Loss of separation

1. The crews were not using the same air traffic control frequency at the same time.
2. An assessment of the controller's fitness for duty did not include due consideration of the cumulative effects of stress.
3. The controller was probably unaware of the harmful effect of stress on individual performance.
4. The management of staff by a number of different managers did not aid Airservices Australia to readily identify the controller's fitness for duty.
5. The operation of traffic alert and collision avoidance systems and aircraft secondary surveillance radar transponders by the crews were valuable defences for the aviation system.

There were opportunities for both crews to be alerted that they were assigned the same level. However, these were missed because:

- the B767 crew had been instructed to monitor the Woomera sector frequency and did not hear the controller approve the A320 crew to climb to FL280,
- the controller's transmission about the A320 interrupted the B767 crew's on-frequency report that would have included their level,
- the controller did not include the assigned level for the opposite direction aircraft in the sighting and passing report transmission to both crews, and
- the presentation of system data reinforced the controller's view that vertical separation had been applied between the aircraft.

High levels of stress can increase the likelihood of individuals failing to complete actions or recognise a situation. For example, stress promotes slips and lapses by narrowing the focus of attention. Slips most often occur when an individual is performing an automated task in familiar surroundings, but is distracted by external events or internal thoughts. A preoccupation with personal problems can divert attentional resources, particularly at times of low workload. Such conditions are ideal for inducing a slip.

The controller's inability to appreciate the use of the wrong level and its relationship with his traffic management plan, and the indicators available to him from the air traffic system was consistent with him being preoccupied. An increased state of stress was probably the prime reason for the lack of concentration or preoccupation by the controller. This then led him to instructing the A320 crew to climb to the wrong level and causing the separation standard to be infringed. This was consistent with the controller's stressful state while under a low workload.

Research literature on occupational stress strongly supports giving priority to person-job fit issues as a strategy to prevent work stress. Also, a change to the work environment has the potential to cause health problems if not managed well. Airservices Australia had not been able to address the controller's concerns about occupational opportunities. Nor were they able to either understand or reduce the controller's increasing reaction to these stressors. The result was an organisational climate that contributed to the controller's stress levels. Concurrently, the controller had significant difficulty adjusting to the changes in vocational circumstances. These organisational and personal factors combined to produce increased stress in the controller.

The impact of stress on the controller's performance was one that needed consideration and action as advised in the CATSOAM. Airservices knew the controller had personal and vocational problems but seemed unable to reduce the individual's occupational discontent. It is probable the change in management staff during the TAAATS transition limited the organisation's ability to recognise the extent of the controller's stress and consequent potential effect on safety. This lack of organisational understanding and the use of various managers, in addressing the issues, may have limited the centre manager's ability to decide on the controller's fitness for duty. Without a full understanding of the controller's situation and current susceptibility to stress related conditions the manager had to assess whether the controller was able to work. The ability to readily access a controller's complete history or the use of a protocol, that ensured vocational and personal details were considered with any commensurate safety risk, would have enabled the manager to make a more informed decision. As it was, the centre manager tried to mitigate the situation by briefing the flight information region manager and the team leader. However, the flight information region manager and the team leader lacked a full understanding of the situation and were less able than the centre manager to assess the controller's fitness for duty.

Had either a more encompassing risk protocol or a better system of monitoring a controller's career been in place it is likely the cumulative stress from the controller's job discontent, the two recent unfavourable briefings and his family problem would have been recognised. If there had been a better understanding of either the controller's situation or the potential risk to safety, the centre manager would have probably removed the controller from duty until a formal evaluation of fitness for duty was done.

Accurate work and rest times were not available to assess the level of fatigue for the controller but it is likely the controller was tired before the occurrence. The controller's overall heightened stress, preoccupation and fatigue made him unlikely to be capable of making a coherent and rational decision about his fitness for operational tasks.

Radio facilities to enable crews under the jurisdiction of the Alice Springs sector to maintain continuous radio communications with that controller would reduce the possibility for similar events.

1. The Inverell controller was experiencing personal stress factors that may have affected his work performance.
2. The Inverell controller had minimal recency time on the sector, which may have affected his performance.
3. The crew of CZA requested and were approved to climb to non-standard level FL370.
4. The Inverell controller did not implement a separation assurance plan when he became aware of the likelihood of a conflict.
5. The Inverell controller allowed secondary considerations to override separation actions.
6. The short-term conflict alert activated in sufficient time to allow remedial action by the controller.
7. The TCAS on each of the aircraft provided each crew with an alert indicating a required positive separation action.

