Loss of separation

Both aircraft were inbound to Sydney on different but converging routes, cruising at the same level. The Sydney Flow Controller requested that VH-TJL be positioned 15NM behind VH-TJD to facilitate their arrival. The controller responsible for the aircraft instructed VH-TJD to turn right (to place it on the same track as VH-TJL) and to descend. As VH-TJD converged on the track flown by VH-TJL the required lateral separation of 5NM was lost before the required vertical separation was achieved.

VH-FCU was enroute Gladstone-Brisbane and was being radar vectored for a left base for runway 01. VH-CZM had departed Brisbane on a standard instrument departure off runway 01. This involves the aircraft intercepting and tracking the 360 degree radial from Brisbane and, at 10 DME Brisbane, turning left and tracking for Kilcoy. Both aircraft were under the control of Brisbane Approach. At the time of the occurrence, the Approach position was occupied by a trainee controller under the supervision of a training officer.

The training officer alerted the trainee to the potential confliction between the two aircraft as VH-CZM turned left for Kilcoy. However, as the trainee was about to instruct VH-FCU to turn, a number of radio transmissions prevented him transmitting the instruction until a loss of separation was unavoidable. The aircraft came within four miles of one another while vertical separation did not exist. The trainee was an experienced approach controller from another centre and had only recently begun training in Brisbane. The training officer was aware of the trainee's experience level and delayed his intervention on this basis.

The following factors were considered relevant to this occurrence:

1. The Approach position was occupied by a trainee controller under the supervision of a training officer.

2. The training officer delayed his intervention in the process because of the trainee's previous experience.

3. Other radio traffic delayed the trainee issuing instructions to the aircraft which would have ensured separation.

9V-STT was en-route from Perth to Singapore and tracking via air routes B469 then R586 after Carnarvon, cruising at flight level (FL) 350. VH-EAL was also Perth for Singapore but tracking via air route G337 at FL350. The routes flown by the two aircraft are initially laterally separated but, because the cross-track tolerance changed as the aircraft proceeded north, it was necessary to apply separation to ensure that the standards were not infringed. This action was not completed by the air traffic controller responsible for the aircraft by the required point along the routes, and the standards were infringed.

About 30 minutes prior to the incident there was a change of controller on the sector responsible for the aircraft. The controller commencing duty thought that he had confirmed that correct separation existed. The handing over controller believed the other controller had referred to two different aircraft when they discussed the adequacy of the separation during the handover. The controller assuming responsibility for the aircraft had recently returned from a period in the Perth Control Tower and may have forgotten, because of a lack of recent experience, that the cross-track tolerances of the two routes vary.

The handing over controller was recently rated in the position and assumed that the more experienced controller was aware of the lack of separation between the routes. Safety Action: The Civil Aviation Authority is redesigning the route structure, and this should remove the potential for separation reductions in this area of airspace.

When VH-DLF was descending through 7,500 feet, 65 km from Adelaide, Adelaide Departures advised the pilot of an unidentified aircraft converging from his right. Departures continued to update the pilot of VH-DLF on the position of the other aircraft (which he was unable to sight, probably due to haze and the position of the setting sun) but were unable to advise the altitude of the other aircraft. A short time later the pilot of VH-DLF observed a Piper Pawnee aircraft, with a glider in tow, pass about 200 ft overhead.

At the time VH-DLF was descending through 6,300 ft. The pilot advised Departures of the near collision, and Departures confirmed that his aircraft was in controlled airspace. Attempts to contact the tug and glider by radio were unsuccessful. Subsequent investigation revealed that VH-IGR, a Piper Pawnee, had towed a glider (VH-IKO), cruising at about 6,500 ft, from Port Pirie to Murray Bridge at about that time. The lower limit of controlled airspace on their track was 4,000 feet and an airways clearance had not been requested by the tug/glider combination. Both pilots of the tug/glider combination advised that they had not seen VH-DLF and were unaware that they had violated controlled airspace.

Two B737 aircraft were tracking south on air route A464 at FL330, initially having a 10 minute separation standard, which reduced to 8 minutes before an RNAV distance standard could be achieved. Both aircraft then required diversion left of track to avoid weather, and as an RNAV distance separation standard could no longer be applied, a minimum separation of 10 minutes, or a change of cruising level for one aircraft was now required. Descent to Flight Level (FL)310 was not available due to crossing traffic, and neither aircraft could accept FL350.

A decision was made to descend the second aircraft to FL290, requiring the traffic on the crossing route to descend to FL280. Co-ordination of all possible separation methods with Brisbane Control required some time, especially as they were experiencing high workloads. The actual separation of the aircraft was continually monitored, and never less than 105km. As the lead aircraft had only been 8 minutes ahead of the other, a breakdown of separation occurred until vertical separation could be achieved.

