Separation issue

At approximately 2200 Eastern Daylight-saving Time on 15 December 2005, the pilot of a Piper PA-31-350 Chieftain aircraft, registered VH-HJS, was approaching Bankstown Airport to land on Runway 11.  Following touchdown, the pilot of the Chieftain noticed another aircraft on the right side of the runway and took avoiding action by manoeuvring to the left, off the runway sealed surface and on to the grass, to prevent a collision.  The pilot reported that the other aircraft appeared to have only a dim taillight on at the time.

Subsequently, the other aircraft was identified as a Piper PA-28-151 Cherokee aircraft, registered VH-LMY.  The pilot of the Cherokee had completed a number of night circuits at Bankstown and reported that he was taxiing on runway 11 following a full stop landing.

At the time of the incident, Bankstown Airport was operating as a Common Traffic Advisory Frequency (R) (CTAF - radio required to be carried and used).

The Australian Transport Safety Bureau reviewed the recorded frequency channels for the Bankstown CTAF (R).  The recordings indicated that the pilot of the Chieftain had broadcast his intentions on the Bankstown CTAF (R) during the approach. In addition, transmissions from other traffic and the Bankstown Aerodrome Frequency Response Unit (ARFU) were recorded.

The pilot of the Cherokee reported that he had listened to the information from the aerodrome Automatic Terminal Information Service (ATIS) prior to commencing circuits. During the circuit training, he reported that he had made all the normal transmissions on the Bankstown CTAF (R), but did not hear any transmissions from other aircraft or the Bankstown ARFU. However, there were no recorded transmissions from the pilot of the Cherokee on the Bankstown CTAF (R).

The owner of the Cherokee reported that the aircraft radio equipment functioned correctly with no unserviceability reported and that the aircraft was equipped with functional lights, including navigation, anti-collision and strobe lights.  The pilot of Cherokee reported that all lights were on at the time of the incident.

ATSB Comment

The investigation was unable to determine why the transmissions from the pilot of the Cherokee were not recorded on the Bankstown CTAF (R), but it is likely that that an inadvertent mis-selection of the radio was made at some time after the pilot listened to the Bankstown ATIS. The investigation was also unable to rationalise the difference in evidence from the pilots of each the aircraft regarding the aircraft lighting on the Cherokee.

At approximately 2200 Eastern Daylight-saving Time on 15 December 2005, the pilot of a Piper PA-31-350 Chieftain aircraft, registered VH-HJS, was approaching Bankstown Airport to land on Runway 11.  Following touchdown, the pilot of the Chieftain noticed another aircraft on the right side of the runway and took avoiding action by manoeuvring to the left, off the runway sealed surface and on to the grass, to prevent a collision.  The pilot of the Chieftain reported that the other aircraft appeared to have only a dim taillight on at the time.

Subsequently, the other aircraft was identified as a Piper PA-28-151 Cherokee aircraft, registered VH-LMY.  The pilot of the Cherokee had completed a number of night circuits at Bankstown and reported that he was taxiing on runway 11 following a full stop landing.

At the time of the incident, Bankstown Airport was operating as a Common Traffic Advisory Frequency (R) (CTAF - radio required to be carried and used).

The Australian Transport Safety Bureau reviewed the recorded frequency channels for the Bankstown CTAF (R).  The recordings indicated that the pilot of the Chieftain had broadcast his intentions on the Bankstown CTAF (R) during the approach. In addition, transmissions from other traffic and the Bankstown Aerodrome Frequency Response Unit (ARFU) were recorded.

However, there were no recorded transmissions from the pilot of the Cherokee on the Bankstown CTAF (R).

Although there was no applicable minimum distance standard specified for visual separation, the controller was unable to maintain continuous visual separation between the 152 and the 717. The decision by the pilot of the 152 to turn directly onto the base leg of the circuit, and not continue on the downwind leg as instructed, contributed to the 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.

The controller did not provide the pilots of the 152 or the crew of the 717 with traffic information, or a number in the landing sequence as required by the Manual of Air Traffic Services (MATS). The provision of traffic information was not mandatory and the MATS did not provide any guidance to controllers on the circumstances under which the provision of traffic information would be appropriate. While the controller had intended to provide this information to the pilot of the 152, he relied on a pilot report prior to turning base as a prompt, and this report was not received.

Without the timely provision of traffic or sequence information, the situational awareness of the pilots of both aircraft was reduced. They were effectively excluded from participating in the separation process as described in the Aeronautical Information Publication (AIP) and the MATS. Consequently, the pilots of the 152 were not aware of the broader consequences of their actions once they turned their aircraft onto the base leg. They simply did not recognise that a potential conflict between their aircraft and the 717 existed.

While the flight crew of the 717 was not provided with directed information by the controller, they had been monitoring the radio transmissions between the controller and the pilots of other aircraft in the area. That, together with active scanning of the circuit area for traffic using the traffic alert and collision avoidance system and visual observations, assisted in the resolution of the situation.

