Loss of separation

A Saab SF340B aircraft (Saab) departed Tamworth aerodrome and was tracking to the southeast on climb to flight level (FL) 120. A Mooney Aircraft Corporation M20J (Mooney) was travelling in the opposite direction en route from Bankstown to Inverell via Scone and Tamworth at 8,500 ft. The Mooney was in Tamworth class "C" controlled airspace. The Saab crew received a traffic alert from that aircraft's traffic alert and collision avoidance system as the Saab was approaching 8,000 ft. The Saab crew levelled their aircraft at 8,200 ft and rolled the aircraft to the left to avoid the Mooney. The pilot of the Mooney did not request or obtain an airways clearance from the Tamworth Aerodrome Controller (ADC) to enter Tamworth control area prior to the occurrence. The Saab passed within 1.8 nautical miles (NM) horizontally and 300 ft vertically of the Mooney. The required separation standard was either 1,000 ft vertically or a minimum horizontal distance determined using the appropriate "Lateral Separation" table in the Manual of Air Traffic Services (MATS). There was an infringement of separation standards.

Tamworth Air Traffic Control (ATC) provided a non-radar, or procedural control, service to aircraft operating within the Tamworth control area and control zone (CTR). Controllers used non-radar information to establish and maintain procedural separation standards in accordance with MATS. Tamworth class "C" control area steps extended to 36NM when above 6,500 ft AMSL to the south-southeast of the Tamworth aerodrome in the area that encompassed the flight path of the Mooney. Class "G" non-controlled airspace surrounded the Tamworth CTR and control area.

The Saab crew was conducting a scheduled fare-paying passenger flight under instrument flight rules (IFR) and had been cleared by the Tamworth ADC to climb to FL120. The standard altitude Tamworth ATC could assign to aircraft leaving Tamworth control area and entering the overlying Brisbane sector in accordance with the letter of agreement between Tamworth ATC and Brisbane ATC, was FL120 (subject to other aircraft). Otherwise, a procedural separation standard was applied by Tamworth ATC and coordinated with the Brisbane sector controller, or responsibility for separation was specifically assigned to the Brisbane sector controller.

The pilot of the Mooney was an IFR pilot and was normally provided with radio frequency management instructions by ATC along the route. On this flight however, the pilot of the Mooney was operating under visual flight rules (VFR) and no such advice was provided. All aircraft crews that planned to enter class "C" controlled airspace, whether operating under IFR or VFR, were required to establish two-way radio contact with ATC and obtain an airways clearance prior to entering class "C" airspace. The Tamworth visual terminal chart (VTC) depicted the lateral boundaries of class "C" control area surrounding Tamworth aerodrome above 4,500ft AMSL. However, neither the VTC nor the Aeronautical Information Publication (AIP) En-Route Supplement Australia (ERSA) specified the vertical boundary between Tamworth control area and the overlying Brisbane sector. The pilot later reported that he had studied the Jeppessen low altitude en-route chart relevant to Tamworth, prior to the flight, and believed that he would not require an ATC clearance to enter Tamworth control area above 6,500ft AMSL.

Airservices Australia reported that the Brisbane sector controller had seen the occurrence on that controller's air situation display (ASD) in the Brisbane Air Traffic Services Centre and had received a short term conflict alert (STCA) from The Australian Advanced Air Traffic System (TAAATS). The Brisbane sector controller reported that STCA's between aircraft operating within Tamworth control area were common but did not necessarily indicate a potential infringement of separation standards. Short term conflict alerts, in those circumstances, occurred when the procedural separation standard being used by Tamworth ATC was less restrictive than the STCA activation parameters used in TAAATS.

Although the track symbol and a label, showing the secondary surveillance radar code and the altitude from the Mooney, were displayed to the Brisbane sector controller on the ASD, the Brisbane sector controller had no control or jurisdiction over the Mooney and was not aware that the pilot had not established two-way radio contact with Tamworth ATC. The Brisbane sector controller was also not aware that the pilot had not received an airways clearance to enter Tamworth control area. The Brisbane sector controller believed both aircraft were under the control of Tamworth ATC because both aircraft were within the Tamworth control area. The Tamworth ADC was unable to provide a separation service to the Saab in relation to the Mooney as he had no information on that aircraft.

