Loss of separation

A Boeing 727 and a Boeing 737 were both on descent to Brisbane from the north. The B727 was ahead of the B737 and was instructed to reduce speed to 230 knots for sequencing. As the speed reduction took effect the following B737 closed rapidly, and the situation was not observed until both aircraft were at flight level 255 and 3.1 nm. Minimum separation is 5 nm. The B737 was turned onto a heading of 180 degrees for avoidance, and there was no further closure.

The traffic situation at the time was light, following a relatively busy period. There was no underlying reason for the controller's failure to maintain the required separation standard. There appears to have been a lapse in concentration following a period of heavy traffic.

FACTUAL INFORMATION

History of the flight

A Boeing 737 (B737) aircraft was tracking via the 320 degree radial of the Hobart VHF omnidirectional radio range (VOR) beacon, on a flight from Melbourne to Hobart. In accordance with standard procedures, Air Traffic Control had transferred the aircraft from Melbourne Sector control to Hobart Tower control.

The Hobart Aerodrome Controller (ADC) cleared the B737 crew to descend to 6,000 ft. This altitude assignment was a restriction due to opposite direction IFR traffic, an Aero Commander (AC500) aircraft which was to track outbound on the Hobart VOR 320 radial.

The AC500 departed Cambridge for Devonport and was on climb to an initial altitude of 4,000 ft for a planned cruise altitude of 8,000 ft. The pilot was advised that there were two B737 aircraft inbound to Hobart on the 320 radial (the second B737 was approximately 20 NM behind the first). He was subsequently instructed by the ADC to climb to 5,000 ft.

At 0916, the ADC carried out a distance measuring equipment (DME) check between the B737 and the AC500 which established that the AC500 was 6 DME and the B737 40 DME from Hobart. The ADC then provided traffic information to both pilots which included a request that each report sighting the other aircraft. While providing the traffic information to the B737 crew the ADC advised that they "should pass [him/in] round about 15 to 16 miles".

At 0918 the B737 crew reported at 24 DME and were advised by the ADC that the AC500 should be in their 1 o'clock to 2 o'clock position and low. Immediately following this transmission, the AC500 pilot reported at 10 DME and maintaining 5,000 ft.

Thirty seconds later, the ADC authorised the B737 crew to track towards a right base for runway 30. He did not specify the time at which they should commence the left turn.

At 0919 the ADC informed the B737 crew that he had the two aircraft in sight, and that as the B737 had turned and was diverging, instructed them to descend to 3,000 ft. He then immediately instructed the AC500 pilot to climb to 6,000ft. The pilots of both aircraft acknowledged their instructions.

The AC500 pilot then reported sighting the B737. This was acknowledged by the ADC but no new instruction was issued other than a request to report sighting the second B737.

The crew of the B737 had not sighted the AC500, but accepted the ADC's judgement that they were clear of that aircraft and commenced a descent from 6,000 ft. At 0919.54 they declared an emergency with a 'PAN' broadcast indicating that the AC500 had passed directly overhead and within 400 ft.

Information later provided by the pilots of both aircraft and a passenger seated on the left side of the AC500 indicated that the AC500 had passed approximately 400 ft above and marginally to the left of the B737 such that their wing tips probably overlapped.

At the time of the confliction, the aircraft were approximately 14 NM from Hobart on the VOR 320 radial, with the AC500 maintaining 6,000 ft. The B737 had just left 6,000 ft in a shallow descent of approximately 600 to 800 ft/min and had commenced the left turn.

Aerodrome controller

The ADC was controlling five aircraft and could not constantly monitor the B737 and the AC500. In accordance with the required standards he initially applied vertical separation of 1,000 ft by maintaining the AC500 at 5,000 ft and the B737 at 6,000 ft. Using binoculars he observed both aircraft and considered that the AC500 was tracking left of the radial. He therefore expected to see the B737 to the right of that aircraft and subsequently assessed that they were just to the right of each other and issued traffic information to that effect.

