Loss of separation

Two Cessna 150s, VH-MHD and VH-KMC, were conducting visual flights from Canberra at the same time. 

VH-MHD was commencing circuits on runway 30 and VH-KMC was departing on a city joy flight from runway 35.  At 1533 (all times are EST) VH-MHD was cleared for take-off from runway 30 with a requirement to maintain runway heading.  At 1535.20 VH-KMC was cleared for take-off from runway 35 and cleared for a left turn after take-off.

At 0535.55 VH-MHD was instructed to make a right circuit, early turn onto crosswind and downwind, traffic to sight is a Cessna heading for the racecourse, airborne off 35 over the old farmhouse.  At 0536.30 VH-MHD was instructed to continue the right turn to the tower and asked do you have that traffic now, it’ll be coming up in your 10 o’clock position. The pilot of VH-MHD responded uh MHD looking, do you want us to turn right now.  VH-MHD was then instructed continue the right turn to track to the tower, that will put you well inside the traffic

At 0536.55 VH-MHD was again asked if the traffic was sighted.  The pilot replied MHD negative.  VH-MHD was then instructed to......tighten the turn thanks, it’s in your 12 o’clock position half a mile continue the right turn now thanks straight to the tower.  At 0536 VH-MHD was advised ... you are clear of that traffic now continue downwind........

Meanwhile, after becoming airborne from runway 35, the pilot of VH-KMC contacted Canberra Departures Control at 1536.42 and advised .......KMC in a left turn on climb to 4000 left 2500.  VH-KMC had not been given any traffic advice by Canberra Tower on VH-MHD and there had been no coordination with Canberra Departures Control to pass any traffic advice.

At 1537.08 the pilot of VH-KMC advised departures we seem to have an aeroplane in the circuit area still heading towards us, do you have anything on it.  Departures advised VH-KMC of traffic in his 12 o’clock position and to maintain separation with that traffic.  VH-KMC then advised he’s just passed over me at about a 100ft.

When the aerodrome controller instructed VH-MHD to make a right circuit off runway 30 this put the aircraft in the same quadrant of airspace as VH-KMC making a left turn off runway 35.  The controller thought that instructing VH-MHD to tighten the right turn would resolve any potential conflict.  However, VH-KMC turned left earlier than expected whereas VH-MHD turned right later than expected.  The controller continued to give traffic to VH-MHD and instructed the pilot to track towards the tower but at no time passed traffic to VH-KMC nor instructed VH-KMC to remain on tower frequency.  The pilot of VH-MHD never sighted VH-KMC.

The traffic information passed to VH-MHD may not have been sufficient for the pilot to sight VH-KMC as it pre-supposed that the pilot was familiar with local landmarks and no relative height was passed. Additionally, the controller did not think it was significant that the pilot of VH-MHD may have had difficulty seeing VH-KMC in that he was flying a high wing aircraft, in a right turn, with the traffic approaching from his four to five o’clock position.

When it became apparent that VH-MHD could not sight VH-KMC and was not tracking towards the tower, the controller failed to issue a positive control instruction to VH-MHD, such as turn right immediately, to ensure that separation was maintained between the two aircraft.

An examination of the recorded radar data for Canberra terminal area indicated no apparent traffic reason why VH-MHD was instructed to make a right circuit.  The controller advised that he thought there was inbound traffic which may have conflicted if VH-MHD flew a left circuit.  At the time of the occurrence the aerodrome controller was under a moderate workload.  There were two aircraft operating in the circuit, VH-KMC departing on a scenic flight and a BA146 on final for runway 35.

Significant Factors

The following factors were considered relevant to the development of the incident:

1. The aerodrome controller placed two VFR aircraft in the same small quadrant of airspace and on different radiofrequencies, when an alternative was available.
2. The aerodrome controller did not pass mutual, timely and effective traffic information.
3. The aerodrome controller did not take positive steps to ensure separation.
4. The pilot of VH-MHD was slow in complying with an instruction to make a right turn.
5. The pilot of VH-KMC made an earlier than expected left turn.

VH-EBX was tracking eastbound on air route Y44, and VH-HYG was tracking westbound on air route Q34B, both aircraft were at FL 370. These air routes diverge from Adelaide towards the west.

