Loss of separation

On 17 November 2005, at 1327 Central Standard Time, a Boeing Company 717-200 (717) aircraft, registered VH-NXE, was inbound to Alice Springs, NT, on the 055 degree radial of the very high frequency omni-directional radio range (VOR) ground-based navigation aid, on a scheduled flight from Cairns, Qld. The 717 crew reported to the Alice Springs aerodrome controller that they passed almost overhead and within about 1,500 ft of a Cessna Aircraft Company C310R (C310) that was outbound from Alice Springs.

The C310, registered VH-JOI, was being operated on a pilot instrument rating renewal flight and, after becoming airborne off runway 12, was being tracked via a  GAFER ONE standard instrument departure (SID), on climb to operate in a training area located to the east of Alice Springs (Figure 1), not above 6,000 ft. A SID is a published instrument flight rules departure comprising obstacle clearance data to the minimum safe altitude and tracking data until the aircraft reaches a specified point on its air traffic control cleared route. The GAFER ONE SID provided altitude requirements and heading instructions that enabled the pilot to intercept the 055 degree radial from the VOR.

Figure 1:   Alice Springs visual terminal chart showing the 055 degree radial, the approximate track of the GAFER ONE standard instrument departure and the Eastern Training Area

The approved testing officer, who was also the pilot in command on board the C310, later reported that when the aircraft was at about 15 NM from Alice Springs, the pilot being tested turned the aircraft right, off the 055 degree radial of the VOR, and tracked towards the training area.

The Alice Springs airspace was not serviced by radar and as such the Alice Springs aerodrome controller was required to apply non-radar (procedural) control, in accordance with published procedures. The aerodrome controller was responsible for controlling aircraft operating within the Alice Springs control zone and control area up to 8,500 ft. Procedural control is achieved by the use of information from sources other than radar. The aerodrome controller intended to establish a lateral separation standard between the C310 and the inbound 717 once the C310 was established in the training area.

The aerodrome controller initially applied a vertical separation standard of 1,000 ft between the C310 and the 717. At 1326, the pilot of the C310 reported, to the aerodrome controller, that the C310 was established in the training area to the east of Alice Springs. That training area was procedurally separated, using a lateral separation standard, from the 055 degree radial from the Alice Springs VOR. Once the pilot of the C310 reported established in that training area, the C310 was laterally separated from the 717. The aerodrome controller then cleared the crew of the 717 to descend below the altitude of the C310.

About a minute later, the crew of the 717 reported that they had the C310 in sight about 1,500 ft below their aircraft. The pilot in command of the C310 later reported that he heard that broadcast made by the crew of the 717, but despite conducting a search for the 717, did not see the aircraft.

On 17 November 2005, a Boeing 717-200 (717) aircraft departed Cairns, Qld, on a scheduled flight to Alice Springs, N.T. It reportedly passed within 1 NM horizontally and 1,500 ft vertically of a Cessna Aircraft Company C310R (C310) aircraft that was outbound from Alice Springs, NT, on an instrument flight rules (IFR) training flight.

The information provided to the Alice Springs aerodrome controller indicated that a lateral separation standard existed between the aircraft. On that basis, the aerodrome controller cleared the crew of the 717 to descend below the level of the C310. However, the position of the Cessna C310 relative to the 717 at the time the aerodrome controller cleared the 717 to descend through the level of the C310 meant that no procedural separation standard existed between the two aircraft. The aerodrome controller had no indication that the information from either source may have been erroneous.

Alice Springs airspace is not serviced by radar, and there was no recorded information on board either aircraft that could be used to determine the exact proximity between the two aircraft at their closest point. It was not possible for the investigation to determine why the lateral separation standard applied by the aerodrome controller was compromised.

Pilots should be mindful of the importance of the accuracy of the information they provide to air traffic control, in maintaining the integrity of published separation standards.

The pilots of the Saab were instructed by air traffic control to depart Townsville on a heading of 350 degrees magnetic, to ensure a lateral separation standard existed between the Saab and the arriving Cessna 310. However, on departure, the pilots of the Saab continued their left turn, through the assigned heading. That reduced the lateral separation between the Saab and the inbound Cessna and resulted in an infringement of separation standards. This analysis examines the development of the occurrence and highlights the safety issues that became evident as a result of the investigation.

Despite a correct readback to the controller and recording the assigned heading on the Takeoff and Landing Data Card, the copilot forgot about the heading instruction as he commenced the turn after takeoff. The pilot's familiarity with the visual departures to the west, and his expectation of a visual departure on this occasion, may have contributed to the occurrence.

A person's capacity to remember to perform a task in the future can be adversely affected by workload, distraction, lack of mindfulness because of familiarity, and the elapsed time between any instruction and the proposed activity. Although the sterile cockpit policy practiced by the crew reduced the risk of distractions, the takeoff necessarily involved a high workload and there was a period of elapsed time between the assignment of the heading and initiation of the turn. Application of a cue such as setting the assigned heading on the EHSI heading selector or on the course deviation indicator would reduce the risk of flight crew forgetting assigned headings.

