Separation issue

Had the pilot of the Strikemaster confirmed what fare paying passenger operations were expected for the period of the joy flight before leaving the terminal the occurrence possibly would have been prevented.

There were a number of following events that probably prevented the pilot of the Strikemaster either hearing or appreciating what was being broadcast on the common traffic advisory frequency (CTAF) by the pilot of the taxiing 717. It is possible that the pilot of the Strikemaster missed some broadcasts because of intermittent radio operation or reception anomalies. The investigation was unable to determine whether the initial missed calls were due to a technical problem or as a result of shielding of some of the other pilots' radio broadcasts. However, the coincident radio transmission from the pilot of the 737 to the pilot of the Strikemaster, as the pilot of the 717 reported that he was taxiing for runway 30, probably over rode the latter's radio transmission. Consequently, the pilot of the Strikemaster remained unalerted about the fact that after departure the 717 would track to the south. Had the pilot of the Strikemaster become aware at that stage that the 717 was going to track to the south he most likely would have amended his area of operation to avoid the 717's departure track.

The pilot of the Strikemaster had another opportunity to understand that the 717 may possibly conflict with his operations when the pilot of that aircraft broadcast intentions, when lining up and after departing. On lining up, the pilot reported his intention to turn left and following departure he reported that the aircraft was turning left to intercept the one six one non-directional beacon navigation aid track. At that time the pilot of the Strikemaster had commenced his manœuvres at an altitude that prior to 24 November 2006  would have been outside the CTAF. It is possible that because he thought he was outside the CTAF area, the pilot was less attentive to radio transmissions on the CTAF. This aspect combined with distraction due to passenger commentary and workload requirements may have prevented the pilot from appreciating the developing situation.

The pilot of the Strikemaster could have clarified the intention of the pilot of 717 when he saw the aircraft turning left instead of right, as he expected. Again however, his high workload in the conduct of the flight probably caused him to dismiss the issue as a low priority at that time, and the situation remained unresolved. Overall, the pilot's situational awareness of the 717's departure track was diminished.

The pilot of the 717 also had an opportunity to query the pilot of the Strikemaster regarding his operation. However, that lack of any query by the pilot of the Strikemaster, with respect to the 717 CTAF broadcasts, indicated that there was no apparent problem with the 717's intended track. Compliance with the radio broadcast procedures by all pilots did not prevent the occurrence.

The limited radar coverage was not a factor in the occurrence. The Brisbane Centre controller had sufficient information from the Strikemaster radar returns to provide traffic information about the Strikemaster to the pilot of the 717. That traffic information, in conjunction with information from the Traffic Alert and Collision Avoidance System (TCAS) fitted to the 717, assisted the pilot to prepare to avoid the Strikemaster when alerted by the TCAS. Similarly, had the pilot of the Strikemaster requested a radar information service (RIS) it is likely that the controller would have provided traffic information on the departing 717 to the pilot in time to avoid that aircraft's track. A request by the pilot of theStrikemaster for a RIS was a preventative risk control that was available subject to controller workload. While provision of a RIS might possibly increase the pilot's workload, it should, when used in conjunction with monitoring of the CTAF frequency, improve situational awareness.

The Strikemaster pilot's original intention to avoid routes likely to be used by other aircraft during a local flight was the preferred risk mitigator for the operation. However, on the day the pilot's lapse in not checking the expected operations of fare paying passenger flights to/from Newcastle Airport, his misperception about the 717's destination and the radio problems negated that mitigator. A near collision was prevented by the combined use of radar based traffic information and TCAS.

On 18 December 2005 at 0954 Eastern Daylight-saving Time ¹, shortly after departure from Williamtown (Newcastle) Airport, NSW, while operating under common traffic advisory frequency (CTAF (R)) procedures, the pilot of a Boeing Company 717-200 (717) aircraft reported receiving a traffic collision and avoidance system (TCAS) resolution advisory (RA). The other aircraft was subsequently identified as a British Aircraft Corp 167 Strikemaster aircraft. A review of recorded radar data showed that the RA activated when the 717 was 9 km south of the airport. The radar data, in conjunction with TCAS data from the 717, showed that the minimum horizontal distance between the aircraft was 1,500 m with the Strikemaster about 250 ft above the level of the 717. There had been an AIRPROX ².

The airspace surrounding the airport was non-controlled Class G designated as CTAF (R). The R indicates that the carriage and use of a radio is required for aircraft that operate into or out of the airport and that a pilot is required to monitor and broadcast on the CTAF frequency prior to, and within 10 NM of the airport. Pilots of aircraft operating in a CTAF (R) are required to make specific radio broadcasts when operating in the vicinity ³ of a non-towered airport. In visual meteorological conditions ⁴ (VMC) pilots use those broadcasts to visually identify and adjust flight paths to avoid other aircraft in the area of the airport.

Runway 30 was in use at Williamtown Airport and the Strikemaster departed about 3 minutes before the 717. At the time the traffic situation included a Cessna 150 being used to conduct circuit training, a Boeing Company 737 (737) was inbound from the south, a New Zealand Aerospace Ind Ltd CT4 was inbound from the south following a flight over Newcastle City, a Cessna 172 was inbound from the south and a Fairchild Metro 23 was taxiing for a departure to Sydney. Pilot reports of the weather indicated that it was VMC with little or no cloud present.