About 25 seconds after advising the B737 crew that they could turn left off the 320 radial, the ADC assessed that the aircraft had begun the turn and was therefore diverging and providing azimuth separation. He believed that he saw a change in the aircraft profile with only one landing light visible, caused by the nose of the aircraft shielding the other light as the aircraft commenced its anticipated turn. However, the aircraft did not begin the turn until almost a minute later.

Having assumed that the aircraft were diverging, the ADC elected to cancel the vertical separation and issued descent instructions to the crew of the B737. He then instructed the AC500 pilot to climb and was advised by the pilot that he had sighted the B737. The ADC later said that he then automatically transferred the responsibility for separation of the two aircraft to the AC500 pilot and did not continue to monitor their separation. In fact, the ADC did not assign separation responsibility to the AC500 pilot.

Shortly after the occurrence the ADC asserted that the automatic transfer of separation responsibility to the AC500 pilot was valid once the pilot had reported sighting the B737. This was not in accordance with the Australian Manual of Air Traffic Services (MATS) and the ADC subsequently accepted that his position had been incorrect.

MATS provides instructions on the controller's responsibilities when applying visual separation. These instructions include:

- when providing visual separation, controllers shall rely primarily on azimuth;

- corroborative evidence from the pilot of one aircraft on the relative position of another aircraft shall be obtained whenever possible before providing visual separation; and

- visual separation may be achieved when a pilot reports sighting another aircraft and is instructed to maintain separation from that aircraft.

During the period surrounding the occurrence, some phraseologies used by the ADC were not in accordance with the MATS. There were non-standard and/or inappropriate phrases transmitted which led to confusion and added to the airtime taken by the ADC. These included:

- The ADC's advice to the B737 crew, of the opposite direction traffic, was not clear in that he had intended to indicate that the aircraft would pass at about 15 to 16 DME from Hobart, but the crew understood it to mean in 15 to 16 NM from their present position (therefore at about 24 DME). The use, by the ADC, of the non-standard word "him", which the B737 crew heard as "in", in the statement referring to the distance at which the AC500 should pass, contributed to the confusion.

- The ADC, when issuing the instruction for the B737 to divert from the Hobart VOR 320 radial was not specific, in that he said only "CZH you can track towards right base", and did not include a time requirement.

- The ADC's later remark concerning the B737 making a turn and thereby diverging, was not recognised by the crew.

The controller stated that visual separation is a subjective judgement, and, in this case, he was satisfied with the separation provided. He did not enter the details of the PAN radio transmission into the tower log because he did not consider that an incident had occurred. MATS states that such an entry must be recorded, even though subsequent transmissions indicated that the B737 had resumed normal operations within a few seconds of the PAN broadcast.

B737 Crew

As the B737 approached 6,000 ft, it was maintaining approximately 250 kts, and the crew had not yet sighted the AC500. The aircraft was being levelled at 6,000 ft when the ADC informed the crew that he had both aircraft in sight and issued instructions for a further descent to 3,000 ft. They believed that they could descend and turn at their discretion. Consequently, the crew elected to maintain 6,000 ft for a short time while continuing to look for the AC500 in their 1 to 2 o'clock low position as advised by the ADC.

The crew could not recall hearing the ADC's reference to their aircraft having turned left, nor the clearance for the AC500 to climb to 6,000 ft. They considered that transmissions by the ADC had been generally vague and had contained unnecessary chatter.

At approximately 15 DME, and within one minute of the instruction to descend, they decided to continue descent. Earlier in the flight they had expected that the point of passing would be at about 24 DME. However, this was reassessed when at 24 DME the AC500 reported at 10 DME. Confident that the ADC was maintaining separation and that the AC500 must by then have passed, they commenced the descent and the left turn towards the runway 30 base leg. The AC500 then passed close over their aircraft.

AC500 pilot

Although the flight was a single pilot operation, a second pilot (travelling as a passenger) was seated in the right front seat. During the period of the occurrence, the passenger assisted the pilot to monitor the proximity of the other aircraft and to assess possible responses to the situation.