The crew of the Beech 1900D (B1900) aircraft had been cleared by the aerodrome controller (ADC) 1 to descend to 4,000 ft and were instructed to join the circuit via a left base leg for runway 12L (northern) circuit at Tamworth. The ADC 1 also instructed the crew to report approaching 4,000 ft. The crew acknowledged the clearance but later advised that they were unable to report approaching 4,000 ft due to frequency congestion. The ADC 1 had assigned 4,000 ft so that a procedural separation standard of 1,000 ft could be maintained with other aircraft until he could see the B1900 and apply visual separation.

The crew of the B1900 subsequently requested confirmation of their assigned level and reported their position when at 2NM north of the aerodrome. The controller then sighted the aircraft and cleared the crew to make a visual approach. The controller also asked the crew if they would need to extend through the centreline of runway 12L. The crew acknowledged the visual approach but did not advise the controller that they would need to extend through the centreline. The visual approach clearance authorised the crew to descend below 4,000 ft.

The crew of the B1900 could not comply with the instruction to join the circuit via a left base for runway 12L because the aircraft was too high and too fast. The co-pilot was the non-flying pilot and was undergoing command upgrade training. He had acknowledged the clearance for the visual approach and had previously acknowledged the instruction to join the northern circuit via a left base leg for runway 12L. At the time that the visual approach clearance was issued, the pilot-in-command was aware that they could not comply with the circuit entry instruction. He believed that the controller would have been aware that they were unable to enter the circuit on a left base leg, given their proximity to the circuit at that time and the height and speed of the aircraft. He also believed that the controller would have taken those circumstances, and the disposition of traffic in the southern circuit, into account when issuing the visual approach instruction.

The B1900 continued to descend and entered the runway 12R (southern) circuit airspace which was under the control of the ADC 2. The ADC 1 notified the ADC 2 that the B1900 had entered the southern circuit. The ADC 2 advised the ADC 1 that there was a Pacific Aerospace Corporation Airtrainer CT4B (CT4) on a right base leg for runway 12R. The ADC 2 provided traffic information about the B1900 to the crew of the CT4. There were four other CT4 aircraft inbound to the circuit at that time but the ADC 2 had not sighted them. The ADC 1 was unable to apply the required separation standard, either 1,000 ft vertically, or a minimum horizontal distance determined using the appropriate `Lateral Separation' standard from the Manual of Air Traffic Services (MATS), between the B1900 and the traffic inbound to the southern circuit. There was an infringement of separation standards.

The ADC 1 reported that he had sighted the CT4 on right base. He then instructed the crew of the B1900 to make a right turn and track via a right base leg for runway 12L. He believed that the B1900 was visually separated from that CT4 and that he could maintain visual separation between the B1900 and that CT4 with that instruction. He did not think that there had been an infringement of separation standards between the B1900 and the aircraft inbound to the southern circuit because all parties had enough time to ensure that visual separation was not infringed.

A Fairchild SA227-AC (Metro) aircraft was cleared to take off from runway 16R at Sydney. A Cessna 404 (Titan), holding at the Foxtrot intersection with runway 16R, was cleared to line up behind the departing Metro. The Titan pilot acknowledged the conditional clearance and reported lining up at Foxtrot. As the Metro was rolling under take off power, the pilot noticed the Titan moving onto the runway. The Metro pilot veered the aircraft sharply to the right to avoid a collision and passed the Titan at 77 kts. The wingtip clearance between the two aircraft was estimated to be 6m. The take-off was aborted.

The investigation revealed that the Metro was cleared by air traffic control at 22:44 Eastern Standard Time to taxy to the Bravo One holding point on runway 16R. The aircraft was on a night freight operation to Brisbane and had two pilots on board. Bravo One was the most northerly of the taxiways on the eastern side of the north-south runways. The Bravo One taxiway entered runway 16R at the threshold allowing the full length of the runway for take-off. The co-pilot reported ready for take-off at 22:47:51, which was approximately 12 minutes before curfew. The Metro had taxied behind a Boeing 727 that was to depart first.

