CPDLC. Initial training on the use of CPDLC was incorporated into a one-week training program in Papeete that included CPDLC with other local training requirements. Ongoing CPDLC training was incorporated into on-the-job training which could take controllers around 57 weeks to complete. Controllers reported that the initial training was essential but they had not been exposed to the system sufficiently during training to master all aspects of the system.

The controller involved in the occurrence had been working Tahiti Oceanic Controlled Airspace for approximately three and a half years and was qualified on the three ATC positions being managed at the time of the occurrence.

The pilot of a Saab Aircraft AB SF-340B (Saab), on final to runway 16R at Sydney airport, was instructed by the aerodrome controller (ADC) to turn right heading 240 degrees M due to an unidentified aircraft in the control zone. The unidentified aircraft was observed to turn north and pass the Saab with 2 NM lateral and 400 ft vertical displacement. The required separation standard was either 3 NM laterally or 1,000 ft vertically. The unknown aircraft was subsequently identified as a Piper Aircraft Corporation PA-23-250 (Aztec). The Aztec pilot had entered the Sydney control zone without a clearance, resulting in an infringement of separation standards.

The Aztec pilot had intended to conduct a visual flight rules (VFR) flight from Bankstown, located 9.5 NM west of Sydney, to Grafton. He had recently purchased the Aztec and this was the first significant trip in that aircraft. The pilot had previously flown a Beech Baron and had completed a flight check on the Aztec.

Flights under the VFR conducted below 10,000 ft required a pilot to operate in the following meteorological conditions:

• flight visibility greater than 5,000 m;
• clear of cloud when in a general aviation control zone; and
• 1,500 m horizontally and 1,000 ft vertically from cloud while en route.

The Bankstown terminal area forecast, issued at 0433, covering the period from 0600 to 1900 Eastern Summer Time forecast a flight visibility of 5,000 m in smoke and a few (1 to 2 OKTAS) clouds at 3,000 ft. The forecast indicated that visibility was expected to increase to greater than 10 km by mid afternoon. The actual meteorological conditions reported at Bankstown during the morning of the occurrence were:

• 0900: westerly wind at 9 kts with visibility of 8,000 m, no cloud below 12,500 ft and temperature of 18 degrees C;
• 0900 report was amended at 0919: westerly wind at 9 kts with visibility of 3,000 m in smoke, no cloud below 12,500 ft with the sky obscured and temperature of 18 degrees C;
• 0930: south-westerly wind at 6 kts with visibility of 6,000 m, no cloud below 12,500 ft and temperature of 18 degrees C;
• 1000: wind was calm with visibility of 7,000 m no cloud below 12,500 ft and temperature of 19 degrees C; and
• 1030: north-westerly wind of 4 kts with visibility of 7,000 m, no cloud below 12,500 ft and temperature of 20 degrees C.

The actual reported weather conditions at Sydney during the morning were:

• 0900: southerly wind at 7 kts with visibility of 4,000 m in smoke and scattered (3 to 4 OKTAS) clouds at 1,600 ft and temperature of 18 degrees C;
• 0920: southerly wind at 7 kts with visibility of 6,000 m in smoke with a few (1 to 2 OKTAS) clouds at 4,500 ft and temperature of 18 degrees C;
• 0930: southerly wind at 7 kts with visibility of 6,000 m in smoke with a few (1 to 2 OKTAS) clouds at 4,500 ft and temperature of 18 degrees C; and
• 0955: south-easterly wind at 7 kts with visibility of 7,000 m in smoke with a few (1 to 2 OKTAS) clouds at 4,500 ft and temperature of 19 degrees C.

Weather conditions at the time were visual meteorological conditions that had been affected by bushfires in the Sydney basin.

The pilot had delayed departing from Bankstown in anticipation of the weather conditions improving and subsequently departed at about 0945. Immediately after take-off, while the pilot was still monitoring the Bankstown ADC frequency, the right engine commenced to `run rough'. The pilot reduced power on that engine and attempted to identify the cause of the problem. The pilot decided to return to Bankstown as the situation could not be rectified and the aircraft was vibrating. As he manoeuvred to return, the flight visibility was such that the pilot could not see Bankstown airport. He was aware of the aircraft's proximity to the Sydney control zone and reported that he was about to call Sydney air traffic control for a clearance when he was advised by the departure south controller that the Aztec had infringed the control zone.