The MATS provided no guidance as to whether routinely issued sequencing and separation instructions, such as 'continue downwind', required a read-back. While it may be impractical for the controller to obtain a read-back for every circuit instruction, emphasis should be placed on obtaining a read-back of safety critical instructions. Had the controller requested a read-back of the instruction to continue downwind, and provided a reason for the action, the likelihood of any misunderstanding would have been significantly reduced.

The investigation could not establish whether any aspect of the occurrence sequence could be attributed to the effects of fatigue. However, due to the instructor's non-aviation working commitments, the possibility that fatigue contributed to the occurrence could not be discounted.

On 30 April 2005, at about 1033 Eastern Standard Time¹, a Cessna Aircraft Company A152 (152) aircraft, registered VH-PVX, was being operated on circuit flying training at Hobart Airport, Tasmania. On board were a student pilot and an instructor. The pilot had been issued with a clearance by the aerodrome controller (controller) to conduct right circuits from runway 30 and to operate not above 1,000 ft above mean sea level.

At 1037, a Boeing Company 717-200 (717) aircraft registered VH-LAX, en route from Melbourne to Hobart, commenced the final leg of the Hobart runway 30 VOR² instrument approach. The crew had been issued a clearance by the controller to conduct the final approach. The aircraft was being operated under the instrument flight rules. At about the same time, the controller instructed the pilot of the 152 to make a left orbit to enable the controller to visually separate the 152 with both the inbound 717 and another jet aircraft departing from runway 30. At that time the 152 was at the end of the downwind leg of the circuit.

At about 1039, the pilot of the 152 reported that the orbit was complete and the controller instructed the pilot to continue on the downwind leg and to report prior to turning on to the base leg. However, the pilot had not completed a full orbit but had 'rolled out' of the orbit after completing only a 270 degree turn, directly onto the base leg of the circuit. At that time the 717 was on the final approach leg of the circuit, 90 degrees to the left of the flight path of the 152 and converging.

At about 1041, when the pilots of both aircraft became aware of the potential conflict, the minimum horizontal distance between the two aircraft had reduced to between 400 and 500 m. The 717 was about 300 ft below the 152, and the pilots of both aircraft commenced avoiding action.

The Manual of Air Traffic Services (MATS) 4.5.1.1 stated that visual separation shall be achieved by the use of visual procedures, or by assigning visual separation responsibility to a pilot. The MATS did not specify any minimum distance requirement for the application of visual separation. As the controller was unable to continue to visually observe separation between the 152 and the 717, and had not assigned separation responsibility to the pilot of the 152, there was an infringement of separation standards.

The routine aerodrome weather report (METAR) for Hobart issued at 1030, recorded the cloud as few³ at 3,000 ft and broken at 5,000 ft with visibility greater than 10 km. The pilot in command of the 717 and the controller later reported that the cloud was scattered at 4,000 ft while the 152 instructor believed the cloud base to be broken at about 3,000 ft.

The 152 instructor reported that he had a total of about 270 flying hours including 15 to 20 hours as an instructor. He was relatively new to Hobart and worked about 5 to 10 hours a week flying. He also worked casually in another non-aviation position with shifts that finished late at night or in the early morning.

The instructor believed that his inexperience, together with the cockpit workload involved in instructing the student pilot and controlling the aeroplane in moderately difficult crosswind conditions, reduced his situational awareness. He reported that he was not aware of the 717 on final, and believed he would not have had any opportunity to observe the aircraft until it was established on the final approach because of the low cloud in the area.

The controller had extensive experience in the provision of aerodrome control services at Hobart, and reported that the workload at the time of the occurrence was both moderately busy and complex.

The controller reported that he was applying visual separation between the 152 and several other aircraft. The MATS specified that:

The requirement to provide traffic information was changed from 'mandatory (and)' to 'optional (and/or)' by Airservices Australia in April 2003. On 1 September 2005, Airservices Australia amended the MATS to completely remove the previously amended section 4.5.2.3 relating to the provision of aerodrome traffic information, with the concurrence of the Civil Aviation Safety Authority (CASA), to remove ambiguity over separation responsibilities in the aerodrome traffic zone.

On 16 September 2005, the Civil Aviation Safety Regulation Part 172 Manual of Standards was amended, after agreement between CASA and Airservices Australia, to state:

When aircraft are operating visually as aerodrome traffic ATC must issue 1 or more of the following:

(a) clearances designed to maintain separation

(b) sequencing instructions

(c) relevant traffic information

The issue of the provision of traffic information is subject to an Australian Transport Safety Bureau (ATSB) safety recommendation⁴ (see also Safety Actions section of this report).

The Aeronautical Information Publication (AIP) GEN 2.14.3 also specified that:

ATC will provide relevant traffic information to aerodrome traffic to enable pilots, while complying with ATC instructions, to maintain separation from other aircraft.

The controller reported that the initial orbit instruction given to the pilot of the 152 was to allow for the departure of another jet aircraft and his plan was for the 152 to then extend on a downwind leg until it was possible for the 152 to safely follow the 717 on final. He had intended to pass the pilot of the 152 a number in the landing sequence when the pilot reported prior to turning base. However, as this report was not received, the pilot was not provided with either a number in the sequence or traffic information.