Both aircraft crews acted in accordance with company procedures and followed the TCAS resolution advisories.

The incident occurred at a well-known point of conflict within an en-route sector. The experienced controller was operating a control position with which he was familiar and with traffic volume and complexity that should have been well within his ability. However, he did not take action to ensure separation between two aircraft that he had earlier recognised as being in potential conflict. That was most likely the result of a number of predisposing factors, including the effects of stress, limited recency, distraction, and not using a memory aid.

At the time of the occurrence the controller was testing the serviceability of the secondary radio transmitter. That was a routine and relatively unimportant task. However, the controller felt under pressure, largely self-imposed, to complete it without undue delay. Initially the controller was mistaken as to the exact test required and that led to some confusion on his part. Consequently, he was distracted for longer than anticipated and his awareness of the developing traffic situation was compromised.

Correct prioritisation is fundamental to any complex operating task. The controller was faced with several competing demands for his attention. In addition to monitoring the conflicting aircraft he carried out routine tasks such as issuing instructions to aircraft and transferring aircraft to or from other control sectors, and attempted to assist the supervisor with the radio checks.

Inadequate prioritisation committed the controller to remembering the unresolved aircraft conflict in order to deal with it in a timely manner. However, distraction and a subsequent memory lapse left the conflict unresolved until the activation of the short-term conflict alert.

There were other aspects of the controller's performance that, while not significant in isolation, were possibly indicative of a lower general level of performance at the time of the occurrence. When the southbound aircraft entered the controller's airspace, he did not appreciate that the aircraft was at a non-standard flight level, even though he was well familiar with the airways route. Later, approximately 10 minutes before the incident, the controller did not correct an oversight by a Dash 8 crew. Further, when asked by the supervisor to carry out a frequency check, the controller had some difficulty with what was a relatively straightforward task.

Recency

Using low traffic movement periods to combine sectors in order to maintain adequate work levels may result in an individual meeting the formal recency requirements, but not actually having sufficient exposure to a particular sector to warrant meaningful traffic practice. In this occurrence, the amount and type of recent control work that the controller had completed may not have been sufficient to ensure performance to the standard that the recency requirements were intended to maintain; taking into consideration the variation in the level/complexity of traffic during different periods. It is possible that the controller's performance was affected by the relatively short time he had worked in the control position during the previous fortnight.

Stress and performance

At the time of the incident the performance of the Inverell controller may have been adversely affected by stress. Recent significant personal factors may have been likely to cause him considerable chronic stress. The extent to which stress related to non-work factors can affect work performance is often underestimated. Major life events can markedly affect stress levels. In addition, the acute stress of leaving work to attend to an urgent personal matter may also have influenced the controller's performance.

Individuals are often unaware of the extent to which their performance is affected by stress. They may try to "work on" despite problems or difficulties. Individuals may be reluctant to admit, even to themselves, that they are suffering from stress because of a perception that this will be seen as a sign of weakness or failure.

Research has shown that stress can produce errors such as inappropriately delaying necessary actions and forgetting to carry out required actions at a time of high workload or distraction. Stress can result in perceptual and cognitive narrowing, where attention and decision making are focussed on a restricted range of information and tasks. For example, a controller's scan pattern may be disrupted. Stress can also lead to task shedding. This can result in the neglect of crucial matters while time may be spent on tasks of lesser importance. Memory can be significantly inhibited by stress.

In this incident, stress may have reduced the controller's capacity to handle what would normally have been a moderate workload. The controller allowed himself to be distracted by testing the radio equipment, to the detriment of his primary task; that of managing air traffic.

Memory aids

Because the potential conflict remained unresolved, the controller had to keep the task in short-term memory. Omitting to carry out planned actions - a failure of prospective memory - is one of the most common forms of memory lapse. A necessary condition for a memory lapse to occur is that attention is captured by either an external distraction or an internal preoccupation. The use of an appropriate memory aid would have guarded against the separation task being forgotten and a number of such methods were available to controllers.