The Aztec pilot had selected code 1200, the nominated code for a VFR flight operating in non-controlled airspace (and not participating in a radar information service) and was operating the aircraft's transponder.

The Sydney Aerodrome and Director West controllers saw, on the air traffic control radar, that the Aztec was in the control zone, northwest of Canterbury racecourse, and likely to conflict with aircraft on final to runway 16R. The ADC instructed the pilot of the Saab to turn right to avoid the Aztec. The pilot of another aircraft was similarly instructed.

Before the pilot could return to Bankstown, the right engine on the Aztec started to operate normally. The pilot decided to continue the flight and tracked to the north to vacate the control zone and to join the VFR lane. The flight continued uneventfully to Coffs Harbour where the pilot refuelled the aircraft. Subsequently, after take-off from Coffs Harbour, at approximately 300 ft, the right engine surged and the pilot landed the aircraft on the remaining runway. The pilot taxied the aircraft to a hangar for maintenance action and on reaching the hangar the left engine stopped. Inspection by a licensed aircraft maintenance engineer (LAME) found that the cooling flaps on both engines were inoperative and had caused the engines to overheat. The LAME re-rigged the cowl flaps for maximum cooling and a subsequent engine ground run confirmed normal operation.

A Boeing 767-338ER (B767) was maintaining flight level (FL) 370 and had been assigned FL330 to maintain separation with a Cessna Citation 500 (C500), maintaining FL310, that was crossing the track of the B767. The controller entered FL330 into The Australian Advanced Air Traffic System (TAAATS). He subsequently, and unintentionally, assigned the crew of the B767 descent to FL300. The controller received a cleared level adherence monitor alarm (CLAM) when the B767 descended through FL326. Vertical separation between the B767 and the C500 reduced to 700 ft, and horizontal separation reduced to 3.4 NM. The required separation standard was 2,000 ft or 5 NM. There was an infringement of separation standards.

The controller had initially cleared the crew of the B767 to descend from FL370 to FL330. The descent was to be commenced at the discretion of the crew. He then entered FL330 into TAAATS as the new cleared flight level (CFL). The controller reported that he had made the necessary TAAATS entries on receipt of the correct level read back from the crew of the B767. There were no subsequent TAAATS entries required in relation to FL330 being assigned to the crew of the B767.

The B767 crew reported leaving FL370 approximately five minutes after they had been assigned FL330. The controller reported that he had intended to confirm FL330 as the cleared flight level with the B767 crew at that time, but he unintentionally assigned FL300. The crew of the B767 read-back FL300 and continued descent through FL330. The controller did not detect from the read back that he had assigned an incorrect flight level. There were no subsequent opportunities for the controller to realise the error until the CLAM alarm from TAAATS.

The controller indicated that he considered his workload at the time of the occurrence to be light. He was responsible for two sectors of airspace but he did not believe that the increase in workload caused by the combination of the two sectors contributed to the error. The replay of the voice recording indicated that the controller had up to ten aircraft under his control at the time of the occurrence. Three of those aircraft had requested a shorter route. The controller accommodated the requests because some of those aircraft were involved in bush fire fighting operations. When the routes for those aircraft were amended, the associated flight data record in TAAATS also needed to be amended and the changes needed to be coordinated with adjacent sectors. The controller did not believe the extra workload generated by those tasks contributed to the occurrence.

The controller was also the team leader on the shift. He reported that there were no distracting team leader issues at the time of the occurrence. The controller did not believe he was fatigued.

A Lockheed C-130J Hercules (Hercules) had conducted a practice Instrument Landing System (ILS) approach to runway 35 at Canberra followed by a touch and go landing and departure to the northwest. A Boeing 737-376 (B737) was simultaneously radar vectored for an instrument approach to runway 17. Both aircraft intended to use the same runway surface but in opposite directions. The Canberra Approach East (CBE) controller became concerned that the separation standard of either 3 NM laterally or 1,000 ft vertically would not be maintained between the aircraft and issued turn instructions to both crews. While complying with the instructions the two aircraft passed with approximately 2 NM and 100 ft separation. There was an infringement of separation standards.

