Loss of separation

The crew of an international Boeing 747 departing runway 16 had been cleared for a KEPPA 1 Standard Instrument Departure. Departure instructions had been issued by Departures North for the aircraft to initially maintain 3,000 ft due to overflying traffic. The B747 was rolling when the tower controller called Departures North to obtain departure instructions for a Boeing 767, that had been cleared for a DOSEL 6 Standard Instrument Departure from runway 27. The Departures North controller indicated he would call back shortly with departure instructions in respect of the B767.

The departures controller observed the B747 airborne and cleared the aircraft to continue its climb, having established that separation existed with the overflying traffic. At this stage the departures controller called the tower controller and issued departure instructions for the B767 to maintain 3,000 ft. This would allow the B767 to pass under the path of the departing B747.

The departures controller subsequently observed the B767 to be airborne, with the B747 still south-west of the aerodrome, in a left turn climbing slowly through 2,800 ft. As the controller considered the B747's rate of climb insufficient to preserve vertical separation between the two aircraft, the departures controller turned the B747 onto a westerly heading and passed traffic information to the crew in respect of the B767.

Horizontal separation between the two aircraft reduced below the 3 NM standard before vertical separation standards were established. The two aircraft were further processed without incident.

Report not released due to no IP comment from the Indonesian agency on this joint investigation report. ASOR hard copy held on occurrence file. Report not to be released without the approval of the Director or a DD Signed Alan L Stray Deputy Director Investigations 6 July 1998

A Cessna Conquest, VH-LBY was flying at FL240 from Plutonic to Perth via the Meekatharra and Mount Magnet radio navigation aids. A BAE 146, VH-NJZ was flying at FL250 from Perth to Paraburdoo via the Rusty reporting point and the Meekatharra aid. A second Conquest, VH-NFD was tracking to Mount Magnet, also via Rusty and the Meekatharra aid at FL250. At 0931, the pilot of LBY requested a climb to FL260 but the sector air traffic controller advised that the level was unavailable due to opposite direction traffic, (a second BAe 146, VH-NJN). VH-NJN was behind NJZ at the non-standard level of FL260. The Sector controller then cleared NJN to climb to the standard FL270. At 0935, the controller cleared LBY to climb to FL260 with a requirement for the aircraft to achieve the level by 0940. Approximately 1 minute later, the controller apparently recognised that a breakdown in separation may be possible between LBY and NJZ. He requested a level check from the pilot of LBY that indicated that the aircraft were vertically separated by 300 ft. He did not issue traffic information to either aircraft. Soon after, the pilot of NJZ requested the position of LBY and the aircraft were 17 NM apart and closing. The controller then instructed LBY to expedite the climb. Three seconds later, the pilot of LBY reported maintaining FL260.

The pilot of NJZ subsequently reported that he had calculated a time of passing with LBY of 0940 and when he heard LBY was cleared to climb, he began an immediate visual scan of the area ahead and turned the aircraft's landing lights on. The pilot of NJZ reported that approximately 1 minute after the pilot of LBY reported maintaining FL260, he sighted LBY. The pilot of LBY reported that approximately 2 minutes after establishing FL260, he sighted NJZ passing below. A breakdown of separation had occurred because the required 10 minutes separation, based on the time the aircraft expected to pass each other, had not been achieved. The aircraft had not been issued with the required traffic information when the controller became aware that the aircraft were in unsafe proximity to each other.

Whilst the controller had 3 sectors combined at the time of the incident, the workload was reported as being light to moderate and not considered excessive. Personnel were available to provide assistance if requested and evidence indicated that the controller was not under pressure at the time of the incident. The controller had not calculated a time of passing between LBY and NJZ because the aircraft were to be separated by 1,000 ft and were not in conflict. The controller's requirement for LBY to reach FL260 by 0940 was 10 minutes before the calculated time of LBY passing NFD, therefore, it is likely that the calculated separation requirement was based on LBY's passing of NFD, not NJZ.

The controller appears to have been preoccupied with ensuring separation between LBY and NFD. Therefore, he overlooked separation requirements with NJZ. This oversight subsequently led to a breakdown in separation between NJZ and LBY.