The pilot advised that the aircraft was established on the 320 radial and climbed to 6,000 ft in accordance with the ADC's instructions. At about one minute prior to passing he sighted the B737 and was expecting the ADC to transfer the responsibility for separation to him. This did not occur and the pilot assumed that the ADC continued to provide separation. As the two aircraft came closer (about 20 seconds prior to passing), both he and the passenger observed that the B737 was at about their height. They considered that avoiding action was not necessary as the aircraft was descending and would pass beneath. They assessed that the B737 passed about 400 ft below and slightly to the left.

Although the AC500 pilot said that he was used to seeing other aircraft pass within about 500 ft, he had not seen a jet at that distance before. He commented on the unexpectedly high closing speed of the aircraft.

The aircraft were closing at approximately 7 NM/min.

"See-and-avoid"

Previous BASI reports have described in detail the limitations of the "see-and-avoid" principle in circumstances such as those applying to this occurrence. The time required to detect the traffic, decide upon and execute evasive action, combined with the delay of the aircraft in taking up the commands, can require approximately 8-12 seconds. However, "see-and-avoid" without accurate traffic position alerting is likely to be ineffective.

ANALYSIS

Separation

The controller initially applied vertical separation procedures but reverted to visual procedures when he assumed that the B737 had begun to turn from the 320 radial. Having earlier assessed that the aircraft were to the right of each other, he was satisfied that the left turn he believed had been commenced would provide azimuth separation. However, the required separation did not exist, as the aircraft were being flown on track and the B737 had not commenced the turn until immediately prior to the time of passing.

The pilots were given misleading advice regarding the relative positions of the aircraft. However, having sighted the B737 about one minute before passing, the AC500 pilot did not advise either the ADC or the other crew of the positions of the aircraft. Consequently, the potential benefit of alerted "see-and-avoid" was reduced, with the B737 crew continuing to search for traffic in the 1 to 2 o'clock low position, when the AC500 was directly ahead and at about the same level.

The ADC's judgement that visual separation had been achieved was based on a very short monitoring period. Human visual acuity is degraded when anticipated tracking time and target exposure time are both short. He was providing visual monitoring at distances greater than 15 NM. Without the aid of binoculars, details such as aircraft profile and the number of lights would not be adequately distinguishable. However, using binoculars to see such detail severely restricts the field of view, to the extent that the opportunity of using distinct visual reference cues, such as the attitude of the B737 in relation to the ground or to the AC500, would have been reduced.

Expectation is a powerful influence when information detail is incomplete. In this case, it is possible that the ADC allowed his expectancy to fill in the gaps when, due to other work-related tasks, he was unable to constantly monitor the progress of both aircraft.

Radio phraseology

It is likely that the use of standard, precise phraseology by the ADC would have resulted in less confusion and a greater awareness of the situation by all involved. The ADC had, on previous occasions, been made formally aware of the need for the disciplined use of standard phraseology.

B737 crew

The B737 crew commenced their descent from 6,000 ft within the required time frame. However, the timing of their descent was slightly delayed, reflecting their uncertainty resulting from the ADC's poor application of phraseology and separation procedures. The crew's mis-understanding of the ADC's earlier estimated point of passing together with their inability to visually acquire the traffic, and the vague instruction to turn left, were all potentially distracting. The words "..you can...", used in the instruction, were the subject of some discussion by the crew and were considered by them to mean "when ready". At a time of heightened cockpit workload, these sources of ambiguity may have degraded the crew's ability to assimilate other information transmitted by either the ADC or other aircrew.

It is likely that this environment contributed to the apparent lack of recognition by the crew concerning the ADC's comment that the aircraft had turned and was now diverging. Had the crew recognised the error and corrected the ADC, he may have applied more appropriate separation procedures. However, the crew accepted the ADC's assertion that he was able to visually separate both aircraft.

The instruction to the AC500 pilot to climb to 6,000 ft probably did not appear to be significant to the B737 crew at that time as they assumed that the aircraft had already passed.

The AC500 pilot

Although the pilot was aware that the ADC's traffic advice was incorrect, he did not recognise the value of an accurate position alert to the B737 crew to help ensure their safe passing. It is likely that this was because he believed that the ADC was providing the separation. However, he did not consider questioning that the separation was adequate.