VH-NJC, was en route from Cairns to Ayers Rock cruising at flight level (FL)280. VH-TAJ was en route Darwin to Adelaide, cruising at FL330. The crossing point of the two tracks was approximately 100 nautical miles east of Alice Springs. Near the intersection of the two tracks, the pilot of VH-TAJ requested a clearance from air traffic control to descend to FL270. This was approved.

Subsequently the air traffic controller realised VH-TAJ did not have the required lateral separation from VH-NJC. He checked the level of VH-TAJ, but the aircraft had left FL285 on descent. The aircraft was allowed to continue the descent as both aircraft were well clear of each other, but within the area of lateral confliction.

VH-EXF which had been operating on a low-level training exercise was cleared by Darwin Approach for a visual approach and instructed to track to a left base for runway 36, and to contact the tower. The aircraft joined left base at about 800 feet AMSL and contacted the tower as instructed.

After completing prelanding checks the pilot became aware of another aircraft on a converging track at a similar attitude. The separation between the aircraft was about 200 metres horizontally and less than 100 feet vertically. The other aircraft, VH-AHX, had just become airborne from runway 18 and was making a right turn, as instructed, onto a heading 240 degrees and climbing to 1000 feet.

Aircraft under the control of Brisbane Approach (North) in the period leading up to the occurrence included:

- VH-TNG which was on left downwind at 3000 feet for a runway 01 ILS approach.

- VH-CZL which was about 20 nm final for runway 01 ILS approach.

- VH-JEX, a VFR aircraft out of Brisbane, heading north-west on climb to 4500 feet.

- An Army helicopter transiting Approach (North) airspace south of the aerodrome.

- A number of aircraft inbound from the north-west.

As VH-JEX climbed, it experienced problems maintaining visual flight and required a number of clearance amendments with respect to both heading and altitude. During this process, the Approach (North) controller became aware of another aircraft, VH-CFQ orbiting Archerfield at 1000 feet awaiting clearance into Approach (North) airspace. The controller involved said that he was not sure how long VH-CFQ had been airborne at Archerfield but was aware of feeling under some pressure to issue a clearance.

He cleared VH-CFQ to enter controlled airspace on climb to 3000 feet west of the Mt Coot-tha TV towers and returned his attention to VH-JEX and the inbound traffic from the north. A short time later, he looked to check the progress of VH-CFQ and saw a conflict with VH-TNG developing. He instructed both aircraft to turn left. However, this resulted in VH-TNG conflicting with VH-CZL. The controller then told VH-TNG to turn right for separation with VH-CZL. The 5 nm separation standard was infringed in both instances, the radar recording indicating the minimum distance to have been 3.5 nm each time.

SIGIFICANT FACTORS

1. Controller workload was significantly increased by the clearance amendments necessary for VH-JEX.

2. The controller felt under pressure to clear VH-CFQ from Archerfield into controlled airspace.

3. The clearance given to VH-JEX was inappropriate and led to the losses of separation.

Both aircraft were operating scheduled regular public transport flights on crossing air routes within controlled airspace under the jurisdiction of Sydney control sector 5 (SEC 5). 5W-ILF was maintaining FL330. Although SEC 5 was providing control and separation services, Sydney Flight Information Service, International (FS INT) was responsible for providing communications on high frequency (HF) aeromobile facilities. Subsequently 5W-ILF requested FL370 via FS INT. This request was co-ordinated with, and approved by, SEC 5 for a descent to FL270. 5W-ILF was then instructed to descend to FL270 and report when maintaining.

At approximately 1627 EST, 5W-ILF acknowledged the level change and read back maintain FL370. A short time later, a level check of 5W-ILF was initiated by SEC 5 and the pilot of 5W-ILF reported maintaining FL370. It was then realised that 5W-ILF had climbed to FL370 instead of descending to the amended level of FL270 as cleared. SEC 5 calculated that 5W-ILF may have entered the lateral conflict area of crossing traffic, which was VH-EBR at FL350, at 1631. It has been established that 5W-ILF would have taken approximately four minutes or less to climb from FL330 to FL370 and reached FL370 at approximately 1631.

It was not possible to determine if the prescribed separation standards of at least 2,000 feet in altitude or 10 minutes prior to crossing were actually infringed. The pilot-in-command was performing all communications duties and believed that the clearance instruction, which was mutually confirmed by the co-pilot was 5W-ILF, recleared FL370. However, neither pilot could recall hearing the phraseology to descend used in the clearance provided. The aircrew had an expectation for a control instruction to climb.