AIP GEN 4.4.1 specified that 'pilots must transmit a correct read-back of ATC clearances, instructions and information which are transmitted by voice' and ensure 'sufficient detail is included to indicate compliance'. The MATS 6.1.13 specified that ATC 'shall ensure that a correct read-back in sufficient detail is obtained'.

Both documents indicated that only key elements relating to certain clearance items must be read back, including 'level instructions, direction of turn, heading and speed instructions'.

The pilot of the 152 did not read back the instruction to continue on the downwind leg, nor did the controller request the read-back. There was no specific requirement in either the AIP or the MATS for the read-back to be provided.

The controller later acknowledged that a sequence number and traffic information should have been provided to the pilot of the 152 and that he believed that a read-back of the downwind instruction would have been beneficial.

Airservices Australia had an annual refresher training program for tower controllers that detailed several mandatory and optional training modules. One mandated module relating to separation assurance was not available to the Hobart controllers at the time of the occurrence.

The 717 flight crew was not provided with traffic information by the controller, but reported that they had been monitoring the radio transmissions between the controller and other pilots. Additionally they had observed the 152, initially on the traffic alert and collision avoidance system (TCAS),⁵ then visually, before commencing avoiding action.

1. The 24-hour clock is used in this report to describe the local time of day, Eastern Standard Time (EST), as particular events occurred. Eastern Standard Time was Coordinated Universal Time (UTC) + 10 hours.
2. Very high frequency omnidirectional radio range.
3. Cloud amounts are reported in oktas. An okta is a unit of sky area equal to one-eighth of total sky visible to the celestial horizon. Few = 1 to 2 oktas, scattered = 3 to 4 oktas, broken = 5 to 7 oktas and overcast = 8 oktas.
4. ATSB occurrence investigation report 200205540 and associated safety recommendation R20040063 available at www.atsb.gov.au.
5. TCAS is an independent onboard collision avoidance system. It is designed as a backup to the ATC system and the 'see and avoid' concept.

On 30 April 2005, the pilot of a Cessna Aircraft Company A152 (C152) aircraft was conducting circuit training at Hobart. The C152 was on the downwind leg of the circuit when the crew of a Boeing Company B717-200 (B717) aircraft commenced the final leg of an instrument approach to the same runway.

The Hobart aerodrome controller was applying visual separation standards and had instructed the pilot of the C152 to make an orbit, and then continue downwind, to separate the C152 from other aircraft. The C152 pilot did not complete a full orbit, but turned onto the base leg of the circuit when the B717 was on final approach. The minimum distance between the converging aircraft reduced to between 400 and 500 m horizontally and 300 ft vertically and required the pilots of both aircraft to commence avoiding action. There was an infringement of separation standards.

The pilot of the C152 did not read back the instruction to continue on the downwind leg to the controller, nor did the controller request this read-back. There was no specific requirement in published documents for the read-back to be provided.

The controller did not provide the pilot of the C152 or the B717 with traffic information, or a number in the landing sequence as required by published documents. This led to a reduction in the situational awareness of the pilots of both aircraft and excluded them from participating effectively in the separation process.

Airservices Australia has advised that they are addressing the issue of obtaining read-backs, when necessary, through controller education and have developed a roving check and standardisation programme for regional towers. As part of that programme, check and standardisation officers place emphasis on the use of correct phraseology and read-back.

The ATSB issued a safety recommendation to Airservices Australia to enhance pilot situational awareness.

On 31 August 2004, at about 1000 eastern standard time, the pilot of a Piper Aircraft Corporation PA-31 (Chieftain), registered VH-LTW, was conducting an instrument flight rules (IFR) flight from King Island to Devonport. The aircraft was maintaining 7,000 feet in visual meteorological conditions. The pilot saw an aircraft flying in the opposite direction passing between 100 ft and 200 ft, down the left of the Chieftain. He took immediate avoiding action. The pilot estimated that there was two to three seconds between initially seeing and then passing the other aircraft.

The other aircraft was later identified as an Aero Commander 500-S (Shrike), registered VH-LET, on a visual flight rules (VFR) flight at 6,500 feet from Launceston to King Island via overhead Devonport. The presence of the Shrike was established when the pilot of that aircraft advised the pilot of a third aircraft, that was east of King Island en route to Wynyard at 7,000 ft, of his position and altitude of 6,500 ft. The pilot of the third aircraft (tracking to Wynyard) had broadcast his intention to descend from 7,000 ft and that radio transmission alerted the Shrike pilot to a possible conflict. The pilots agreed to maintain their respective altitudes until the aircraft had passed. Neither the pilot nor the passenger in the Shrike saw the Chieftain.

At 1001, the Chieftain was 61 NM from Devonport and the pilot advised the controller of the occurrence and requested traffic information on any other aircraft in the area. The controller replied that there was no observed traffic [displayed on the radar].

Both pilots reported operating their aircraft transponders, including the Mode C altitude function, as required by the Aeronautical Information Publication (AIP) procedures. A review of the recorded air traffic control radar data confirmed that both aircraft were cruising at their reported altitudes about 20 minutes before they passed. However, the area where the aircraft passed was not within radar coverage and the investigation could not confirm the altitudes of the aircraft when they passed.