Monitoring by the pilot not flying is a means of early identification of heading deviations. In this case, the pilot in command was busy adjusting the power levers and did not immediately realise that the aircraft's heading had diverged from the assigned heading. Although the pilot not flying has a number of actions to perform during a departure, monitoring critical phases of the flight should be a high priority.

The controller's plan for separating the Saab and Cessna was based on establishing a lateral separation standard based on the Saab's assigned heading of 350 degrees and the Cessna's inbound radial of 329 degrees. The aerodrome controller and approach controller both became aware that the Saab had turned through the assigned heading, but were initially uncertain of the crew's intentions. It was a dynamic situation and the approach controller attempted to mitigate the effect of the Saab's displaced departure track by alerting the Cessna pilot to the proximity of the Saab and instructing him to conduct an orbit.

Although there was an infringement of separation standards, the Cessna's orbit delayed the closest point of approach, allowing time for the Saab to climb, which increased the vertical distance between the two aircraft as they passed. The left orbit would have been more effective had the Saab continued onto the flight planned track. However, there was little time for the approach controller to establish radio contact and ascertain the crew's intentions before the Saab crew turned their aircraft back to a heading of 350 degrees and passed almost overhead the Cessna with about 500 ft vertical separation between them.

The Traffic Alert Collision Avoidance System (TCAS) alerted the Saab crew to the proximity of the Cessna and reduced the risk of collision.

Sequence of events

On 13 October 2005 at 0618 Eastern Standard Time, a Saab Aircraft AB 340B (Saab), registered VH-UYA, departing Townsville Airport, Qld, came within the minimum separation standard of 1,000 ft vertically and of 3 NM horizontally of an inbound Cessna Aircraft Company 310R (Cessna), registered VH-TFP. There was an infringement of air traffic control separation standards.

The crew of the Saab was operating a scheduled passenger flight to Trepell, in central Queensland, in accordance with the instrument flight rules. Air traffic control issued a clearance to the crew to track via waypoint CATEY, a track of 243 degrees magnetic from Townsville. At 0611, the crew taxied for runway 01 ¹. After the crew reported ready for departure, the aerodrome controller issued the crew with a departure clearance, including an instruction to turn left, heading 350 degrees, visual. The pilot in command confirmed the instruction with the copilot. The copilot, who was the pilot flying, then read back the clearance to the aerodrome controller. Both pilots reported that the departure clearance was written on the Take-off and Landing Data Card before completing the pre-take-off checks and commencing the take-off roll.

Recorded information showed that as the Saab was rolling, the Cessna was 9 NM from Townsville, inbound from Cairns, Queensland on the 329 radial, and was on descent to 1,800 ft.

When the Saab had reached an altitude of between 600 and 700 ft the copilot commenced a left turn, engaged the autopilot, and selected a heading of about 210 degrees to intercept the planned outbound track of 243 degrees. At about 1,000 ft the pilot in command, as the pilot not flying, set the power levers to climb power. During the turn, at a heading of about 290 degrees, the pilot in command realised that they had flown through the assigned heading and alerted the copilot. The copilot reported that at about the same time he also realised they had flown through the assigned heading. He disengaged the autopilot and quickly turned the aircraft to the right onto the assigned heading of 350 degrees. During the turn, the Saab's Traffic Alert and Collision Avoidance System (TCAS) produced a traffic advisory (TA) consisting of an aural alert of 'traffic traffic' and the crew observed an amber symbol on the TCAS display that was indicating traffic below them. The crew attempted to sight the traffic but were unsuccessful. The pilot in command advised the approach controller that they were turning onto 350 degrees.

Both the approach controller and the aerodrome controller noticed that the Saab appeared to be turning as if to intercept the flight planned 243 radial. The aerodrome controller attempted to contact the Saab crew however, at that time the crew were in the process of transferring to the approach frequency and had not yet established contact. The approach controller provided traffic information to the pilot of the Cessna and when the pilot reported that he had not sighted the Saab, the approach controller instructed him to make a left orbit. The Saab crew subsequently contacted the approach controller and advised that they were turning onto a heading of 350 degrees visual, passing 2,200 ft. The approach controller passed updated traffic information to the Cessna pilot who then sighted the Saab about 500 ft above him and 0.5 NM ahead.

Recorded radar information indicated that when the horizontal distance reduced to approximately 3 NM between the closing aircraft, there was 400 ft vertical separation, and the Saab subsequently flew about 500 ft directly over the Cessna.

Operational aspects

Prior to engine start, the Saab crew set the Electronic Horizontal Situation Indicator (EHSI) heading selectors to the runway heading and the course deviation indicators to the outbound track of 243 degrees.