The Hercules crew had requested an ILS approach to runway 35 at Canberra for instrument flight rules (IFR) flying training purposes. An ILS is a precision instrument approach that provides centreline and glideslope guidance to the pilot, aligned to the landing runway and is primarily used during periods of low cloud and/or poor visibility. Runway 35 was the only runway serviced by an ILS approach at Canberra. The En Route Supplement Australia (ERSA) required crews planning instrument training at Canberra to contact Canberra air traffic control to book an approach time slot for air traffic control traffic management purposes. The service provider reported that the Hercules was running late and had missed its booked slot.

The CBE controller planned to sequence the Hercules between the second of two aircraft taxiing for a departure from runway 17 and the B737, which was the first of a number of arrivals for that runway. The CBE controller was aware that the Hercules' practice ILS approach might be delayed by up to 45 minutes if he was unable to sequence that aircraft ahead of the B737. The CBE controller stated that he felt some pressure to provide a good service to the Hercules crew.

On first contact with the CBE controller the B737 crew was told to expect a Very High Frequency Omni-Directional Radio Range/Distance Measuring Equipment (VOR/DME) approach to runway 17. At various times thereafter, the crew was assigned radar vectors and a speed restriction of 270 knots indicated airspeed to position the B737 behind the Hercules and provide sufficient spacing to maintain the required separation standard. The CBE controller had asked the crew of the Hercules to maintain their best speed once established on the final approach track. That was a request only and the crew of the Hercules was under no obligation to comply. From that point, the Hercules was capable of maintaining an indicated airspeed between 10 to 20 kts faster than that which it would routinely maintain. The crew of the Hercules could not recall whether they had complied with the request.

The CBE controller was concerned about the application of separation between the B737 and the Hercules after the Hercules had completed its touch and go landing. The Letter of Agreement between the Canberra Tower and Canberra Approach Control stated that the approach controller was responsible for separation assurance during reciprocal runway operations unless it was assigned to the ADC "by mutual agreement". The CBE controller advised that his contingency plan was to instruct the B737 crew to terminate the approach if it became apparent that separation between the two aircraft may be infringed.

The CBE controller reported that he had based the traffic management plan on his expectation that the Hercules would be operated at a higher speed along the final approach path. The CBE controller also reported that the Hercules had commenced its turn later than he had expected following the touch and go landing. He could not see the Hercules on radar following the touch and go landing and was unsure of the position of that aircraft in relation to the B737. The CBE controller asked the ADC to confirm when the Hercules had commenced the turn and was visually clear of the inbound path of the B737. The ADC was unable to provide visual separation between the B737 and the Hercules before radar and vertical separation were infringed.

The CBE controller reported that his workload was very high at the time of the occurrence. He did not provide either crew with a safety alert in respect of the other aircraft, as required by MATS, despite the distance between the two aircraft being less than that prescribed by the applicable separation standards.

Both aircraft were equipped with a traffic alert and collision avoidance system (TCAS). The crew of the Hercules did not receive either a traffic advisory (TA) or a resolution advisory (RA) on the TCAS. The crew of the B737 did not report receiving a TA or a RA either during, or after, the occurrence.

A Boeing 767-336 (B767) was en route from Sydney to Melbourne and was maintaining flight level (FL) 350. A Boeing 737-800 (B737) was en route from Melbourne to Nadi, Fiji, and was also maintaining FL350. The aircraft were on segregated routes that provided lateral separation until the crew of the B737 was provided with track shortening. That decision placed the two aircraft on conflicting flight paths at the same level. The Eildon Weir/Benalla (ELW/BLA) sector controllers saw the impending conflict and alerted the Wollongong/Jervis (WOL/JVS) controller. Both controllers then issued traffic information and instructions to the crews for avoiding action. Both crews received Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisories (RA) and the controllers received a Short Term Conflict Alert (STCA) from The Australian Advanced Air Traffic System (TAAATS). The aircraft passed within 4.8 NM laterally and 800 ft vertically. The required separation standard was either 5 NM laterally or 2,000 ft vertically. There was an infringement of separation standards.