The Chieftain was fitted with two altimeters and the pilot reported that he had set both subscales to the area QNH and had engaged the autopilot. The Shrike was fitted with two altimeters and the pilot reported that he had set both subscales to either the local QNH or, when outside the mandatory broadcast zone (MBZ), the area QNH. The pilot had also engaged the auto-pilot. Both aircraft were maintained to IFR equipment requirements.

The AIP detailed altitudes to be used for aircraft on IFR/VFR flights in an easterly or westerly direction. The altitudes reported by the pilots were in accordance with the AIP.

Both aircraft were fitted with dual very high frequency radios. From about 0954, both pilots were simultaneously monitoring the Melbourne Centre area frequency on 122.6 MHz and the Devonport/Wynyard MBZ frequency on 126.9 MHz. At that time, the Chieftain pilot reported his Devonport estimate of 1020 to the Melbourne Centre controller. The pilot of the Shrike was monitoring Melbourne Centre on 122.6 MHz, but could not recall hearing the position report by the Chieftain pilot to Melbourne Centre. The pilot of the Shrike was not required to make any radio broadcasts. Also, pilots were not required to make any radio broadcasts when leaving an MBZ. A replay of the Melbourne Centre on 122.6 MHz revealed some interruptions by other pilots on that frequency.

Apart from the mandated broadcasts, the procedure in MBZ and Class G airspace is for pilots to listen on the appropriate frequency and to make a radio broadcast if there is the potential for aircraft to come into conflict.

In Class G airspace, air traffic control (ATC) provides traffic information to pilots of aircraft operating an IFR flight about other aircraft operating as IFR flights and military jet aircraft. Pilots operating a VFR flight may request traffic information from ATC, but it is provided subject to workload at the time. Pilots may make additional radio broadcasts at their discretion.

Neither aircraft was fitted with a traffic alert and collision avoidance system, nor was there any legislated requirement to do so.

On 31 August 2004, at about 1000 eastern standard time, the pilot of a Piper Aircraft Corporation PA-31 (Chieftain), registered VH-LTW, was conducting an instrument flight rules (IFR) flight from King Island to Devonport. The aircraft was maintaining 7,000 feet in visual meteorological conditions. The pilot saw an aircraft flying in the opposite direction passing between 100 ft and 200 ft, down the left of the Chieftain. He took immediate avoiding action. The pilot estimated that there was two to three seconds between initially seeing and then passing the other aircraft.

The SMC did not anticipate the aircraft conflict. The night environment, lack of aircraft taxi lights and the distraction with entering system data may have limited his situational awareness despite the availability of the SMR. The use of a segmented or a conditional clearance to the pilots of either aircraft would have assured that the aircraft would not conflict. Alternatively, the SMC could have requested assistance to either enter the system data, or to monitor the taxiing aircraft visually or by using the SMR.

The situational awareness of all the pilots was reduced by the lack of specific traffic information on the other potentially conflicting aircraft. They did not see the other aircraft's taxi light as they carried out their standard lookout. Even if the other aircraft had been seen, it may have been interpreted as an aircraft holding, or even preparing to give way, because the taxi light was selected off.

Both aircraft were being taxied at speeds that limited either crews' ability to react to a conflict in a known busy area of the airport. In this instance a slower taxiing speed would have been appropriate. However, a limiting factor for both aircraft was the lack of a practical means to monitor or assess groundspeed, especially at night.

The SMC's use of the term 'expedite' and a perceived need to not delay taxiing, to assist both pilots and controllers, may have led the pilots of both aircraft to taxi faster than they might normally do. While high taxi speeds may be acceptable in some locations on the movement area, reduced taxi speeds are warranted in potential areas of conflict near runway exits, multiple crossing taxiways and apron access areas. The use of slower taxi speeds in those areas should assist pilots to more readily maintain separation from other taxiing aircraft while complying with air traffic control clearances.

The requirement for pilots to monitor the company frequency throughout the taxi period is a potential distraction for pilots, when the priority for their attention should be on operational duties. Procedures for non-operational radio usage could be better managed so as to have minimal impact on operational duties. This is particularly relevant in a busy taxiway environment like Sydney Airport. If the Dash 8 copilot had also been busy or distracted, a collision may have occurred.

On 2 July 2004, at about 1805 Eastern Standard Time, a de Havilland DHC-8-315 (Dash 8) was taxiing at Sydney Airport for a night departure on a scheduled fare-paying passenger flight to Williamtown, NSW. The Sydney Tower Surface Movement Controller (SMC) had issued a clearance for the crew of the Dash 8 to taxi from Domestic Terminal 1 to the holding point for runway 16 Left. The clearance was via taxiway Charlie, across runway 25 to Bravo 10.

Figure 1: Sydney aerodrome chart

A Saab 340F (Saab) had landed on runway 16 Right and taxied from that runway via runway 25 then turned left onto taxiway Bravo. The SMC issued a clearance for the crew to taxi via taxiway Golf to the apron. The SMC had instructed the crew of the Saab to expedite. The dictionary definition of expedite means to 'speed up the progress of, or to hasten'. The Saab pilot in command (PIC) interpreted this instruction as to not waste any time and to keep the aircraft moving but at a safe speed.