SIGNIFICANT FACTORS

1. The ADC's perception of the relative positions of the two aircraft was incorrect.

2. The ADC used poor and confusing phraseology.

3. The ADC's uncertainty of the rules relating to visual separation by pilots resulted in no positive separation being applied at the time of the occurrence.

4. The pilots of both aircraft missed opportunities to resolve confusion and errors in the ADC's management of the separation of the aircraft.

SAFETY ACTION

As a result of the investigation of this and other occurrences, the Bureau of Air Safety Investigation reviewed aspects of visual separation guidance in MATS.

Airservices Australia have advised that references in MATS to visual separation are to be consolidated into a new section titled 'Visual Separation'. A controller's guide to the application of visual separation procedures is also to be produced.

A British Aerospace 146 (BA146) was established in the holding pattern at YANGO, 60 NM NNW of Sydney, maintaining Flight Level 190 (FL190). A Boeing 737 (B737) entered the holding pattern at FL200, and was given traffic information on the BA146, which was directly below.

Holding was cancelled for the BA146, which was cleared to descend to FL180. The Arrivals North radar controller subsequently observed the BA146 height readout indicating FL200, and was about to query the level of the aircraft, when the crew of the BA146 advised the aircraft had climbed to FL200 but was now on descent to FL190. The lateral separation between the two aircraft was reduced to 1.53 NM, with a vertical separation of 800 ft. The minimum required separation was 5 NM laterally, or 1,000 ft vertically.

The Captain of the BA146 subsequently reported that the aircraft was being controlled by the autopilot at the time of the occurrence. FL180 had been correctly set on the altitude controller, but the aircraft had inadvertently climbed without warning. The captain, who was the non-flying pilot, had earlier given permission to permit three children to stand near the flight deck door. As the aircraft commenced descent to FL180 he had turned around briefly to say goodbye. When he turned back, he saw the aircraft was climbing at about 1,500 fpm, approaching FL200. He immediately instructed the co-pilot to descend to FL190, and alerted ATC.

FACTUAL INFORMATION

A Boeing 737 was maintaining FL200 while enroute from Sydney to Canberra via Wollongong and a Beech 1900 was maintaining FL190 while enroute from Cooma to Sydney via Shelleys and Bindook. A Saab 340 was enroute from Sydney to Canberra at FL200 tracking via PAGER and Shelleys. The crews of all three aircraft were operating on the same air traffic control frequency when the crew of the Beech 1900 requested climb to FL200 due to turbulence.

The air traffic controller had previously assessed that the Beech 1900 would be a possible restriction for the descent of the Boeing 737 into Canberra. However, when the pilot of the Beech 1900 requested climb to avoid the turbulence, the controller forgot this potential conflict.

There was a route limitation of not above FL190 without prior coordination with the northern adjacent sector. The controller subsequently assessed that there were no restrictions for the climb and that the Beech 1900 would pass safely in front of the Saab 340. Consequently, the controller coordinated a clearance for the Beech 1900 to climb to FL200 and instructed the pilot to climb to that level. As the pilot of the Beech 1900 commenced climb to FL200 the horizontal distance between that aircraft and the Boeing 737 was approximately 6 NM. The controller then recognised the potential conflict between the Boeing 737 and the Beech 1900 and immediately instructed the pilot of the latter aircraft to maintain FL190. However, the pilot of the Beech 1900 had already left FL190 on climb and was unable to descend back to the original level before the aircraft reached FL195. The horizontal distance between the Beech 1900 and the Boeing 737 at the time was 3 NM. The required separation was 5 NM horizontally or 1000 ft vertically.

The controller instructed the crew of the Boeing 737 to turn left onto a heading of 180 degrees to maintain horizontal separation. However, before turning the aircraft, the crew reported that they had the Beech 1900 in sight. The controller provided an update of the position of the Beech 1900 as the aircraft passed 4 NM abeam. The pilot of the Beech 1900 reported at FL190, and the crew of the Boeing 737 reported turning left onto the radar heading. There was a breakdown in separation.