At approximately 75 km west of Adelaide, when the two aircraft were within 22 km of each other, with a track angle difference of 20 degrees, which was increasing as they came closer, VH-EBX experienced a traffic alert and collision avoidance system (TCAS) traffic advisory (TA). This was followed 20 seconds later by a resolution advisory (RA).

The pilot of VH-EBX responded immediately to the RA, initiating a climb to FL 380 followed by a descent back to FL 370. Neither aircraft changed heading.

The aircraft passed with a separation of 8.5 km horizontal and 900 ft vertical.

The air traffic controller had been monitoring the situation for some time and was confident that the aircraft would pass with at least the minimum required horizontal separation standard of 9 km.

The recommended vertical deviation following an RA should be no more than 700 ft. The pilot of VH-EBX climbed the aircraft through 1,000 ft before returning to the assigned level.

That aircraft was Hudson 540, a type which is normally slower than a SAAB. Both aircraft were in visual conditions above broken cloud when the loss of separation occurred. Hudson 540 later advised experiencing a 290-knot groundspeed from Deniliquin to Albury. A check of recorded radar data and automatic voice recordings showed that Hudson 540 was actually only 12 miles from Albury when the pilot reported at 15 DME. The recorded radar data confirmed a cruise groundspeed for Hudson 540 of 290 knots from 70 miles west of Albury to commencement of descent at 30 miles west. From that point groundspeed decreased and was last observed to be 255 knots at loss of radar contact 12 miles west of Albury.

Radar contact with VH-SBA was lost when the aircraft was 23 miles south of Albury. At that point, its groundspeed was 250 knots. For comparison purposes, at that time Hudson 540 was 20 miles west of Albury with a groundspeed of 260 knots. This information was from a Melbourne radar tape. Albury tower is not radar equipped.

With VH-SBA inbound on the 207 radial and Hudson inbound on the 240 radial the aircraft were laterally separated until eight DME. It was the controller's intention to apply a specific longitudinal separation standard after the first aircraft passed the eight-mile lateral separation point. That standard allows for five miles longitudinal separation between two arriving aircraft provided that:

.  Angular difference between tracks is less than 45 degrees

.  No closing indicated airspeed

.  Leading aircraft within 15 miles of the aerodrome

.  Aircraft are assigned levels which are vertically separated.

Based on the respective DME reports of 15 DME and 11 DME for Hudson 540 and VH-SBA, the controller assessed that five miles longitudinal separation would exist when VH-SBA got to eight miles (loss of lateral separation point). This assessment was based on the assumption that the SAAB would be faster than the HS 748 and that the HS 748 would fly at least two extra track miles manoeuvring to track direct to the VOR.

Although the controller cleared Hudson 540 to divert right of track but no further south than the 240 radial, he did not actually check what radial the aircraft was tracking to the VOR. The aircraft could have tracked in on the 260 radial (only a minor diversion right of track) and commenced the VOR/DME approach without making a sector entry. If the HS 748 did track in on the 260 radial, then the two tracks would have been separated by more than 45 degrees which would mean the controller was applying an inappropriate separation standard. In addition, he did not check if there was a closing indicated airspeed. It would be reasonable to expect the SAAB, on a DME approach, to be slowing down, whereas the HS 748 could well be expected to maintain airspeed until over the VOR outbound because there is plenty of time to slow down on the outbound leg.

Factors

The following factors were considered relevant to the development of the incident:

1. The controller did not take all necessary steps to ensure he had full situational awareness in that he did not establish on what radial the HS 748 would track to the VOR nor the extent to which the aircraft would manoeuvre to the right of track.
2. The controller did not establish that all the conditions existed that were applicable to the specific separation standard he was using.
3. The HS 748 reported at 15 DME when it was actually only 12 miles from the aerodrome. (Albury DME is located on the aerodrome)

The pilot of VH-LDK was conducting a private business flight under instrument flight rules.  Enroute, he encountered more cloud than forecast.  The forecast freezing level was 6,000 ft.  The aircraft was not equipped for flight in icing conditions.

According to the pilot, the forecast cloud tops were 7000 ft.  He initially climbed to 8,000 ft but, to remain clear of cloud before reaching Wee Jasper, he climbed firstly to 10,000 ft and then to 11,500 ft.  Rather than continue climbing to 12,000 ft (without oxygen) to avoid cloud, he elected to divert from Popla to Wagga and descend to the lowest safe altitude of 4,500 ft. During the descent, the airframe and the propeller accumulated a lot of ice. Widespread icing conditions were also reported in the Wagga area above 5,000 ft.