Aircraft movements between 23:00 and 06:00 local time at Sydney's Kingsford Smith Airport were regulated by the Sydney Airport Curfew Act 1995, the Sydney Airport Curfew Regulations and the Air Navigation (Aerodrome Curfew) Regulations. Aircraft that received a taxi clearance prior to 23:00 but subsequently departed after commencement of the curfew period were able to use the full length of the runway and were not required to reposition south of the intersection of runway 16R and taxiway Golf. However, aircraft that taxied after 23:00 were required to take-off from a position south of the intersection of taxiway Golf. That requirement was to minimise environmental noise in the residential areas to the north of the airport.

At 22:52:15, the controller had cleared the Boeing 727 to take off when the pilot of the Titan called for a taxi clearance. The Titan was scheduled for a single-pilot night freight operation to Canberra. The pilot of the Titan was instructed by the controller to stand by. Due to arriving traffic on runway 34L, the crew of the Metro was advised that a delay of around 10 minutes was probable. The Titan was then cleared to the taxiway Golf intersection, which was immediately changed to the taxiway Foxtrot intersection. The Foxtrot intersection was closer to the Titan's parking bay. Because the clearance was issued 7 minutes prior to curfew, taxiway Golf was not required for noise abatement purposes. The B727 departed and the controller then processed two other aircraft for landing on runway 34L before the Metro was cleared to line up at 23:04:12.

During the approximately 17-minute period that the Metro was waiting at the holding point, the controller made numerous transmissions to seven other aircraft, as well as the coordination with controllers in the Sydney terminal control unit. In addition, the controller had discussed the closures of taxiways with the safety officer in "Car 3" and had cleared an aircraft under tow on the north-east apron. All transmissions made by the controller sounded professional and confident.

When the second of the two landing aircraft had vacated runway 34L at taxiway Bravo 9, the Metro was cleared to take off from runway 16R. The clearance was issued at 23:05:00 and acknowledged by the pilot. Almost immediately, at 23:05:07, the pilot of the Titan was issued a conditional clearance to line up. The clearance was: "CSV, Metro departing, behind that aircraft line up". The pilot replied "Behind the Metro holding clear Foxtrot at the moment, lining up, CSV". Less than 30 seconds later, the Metro crew reported aborting take-off.

Neither the Metro pilot nor the controller understood that the Titan pilot had said that he was lining up at the Foxtrot intersection during the read back of the conditional clearance. The replay of the audio recording confirmed that the Titan pilot's readback transmission was difficult to understand.

The controller in the tower was working the combined position of aerodrome control and surface movement control. Although five controllers were rostered for duty until 2300, three had stood down earlier; after the last international aircraft had departed. Such a reduction in staff coverage was common practice. The other controller was absent from the tower on a break. The occurrence happened at night in clear weather conditions. Works in progress on the north-east sector of the airport increased the normal lighting within the area with a number of flashing lights and lighting towers.

The Titan pilot said that he understood that the line up clearance was a conditional clearance but had thought the Metro was departing from taxiway Golf because they were in the curfew period. He looked right and left when entering the runway but did not see the Metro, which was using the full length of the runway. The Foxtrot intersection was 462 m from the runway threshold and the Golf intersection was 1039 m from the runway threshold.

During discussions with the Metro crew, the pilot in command reported that all of their aircraft's external lighting was selected on, including the landing lights. After veering to the right to miss the Titan that was lining up, the pilot considered the take-off to be unstable and, therefore, aborted the departure. The aircraft returned to the apron.

The Manual of Air Traffic Services (MATS) stated at 6.3.8.2:

"An aircraft delayed by the traffic situation shall be issued traffic information if appropriate, and instructed to hold position off the runway, or shall be issued a conditional line-up clearance".

MATS did not provide a definition of a conditional clearance. However, there was a definition a conditional line-up clearance in the Aeronautical Information Publication (AIP).

The AIP 3.4 - 14 at paragraph 4.5 defined Conditional Clearances as follows:

"Phrases such as "behind landing aircraft" or "after departing aircraft", will only be used for movements affecting the active runway(s) when the aircraft or vehicles concerned are seen by the appropriate controller, pilot or vehicle driver. In all cases, a conditional clearance will be given in the following order and consist of:

1. identification;
2. the condition (specify); and

3. the clearance, eg:

   ATS: "(aircraft callsign) A340 ON SHORT FINAL, BEHIND THAT AIRCRAFT LINE UP".
   Pilot: "BEHIND THE A340 LINING UP (aircraft callsign)".
   Note: This implies the need for the aircraft receiving the conditional clearance to identify the aircraft or vehicle causing the conditional clearance."