The B737 crew had flight planned, and had been initially cleared to, FL330. That was a standard level for the direction of flight. Levels were assigned depending on a number of factors including the track of the aircraft in accordance with the Table of Cruising levels in the Manual of Air Traffic Services (MATS). Aircraft operating at altitudes and flight levels in accordance with the table were considered to be at standard levels, and those operating at altitudes and levels not in accordance with the tables were considered to be at non-standard levels. The crew of the B737 subsequently requested, and was assigned, non-standard FL350. At the time of the level change, both the crew of the B767 and the crew of the B737 had been issued airways clearances on one-way, designated air routes that formed a segregated race-track pattern between Sydney and Melbourne.

The controller responsible for the ELW/BLA sectors was instructing a student training on the ELW sector. Both were concentrating on the sequencing and separation of two jet aircraft in a step descent into Melbourne. A step descent allowed aircraft to simultaneously descend to vertically separated levels provided that the higher aircraft was progressively assigned levels that provided vertical separation with the lower aircraft. The step descent was occurring in the bottom left quadrant of the controller's Air Situation Display (ASD). The instructor was positioned behind and to the left of the student so that he could readily view the ELW sector and the traffic on the left side of the ASD. The B767 and the B737 were displayed in the top right quadrant of the ASD. Once the step descent had been established, the instructor noticed that the B737 was about to conflict with the B767. By that time the B737 was within the WOL/JVS combined sector and the instructor advised the WOL/JVS sector controller that he would be turning the B737. The student instructed the crew of the B737 to turn right to avoid the opposite direction B767.

The B737 was within the WOL/JVS control area, but under the jurisdiction (and control) of the ELW/BLA controller. The Australian Advanced Air Traffic System Human Machine Interface (HMI) used different coloured tracks and labels to aid situational awareness. The track and label colour of the B737 was green to the ELW/BLA controller, and blue to the WOL/JVS controller. The B737 crew should have been transferred to the WOL/JVS sector prior to crossing the boundary between those sectors and the ELW/BLA sectors. Had the transfer been made the track label and symbol would have been green on the WOL/JVS ASD.

The WOL/JVS controller initiated the amended route clearance for the B737 at FL350, and verbally coordinated the change with the ELW/BLA controller. At that time, the southbound B767 was on climb to FL280. The crew of the B767 contacted the WOL/JVS controller and was assigned FL350. The amended route for the B737 was direct to ALLOC, a waypoint located 77NM east-north-east of Sydney.

The workload on the WOL/JVS sectors at the time of the occurrence was reported by the controller to be low. The controller was providing a directed traffic information (DTI) service to military helicopters operating under visual flight rules (VFR) in class "G" airspace. The provision of DTI to VFR aircraft was available on request and subject to controller workload.

The WOL/JVS controller reported that the ASD at that console had recently "been faulted" and found to be outside the acceptable parameters for contrast. Subsequent to the occurrence, the screen was again checked and was found to be below acceptable contrast parameters. The controller reported difficulty distinguishing the blue track colours from the grey background of the screen; they appeared faded. A track was blue to indicate that the track was about to become the responsibility of that controller. Tracks will appear (in this case) blue to only one console at a time. The same track symbol and label was a different colour to all other controllers to indicate the relevance of that track to each control position.

The WOL/JVS controller stated that a smaller information screen that was open on the WOL/JVS ASD initially obscured the track symbol and label of the B737. He had used the route function available in TAAATS to determine the cleared route of the B737. The controller also stated that he normally used the text message box in TAAATS as a visual reminder of potential conflicts. He did not use the text box on this occasion. He also indicated that he would have preferred using strips because they better facilitated the acquisition and maintenance of the traffic picture.

The ELW/BLA instructor commented that the angle of view from behind and to one side of a student made it difficult to see some areas of the ASD and to monitor keyboard entries.

The Cessna Aircraft Company 402C (Cessna) was established on final for a straight-in approach to runway 15 at Cairns. The pilot had been assigned a descent to 3,000 ft, due to a de Havilland Canada Dash 8 (Dash 8) that was on final approach ahead of the Cessna and was at or below 2,000 ft. The assigned altitude ensured that the 1,000 ft vertical separation standard was maintained. The Approach One controller did not notice the Cessna descend through 3,000 ft or that the ground speed of the Dash 8 had reduced such that the spacing between the aircraft was less than the required 3 NM radar separation standard. There was an infringement of separation standards. The weather was reported to be fine with a clear sky.