As both aircraft were about to enter the intersection of taxiways Golf and Charlie (see figure 1), the copilot of the Dash 8, seated on the right of the cockpit, saw the Saab. He called for the PIC to stop. The PIC brought the Dash 8 to an abrupt stop as the Saab taxied through the intersection at a constant speed. Neither of the PICs saw the other aircraft until they had crossed at the intersection. The copilot of the Saab was busy with after-landing duties during the initial taxi period after vacating the runway. As the Saab taxied through the intersection he was occupied with a company frequency transmission on the radio and did not see the Dash 8 at any stage. The SMC was unaware of the situation until queried by the PIC of the Dash 8 regarding which aircraft had right of way.

A replay of the surface movement radar (SMR) of the occurrence showed that the taxi speed of the Saab was 26 knots. At 1800:08, the taxi speed of the Dash 8 was 21 knots. It then decelerated and stopped at 1800:11 with the Saab passing directly in front, moving from right to left. The SMR displayed the distance between the aircraft as 42 m. The impression of both PICs was that the aircraft passed in very close proximity.

Normally, pilots use a technique of scanning left and right to check for other aircraft that may conflict with them at intersections. Generally pilots on the left observe the left side and those on the right observe the right side. The standard procedure for the Saab operator was for pilots to survey the area to both the left and the right of the aircraft before entering or crossing any taxiway.

Neither operator had a policy or general guidance material in relation to aircraft taxi speeds, although the Saab operator did require turns in the aircraft to be 'at a speed below 20 kts'. Consequently, the speed at which aircraft were taxied varied between pilots.

Pilots recalled taxi speed limits from their training as 'fast walking pace' but agreed that this was impractical over the long distances involved at Sydney Airport. Neither the Civil Aviation Regulations (CARs) nor the Aeronautical Information Publication (AIP) nominate any specific aircraft taxi speed limits.

A groundspeed readout (utilising a Global Positioning System (GPS)) is available in the Dash 8, but its use is limited by its position on the lower pedestal in the cockpit. This makes it difficult for pilots to cross refer to it while taxiing and looking out for other taxiing aircraft. Similarly, a groundspeed readout is available in the Saab from either the lower pedestal or from the electronic horizontal situation indicator. In addition, a pilot's ability to estimate speed is more difficult at night due to the lack of visual cues used in judging the relative motion of the aircraft with other objects.

The Saab taxi light was unserviceable. The pilots' recollection of the event indicates that the Dash 8 taxi light was not illuminated. Some pilots turn taxi lights off while holding or passing close to other aircraft, to prevent a dazzling effect. The navigation lights and rotating beacons of both aircraft were operating. Both aircraft were backlit to some degree by the domestic terminal and suburban lighting on the far side of the airport.

The SMC was operating the SMC east and SMC west positions on combine. Immediately prior to the occurrence, the controller was endeavouring to manage the arrival and departure from the apron of four aircraft while updating radar system data. System updating included the assigning of labels to radar returns on the SMR for aircraft taxiing for departure. The controller was aware of the disposition of aircraft but his impression, at the time he issued the clearance to the crew of the Dash 8, was that the Saab would pass through the intersection before the Dash 8 would be near that intersection. Consequently, he did not provide traffic information on the other aircraft to either crew, nor did he assess that there was a need to use a segmented or a conditional clearance to either crew. A segmented clearance enables a crew to taxi and to stop at a possible point of conflict, while a conditional clearance enables a crew to taxi subject to specified requirements. In this situation, the Dash 8 crew could have been instructed to taxi and to hold short of the intersection of taxiways Charlie and Golf; or alternatively, to taxi to the runway, with a requirement to pass behind the Saab on taxiway Golf.

Even though crews receive and acknowledge clearances to taxi on the aerodrome manoeuvring areas, the CARs and AIP put the onus on the PIC to maintain a good lookout and observe other traffic to avoid collision.

Both airlines have a procedure where one or both of the pilots listen (and transmit as required) to the company frequency from taxi to just before take off and, from after landing till after shutdown at the parking bay. The Saab copilot was the only pilot reported to be operating a radio at the time of the occurrence.

Throughout this analysis it should be noted that the pilots of both aircraft and the ATS controller involved in the occurrence complied with the rules and procedures for operation in Class E airspace associated with the NAS phase 2b, implemented on 27 November 2003.

The regulations, procedures and educational material associated with that implementation stated that there was a shared responsibility by pilots of IFR and VFR flights to see-and-avoid each other in Class E airspace.

Prior to the implementation of NAS phase 2b on 27 November 2003, both aircraft involved in this occurrence would have been operating in Class C airspace. As such, they would have been subject to an ATS airways clearance and would have been provided with separation in accordance with Class C airspace rules and procedures. In order for two aircraft to pass in close proximity at these flight levels in Class C airspace, those rules and procedures would need to have been compromised. As the pilots of both aircraft and the ATS controller complied with the rules and procedures for Class E airspace under NAS, those rules and procedures do not preclude an IFR high performance, high-capacity regular public transport aircraft from passing within such close proximity as to generate a TCAS RA on either known or unknown VFR traffic.