The aerodrome controller had cleared one aircraft for take-off. The intention was to have the following aircraft cleared for take-off when the first aircraft had gained adequate distance and was turning away from the initial departure track. The second aircraft was allowed to depart about one minute after the first. This was too soon as the controller was unable to maintain visual separation due to both aircraft being on the same track, away from the controller's view. There was also insufficient distance between the two aircraft to provide the required radar separation standard. Separation was positively established after the first aircraft turned to a new heading.

FACTUAL INFORMATION

A Dash 8 had departed Melbourne on a flight to Wynyard, Tasmania and was climbing to flight level (FL) 170 via Cowes, a reporting point and navigational aid south-east of Melbourne en-route to Wynyard. A SAAB 340 had departed Wynyard for Melbourne and was also tracking via Cowes, maintaining FL160. The crew had been instructed to descend to FL130 "when ready". The Melbourne Inner South controller had recognised that there was a potential conflict and coordinated, with the departures controller, for the Dash 8 to be radar vectored to the left of track. This action would provide radar separation between the aircraft to allow both crews to change level without infringing the separation standards.

The crew of the Dash 8 requested, and were given, permission to climb to and maintain FL150. When the controller issued the left turn instruction, the crew of the Dash 8 asked if a right turn was available. The controller checked the route on a longer range radar scale and decided that the direct track to Wynyard would be adequate for radar separation purposes and would be a shorter track for the aircraft. A new instruction was then issued for the crew to track from their present position direct to Wynyard. The controller had, in fact, looked at the wrong track and destination when changing radar range scales and had mistaken a geographical point on the Tasmanian coast as Wynyard, when it was actually Smithton.

Wynyard was not displayed on the map. When the Dash 8 commenced the turn, the controller realised the mistake but thought that there would be sufficient distance between the crossing flight paths to provide the required radar separation. Meanwhile the crew of the SAAB 340 had commenced descent and, as vertical separation was reducing, the controller realised that the aircraft would pass with less than the required standard. The crew of the SAAB 340 were instructed to turn right 20 degrees in an attempt to maintain the standard but the turn proved to be insufficient and a breakdown of separation occurred. Radar analysis indicated that the aircraft passed with 4.7 NM horizontal separation and a vertical separation of 900 ft.

The required standard was either 5 NM horizontally or 1,000 ft vertically. A second SAAB 340 had departed Devonport, Tas. on a flight to Melbourne and was on a similar track to the first SAAB 340. This SAAB was approximately 15NM behind the leading SAAB and the crew had just commenced descent from FL160 in accordance with the controllers instructions. Approximately 80 seconds after the initial incident, the radar separation between the Dash 8 and this second SAAB 340 reduced to 3.5NM with the radar indicating that the aircraft had passed each other with 900 ft vertical difference.

The controller had issued descent instructions to the crew of the SAAB 340 which negated the existing vertical separation standard of 1,000 ft. However, the crews had reported sighting and passing each other's aircraft. The controller had established this visual contact by obtaining a report from the crew of the Dash 8 that "we have the SAAB passing on our left hand side".

ANALYSIS

The radar map showed a track and destination for Smithton but no information at all for Wynyard. Whereas Wynyard was a regular destination, traffic to Smithton was infrequent in controlled airspace. Although the controller promptly detected the initial error, track shortening was allowed to proceed. Although actively monitoring the developing situation, the decision to allow unrestricted climb and descent did not provide separation assurance. When the track of the Dash 8 also conflicted with the second SAAB 340, there was a vertical separation standard in existence.

However, the controller authorised descent for the SAAB 340 with only minimal radar separation. The controller had based separation on pilot reports of mutual sighting. Whereas the SAAB crew could have been safely expected not to miss-identify the Dash 8 aircraft, the Dash 8 crew may have been referring to the first of the two SAAB aircraft.

SIGNIFICANT FACTORS

1. The Inner South controller mistakenly identified the location of Smithton on the radar display map as that of Wynyard.

2. The Inner South controller did not use adequate separation assurance techniques.

3. The Inner South controller misjudged the radar separation between the Dash 8 and SAAB 340 aircraft.

SAFETY ACTION

As a result of the investigation, Airservices Australia Southern District management put in place amendments to the radar display map that indicate the location of Wynyard for controller reference.