ANALYSIS

The controller could offer no reason for not including the Boeing 737 in his considerations. Possibly, he became focused on providing relief from the turbulence for the Beech 1900 to the detriment of his overall control. Also, the need to ensure that coordination with the adjacent sector was completed prior to approving the climb for the Beech 1900 may have diverted his attention.

SIGNIFICANT FACTORS

The controller forgot the relative positions of the aircraft when he approved the pilot of the Beech 1900 to climb to FL200.

FACTUAL INFORMATION

An Airbus A320 and a B737 departed from Melbourne enroute to Sydney approximately two minutes apart. The A320 departed first and the crew had planned to cruise at FL370, while the crew of the B737 had planned at FL330. Both crews were cleared by air traffic control to climb to their respective levels. Airspace procedures for Instrument Flight Rules (IFR) flights operating in Class C airspace below 10,000 ft require aircraft to operate not above 250 kts indicated airspeed (IAS). Air traffic control may amend or cancel the speed restriction if the reduced speed is not required.

After the two aircraft were airborne, the Departures North (DEPN) radar controller cancelled the speed restriction by instructing both crews to climb at their desired speed. The DEPN controller did not notify any other air traffic control position that the speed restriction had been cancelled. Both aircraft transferred to the Inner North radar controller and, soon after, the crews were each instructed to reach FL330 by 80 NM Melbourne. Both crews acknowledged and readback the level requirement. The Inner North position had assumed control responsibility for the Inner West radar position due to the low level of traffic. This was normal practice when traffic levels at a position reduced such that the responsibility for the position could be consolidated with another control position. The controller was controlling one additional aircraft that was not normally the responsibility of the Inner North position. Horizontal separation between the aircraft was approximately 12 NM with the ground speed of the A320 approximately 10 kts faster than the groundspeed of the B737.

The air traffic control radar display provides a readout of aircraft ground speed which is used by controllers to assist in separating aircraft. During the next ten minutes the Inner North radar controller became involved in coordinating the separation and sequencing of a number of other aircraft and the horizontal distance between the A320 and the B737 reduced to 8 NM. The groundspeed of the B737 had increased and was approximately 100 kts faster than the groundspeed of the A320. A controller from another radar position contacted the Inner North radar controller and queried him with regard to the high rate of closure between the two aircraft and the potential to lose horizontal separation. The Inner North radar controller instructed the crew of the B737 to turn left 30 degrees and to report the new heading. The B737 crew reported that the heading would be 360 degrees and asked the controller for the type of the aircraft ahead of them and, shortly after, for the level of that aircraft. The horizontal distance between the two aircraft was then 5 NM, with the groundspeed of the B737 just over 100 kts faster than the groundspeed of the A320.

The Inner North radar controller advised the crew the aircraft ahead was an A320 at FL310. The controller was required to acknowledge and transmit instructions to a number of aircraft before being able to return his attention to the A320 and the B737. The horizontal distance between the aircraft had reduced to 4 NM, with vertical separation of 700 ft and the groundspeed of the following B737 approximately 90 kts faster than the groundspeed of the A320. The crew of the B737 reported sighting the A320 and vertical separation of 2,000 ft was achieved after another two minutes. The controller cancelled the radar heading for the crew of the B737 and instructed them to rejoin their planned route. There was a breakdown in separation.

ANALYSIS

The cancellation of the speed restriction for aircraft below 10,000 ft by the Departures radar controller was not in itself a significant factor in the incident. However, the fact that the Departures radar controller did not advise the next control position that he had cancelled the restriction was significant, as the Inner North radar controller was not alerted to the possibility of a ground speed differential between the two aircraft. Had the Inner North radar controller known of the cancellation of the speed requirement, he may have more closely monitored the progress of the two aircraft.

A significant portion of the inner North radar controller's time was spent co-ordinating with another control position and this would appear to have diverted him from adequately monitoring aircraft separation on the radar display. The inquiry by another controller alerted the Inner North radar controller to the proximity of the two aircraft and the high rate of closure due to the different groundspeeds. The Inner North radar controller became aware that the minimum horizontal and vertical separation was not going to be maintained and instructed the crew of the B737 to turn the aircraft away from the A320.