A Boeing 747 (B747) had departed Avalon, Victoria on a track that passed over Melbourne and then to the northeast. The crew had been issued with a requirement to initially maintain flight level (FL)200. The departures controller had imposed the limitation in accordance with standard operating procedures that required the "cap" to be placed on all aircraft that had planned to a higher flight level. Airspace above FL200 was under the jurisdiction of a sector controller. In addition, the horizontal boundary between the two controllers' airspace below FL200, was 30NM from Melbourne airport.

The departures controller had a Piper Navajo aircraft tracking ahead of a sequence of three jet aircraft departing from Melbourne airport. That situation required that the jet aircraft be radar vectored around the slower aircraft before they could be placed on their flight planned tracks. The first of the jet aircraft was a Boeing 737 (B737) for Brisbane, which departed approximately the same time as the B747.

The tracks of the aircraft were such that the B737 was initially to the left of the B747, but at approximately 30NM northeast of Melbourne they crossed and thereafter diverged.

The sector controller had noticed that the B747 would probably reach FL200 while still in departures airspace and, as a consequence, be forced to maintain FL200. In order to provide the crew of the B747 with an unrestricted climb profile, he coordinated with the departures controller to authorise the crew of the B747 to climb to FL370.

At that time, the B737 was below and approximately 30NM ahead of the B747. However, the ground speed of the B747 was approximately 70 knots faster than the B737, and the B737 had a greater rate of climb.

As the B737 approached the horizontal airspace boundary, the departures contoller handed over the aircraft to the sector controller while it was passing FL170. The sector controller then approved the crew to climb to FL370, as he believed there was sufficient distance between the aircraft to maintain separation. A short time later, the B747 was also handed over to the sector controller as it was passing FL200 and approximately 8 NM behind the B737. At that moment, The Australian Advanced Air Traffic Control System (TAAATS) Short Term Conflict Alert (STCA) activated and the controllers immediately attempted to prevent an infringement of separation standards. However, the crew of the B747 was on his frequency and asked the departures controller to maintain that aircraft at a lower level. The departures controller issued an instruction for the crew of the B737 to maintain FL190 but did not receive a reply, because that crew was on the sector frequency as instructed.

At that moment, the crew of the B747 made radio contact with the sector controller who immediately issued an instruction for them to turn the aircraft. There was a delay as the crew questioned the instruction, but they commenced the manoeuvre when the controller issued the instruction a second time using the word "immediately". He then instructed the crew of the B737 to level out, which they did.

The aircraft passed approximately 2.5NM apart while the vertical separation standard of 1,000ft did not exist. The required radar separation standard was 5 NM.

A Boeing 747-400 was enroute from Los Angeles to Melbourne, cruising at FL320 in a south-westerly direction on airway R576 in the Oakland Flight Information Region. A McDonnell Douglas MD-11 departed Honolulu and was climbing in an easterly direction to FL350. The aircraft were on converging tracks.

The Honolulu Air Traffic Control Facility was responsible for providing positive separation between the aircraft. The radar separation standard was 5 NM lateral, or 1,000 ft vertical.

A controller subsequently recognised that, without intervention, the tracks of the two aircraft would come within 5 NM when less than the required vertical separation existed. The controller instructed the MD-11 crew to "...fly heading 020 ..." to change the aircraft's track. The crew responded by broadcasting "... Air 205, now right, right heading...". The controller did not clarify the required turn direction, and then instructed the B747 crew "...fly 180, vectors on traffic". Those instructions were passed 60 seconds and 49 seconds respectively, prior to the aircraft passing each other.

The B747 crew correctly read back the instruction to fly 180 degrees. Immediately after receiving that instruction, the crew received a traffic advisory alert from the aircraft's traffic alerting and collision avoidance system (TCAS). The alert was followed by a TCAS annunciation of a resolution advisory (RA) to descend. The crew advised air traffic control, "... heading 180 and TCAS descent", as they descended the aircraft, in accordance with standard operating procedures. The crew had the MD-11 in sight for the duration of the incident.