FACTUAL INFORMATION

A Beech 1900 (B1900), callsign SMH, had departed Sydney on a flight to Port Macquarie, NSW. The crew were maintaining the aircraft at flight level (FL) 210 in controlled airspace. The airspace was controlled by Brisbane Sector 15C.

A Swearingen SW4 (Metro) had departed Port Macquarie on a flight to Williamtown, NSW. The crew were maintaining the aircraft at FL140. The base of controlled airspace was 12,500 ft.

At 1433 EST, the crew of the B1900 commenced descent into Port Macquarie but were restricted to FL150 by sector control due to the opposite direction conflict with the Metro.

A second B1900, callsign IMH, was also on the control frequency and had commenced descent into Williamtown. This aircraft was well south of the conflicting aircraft and there were no restrictions for its descent.

At 1436, the sector controller instructed the crew of IMH to contact Sydney flight service (FIS5) on frequency 121.6 Mhz. The reply was broken but included the words "? Mike Hotel". Almost immediately the crew of IMH asked the controller to confirm that the previous transmission was for them. This was confirmed by the controller and the crew transferred to FIS5 as instructed.

The controller then noticed on radar that SMH had descended below its assigned level and attempted to contact the crew to instruct them to return their aircraft to FL150 for separation with the Metro. There was no answer.

The crew of SMH had acknowledged the frequency transfer instruction issued to IMH and had contacted FIS5. The flight service officer had received the correct coordination on SMH and had informed the crew that there was no instrument flight rules traffic conflicting with their intended flight path in his area of responsibility. FIS 5 provided a traffic information service for aircraft travelling outside controlled airspace; that is, at and below 12,500 ft.

The crew of SMH decided that, as control had transferred them to FIS5, the controller was approving their descent from controlled airspace. Consequently, they elected to continue descent from FL150. However, they had not received a specific clearance to descend below FL150.

As soon as the controller realised that the crew of SMH was not on the control frequency, he coordinated with the flight service officer to broadcast on 121.6 Mhz an instruction to the crew to maintain FL150. On receiving this instruction, the crew maintained FL140 because that was the level they were passing at the time. They reported to FIS5 that they were maintaining FL140 and this information reached the controller at approximately 1438, by which time the two aircraft had passed each other. The controller then issued a clearance for the crew of SMH to leave controlled airspace on descent.

Neither crew saw the other aircraft and radar analysis indicates that they passed at 1437 with approximately 2.6 NM horizontal separation and 200 ft vertical separation. The appropriate standard was 5 NM or 1,000 ft.

There was a breakdown of separation standards.

ANALYSIS

The crew of SMH were expecting further descent as they approached FL150 and, hearing such an instruction issued to IMH, misheard the callsign and acted as if the instruction was issued to them. The controller only heard the last two letters of the response and, therefore, did not instantly recognise that the wrong crew had responded. As soon as the crew of IMH questioned the callsign for whom the instruction was intended, the controller commenced action to contact the crew of SMH to re-introduce positive separation that he had observed to have been contravened.

Because they believed that they had been transferred from control frequency, the crew of SMH incorrectly assumed that sector control had no further restrictions for them and descended. They did not receive, nor did they seek, a clearance to descend from FL150.

SIGNIFICANT FACTORS

1. The crew of SMH misheard the callsign "IMH" when an instruction for descent was issued to the crew of IMH.

2. The crew of SMH descended below their assigned level.

FACTUAL INFORMATION

The B737 had departed from Melbourne on a flight to Cairns, Qld, two minutes ahead of the B747 on the same route. Both aircraft were climbing to an initial altitude of flight level (FL) 200 and the crews had been approved to vary their climbing speed. The crew of the B737 increased their speed to 300 knots and the crew of the B747 were approved to conduct a "high speed climb". Due to an active restricted area, both aircraft were given a radar vector to take them off track and jurisdiction for the flights was transferred to the Inner North controller. Both crews contacted the Inner North controller whilst on a heading of 010 degrees, with the B747 paralleling the track of the B737 to the right with approximately 8 - 10 NM separation.