The controller's relatively low workload and other factors, such as the Lancair pilot submitting flight notification details, and broadcasting his departure from Maroochydore, assisted the ATS controller to detect a possible conflict. Although there was no requirement for the controller to pass traffic information to the pilot of the Lancair about the location of the 737 under NAS Class E airspace procedures, the controller provided traffic information to both aircraft with respect to each other.

Part 5 of MATS also stated that controllers shall issue a safety alert when, in the consideration of the controller, such an advice was warranted to avoid conflict. In the circumstances of this occurrence, the controller had provided traffic information to the crews of both aircraft, and the Lancair pilot had broadcast that he had the 737 in sight. Accordingly, the onus was then on the Lancair pilot to avoid the 737. In those circumstances, the provision of a safety alert, which may have included a suggested course of action, may also have complicated the situation, if that suggestion was contrary to what the pilots of each aircraft considered necessary.

Provision of a safety alert, in the circumstances of this occurrence, was not required. However, MATS did not provide any guidance to controllers on the circumstances under which the provision of a safety alert would be appropriate. Publication of those guidelines may assist controllers to determine when a safety alert should be issued.

In Class E airspace, the provisions of CAR 163A required the crews of both aircraft to 'see and avoid' each other. The 737 crew were unable to see the Lancair despite their attempts to do so. The Lancair pilot reported that he had the 737 in sight. When the 737 crew observed the position of the Lancair on the TCAS navigation display, they commenced action to avoid a confliction prior to receipt of both the TCAS TA and RA. In concert with that action, they continued in their attempts to visually acquire the Lancair, in accordance with Class E airspace see-and-avoid requirements.

In the circumstances of this occurrence:

• the ATS controller took more actions than those required by the published requirements for Class E airspace and MATS;
• the Lancair pilot took more actions than those required by the published procedures for Class E airspace under NAS;
• the early action taken by the 737 crew to avoid the conflict was not contrary to the published procedures for Class E airspace under NAS.

Despite those actions, the two aircraft came into such proximity that a TCAS RA was generated in the 737.

Both aircraft were operating in Class E airspace that was introduced as part of the National Airspace System (NAS) phase 2b on 27 November 2003. (An ATSB research report titled National Airspace System Stage 2b: Analysis of Available Data was released in July 2004.) As no prescribed separation standards are applicable in these circumstances, there was no infringement of separation standards. However, ATS audio tapes, radar and TCAS data, and information obtained from the air traffic controller and the pilots of both aircraft were consistent and indicate that the aircraft came into such close proximity that a threat to the safety of the aircraft may have existed. Therefore, the incident has been classified by the ATSB as an airprox event.

The incident at Canty IFR reporting point, on 3 December 2003 (ATSB report 200304963) was also classified as an airprox event.

Unlike this incident north of Brisbane on 7 April 2004 and the Canty incident, the occurrence near Launceston on 24 December 2003 (ATSB report 200305235) was classified as a serious incident due to the lack of radar coverage in the Launceston area and the absence of radio broadcasts from the pilot of the Tobago, which created an unalerted see-and-avoid environment for the crew of the Boeing 737. The air traffic controller also was unaware of the Tobago.

On 7 April 2004, a Boeing 737-7BX (737) aircraft registered VH-VBT, operating under the instrument flight rules (IFR), was en route from Townsville and descending for a landing at Brisbane. A Neico Lancair IV-P aircraft registered VH-LDJ, operating under the visual flight rules (VFR), was en route from Maroochydore to St George, on climb to flight level (FL) 165¹. Both aircraft were operating in radar Class E airspace at the time of the occurrence.

The Lancair pilot reported to the Air Traffic Services (ATS) controller that he had departed Maroochydore at 0718 Eastern Standard Time², although in accordance with the National Airspace System (NAS) procedures, there was no requirement for him to do so³. The controller issued the Lancair pilot with a discrete transponder code to assist with his situational awareness. Published NAS procedures stated that:

In Class E airspace, IFR and VFR flights are permitted. IFR flights are provided with an air traffic control service, are separated from other IFR flights, and receive traffic information on VFR flights as far as is practicable. VFR flights receive a Radar Information Service (RIS)⁴ on request.⁵

Those procedures also stated that, for VFR aircraft operating in Class E airspace, '…no flight notification was required…' and pilots of aircraft conducting operations under the VFR were required to monitor the appropriate radio frequency. The Lancair pilot submitted flight notification details to ATS prior to departure from Maroochydore, and reported that departure to ATS.