Analysis of the continuous data recording plot indicated that the MD-11 did not commence the left turn until after the aircraft had passed. There was no indication that the B747 turned onto a heading of 180 degrees. A minimum of 700 ft vertical separation existed when there was less than 5 NM between the aircraft. An infringement of separation standards had occurred.

The MD-11 crew apparently misunderstood the instruction to fly heading 020 as "right" heading 020 degrees. A right turn would have been confusing to the crew as the aircraft was tracking approximately 090 degrees. The direction of turn onto a new heading would normally be flown via the shortest arc distance; in this case, a turn onto a heading of 020 degrees would normally involve a left turn. Thirty-three seconds later the MD-11 crew sought confirmation that a left turn to heading 020 degrees was required.

The crew of the B747 may not have turned as instructed due to their response to the TCAS alert, and their visual observation of the conflicting traffic. It could not be established whether standard separation would have been achieved if one or both crews had turned their aircraft when instructed to do so.

There was no evidence that the controller applied the principle of separation assurance in the control of these aircraft. Rather, the controller had relied on aircraft performance to achieve separation.

A Cessna 340 (C340) was maintaining 8,000 ft while tracking direct to Williamtown from a position bearing 020 degrees at 25 NM. A Royal Australian Air Force Macchi was conducting a Tacan approach and intended to perform a missed approach procedure before climbing to 10,000 ft to continue the training sortie. The sector controller had issued the approach controller with a restriction for the Macchi to maintain 7,000 ft on departure to ensure separation with the C340. The approach controller was a trainee being supervised by a rated officer.

While the Macchi was on final approach to runway 30, a further restriction of 2,000 ft was placed on the departure due to other conflicting aircraft. The crew of the Macchi contacted the approach controller on departure and advised that they were maintaining 2,000 ft. The controller took appropriate action to resolve the confliction and then cleared the crew of the Macchi to climb to their planned level of 10,000 ft. The controller had omitted to issue the 7,000 ft restriction even though it was still a requirement to ensure separation with inbound aircraft.

As the Macchi climbed through 8,000 ft, while approximately 6 NM south of Williamtown, it passed within 1 NM of the C340. There was an infringement of separation standards.

The investigation by the Directorate of Flying Safety - Australian Defence Force revealed that the Australian Defence Air Traffic System (ADATS) was being trialled at the time but that the older surveillance radar (SURAD) equipment was actually in use by the approach controller. The SURAD did not have identification labels or height information (facilities that were available on more modern equipment) and that limitation increased the workload on the controller. Additionally, the SURAD was unreliable in its ability to provide constant, accurate position information within 10 NM of Williamtown. The controllers were aware of those restrictions as they were documented in aeronautical publications.

The military sector controller was using the Interim Radar Display System (IRDS). Although that system had labels and a Mode "C" height reading capability, the Macchi was not equipped with a Mode "C" capability. Consequently, the sector controller did not have a radar indication of the height of the Macchi.

The airspace was divided vertically between approach/departures and sector control with ground level to 5,000ft being owned by approach/departures, and sector control the airspace above. The coordination had been adequate but flight progress strip management made the task of remembering an additional restriction more complicated. Consequently, the trainee approach controller forgot to issue the 7,000ft requirement to the crew of the Macchi. The rated officer did not pick up the error until the Macchi was actually passing the level of the C340. The training officer said that he looked at the radar and the strips but was feeling tired and may have missed the information.

The management of the flight progress strip, which was very crowded and difficult to read, was different from that used in other Australian locations. Local procedures were being taught where level restrictions were placed in box 11, whereas in all other air traffic control units the box for such notation was box 4, as specified in the Manual of Air Traffic Services (MATS). That local procedure had been in use at Williamtown for some time, but the investigation was unable to find any documentation specifying such action. In addition, the strip had been divided into four quarters to cater for four separate approaches that the crew of the Macchi had intended to carry out. That action made the writing very small and difficult to read. The alternate method was to use separate flight progress strips for each approach, resulting in more writing space being available for instructions and, therefore, allowing for larger print.