A Boeing 737, VH-CZA, departed Maroochydore for Sydney and was tracking southbound on air route H62. The crew planned to cruise at flight level (FL) 350 but experienced turbulence at that level. Several other aircraft had encountered turbulence at various levels up to FL350 but those at FL370 were in smooth flying conditions. Consequently, the crew had obtained approval from air traffic control to climb to non-standard level FL370 well before the aircraft was handed off to the Inverell sector controller.

Although the Keppel controller intimated to the crew of UUQ that a minor reduction in airspeed would assist in the sequencing into Mackay, the crew either did not appreciate the need for a speed reduction or ignored the information. If the Keppel controller had advised the Swampy controller that the crew of UUQ had been forewarned of the likelihood of the imposition of a speed reduction, or had the crew reduced speed when advised, the loss of separation and subsequent situation may not have occurred.

The Swampy controller, despite not being endorsed for the sector, was only required to use standards and procedures that were commonly used on the Daintree sector. However, he did not issue the required instructions to either crew to establish separation using pilot visual separation procedures. The infringement of separation standards may not have occurred if the controller had issued instructions to either crew to follow the other aircraft and confirmed their ability to do so, or alternatively, issued control instructions to ensure that either vertical or radar separation was maintained until at least one aircraft exited controlled airspace.

The controller was probably more fatigued than he believed and he may have endeavoured to operate with minimal restrictions and/or control instructions in a situation which he thought was readily appreciated by both crews. The time of the day (early morning) and the resultant circadian disrhythmia may have also adversely affected his performance. As a consequence of the small amount of aircraft activity in the area at the time, the controller may have also been lulled into being less situationally aware than normal.

The lack of appreciation of the situation by the crew of UUG following the Keppel controller's suggestion to reduce speed and the acceptance by both crews of visual separation responsibility when the forecast indicated IMC in the area, were indicators that their performance may have also been sub-optimal. The limited guidance available in the AIP in relation to the assignment of separation responsibility to pilots does not clearly indicate pilot and controller responsibilities in such cases. Provision of additional information in the AIP would probably assist pilots in their decision making regarding the safe conduct of a flight.

The perceived benefit of transferring separation responsibility to pilots rather than imposing alternate air traffic control standards and procedures requires careful consideration by both controllers and pilots. This is more so the case for night operations when en route or terminal weather conditions may not be readily apparent. Additionally, any forecast that indicates that instrument meteorological conditions may be encountered should be an alert to crews and controllers to be wary of the use of visual separation procedures. The issued forecasts indicated potential instrument meteorological conditions for the Mackay terminal area and it would have been prudent for the pilots to request alternate separation procedures from the controller.

A Brasilia VH-XFZ was on a visual approach to Cairns from the south-east and was tracking to left base runway 15. A Citation VH-PSU departed from runway 15 without a separation standard being applied by the approach controller between the two aircraft. The aircraft passed with approximately 500 ft difference in their altitudes when there was less than 3 NM laterally between them.

The controller could have either coordinated the use of a visual separation procedure with the aerodrome controller or the crew of either aircraft, or employed a 1,000 ft vertical separation standard or a 3 NM radar standard to ensure that the aircraft were separated.

The investigation revealed that the controller was aware that a separation standard was required. This was confirmed by the controller's request to the pilot of XFZ to report seeing PSU that was shortly to depart. However, subsequent actions limited the ability of the pilot of XFZ to comply with the request. If this sighting had been achieved, it would have allowed the transfer of separation responsibility from the controller to the pilot. This procedure did not provide separation assurance and was a "fail-unsafe" procedure as it relied solely on the pilot's ability to see the other aircraft and limited the controllers' options should the pilot be unable to comply.

When the crew of XFZ were unable to sight PSU the controller then monitored PSU's climb performance and assessed that the aircraft would not collide. However, vertical separation reduced to less than the standard when there was no radar standard being applied.

The use of a radar standard was constrained by the disposition and intended tracks of the aircraft. In this situation, the Manual of Air Traffic Services (MATS), 9-2-1, paragraph 19 states that a procedural separation standard shall be applied. Also, the controllers actions did not comply with MATS 4-1-1, paragraphs 4-6 which places greater emphasis on traffic planning and conflict avoidance by controllers instead of conflict resolution. The use of separation assurance techniques by the controller would have ensured the separation of the aircraft.