This aircrew expectation was further compounded by the absence of any challenge to indicate that an incorrect level had been readback to FIS INT, who, expecting to hear FL270, misheard FL 370. The pilot-in-command of 5W-ILF had also omitted to report vacating FL330 and was not queried by FIS INT. It is probable that aircrew operating through differing Authorities may have become conditioned to expect variations and incorrect applications of standard operating procedures and phraseology used by air traffic service officers.

For example, anecdotal evidence suggests that some Authorities are using phraseology such as recleared for level change instructions in lieu of the correct phraseology for climb and descent manoeuvres. The importance of using correct phraseology which clarifies change of level instructions such as climb, descend (for vertical manoeuvres) compared to change of route instructions which contain the phraseology recleared (for horizontal manoeuvres) is evidenced in this occurrence. Analysis The air traffic system safety defences had been progressively breached.

The first system failure was when the FIS INT operator misheard and co-ordinated an incorrect change of level request. The next system failure was the expectation for the requested level (FL370) and readback of that level by the aircrew. The incorrect readback expectation of FIS INT was the final system failure. The departure from published standard operating procedures and phraseology combined with the absence of a standard phraseology which indicates the vertical manoeuvre being executed renders the system fail unsafe. This is particularly relevant when vacating previously maintained flight levels and exposes the air traffic system to the potential for incorrect level assignment expectations by both aircrew and air traffic service officers.

Significant Factors

1. The FIS INT misheard and co-ordinated an incorrect change of level request.

2. There was an expectation for the change of level request to climb to FL370 by the aircrew to be approved.

3. FIS INT did not detect the incorrect level readback by the aircrew nor the absence of advice that the assigned cruising level had been vacated.

4. There is no standard phraseology specified to verify the vertical manoeuvre associated with a change of level.

SAFETY ACTION 

In relation to this and two other occurrences (BO9102639 near Mount Isa and BO9301823 north of Brisbane), the published procedures and phraseologies did not prevent poor listening technique, by both ATS officers and pilots, leading to a breakdown in separation. In this occurrence a flight service officer and two pilots all misheard the flight level given by the other.

Incorrect phraseologies were used, and no indication of climb or descent was included by either party. This resulted in a safety net being absent and the aircraft climbing when ATC expected it to descend.

In occurrence 9102639, four controllers all either misheard the co-ordinated flight level or did not recognise that an amended level had been given. In this case two training officers did not notice the error.

In occurrence 9301823, the trainee inadvertently gave descent instructions to an aircraft without realising that he had given such descent, even though the pilot correctly read back the assigned level. The training officer did not notice the error.

Simultaneous runway operations were in progress at Sydney when VH-MJH was cleared to land on runway 34, with a requirement to hold short of runway 07. After the aircraft had landed the aerodrome controller (ADC) instructed the pilot to take the first available taxiway to the right. This was taxiway 'Hotel', located about 238m from the runway 07 simops lights. VH-RMN, a Boeing 727 which was lined up on runway 07 was then cleared for take-off. VH-MJH was then observed to have passed the taxiway and was told to hold short of runway 07. The aircraft continued rolling and when the ADC became uncertain that the aircraft was going to stop before runway 07, he instructed the aircraft to stop.

The aircraft did not stop, and the ADC then cancelled the take-off clearance for VH-RMN which was now rolling for take-off. Another aircraft was sent around from a 2-mile final approach for runway 07. VH-MJH came to a stop mid-way between the stop lights (holding point) and the edge of runway 07. After VH-RMN had stopped VH-MJH was cleared to cross runway 07 and vacated runway 34 on taxiway Bravo 6. The first officer was the pilot flying VH-MJH until after the aircraft had touched down. The captain then took the controls during the landing roll, and the first officer turned his attention to the after landing checks. The captain stated that he had not heard the transmissions to stop.

It was not until he noticed that the first officer was transmitting that he realised that he was not monitoring the tower frequency. After he had received a landing clearance, he had intended to deselect the company frequency and had inadvertently deselected the tower frequency. The first officer had subsequently acknowledged the transmissions from the tower and was not aware that the captain was not monitoring the tower transmissions. The first officer realised that the captain was not following the towers instructions when the aircraft passed the taxiway and he instructed the captain to stop using both verbal and visual signals. By this time the aircraft was nearing the runway intersection.

The captain misidentified the position of taxiway Hotel due to the position of a large mound of earth which had previously been located south of runway 07, but which was now located north of runway 07. This gave him a false impression as to the location of the taxiway and was not aware that he was approaching the runway until after it was too late to stop before the holding point lights.

Significant factors

1. The captain inadvertently deselected the tower frequency just prior to taking control from the first officer.

2. After handing control of the aircraft to the captain, the first officer diverted his attention to the after landing checks.

3. There was a breakdown in crew co-ordination procedures.