In accordance with those procedures, the controller was not providing traffic separation⁶ to either aircraft, and advised the 737 crew accordingly. Recorded ATS audio information showed that the controller provided traffic information about the Lancair to the 737 crew on three occasions between 0721:58 and 0725:08. At 0722:17, and again at 0725:38, the controller provided the Lancair pilot with traffic information about the 737, although there was no requirement for him to do so in Class E airspace. During the first transmission at 0722:17, the controller advised the Lancair pilot that the 737 was crossing traffic which would pass in about 15 NM and was on descent into Brisbane. During the second transmission at 0725:38, the controller advised the Lancair pilot that the 737 was 'in your 2 o'clock at 8 miles now'. ATS radar data showed that, at 0725:38, the 737 was 8.8 NM from the Lancair. At 0725:47, in response to that broadcast, the Lancair pilot advised the controller that he had the 737 in sight.

As the 737 was passing through about FL157 at 0726:01, the crew reported that they observed a traffic alert and collision avoidance system (TCAS)⁷ traffic symbol on the aircraft's navigation display, about the Lancair. They attempted to visually acquire the Lancair, but were unable to see that aircraft. Recorded data from the 737's flight data recorder (FDR) showed that, at that point, the crew commenced manoeuvring the aircraft by reducing the rate of descent. At about 0726:18, the crew received a TCAS traffic advisory (TA)⁸. The crew reported that they were still unable to visually acquire the Lancair and were uncertain of its relative position. Recorded FDR data indicated that at 0726:34, the crew disengaged the autopilot and commenced a right turn away from the Lancair. They subsequently levelled the 737 at FL153 and then climbed to FL154. At 0726:40 and at 15,420 ft the 737 received a TCAS resolution advisory (RA)⁹ aural warning instructing them to climb, in response to the proximity of the Lancair. They subsequently climbed the 737 to FL166 and continued the turn to about 15 degrees right of track. The duration of the RA was 10 seconds and commenced when the Lancair was about 650 ft vertically lower and about 0.7 NM to the left of the 737.¹⁰ Recorded ATS radar data showed that, at about 0726:45, the Lancair altered track 8 degrees to the right away from the 737, just before passing behind and below the 737. The Lancair pilot reported that he did not experience the effect of wake turbulence from the 737. The minimum distance between the two aircraft was about 600 ft vertically at about 0.3 NM laterally.¹¹

The Class E airspace in which the 737 and the Lancair were operating at the time of the occurrence was introduced as part of NAS phase 2b on 27 November 2003. Prior to NAS phase 2b, that volume of airspace was classified as Class C airspace. In Class C airspace, both aircraft would have been subject to an ATS airways clearance and would have been separated in accordance with prescribed standards.

In Class E airspace, the pilots of aircraft operating under the IFR and VFR were required to:

…maintain vigilance so as to see, and avoid other aircraft.¹²

The NAS Implementation Group reference guide, distributed as part of the NAS phase 2b implementation, stated that:

The other important change is that the pilot of a VFR flight should not make broadcasts on ATC frequencies.¹³

It also stated that:

Pilots of VFR flights may monitor the ATC frequency to enhance situational awareness.

Please do not make broadcast transmissions or engage in chatter on an ATC frequency. The safety of others depends on you not doing this.

Pilots are not precluded from responding to any ATC or pilot transmission when they believe their safety is at risk from another aircraft.

Part 2, Section 2, paragraph 2.2.4.1 of the Manual of Air Traffic Services (MATS) stated that:

Before providing a radar service to an aircraft, radar identification shall be established.

Although the controller did not advise the Lancair pilot that the Lancair was radar identified, the controller issued a discrete transponder code to the Lancair pilot and radar-identified the aircraft before providing a Radar Information Service to the pilot.

Part 4, Section 1, paragraph 4.1.1.1 of MATS contained information regarding ATS controller responsibilities for providing aircraft separation. The manual stated that:

Separation shall be provided by ATC using approved separation standards and procedures.

In the circumstances of this occurrence, the ATS controller was not required to provide separation to either aircraft in respect of the other and there were no separation standards applicable in these circumstances in Class E airspace.

Part 4, Section 1, paragraph 4.1.1.3 of MATS stated that:

Nothing in this chapter precludes a controller from using discretion and initiative in any particular circumstance where these procedures appear to be in conflict with the requirement to promote the safe conduct of flight.

In the circumstances of this occurrence, the ATS controller provided traffic information to the crews of both aircraft in respect of the other, although under NAS procedures there was no requirement for him to provide traffic information to the pilot of the Lancair on the location of the 737.

Part 5, Section 1, paragraph 5.1.13 of MATS provided information regarding provision of safety alerts. Paragraph 5.1.13.1 stated that:

A safety alert shall be issued to an aircraft when a controller is aware the aircraft is in a situation which is considered to place it in unsafe proximity to terrain, obstructions, or other aircraft.

The controller reported that, once the Lancair pilot broadcast that he had the 737 in sight, there was no necessity to broadcast a safety alert to either the Lancair pilot or the 737 crew. The controller also reported that if that pilot had not broadcast that he had the 737 in sight, his next option was to issue a safety alert. MATS did not provide any guidance to controllers on what might be considered '…unsafe proximity…', or when to issue a safety alert.

The NAS Implementation Group reference guide contained information for VFR pilots regarding separation from other aircraft when operating in Class E airspace. Page 16 of that guide stated that:

When you are flying in Class E airspace you are responsible for separation from other aircraft. The onus is on you to look out and see and avoid other aircraft.