The closing speed at the time of the transfer was approximately 50 knots. The B737 was then cleared to climb to flight level (FL) 310 and the B747 to FL390. When the aircraft were clear of the restricted area, the controller issued instructions for both crews to track from their respective positions direct to the reporting point TOBOB, at which they were to intercept their planned route. The planned track to TOBOB was approximately 355 degrees and the instruction would result in a turn of approximately 20 - 25 degrees to the left for both aircraft. The controller did not realise that the second aircraft was a B747 and thought it was an aircraft of a lower performance. He was not familiar with this type of aircraft on this route and, although he knew that the registration belonged to a B747, did not reconcile that particular type with the traffic situation.

The wind was from the north-west, with forecast speeds of 70 knots at flight level (FL) 185. The crew of the B737 were the first to turn their aircraft in accordance with the controllers instructions and chose an initial heading that was to the north-west and directly into wind. This turn had the effect of reducing the ground speed by almost 60 knots. The turn was approximately 55 degrees, more than that required to track direct to TOBOB, and an adjustment was made about 1 minute later. Because of the position of the B747 relative to the B737, its turn of approximately 20 degrees resulted in a more northerly heading and, as such, the reduction in its ground speed was less than 10 knots.

These changes resulted in a significant increase in closing speed at a time when the Inner North controller became occupied with other work-related tasks. Consequently, he did not initially notice the reduction in separation. Shortly after, as he conducted a scan of his area of responsibility, he saw that the B747 was closing rapidly on the B737 and that there was less than the required vertical separation with the B747 below. He immediately attempted to achieve vertical separation by instructing the crew of the B737 to maintain FL210. The intent was to allow the B747, which had a greater rate of climb, to achieve the necessary vertical separation before the horizontal separation standard was infringed.

However, it became obvious that this plan would not achieve the objective and he issued a radar vector to the crew of the B747 in order to maintain radar separation. Before the distance between the aircraft started to increase, the horizontal separation had reduced to 4.2 NM while vertical separation was 200 ft. The required standard was either 5 NM horizontally or 1,000 ft vertically. A breakdown of separation had occurred. Examination of the radar data indicated that the closing speed had increased to 160 knots for a short period prior to the radar vector taking affect.

SIGNIFICANT FACTORS

1. The Inner North controller assumed that separation would be achieved through differences in aircraft performance without employing separation assurance techniques.

2. The crew of the B737 executed a turn which was greater than that expected by the controller.

3. The Inner North controller did not immediately recognise the combination of events that led to a significant increase in closing speed.

VH-WNZ was conducting a training flight, and the pilot had advised the Tower of his intention to conduct a simulated engine failure after take-off. The aircraft departed on an initial assigned radar heading of 060 degrees at 1710.40 EST.

VH-OGK was assigned an IBUNA 1 Standard Instrument Departure (SID), departed at 1712.30 EST, and had been instructed to remain on Tower frequency. While turning right as per the SID to heading 180 M, OGK passed behind WNZ. Separation between the aircraft reduced to 2.5 NM horizontally and less than 500 ft vertically. The required separation was 3 NM and 1,000 ft.

Investigation revealed that the aerodrome controller (ADC) did not adequately plan the departure of three aircraft, including WNZ and OGK. An arriving aircraft led to his decision to expedite the departure of OGK, thus compromising the separation between these aircraft. The ADC's decision to retain OGK on Tower frequency until instructed to transfer delayed the opportunity for the departures controller to issue instructions to OGK to ensure separation was maintained.

The controller involved was undergoing a check. MAS135 and VH-HYJ were third and fourth in a six aircraft sequence. Both aircraft were inbound to Brisbane from the north-west with VH-HYJ behind MAS135 and operating at a higher speed. As the sequence progressed, the controller was late in detecting the reducing separation between the two aircraft. VH-HYJ was turned away from MAS135 but not sufficiently early to prevent lateral separation reducing to 4NM. At this time, vertical separation was 500 feet.

FACTUAL INFORMATION

The crew of an instrument flight rules (IFR) Boeing 737 (B737) had been cleared to descend to flight level (FL) 120 while tracking inbound to Darwin. The aircraft was tracking via the 163 radial of the very high frequency omni-directional radio range (VOR) navigation aid. The crew had transferred from Brisbane En Route Control to Darwin Approach Radar Control at approximately 60 NM from Darwin. An IFR Airbus Industrie A320 had departed Darwin for Alice Springs and was tracking via the 163 VOR radial on climb to FL370. The approach radar controller was busy with aircraft operating predominantly within 20 NM of Darwin aerodrome and did not recognise the potential for conflict between the aircraft.