SIGNIFICANT FACTORS

1. The Departures North radar controller did not advise the Inner North radar controller that the speed restriction on the two aircraft had been cancelled.

2. The Inner North radar controller was distracted by coordination with another control position and did not adequately monitor the progress of the A320 and the B737.

3. Action by another controller alerted the Inner North radar controller to the proximity of the A320 and B737 and the significant difference in the groundspeeds.

SAFETY ACTION

The Bureau of Air Safety Investigation is evaluating aspects of separation assurance techniques within air traffic control. The details of this occurrence will be used to support SADN 960051. Any forthcoming recommendations will be published in the Quarterly Safety Deficiency Report.

Numerous aircraft were inbound to Sydney, including a foreign Boeing 747 (B747) aircraft on a Rivet Three Standard Arrival Route (STAR) and a Fokker (FK50) aircraft on an Oakdale Two STAR. Both aircraft were at about the same distance from the airfield, with the FK50 sequenced to overfly to the east for a left circuit to runway 16L. The B747 was sequenced to remain to the west of the airfield for a right circuit to runway 16R. The FK50 was assigned 7,000 ft and the B747 assigned descent to 6,000 ft, with a requirement to reach that altitude by 14 DME. This was in order to achieve vertical separation before losing lateral separation between the two routes as the B747 was turned downwind, through the track of the FK50.

At 18 DME the B747 was observed on radar by the Approach South controller to be passing through 6,900 ft, and to make an authorised left turn. The aircraft was then expected to track 049 degrees until given radar vectors, but the B747 took up a track of 030 degrees, towards the track of the FK50. An instruction was given to the B747 to turn right immediately onto 090 degrees, and traffic conflict information was passed on the FK50. Because the B747 was initially slow to execute the turn a further instruction to turn immediately was passed. As the B747 turned away from the FK50, the lateral separation had reduced to 2 NM, with a vertical separation of 800 ft. The minimum required separation standard was 3 NM laterally, or 1,000 ft vertically.

The B747 crew were of the understanding that a clearance for a visual approach had been given and hence turned the aircraft onto 030 degrees to position the aircraft for a left circuit for runway 16R. The pilot in command of the B747 had the FK50 in sight and assessed that there was sufficient separation.

The two aircraft were on crossing tracks. VH-NJG was cleared to descend initially to flight level (FL)180 to maintain separation with the VH-EKH at FL170. The controller noted successive radar returns indicating NJG was at FL177 and assumed the pilot had continued descent through the cleared level. He decided that the most appropriate course of action was for the aircraft to continue descent through the level of EKH while maintaining separation laterally.

He therefore cleared NJG to descend to FL115 and instructed the aircraft to alter heading 15 deg to the left. However, the groundspeed and descent rate of NJG did not proceed as anticipated and it became evident to the controller that vertical separation would not be established before lateral separation was lost. Accordingly, the controller instructed NJG to expedite descent through FL160 but did not pass traffic information to either aircraft when separation was lost. Radar separation reduced to three miles, instead of the required five miles, before vertical separation was established.

The investigation revealed that when the controller noted NJG's radar return indicating FL177, he assumed the pilot had continued descent through the cleared level. He then decided the conflict would be most quickly resolved by the aircraft continuing descent to below FL160, rather than reversing its flight profile to climb back to FL180. This was why he cleared the aircraft to continue descent to FL115. A replay of the radar tape indicated that the aircraft actually levelled out at FL180 but in doing so had gone down to FL177 in the level out manoeuvre (there were three returns indicating FL177 before FL180 was indicated). Hence the descent instruction resulted in the exact effect the controller was trying to avoid which was a delay due to reversal of flight profile while vertical separation did not exist.

FACTUAL INFORMATION

A Fokker F28 aircraft was conducting pilot training in the Perth terminal area. The terminal information indicated that runway 03 was to be used for departure tracks to the west of the extended runway 21/03 centreline and runway 06 for other departures. Runway 03 was to be used for arriving aircraft. The weather was CAVOK (No cloud below 5,000 ft and with a visibility greater than 10 km). To assist with the management of the training aircraft, the aerodrome controller (ADC) had received an airspace release within 5 NM of the aerodrome to the east of the centreline of runway 21/03 and up to an altitude of 1,500 ft. Traffic levels were moderate and the runway configuration increased the complexity of the traffic sequence. The control tower was normally manned by three air traffic controllers. One of the controllers was required to leave the tower and there was no replacement available. The control tower was then manned by the ADC and the tower co-ordination (COORD) controller.