A Boeing 747-400 (B747-400) was en route from Singapore to Sydney at flight level (FL) 330 on air route A585, and a B747 was tracking in the opposite direction at FL350 from Melbourne to Jakarta on air route G222. The routes A585 and G222 converged at SAPDA, a position approximately 720 NM north-west of Broome on the boundary of the Australian and Indonesian flight information regions (FIRs). Route G222 was north-east of route A585 and the prescribed lateral separation point between the routes was 272 NM south-east of SAPDA. The aircraft were under the control of the Kimberley sector controller of the Brisbane centre.

The B747-400 was estimating SAPDA at 1811 Universal Coordinated Time and the controller approved that crew at 1747 to climb to FL350 at SAPDA. The Kimberley controller was advised at 1806 the B747 was at METUM, a position 252 NM south-east of SAPDA, at 1805 maintaining FL350 and estimating SAPDA at 1836. At 1812, the Kimberley controller received a SAPDA position report from the B747-400 crew and shortly after recognised there was no separation being applied between the aircraft. At 1813, the Kimberley controller instructed the B747-400 crew to descend to FL330, however there was an infringement of separation standards. The B747-400 crew subsequently reported maintaining FL330 at 1819.

The estimated time of passing of the aircraft was 1824. The Kimberley controller needed to establish the vertical separation standard of 2,000 ft between the aircraft before 1809 to ensure separation.

The Kimberley sector was a procedural sector operated by a single controller. Communication between controllers and flight crews could be by either:
- very high frequency (VHF) radio,
- controller pilot data link communication (CPDLC), or
- high frequency (HF) radio through a third-party radio operator.

The controller was communicating with the B747-400 crew using CPDLC and with the B747 crew by third-party HF radio. There was no VHF radio coverage in the portion of the sector in which the aircraft were operating.

The controller commenced the shift at 1300 (2300 Eastern Standard Time) and worked till about 1530 when he had a break. He returned to the position at 1610 and remained there until the occurrence. Normally during the period from 1400 to 1800, the number of aircraft in the sector gradually increased and after that time, traffic numbers decreased. The controller reported that there had been a busy period that had finished just before the occurrence. That was the controller's first shift following a 2-day break.

When the B747-400 crew requested approval to climb to FL350 via CPDLC, the controller checked for conflicting aircraft on the air situation display. At that stage, the B747 on G222 was over Western Australia and the controller reported that he might have perceived that aircraft as part of a group of aircraft on a more northerly route. Consequently, he coordinated a change of level with the adjacent Indonesian sector controller, and then instructed the B747-400 crew to climb at SAPDA.

The air traffic system had a flight plan conflict probe (FPCP) function to assist in the early identification of conflicts outside radar coverage. However, the FPCP had not been included in the commissioning of the new air traffic control system during 1998-99 and was not active.

Darwin air traffic control was using runway 29 as the primary runway. Two BAe Hawk aircraft were approved to depart in the opposite direction from runway 11. A Cessna 310 (C310) had departed from runway 29 prior to the Hawks and was climbing through 3,500 ft while tracking on the 107 radial of the Darwin VOR navigation aid.

The planner controller had coordinated the clearance from runway 11 with the approach controller, who was performing both the departure and approach control functions. However, the flight strip indicated that the Hawks would depart from runway 29. As the aircraft became airborne the error was detected, but the pair were quickly closing on the C310. Traffic information was not passed to any of the crews and the aircraft came within 1.5NM of each before the Hawks had established themselves 1,000 ft above the altitude of the C310. There was an infringement of separation standards, which required a minimum of 3 NM horizontally or 1,000 ft vertically.

The investigation revealed that the planner controller had carried out the correct coordination but had incorrectly written runway 29 on the flight strip for the Hawks. When the aerodrome controller coordinated the correct information to the approach controller (using the phraseology "next runway one one"), the approach controller was not alerted by the words "one one". Consequently, the approach controller issued an instruction for the Hawks to depart on runway heading still believing them to be using runway 29.

When the pilots of the Hawks made radio contact with the approach controller after departure, the controller chose to not initiate evasive action as the rate of climb of the Hawks was far greater than that of the C310, and they had already reached the altitude of the C310. This decision was taken at a time when the horizontal separation had reduced to less than the 3 NM standard. The controller believed that the time taken to issue, and have the pilots respond to, a turn instruction would take longer to attain a horizontal standard than allowing the climb to continue until the vertical standard was achieved.