When the B737 was approximately 45 NM from Darwin, the approach radar controller was alerted to the conflict with the A320 by another controller. The approach radar controller instructed the crews of both aircraft, in separate transmissions, to turn right 30 degrees. In the first transmission, the B737 crew did not understand the instruction and the approach radar controller had to repeat the instruction. Subsequently, the aircraft passed with less than 1,000 ft vertical separation and 2.4 NM lateral separation. The required minimum separation standard was either 3 NM laterally or 1,000 ft vertically. There was a breakdown of separation. B737 crew The B737 was fitted with a traffic alert and collision avoidance system (TCAS) and consequently, the crew was aware of the approaching A320. They were prepared to comply with any avoiding instructions expected from the approach radar controller. However, the initial instruction was given very quickly and was difficult to understand, and the crew had to ask the controller to repeat the transmission. Radar display The approach radar controller was responsible for managing the airspace encompassed by a circle with a radius of 50 NM, centred on Darwin. The approach controller used a main radar display with the range set to 50 NM and a smaller secondary display window with the range set to 100 NM.

The main display was primarily used to separate and manage aircraft close to Darwin while the smaller window was used for the management of aircraft that were further than 50 NM from Darwin. To avoid clutter on the display, the radar display labels on the secondary window were suppressed for all aircraft within 30 NM of Darwin. Just prior to the occurrence, the A320 would have been displayed on the main display with a full label and on the secondary window without a label. The B737 would have been displayed on the secondary window with a full label. Approach radar controller The approach controller was busy with aircraft conducting practice instrument approaches at Darwin and other arriving and departing aircraft. The controller was using flight progress strips and radar to manage aircraft in the area of responsibility. The Brisbane sector 11 controller advised the approach radar controller of the radar identification of the B737 and the assigned level of FL120 when the aircraft was 68 NM from Darwin. The approach controller reported the departure of the A320 to the sector 11 controller during the same coordination exchange. Neither controller remarked on a need to coordinate a separation procedure for the aircraft. Coordination The approach coordinator advised the sector 11 controller when the A320 taxied for departure.

The approach coordination controller and the sector 11 controller did not recognise the potential for conflict between the B737 and the A320. There were no specific procedures to ensure that Brisbane and Darwin controllers coordinated separation procedures for aircraft that were likely to pass near the boundary of the respective areas of responsibility. Generally, the approach coordinator would have alerted the approach radar controller to the potential for conflict before a taxiing aircraft departed. The investigation did not ascertain why the approach coordination controller did not advise the approach radar controller of the situation.

ANALYSIS

It is probable that due to the busy nature of the traffic environment at the time, both the approach coordinator controller and the approach radar controller became so fixated on the coordination and separation of aircraft close to Darwin that they were unable to comprehend the developing conflict. This aspect is further supported by the fact that the approach radar controller did not appreciate the potential conflict when the B737 was handed off from Brisbane and the departure time for the A320 was advised to Brisbane, in the same coordination exchange. The display of the radar tracks and labels for the aircraft on separate windows would have constrained the approach radar controller's ability to develop an appreciation of the potential conflict. However, flight progress strips were available and if they were scanned regularly by the approach radar controller, they should have provided a prompt for that controller. The aircraft passed at a position approximately 35 NM south-south-east of Darwin, which was well within Darwin's area of responsibility. Consequently, while the coordination of a procedure for the separation of the aircraft between Brisbane and Darwin Air Traffic Control would have assisted in the occurrence, it was not considered to be a significant factor.

SIGNIFICANT FACTORS

1. The approach coordination controller did not warn the approach radar controller of the potential conflict between the aircraft.

2. The approach radar controller's attention was diverted from scanning the flight progress strips for potential conflicts.

SAFETY ACTION

Local action

Darwin and Brisbane Air Traffic Control have developed a Manual of Air Traffic Services Supplementary Procedure which requires controllers to coordinate a separation procedure for aircraft that may conflict in the area between 40 NM and 80 NM from Darwin.

Additionally, Darwin Air Traffic Control has established an arrival position to reduce approach radar controller workload.