Cairns approach control was managed by two control positions; Approach One and Approach Two. The Approach One area of responsibility was over the sea and included the final approach for runway 15 and all departures. Approach Two was responsible for the area over land.

The Approach One controller had accepted responsibility for the position about 10 minutes before the occurrence. The controller had been the Flow controller for the afternoon and was due to finish his shift at 1830 Eastern Standard Time. Prior to leaving, he offered to take over Approach One to enable another controller to take a short break. A handover/takeover was conducted at the Approach One position and he was aware of the six aircraft in the arrival sequence; having established the sequence himself while in the Flow position. He was also aware of another four aircraft taxiing for departure.

The initial aircraft in the arrival sequence were a Twin Otter from Mackay, the Dash 8, the Cessna and then an Embraer Bandeirante from the west. The Approach One controller was responsible for the Twin Otter, Dash 8 and the Cessna, while the Bandeirante was under the control of the Approach Two controller.

The Twin Otter crew had been assigned a visual approach for left base to runway 15 but subsequently requested an instrument landing system (ILS) approach. Approval of the ILS would entail additional track miles and reduce the spacing between the Twin Otter and the Dash 8. The controller believed sufficient spacing for separation and sequencing would be maintained with some minor track adjustments. He instructed the Dash 8 crew to turn left heading 330 degrees and to descend to 3,000 ft to position the aircraft east of the coast and 6 miles behind the Twin Otter. About two minutes later the controller instructed the Dash 8 crew to descend to 2,000 ft and the pilot of the Cessna to descend to "3000 (ft) visual". Those instructions were appropriately acknowledged by the crews. As the Cessna was from the north, and effectively on a long final, the controller issued the instructions to establish the vertical separation standard between the aircraft.

The Approach One controller instructed the Dash 8 crew to turn left heading 250 degrees and to descend to 1,500 ft. The crew was aware of aircraft arriving from the north. As the Dash 8 approached the runway 15 extended centreline, the crew thought the approach controller may have forgotten them and they discussed whether or not to contact the aerodrome controller direct. The crew could see the terrain to the west and reduced the aircraft's speed to reduce the rate of closure with the terrain. Shortly after, the Approach One controller instructed them to make a visual approach. That required them to turn the Dash 8 approximately 140 degrees to the left and then turn back to the right to establish the aircraft on final for runway 15. The crew did not report, nor were they required to report, their reduced speed. The controller could monitor aircraft ground speeds via the air situation display radar track labels.

The pilot of the Cessna had been assigned a descent to 3,000 ft and was instructed to maintain the best approach speed. He had been given traffic information on the Dash 8 and was expecting to be instructed to sight and follow that aircraft. Use of the sight and follow procedure by air traffic control (ATC) was common and transferred separation responsibility from ATC to a pilot. The Cessna pilot could see the Dash 8 ahead on final and continued descent below 3,000 ft without a clearance. He later stated that he only became aware that he had not complied with his clearance after he transferred to the aerodrome control frequency. The pilot also said that he believed there was no risk of collision.

The Approach One controller was an experienced controller and was also a team leader. He was on the second shift of a four-day cycle and had commenced the shift at 1100. He had worked the same shift the previous day. The controller had two days off duty before commencing this shift cycle. On the day of the occurrence he had visited a relative in hospital at 0800 and then returned home to do some paperwork before going to work. The relative had entered hospital about a month previously and the controller had visited regularly over that period. The controller was aware of the need to not allow the relative's illness to impact on his work. He believed he had achieved a balance between work and non-work commitments.

The controller spoke briefly with the Approach Two controller and gained the impression that the Cessna was under the control of the Approach Two controller. Consequently, as he saw the distance reducing between the Dash 8 and the Cessna he was waiting for a response from the Approach Two controller despite the fact that both pilots were operating on the Approach One radio frequency.

The Aerodrome controller was concerned with the reduction in spacing between the aircraft, and asked the Approach One controller to slow the Cessna to provide sufficient time for the Dash 8 to vacate the runway. That prompted the controller to ask the pilot of the Cessna if he could see the Dash 8. The pilot replied that he could, and was then instructed to contact the Aerodrome controller, who instructed the pilot to make an orbit, as there was insufficient spacing to ensure that the runway standard would be met.