When the pilot of VH-LDK requested a clearance to Wagga from flight service, he was advised twice to remain outside controlled airspace (OCTA) and he acknowledged those instructions. When VH-LDK was transferred to Wagga tower frequency, 37 nm east of Wagga, the tower controller also instructed the pilot to remain OCTA. 

However, the aircraft proceeded towards Wagga entering controlled airspace (CTA)and the Wagga control zone (CTR) until Wagga tower became aware of its position when it was 13 miles from the aerodrome.  There is no radar at Wagga.

According to the pilot, he had advised Melbourne Flight Service of the icing problems, but he did not advise Wagga Tower.  After the incident, he remembered that he had been instructed to remain OCTA.

During the descent, the diversion and the penetration of CTA/ CTR, the aircraft was in cloud.   The pilot advised that he became so preoccupied with the severe inflight icing problems that he thought that Wagga Tower had issued an onwards clearance.  The problems included engine vibrations, never before experienced by the pilot, as the propeller shed ice.

At the time of the penetration VH-WGV, a regular public transport aircraft was conducting an instrument approach at Wagga. On recognition of the potential conflict, the controller instructed VH-WGV to climb to 5,500 and enter the holding pattern.  It was estimated that the separation reduced to 500 ft vertical and 5 to 8 miles head on.

Significant Factors

The following factors were considered relevant to the development of the incident:

1. The pilot encountered more cloud than forecast.
2. The aircraft was not de-icing equipped.
3. The pilot elected to continue rather than turn back.
4. The aircraft accumulated ice on the airframe and the propeller.
5. The pilot became preoccupied with the icing problem which included vibration as the propeller shed ice.
6. While preoccupied with the icing problem the pilot erroneously thought that a clearance had been issued to proceed to Wagga.
7. Without radar coverage at Wagga, the tower controller was unaware of the penetration until the pilot made a radio transmission 13 miles from Wagga.

VH-ANJ was tracking from over Canberra to Sydney at flight level 150. VH-WGW was tracking outbound from Sydney via Pager, Shelleys and over Canberra, climbing to flight level 140. VH-ANJ was turned right onto a heading of 050, for positioning on a 15-mile final approach for runway 34 at Sydney and cleared to descend to flight level 130. This heading change put the two aircraft on opposing tracks.

To maintain separation, VH-WGW was turned to the right 20 degrees. At that time the two aircraft were nose to nose and 15 miles apart with a closing speed of 450 knots. VH-ANJ was then turned a further 10 degrees to the right, however separation reduced to 3.5 miles laterally and 700 feet vertically as the two aircraft passed.

The minimum separation required was five miles horizontal or 1000 feet vertical.

Significant Factors

The following factors were considered relevant to the development of the incident: 

1. The controller did not take the appropriate action to ensure adequate separation was maintained.

FACTUAL INFORMATION

The Boeing 737 had landed on runway 35 and commenced taxiing back to the terminal.  There was a requirement to cross that runway again in order to reach the terminal. After being instructed to hold short of runway 35, the crew were subsequently given a clearance to cross and to hold short of runway 30.  The crew replied to this instruction with the aircraft callsign only, consistent with the requirements of AIP OPS CTL paragraph 15.1 to 15.3 and OPS CTL paragraph 25 - note 1 and 3.

At this time, the crew turned on the navigation and landing lights in accordance with their company Standard Operating Procedures (SOPS) and crossed runway 35.

A Cessna 152 was on final approach for runway 30 and was cleared for a touch-and-go.

After crossing runway 35, the crew of the Boeing confirmed between themselves that they were cleared to cross runway 30 and, in doing so, left the navigation and landing lights on as they proceeded to cross this runway.

Before entering the runway strip, they checked both approaches.  The co-pilot looked to the right while the pilot in command looked to the left, the direction from which the Cessna was approaching.  At this time, the Cessna was in the final stage of its approach.  The pilot-in-command did not see the Cessna. As the two aircraft were transmitting on different frequencies, the crew of the Boeing did not hear the landing clearance given to the pilot of the Cessna.