The crew of the Dash 7, registered VH-UUM, were cleared to track visually via Kuranda. The crew of the Cessna 310, registered VH-XXT, were cleared to track visually via Stoney Creek. Both aircraft had flight planned IFR and were entitled to positive air traffic control separation. Such separation was not provided and the required radar separation standard of 3 NM between the two aircraft was infringed.

The investigation revealed that the strategy for separation employed by the approach controller did not provide a separation standard. Radar separation was an option, in accordance with the Manual of Air Traffic Services (MATS), 9-2-1, paragraph 19, if the departing aircraft had been issued a standard radar departure. Such a departure was not provided. Moreover, because vertical separation had not been employed, the controller had to rely on lateral separation to ensure the aircraft did not conflict. However, lateral separation would not exist if both aircraft were simultaneously east of a line Stoney Creek-Kuranda. There was no separation assurance. MATS 4-1-1, paragraphs 4-6 place significant emphasis on the need for controllers to apply separation assurance techniques.

Realising the potential confliction after take-off, the controller attempted to establish a visual sighting of the departing aircraft by the crew of the arriving aircraft. The visibility was reduced due to smoke haze, making a sighting unlikely. If a sighting had been achieved, it would have allowed the transfer of separation responsibility from the controller to the pilot. The crew of the arriving aircraft were unable to sight the departing aircraft. The controller then requested the crew of the arriving aircraft to make a right orbit but the crew were unable to comply due to high terrain. The crew suggested a left orbit, which was agreed to by the approach controller. The left orbit however, further reduced the distance between the two aircraft.

The crew of the departing aircraft then sighted the arriving aircraft, with the assistance of the aircraft's TCAS, and were able to visually maintain separation. Vertical separation was reduced to 100 ft when the aircraft were 1.5 NM apart. The required radar separation was 3 NM or 1,000 ft vertically.

The disparity between the TAAATS training and management of sector 6, the differences in coordination and management between TAAATS and sector 6 and the limited opportunities to use radar vectoring, all contributed to limit the controller's ability to successfully resolve the conflict.

The ready provision of direct tracking to Mudgee for aircraft that had planned via H29 increased the controller workload. While this factor was minor in comparison to the others previously mentioned, it nevertheless resulted in some additional action by the controller. Had aircraft been required to intercept and rejoin air route H29 prior to the majority of the intersections with other routes it is likely that the complexity of the controller's task would have been reduced.

Prior to the crew requesting the availability of alternative separation methods the controller had formulated a traffic management plan that would ensure separation was maintained between the aircraft. After the crew's query, the controller adjusted his plan to compensate for the changed circumstances but was inadequately prepared to ensure maintenance of separation using the radar.

VH-CZU a Boeing 737 (B737), was en route from Adelaide to Sydney at flight level (FL) 370 on air route H31. VH-CZS a B737 was tracking north on H29 en route from Melbourne to Brisbane at FL370. Their respective air routes intersected at a position approximately 25 NM north-east of Wagga and their relative positions and groundspeeds indicated that the radar separation standard of 5 NM would not be maintained.

The Melbourne Sector 6 controller coordinated with the Canty Sector controller for the crew of CZU to be issued with a requirement to descend to FL350 by 110 NM from Culin, a position approximately 34 NM north of Canberra. The intent of the instruction was to ensure that the vertical separation standard of 2,000 ft was established between the aircraft prior to the lateral distance between them reducing to less than the standard. Following the acknowledgment and read back of the requirement, the crew of CZU queried whether radar vectors would be available, as they preferred to maintain FL370 due to cloud and possible turbulence below that level. The Canty controller advised the crew to stand by and after conferring with the sector 6 controller instructed them to transfer to the sector 6 frequency. Once established on the sector 6 radio frequency the crew was instructed by the controller to turn right heading 130 degrees. The controller issued further instructions to the crew of CZU to turn onto 140 degrees and 150 degrees.

The controller then instructed the crew of CZS to turn right onto 060.