The F28 had completed an instrument landing system (ILS) approach to runway 24. The crew of the F28 requested to overshoot to the left from the approach and then to conduct a right circuit to runway 03. The crew were cleared to overshoot to the left on climb to 1,500 ft and to remain east of the centreline of runway 21/03. The crew of the F28 complied with the clearance and remained east of runway 21/03.

An international Boeing B747 aircraft was being radar vectored for positioning on a left base for runway 03 and a Cessna Conquest C441 was being radar vectored from the east of Perth for a right base to runway 03. The pilot of the C441 had been cleared to descend to 2,500 ft. The ADC determined that there would be insufficient time for the F28 to land on runway 03 before the B747.

The ADC decided to hold the F28 to the southeast until the B747 had landed. The ADC's intention was to then instruct the crew of the F28 to continue for a landing on runway 03. The approach radar east (APPE) controller required the F28 to leave the circuit area to enable the C441 to descend for landing and to assist with the approaches of three following aircraft. The APPE controller co-ordinated with the ADC for the F28 to track to Parkerville, located to the northeast of the aerodrome, on climb to 2,500 ft for another ILS and for the C441 to be positioned on a close right base for runway 03. The ADC agreed with the proposal.

The APPE controller was aware that the F28 and the C441 may conflict. The APPE controller used non-standard phraseology to instruct the ADC to not approve the climb to 2,500 ft for the F28 until the C441 was closer to the aerodrome. The APPE controller also requested advice from the ADC of when the C441 could descend below 2,500 ft. The two controllers did not establish who was to be responsible for separating the F28 and the C441, or co-ordinate suitable clearances which would provide separation assurance between the aircraft.

The ADC believed that if the crew of the F28 received the instruction to track to Parkerville immediately, that the separation with the C441 would be maintained. The ADC instructed the crew of the F28 to track to Parkerville and to climb to 2,500 ft.

After observing the landing of the B747, the ADC returned his attention to the F28. The ADC did not request the COORD controller, who was a rated ADC, to assist in the monitoring of aircraft. The ADC noticed that the F28 had tracked further to the south than expected and that this had placed the aircraft in conflict with the C441. The ADC attempted to contact the crew of the F28 but was unsuccessful, as they had previously been instructed to call the APPE controller. Visual separation from the tower could not be used due to the proximity of the aircraft to each other. The APPE controller instructed the pilot of the C441 to climb to 3,000 ft and to turn to the south for separation. The aircraft passed with approximately 1 NM horizontal separation and 500 ft vertical separation. The required separation was 3 NM horizontally or 1,000 ft vertically. There was a breakdown in separation.

ANALYSIS

As the aircraft would possibly conflict at a point close to the boundary of both controller's area of responsibility, the ADC and the APPE controllers needed to co-ordinate a separation procedure to ensure separation was maintained between the inbound C441 and the outbound F28. The controllers also needed to establish which of them would undertake responsibility for the separation and for both aircraft to be transferred to that controller's radio frequency for the application of separation.

The co-ordination between the controllers did not adequately address these aspects and consequently there was a lack of separation assurance.

The ADC could have utilised the COORD to monitor some of the traffic during the period when visual separation was to be applied. The ADC possibly became distracted with other traffic and did not adequately monitor the flight of the F28 with reference to the C441.

SIGNIFICANT FACTORS

1. The ADC and APPE controllers did not co-ordinate an adequate separation procedure before transferring each aircraft to the other control position.

2. The ADC did not utilise the COORD controller to assist in the monitoring of aircraft.

The Boeing 737 was northbound from Melbourne to Brisbane, via route H29. The air traffic controller cleared the Boeing 737 direct to Mudgee so that it would pass about 15 nautical miles west of Wagga. The Boeing 747 was eastbound from Perth to Sydney on route H31 tracking from Natya to Cullerin. This track crosses H29 about 20 miles north of Wagga. Both aircraft were maintaining flight level 370 as cleared by air traffic control.