The operator did not require any change to the EHSI heading selectors or course deviation indicators in response to heading assignments in departure clearances. The operator required pilots to set the EHSI heading selector to runway heading in readiness for a one engine inoperative situation. It was common practice for the operator's crews to confirm a departure clearance with each other before it was written down and read back to the controller.

The pilot in command indicated that, as the pilot not flying, he monitored the conduct of the flight. However, there was no specific operator requirement for the pilot not flying to monitor the turn and ensure that the pilot flying captured assigned headings.

The Saab crew reported that they were observing the 'sterile cockpit' policy specified in the operator's aircraft operating manual. That policy prohibited discussion about anything except the immediate operation of the aircraft while an aircraft was climbing or descending below 10,000 ft. They indicated that their workload was normal and that they were not rushing. The copilot considered that he was not tired and there was no apparent reason for him forgetting about the assigned heading. He also said that most of the Saab operations from Townsville involved visual departures to the west and the assignment of radar headings with departure clearances was unusual.

The TCAS fitted to the Saab provided aural and visual traffic advisories when an aircraft equipped with a functioning transponder was within about 45 seconds of the projected closest point of approach. When an aircraft was within approximately 30 seconds of the closest point of approach, the TCAS issued aural and visual resolution advisories. The operator's requirements for crew response to a TCAS traffic advisory was: 'Conduct a visual search for the intruder. If successful, maintain visual acquisition to ensure safe separation.'

The weather conditions were reported to be a light wind with greater than 10 km visibility and 1 to 2 eighths cloud coverage at 2,000 ft.

1. Runway 01 heading is 016 degrees M.

On 13 October 2005, at 0618 Eastern Standard Time, a Saab Aircraft AB 340B (Saab), registered VH-UYA, departing Townsville Airport, Queensland, came within the minimum separation standard of 1,000 ft vertically and of 3 NM horizontally of an inbound Cessna Aircraft Company 310R (Cessna), registered VH-TFP. There was an infringement of air traffic control separation standards.

The pilots of the Saab were instructed by air traffic control to depart Townsville on a heading of 350 degrees magnetic, to ensure a lateral separation standard existed between the Saab and the arriving Cessna. However, on departure the pilots of the Saab continued their left turn through the assigned heading, before turning their aircraft back to a heading of 350 degrees. The approach controller provided traffic information to the pilot of the Cessna and when the pilot reported that he had not sighted the Saab, the approach controller instructed him to make a left orbit. The Saab passed almost overhead the Cessna with about 500 ft vertical separation between them.

Despite a correct readback to the controller and recording the assigned heading on the take-off and landing data card, the Saab copilot forgot about the heading instruction as he commenced the turn after take-off. The pilot in command, as the pilot not flying, intended to monitor the conduct of the flight but was busy adjusting the power levers and did not immediately realise that the aircraft's heading had diverged from the assigned heading.

Following the occurrence the operator of the Saab issued two standing orders.

On 25 June 2005, a Bombardier Aerospace Dash 8-315 (Dash 8) aircraft and three Aero Commander Div Shrike Commander aircraft (Aero commander) were in an arrival sequence of seven aircraft to runway 15 at Cairns Airport. At 1705 Eastern Standard Time, when the Dash 8 was on final approach at about 6 nautical miles (NM) from the airport, the crew received a traffic alert and collision avoidance system (TCAS) traffic advisory. The crew saw an Aero commander to the left at about 3 NM, and observed on the TCAS display an aircraft in close proximity. Very shortly after, the crew of the Dash 8 received a TCAS resolution advisory to climb their aircraft, which they followed. The crew reported as they were climbing, they saw a second Aero commander pass to the left of the Dash 8.

An Airservices Australia (Airservices) investigation of the occurrence found that the occurrence was due to the following factors:

• the approach controller made an error of judgement in assessing the new traffic sequence
• the aerodrome controller used non-standard coordination phraseology that was misinterpreted by the approach controller
• the use of sight and follow procedures for the number of aircraft involved increased controller workload
• the approach controller's lack of recent familiarisation with tower visual separation procedures.

The Airservices report made four recommendations to Cairns Terminal and two recommendations to Cairns Tower, for action.

In this occurrence, the controller did not perceive the potential conflict between the aircraft, despite previous refresher training that should have assisted in the task.

Although the presence of the supervisor in the operations room created an opportunity to discuss an operational issue, it was not a priority. The controller should have discussed the issue at some other time when he was not responsible for an operational position. The supervisor was also in a position to defer the discussion with the controller, until a more suitable time or location, which would have reduced the likelihood of compromising operations.

While distraction could not be discounted as a contributing factor, the circumstances of the occurrence are consistent with the findings of the Airservices Australia June 2003 review, particularly with respect to low levels of situational awareness by controllers. Over time, the benefits of the implementation of the review recommendations in helping to develop controller awareness of potential performance limitations should become apparent.