CZU passed 4 miles behind CZS while they were at the same level. There was an infringement of separation standards. The incident occurred during the period when Melbourne air traffic controllers were transitioning from the old centre that used the Australian Computer Air Traffic Control System radar and procedural flight strip bay facilities to new facilities using the Advanced Australian Air Traffic Control System (TAAATS). Sector 6 was in the old centre while Canty sector was a TAAATS position. TAAATS has a number of alarms to alert controllers of potential separation infringements. During the occurrence the short-term conflict alert operated at the Canty position. The sector 6 controller was busy at the time with a moderate level of traffic. The complexity of the management of sector 6 was compounded by weather that was causing flight crews to request advice of weather on various routes and also for diversions to avoid developing weather cells. Sector 6 had two control positions, radar and procedural and was normally operated in the combined configuration. The controller was managing both positions at the time. Another controller was available to assist at the position. This controller was not utilised until after the occurrence.

When CZS entered sector 6 airspace it was approximately 7 NM west of air route H29. This was normal practice as departure controllers were approved to instruct crews to track from their present position, within the terminal area (within 45 NM of Melbourne), direct to Mudgee. This required the sector 6 controller to calculate a specific solution for each potential crossing conflict with aircraft nominally tracking on H29 and aircraft on all intersecting routes. Sector controllers can use lateral separation diagrams to assist in the application of separation. However, the use of such diagrams was dependent upon aircraft operating within the navigational tracking tolerance of the air route being flown. Sector 6 did not have or use lateral separation diagrams.

Sector 6 was a joint radar/procedural sector with a majority of radar coverage. However, the size of the sector and the disposition of air routes within the sector meant that generally conflicts were resolved using procedural control methods. Radar vectoring was used to sequence aircraft for arrival into Sydney but was not generally used to establish separation between aircraft. Consequently, controllers had limited opportunities to practice vectoring techniques.

The controller had returned to the sector 6 staff roster approximately two weeks prior to the occurrence after being rostered for familiarisation training on 30 September and 1 October 1999. The controller had agreed to return to sector 6, following his TAAATS transition training, to enable other controllers to be released for transition training. The sector 6 area of responsibility within TAAATS had been divided into two sectors, Parkes and Bourke. The management of traffic and coordination requirements for these sectors differed considerably from those required for sector 6. The controller had undergone radar vectoring exercises during the transition training but these had focused primarily on developing human-machine interface skills and not traffic management skills. Also, some exercises used nil-wind conditions and thus were not reflective of conditions likely to be experienced on the job. The controller was rostered for two familiarisation shifts on the return to sector 6. However, after five hours during the first period of familiarisation, the controller felt comfortable and believed he was capable of operating at a satisfactory level and was subsequently endorsed to operate the sector.

The crew reported that they believed that the requirement, to descend to FL350 by 110 NM Culin, had been cancelled when the Canty controller advised them to stand by. The sector 6 controller did not instruct the crew to maintain FL370 once he had decided to vector the aircraft. Neither the crew of CZS nor the controller queried or clarified the status of the level requirement until after the occurrence.

1. The approach controller used the "sight and follow procedure" in an inappropriate circumstance. That is, the controller transferred the responsibility for separation to the pilot in a situation where the positive application of a separation standard may have been more appropriate.
2. The approach controller requested the crew of the Metroliner to sight the Jetstream in circumstances where a positive sighting may have been improbable.
3. The crew of the Metroliner sighted and followed the BAe 146 rather than the Jetstream

Notwithstanding the different profiles of the two aircraft, type identification of either aircraft would have been virtually impossible to determine at the ranges of the aircraft, that is beyond 6.8 NM. This is because contrast, which is the difference between the brightness of the aircraft against the brightness of the background, reduces with increasing range. Further, target identification would have been hampered by contour interaction. This phenomenon occurs where the outline of a target aircraft interacts with the contours present in the background. This is a particular problem at lower altitudes where aircraft appear against complex backgrounds.

When the crew of the Metroliner was instructed to report when they could see the other aircraft at 11 o'clock, the BAe 146 was the only aircraft discernible to them. With its side profile, the BAe 146 was at the extreme limit of visual acuity and could easily have been mistaken for a Jetstream, particularly as no other aircraft was in their field of vision. The Jetstream, with its head-on profile and greater range, would have been impossible to see at 9.8 NM. Moreover, because its actual position was not correctly described to the crew, they were looking for the Jetstream in the wrong place.