Part 12, The Rules of the Air, Division 1, of the Civil Aviation Regulations (CAR) 1988, contained information regarding right of way, prevention of collision, operating near other aircraft and responsibilities of flight crew to see and avoid aircraft. More specifically, CAR 161 contained information regarding right of way and stated that:

(1) An aircraft that is required by the rules in this Division to keep out of the way of another aircraft shall avoid passing over or under the other, or crossing ahead of it, unless passing well clear.¹⁴

(2) The pilot in command of an aircraft that has the right of way must maintain its heading and speed, but nothing in the rules in this Division shall relieve the pilot in command of an aircraft from the responsibility of taking such action as will best avert collision.

CAR 162 (1) contained information regarding prevention of collision and stated that:

When 2 aircraft are on converging headings at approximately the same height, the aircraft that has the other on its right shall give way…

Although not specifically referring to converging aircraft, CAR 162 also stated that '…each shall alter its heading to the right…', and when referring to the aircraft other than the aircraft having right of way, '…shall keep out of the way of the other aircraft by altering its heading to the right…'.

CAR 163 (1) contained information regarding operating near other aircraft and stated that:

The pilot in command of an aircraft must not fly the aircraft so close to another aircraft as to create a collision hazard.

The 737 was on the Lancair's right and, in accordance with CAR 161 and CAR 162 (1), had right of way. The Lancair pilot reported that he had the 737 in sight. While the crew of the 737 had observed a traffic symbol on the TCAS display, they reported that they did not see the Lancair, despite attempts to do so.

Information obtained from the crews of both aircraft, the ATS controller, recorded flight data from the 737, ATS audio recordings and radar data, was consistent and indicated that the crews of both aircraft and the ATS controller complied with the published procedures for Class E airspace under NAS.

Based on the factual data, and the definition contained in Regulation 2.2 of the Transport Safety Investigation Regulations 2003, the incident was classified as an airprox event.¹⁵

¹ 16,500 ft with an altimeter pressure sub-scale setting (QNH) of 1013.2 hPa.
² The 24-hour clock is used in this report to describe the local time of day, Eastern Standard Time (EST), as particular events occurred. Eastern Standard Time was Coordinated Universal Time (UTC) + 10 hours.
³ Aeronautical Information Publication (AIP), ENR 1.1, Sections 3.4 and 18.3.2.
⁴ Radar Information Service (RIS) is defined in Part 10, Section 1 of the Manual of Air Traffic Services as:

An add-on ATC service within radar coverage, which provides information to flights, not otherwise receiving a separation service, in order to improve situational awareness, and assist pilots in avoiding collisions with other aircraft.

⁵ AIP, ENR 1.4, Section 2.1.4 and pages 12-13 and 40 of the NAS Implementation Group Reference Guide - How to Operate in the National Airspace System, effective from 27 Nov 2003.
⁶ Separation is defined in Part 10, Section 1 of the Manual of Air Traffic Services as:

A controlled condition using defined standards to prevent collisions between aircraft.

⁷ The Boeing 737-NG Operations Manual, Volume 2, states that:
 

TCAS alerts the crew to possible conflicting traffic. TCAS interrogates operating transponders in other airplanes, tracks the other airplanes by analysing the transponder replies, and predicts the flight paths and positions. TCAS provides advisory, flight path guidance, and traffic displays of the other airplanes to the flight crew. Neither advisory, guidance, nor traffic display is provided for other airplanes which do not have operating transponders. TCAS operation is independent of ground-based air traffic control.

⁸ A traffic advisory (TA) is generated when the other aircraft is approximately 40 seconds from the point of closest approach, dependent upon aircraft altitude.
⁹ If the other aircraft continues to close, a resolution advisory (RA) is generated when the other aircraft is approximately 25 seconds from the point of closest approach, dependent upon aircraft altitude. The RA provides aural warning and guidance as well as manoeuvre guidance to maintain or increase separation from the traffic.
¹⁰ The recorded ATS radar data and the 737 flight recorder data was consistent. However, the accuracy of that information is dependent upon the tolerances associated with both aircraft altimeters and the ATS radar equipment.
¹¹ The horizontal distance was determined using the radar positions that were recorded every 5 seconds. By interpolating between those points to derive a position every second, the minimum horizontal separation was 0.3 NM, dependent upon the tolerances previously mentioned.
¹² Civil Aviation Regulations 1988, 163A - Responsibility of flight crew to see and avoid aircraft.
¹³ Page 8 of the NAS Implementation Group Reference Guide - How to Operate in the National Airspace System, effective from 27 Nov 2003.
¹⁴ The investigation was unable to locate a CAR definition of '…well clear…'.
¹⁵ An airprox event is defined in Regulation 2.2 of the Transport Safety Investigation Regulations 2003, as:

…an occurrence in which 2 or more aircraft come into such close proximity that a threat to the safety of the aircraft exists or may exist, in airspace where the aircraft are not subject to an air traffic separation standard or where separation is a pilot responsibility.