Less than three minutes before the Boeing 747 reached the crossing point of the two tracks, the air traffic controller instructed the aircraft to descend to flight level 350. At the time the Boeing 747 commenced the descent, recorded radar data showed that the aircraft had a groundspeed of 530 knots and was 24 miles from the point of track intersection. At the same time the Boeing 737 had a groundspeed of 480 knots and was 20 miles from the point of track intersection. Subsequently both aircraft passed with 1,600 ft vertical separation and within 3 miles horizontally.

The time allowed by the controller for the descent was inadequate to ensure the required 2,000 feet vertical separation standard for aircraft within five miles of each other was maintained. The controller's resolution of the conflict was deemed to be inappropriate for the situation. Given the short time to the crossing point, a higher degree of expedition would have been required if descending the Boeing 747 was to be the only measure to achieve separation. For separation assurance, once the conflict was recognised, it was not enough for the controller to merely instruct the Boeing 747 to descend. A requirement or a radar vector was necessary given the proximity of both aircraft.

Weather was not a factor in the incident. The air traffic controller's workload was not busy. All facilities and equipment were serviceable. The controller was licensed and current. There was no evidence that the controller was suffering fatigue.

The following factor was considered relevant to the development of the incident:

1. The air traffic controller did not adequately plan ahead to ensure required separation standards were achieved.

Runway 17 was nominated as the duty runway, with runway 12 available for departures only. Aircraft movements included VH-DUP, flying practice Instrument Landing System (ILS) approaches for runway 35, and VH-TQN and VH-KDV, both to depart from runway 35 for Sydney. Before these two aircraft departed the tower co-ordinator attempted to pass to the approach controller a departure clearance for VH-AVS for departure from runway 35.

The approach controller initially did not acknowledge acceptance of this clearance, instead advising he did not hold details for this aircraft. Shortly after the approach controller obtained the details and also got the clearance from the tower controller for AVS. This was for a runway 35 radar departure tracking via Shelleys at 5,000 ft. The approach controller then passed to the tower the departure instructions for AVS which included a 90 deg right turn after take-off.

A short time later the approach controller passed the approach sequence to the tower. This was for TQQ and EKN, both inbound from Sydney for runway 17 and for HYD, inbound from Melbourne for an ILS approach for runway 35. The crew of TQQ were initially told by the approach controller to take up a heading of 260 degrees to position for the final leg of the runway 17 VOR/DME approach. Shortly after this they reported visual and were given a visual approach. TQQ was then flown on a south-westerly track, with the aim of intercepting the extended centreline of runway 17.

AVS then became airborne off runway 35 and when the pilot contacted the approach controller he was instructed to cancel the right turn and to maintain runway heading. The approach controller then realised a conflict existed between TQQ, which was closing on the extended centreline for runway 17 and AVS which was flying on the runway 35 extended centreline in the opposite direction. The controller assessed that TQQ would pass behind AVS, but with less than the required radar separation of three miles. The pilot of AVS reported sighting TQQ but no traffic information was passed to the crew of TQQ. The two aircraft passed with a lateral separation of about two miles and about 400 ft vertical separation. In a situation where the minimum lateral separation of three miles does not exist the minimum required vertical separation is 1000 ft.

The instructions for the use of runways at Canberra did not preclude the use of reciprocal runways. Following this incident amended instructions were issued. These included the following:

'The duty runway nominated on the Canberra ATIS, or the runway nominated in the runway agreement, are the only runways that shall be used for departing and arriving aircraft. The runway agreement shall not include reciprocal ends of the same runway. The only exception to the above shall be an aircraft with an operational requirement to use another runway.'

The existing instructions did not exclude the use of reciprocal runway directions. The traffic situation was significantly complicated by the use of reciprocal runway directions for both departing and arriving aircraft. The procedures adopted by the approach controller did not provide for separation assurance and thus a loss of separation occurred.