The approach controller may have realised that there was potential for mis-identification of the two aircraft. This is suggested by his action of cautioning the Metroliner crew about the BAe 146 that was on short final. However, the controller did not provide additional information that would have provided an assurance that the pilot was following the correct aircraft.

The identification of the potential conflict and action taken by the aerodrome controller was timely and appropriate. The vigilance and prompt action of the aerodrome controller acted as a safety defence to reduce the possibility of collision between the aircraft.

There were three aircraft in the approach sequence to land on runway 19 at Brisbane: a British Aerospace 146 (BAe 146), VH-JJS, on final at 2 NM; a British Aerospace Jetstream 3200 (Jetstream), VH-XFC, on right base at 6.5 NM with approximately 8 track-miles to touchdown; and a Fairchild SA226 Metroliner (Metroliner), VH-TFQ, on an oblique left base at 6.5 NM with approximately 10.5 track-miles to touchdown. Analysis of recorded voice data confirmed that the approach controller advised the crew of the Metroliner that they were number 3 in the sequence and that the Jetstream was number 2. Subsequent to that advice, the controller advised the Metroliner crew that the aircraft they were to follow was at 11 o'clock, 10 NM at 1600 ft. They were instructed to report when they could see that aircraft. The crew responded "Traffic sighted". The crew of the Metroliner was then assigned responsibility for separation from the Jetstream by being cleared to manoeuvre as required to make a visual approach and to "follow the Jetstream". In the same transmission, the crew were cautioned about the BAe 146 "on a very short final runway 19" and instructed to contact the tower.

When the crew transferred to the tower frequency, the aerodrome controller realised that the Metroliner was following the wrong aircraft, the BAe 146, and could be in conflict with the Jetstream. The controller instructed the Metroliner crew to orbit their aircraft on left base to increase its separation with the Jetstream. Analysis of recorded radar data indicated that separation between the Metroliner and the Jetstream had reduced to 1.2 NM when they were at the same level.

The crew of the Metroliner did not recall being cautioned about the 146. The aircraft they had sighted and followed was not the Jetstream but the BAe 146, which resulted in the breakdown of separation.

The Manual of Air Traffic Services (MATS) stated in MATS 4-5-1, paragraph 7:

"Correct identification of the aircraft from which separation must be maintained is essential".

Paragraph 10 stated:

"The traffic information provided shall contain as much as is necessary of the following to assist the pilot in identifying the other aircraft:

1. type, and description if unfamiliar;
2. level;
3. position information either by clock reference, bearing and distance, relation to a geographical point, reported position and estimate, or position in circuit;
4. intentions, or direction in flight."

Analysis of the radar data indicated that at the time the crew was asked to report sighting the aircraft, the BAe 146 was in the Metroliner's 11 o'clock relative position at 6.8 NM and displayed an oblique, side profile. The position of the Jetstream was more to the Metroliner's 12 o'clock relative position, at 9.8 NM and displayed a head-on profile.

1. The similarity of the departure instruction to the noise abatement procedure provided the potential for a human error to occur.
2. The crew of TAF did not comply with the departure instruction issued by air traffic control because the crew confused the instruction with the noise abatement procedure.
3. The enroute controller did not issue TAF with a procedural clearance that would have provided separation assurance with CZV.

The investigation determined that the crew of TAF did not comply with their departure instruction. Separation would not have been infringed if TAF had continued on a heading of 090 M as directed, instead of intercepting the 128 radial. The similarity of the noise abatement procedure to the departure instruction may have diminished the importance of the requirement for the crew to maintain a heading of 090 M after departure.

Although the action of the crew resulted in an infringement of the radar separation standard, the enroute controller did not issue a clearance that assured separation between TAF and CZV. Separation assurance was required in accordance with MATS 4-1-1, paragraph 6. The controller used radar procedures to separate the two aircraft, where non-radar separation should have been applied in accordance with MATS 4-1-1 paragraph 11 because communications and equipment did not allow the application of radar separation.

The enroute controller was not in direct communication with the crews of both aircraft. Moreover, portions of Maroochydore airspace were below radar coverage including airspace below 1,500 ft. Although LOA 98/054 allowed the enroute controller to "assume radar identification of departing aircraft within four minutes of the next call", published procedures did not permit the enroute controller to apply radar separation between radar identified aircraft and aircraft departing Maroochydore that were not identified.