As the Cessna was touching down the Surface Movement Controller (SMC) and the Aerodrome Controller (ADC) observed the Boeing infringe the flight strip of runway 30.  The crew of the Boeing were instructed to hold position by the SMC while the ADC instructed the pilot of the Cessna to stop immediately.

The Cessna stopped at the intersection of runway 35 and 30. The Boeing stopped within the flight strip of runway 30.  The distance between the two aircraft was 525 metres.  The crew of the Boeing were then cleared to cross runway 30, after which, the pilot of the Cessna was given a clearance to take off.

Analysis of the Automatic Voice Recorder (AVR) tape confirmed that the SMC had, in fact, issued the instruction to hold short of runway 30. The tape also indicates that, at the same time the instruction was given to hold short of runway 30, there was a transmission from another station which was not discernible. The over-transmission made it difficult to clearly hear this part of the clearance. The crew of the Boeing were sure, however, that they heard a clearance to cross both runways.  It has been common in the past for such a clearance to be given at Canberra.

The weather at the time of the incident, as described by the Automatic Terminal Information Service (ATIS), indicated light and variable winds with a visibility of more than 10 kms in haze.

ANALYSIS

As it was not uncommon to receive a clearance to cross both runways at Canberra, the crew of the Boeing appeared to have the expectation of such a clearance in this incident.  Even though the transmission from another station may have made it difficult for the Boeing crew to clearly hear the instruction to hold short of runway 30, this expectation appears to have overridden any possible ambiguity the crew may have had about the clearance instruction.  As the crew were sure they had heard the instruction to cross both runways, it was not necessary for them to clarify these instructions.

The acknowledgement of the clearance instruction with the aircraft callsign indicated an understanding of that instruction. ATC would not, therefore, have been alerted to the misunderstanding.

At the time the pilot-in-command scanned the final approach area to runway 30, shortly before the Boeing commenced crossing that runway, the Cessna may have been difficult to sight. At that time, it is likely that the Cessna was below the line of hills to the south-east of the aerodrome.  The haze that existed at the time of the incident would have affected the visibility and may have increased the difficulty in sighting the Cessna.

CONCLUSIONS

Findings

1. The SMC issued the Boeing with a clearance instruction to cross runway 35 but to hold short of runway 30.
2. The instruction to hold short of runway 30 was over transmitted by another station.
3. The crew did not check or confirm the unclear instruction.
4. The crew acknowledged the instruction with the aircraft callsign only.  Such an acknowledgement indicated to SMC that the crew understood the instruction to hold short of runway 30.
5. As the crew of the Boeing was transmitting on a different frequency to the pilot of the Cessna, they did not hear the landing clearance that was given to the Cessna.
6. The pilot in command did not see the Cessna on its final approach to runway 30.

Significant Factors

1. The crew appeared to have had an expectation to be issued with a clearance instruction to cross both runways.
2. The acknowledgement of the instruction with the callsign only did not alert ATC to the misunderstanding.
3. The Cessna may have been difficult to see on its approach path due to a line of hills to the south-east of the aerodrome.
4. The visibility at the time of the incident was affected by haze.

The sector 12 controller was responsible for ensuring separation was maintained between the two aircraft. ZK-NZY was tracking direct from over Sydney to Melbourne and this track took it close to over Canberra. VH-EWI was proceeding via over Cooma and Canberra to Sydney. Both were cruising at flight level 280. It was necessary to retain either 1000 feet of vertical separation or five miles horizontal separation.

The controller considered the option of vectoring in the vicinity of Canberra to ensure separation was maintained. However, she assessed that the lateral separation of five miles would be retained without the need for this. In spite of this assessment the horizontal distance between the two aircraft reduced to 4.5 miles.

Significant Factors

The following factors were considered relevant to the development of the incident:

1 The controller did not take positive action to ensure that the minimum required separation was maintained.

The pilot of VH-UBM called Wagga Tower inbound at 45 miles on the 065 radial of the Wagga VOR, at 8000 feet. He was instructed to maintain 8000 feet and track inbound on the 075 radial. The assigned track was to ensure separation was maintained with VH-SBA, which was tracking outbound on the 053 radial, initially at 7000 feet.