The crew of VH-TAF, a Boeing 737, was taxiing for runway 18 at Maroochydore (MC) for a departure to Sydney. The route was flight planned, and subsequently cleared, via waypoint TRIKI (128 MC at 22 NM) on air route W196 at flight level (FL) 350. The crew had pre-briefed the noise abatement procedure that was detailed in AIP Australia as follows:

"2.2 Departing Runway 18 - MC to TRIKI: UNLESS OTHERWISE DIRECTED BY ATC. (a) Jet aircraft shall at 500FT, turn left heading 090 degrees M to intercept track. If not able to intercept track from this heading, then maintain heading 090 degrees until 4DME before turning to intercept track".

Another Boeing 737, VH-CZV, was inbound to Maroochydore from Sydney and tracking via TRIKI on W196. The Maroochydore aerodrome controller had received prior coordination on this aircraft, which was on descent to 5,000 ft. Although the BURNET enroute controller was responsible for CZV, the crew was not in two-way communication with the enroute controller. The enroute controller had agreed to allow the crew to transfer frequency direct from Brisbane Approach to Maroochydore Tower. The standard frequency transfer point in accordance with Letter of Agreement (LOA) ND 98/054 was TRIKI. CZV was roughly 32 NM from Maroochydore when the aerodrome controller contacted the enroute controller to coordinate a departure clearance for TAF.

Maroochydore was a non-radar tower and the aerodrome controller was responsible for providing procedural separation to aircraft below 4,500 ft. The enroute controller was responsible for the surrounding airspace overlying and to the south of Maroochydore as defined in the AIP Designated Airspace Handbook. The enroute controller was able to use radar or procedural means to provide air traffic control separation. Radar coverage in the Maroochydore area was generally available above 1,500 ft.

At 1236, the enroute controller advised the aerodrome controller to issue TAF a heading of 090 degrees M and to maintain FL150, a level that would provide vertical separation from a third aircraft overflying at FL160. The aerodrome controller recognised a potential conflict between TAF and CZV, and confirmed with enroute that the enroute controller would accept the responsibility for conflict resolution. The enroute controller accepted the responsibility for providing separation.

The aerodrome controller then issued the following departure instruction to the crew of TAF: "TAF, restriction on departure is to turn left and take up a heading of 090 magnetic, maintain FL 150, clear for take-off, make left turn". The crew correctly read back the instruction. The crew later reported that they interpreted the restriction on departure to be the altitude restriction and believed the heading instruction applied to the noise abatement procedure. At 1238, the crew was instructed by the aerodrome controller to contact the enroute controller to make a departure report.

CZV meanwhile, was approaching TRIKI and the crew reported to the Maroochydore aerodrome controller on descent to 5,000 ft. The aerodrome controller directed the crew to continue tracking direct to Maroochydore and to maintain 5,000 ft. Maroochydore tower then contacted the enroute controller seeking an unrestricted descent clearance for CZV, which was granted but not issued to the crew.

At the same time, TAF was turning right to intercept the 128 radial at 4 NM from Maroochydore and was passing 4,400 ft on climb. The crew contacted the enroute controller at 1239 and reported established on the 128 radial, climbing to FL 150. TAF and CZV were then on reciprocal tracks and closing. In an unsuccessful attempt to preserve the required separation, the crew of TAF was told to turn left onto a heading of 090 M and maintain 5,000 ft. The aerodrome controller who was in two-way communication with the enroute controller over the hotline, found that CZV was passing 5,400 ft on descent. The enroute controller asked the aerodrome controller to direct the crew of CZV to turn right on to 090 M, which was actioned. The enroute controller issued traffic information to the crew of TAF about the relative position of CZV and requested the crew to climb to FL 150 and maintain their best rate of climb. Traffic information was not issued to the crew of CZV about TAF.

At 1240, the required vertical separation of 1,000 ft between the two aircraft reduced to 400 ft when the minimum radar separation of 5 NM was infringed. The application of radar separation and the requirements for issuing clearances were detailed in chapters 4, 9 and 12 of the Manual of Air Traffic Services (MATS).

The enroute controller received a short-term conflict alert (STCA) on the radar display. However, neither of the flight crews reported receiving a traffic alerting and collision avoidance system (TCAS) warning from their respective aircraft's display.