Once VH-SBA had passed the lateral separation point for the two tracks, at 10 miles from Wagga, the aircraft was cleared to climb above the level of VH-UBM. Although separation was arranged by procedural means by Wagga Tower, VH-UBM was observed on radar by Melbourne air traffic control crossing the 050 radial from south to north. A loss of procedural separation occurred due to VH-UBM being off track to the north of the 075 radial.

The pilot of VH-UBM made an error in mentally orientating himself with respect to the Wagga VOR and turned right instead of left. In retrospect he was uncertain of how this occurred.

Significant Factors

The following factors were considered relevant to the development of the incident:

1. The pilot made an error in orientating himself with respect to the Wagga VOR.

2. The precise reason for this happening was not determined.

VH-HYG departed Melbourne (ML) for Perth (PH) tracking ML - Yarrowee (YWE) - Q23 - CRENA cleared to climb to flight level (FL) 350. At approximately 20 nm from ML, VH-HYG was recleared direct to CRENA. VH-HYG contacted the Inner West (IW) radar controller 30 nm outbound from ML.

VH-HYA contacted the IW controller 120 nm inbound to ML, tracking from Mount Gambier (MTG) direct ADAMS - ML on descent to FL 210.

The controller was aware of the potential conflict with the tracks and the traffic disposition. He determined that both aircraft would pass with at least six miles separation and, in his judgement, would also have established vertical separation.

When the aircraft were approximately 10 nm apart (nose to nose) the outbound aircraft, VH-HYG, appeared to turn approximately 10 degrees to the right, towards the track of VH-HYA. The controller observed this but assessed that both aircraft would still pass abeam each other with minimum allowable lateral separation of five miles.

Both aircraft entered an area of potential separation conflict at approximately 60 nm from ML. Lateral separation reduced to 3.5 nm with no vertical separation.

The controller had used his experience to assess current/future positions of both aircraft but had failed to alter his plans to ensure separation after VH-HYG altered track. At the time of the loss of separation, the controller did not pass traffic information to either aircraft because of lack of time and because from the 3.5 miles nearest proximity the aircraft tracks began to diverge.

Significant Factors

The following factors were considered relevant to the development of the incident:

1. The controller failed to alter his plans to ensure separation standards were not infringed after VH-HYG turned approximately 10 degrees towards the track of VH-HYA.

SAFETY ACTION

Since this incident, the CAA has reminded all inner sector controllers of the importance of:

1. Separation assurance; specifically, that profile separation is not separation assurance.

2. Radar technique and the action required when using minimum radar separation.

FACTUAL INFORMATION

When the pilot of the Boeing 727 requested a taxi clearance from bay two for runway 17, the Aerodrome Controller (ADC) issued a clearance to the Surface Movement Controller (SMC) for the Boeing to cross runway 12. The SMC cleared the Boeing to taxi and issued the clearance to cross runway 12.

Other traffic requirements then became the priority for both the SMC and Co-ordinator.

While the SMC was issuing the Boeing 727 pilot with the clearance, the Cessna 152 pilot reported ready for take-off to the ADC. As both aircraft were on different frequencies, neither pilot heard the other. After a short delay, the Cessna was cleared for take-off.

Over the 30 seconds following the issuing of the take-off clearance, the ADC became involved in the co-ordination of three other aircraft. He then noticed that the Cessna had not taken off and reissued the take-off clearance. The Cessna pilot replied, 'We've got the 727 crossing'. At this point the ADC saw the Boeing crossing the runway and instructed the Cessna to hold position.

The second ADC was involved in discussions with surveyors during the entire sequence.

Due to the weather conditions, runway 17 was designated for arrivals and departures, while runway 12 was used for circuit traffic. This runway configuration is not often used in Canberra.

ANALYSIS

The ADC forgot that he had issued the Boeing with the clearance to cross. He was distracted by the presence of visitors in the tower and the busy workload.

The non-standard runway configuration may have had some influence on the ADCs procedures, resulting in a runway scan different to that which he normally would use.

FINDINGS

1. The ADC issued the SMC with a clearance for the Boeing 727 to cross runway 12.
2. The SMC issued that clearance to the Boeing 727 pilot.
3. The ADC issued the Cessna 152 pilot with a take-off clearance from runway 12.
4. The Cessna 152 pilot reported holding while the Boeing 727 crossed the runway.

SIGNIFICANT FACTORS

1. The runway configuration was not standard for Canberra.
2. The ADC was distracted due to a busy workload and the presence of visitors.