The Strikemaster is a two seat (side by side) ground attack jet aircraft that was initially developed as a training aircraft. The aircraft involved in the occurrence was civil registered and was being used to conduct a local area joy flight ⁵, including aerobatic manoeuvres, under the visual flight rules (VFR) up to 8,000 ft above mean sea level (AMSL). The pilot had extensive military jet fighter experience and during the flight was required to provide commentary to the passenger. The flight was one of three flight profiles that were available. While the three profiles were essentially standard, the intention was, when possible, to accommodate requests by a passenger for repeat or specific manoeuvres.

After the occurrence, the Strikemaster pilot reported it was the first flight for the day and that he normally checked what fare paying passenger flights were expected to depart and arrive during the period of a flight before he left the terminal. He reported that he would normally amend his area of operation to avoid any routes to be used by other aircraft operating to/from the airport. On the day of the occurrence, he did not check the expected flights but thought that the 717 was bound for Brisbane, Qld.

At 0943, the pilot of the Strikemaster broadcast on the CTAF frequency that the aircraft was taxing for runway 30 for a local flight. That radio call was heard by the crew of the 737 as 30 seconds later they called the Strikemaster pilot requesting his intentions on departure. That radio call was co-incident with another radio call on the frequency. The pilot of the Strikmaster did not respond to the radio call from the pilot of the 737. At 0944, the pilot of the 717 broadcast that the aircraft was taxiing to runway 30 for a departure to Melbourne and at the same time the pilot of the 737 called the pilot of the Strikemaster for a second time. The pilot of the Strikmaster did not respond to the second radio call from the pilot of the 737. The pilot of the 737 reported to the pilot of the 717 that the 737 was 14 miles from the airport. That radio transmission was acknowledged by the pilot of the 717.

At 0945, the pilot of the Strikemaster broadcast on the CTAF frequency that the aircraft was entering runway 30 in preparation for a departure with a 'left hand turn out climbing to eight thousand [ft] overhead the coast'. The Cessna 150 was on final for runway 30 and the pilot of that aircraft attempted twice to call the pilot of the Strikemaster and advise that the Cessna 150 was on short final. The pilot of Strikemaster heard the second radio transmission and manoeuvred the jet off the runway threshold to enable the Cessna 150 to conduct a touch and go. While waiting for the Cessna to land, the pilot of the Strikemaster confirmed the operation of the aircraft's radio with the pilot of another aircraft. He received a response from the other pilot indicating that the radio was operating okay. The pilot of the Strikemaster reported later that the aircraft's two radios operated satisfactorily for all other flights conducted that day.

At 0946, the pilot of the Strikemaster broadcast that the aircraft was lining up on runway 30. At 0948, the pilot of the Strikemaster broadcast that the aircraft was passing 2,500 ft AMSL on departure, on climb to 8,000 ft AMSL and was tracking to the coast.

The 717 was on a scheduled fare paying passenger instrument fight rules (IFR) flight to Melbourne Vic. At 0949, the pilot of the 717 broadcast that the aircraft was lining up runway 30 for a departure to the south. At 0951, the pilot of the 717 broadcast that the aircraft was airborne from runway 30 and that when the aircraft reached 1,500 ft AMSL it would turn left to intercept the 161-track reference the non-directional beacon (NDB) navigation aid. At 0953, while still monitoring the CTAF frequency, the pilot of the 717 reported to the Brisbane Centre sector controller that the aircraft had departed at five one and was on climb to flight level three six zero.

The pilot of the Strikemaster reported later that he saw the 717 airborne, upwind and turning left. At the time he could not understand why it was turning left if it was going to Brisbane. He lost sight of the 717 shortly after and continued with his flight. The pilot also advised that he was monitoring the CTAF but believed that the previous 5,000 ft upper limit for CTAF areas still applied. This was not the case, as new airspace procedures ⁶, implemented 24 November 2005, required pilots of aircraft in Class G airspace, within 10 NM of a CTAF (R), to monitor the designated frequency regardless of the aircraft's altitude.

The sector controller was aware of the departure of the 717 as The Australian Advanced Air Traffic Control System (TAAATS) had correlated the aircraft's flight plan with the secondary surveillance radar (SSR) code assigned to the flight as that aircraft passed 2,400 ft AMSL. The controller reported to the pilot of the 717 that there was a radar return of an unidentified aircraft 3.5 NM in his eleven o'clock position ⁷ (see Figure 1) and its altitude was an unverified ⁸ 6,000 ft AMSL. The controller also received a short-term conflict alert (STCA) on the radar display indicating that the 717 and the unidentified were in potential conflict. The unidentified aircraft was operating on SSR code 1200. That code is designated for use by pilots of aircraft operating a VFR category flight in Class G airspace. About 8 seconds later, the pilot of the 717 received a TCAS RA to adjust the rate of climb of the aircraft to no greater than 2,000 feet per minute. The pilot complied with the advisory and at 0953:40 the RA ceased.

Figure 1: Replay of recorded radar data at 0953:30 ⁹

After the occurrence, the pilot and copilot of the 717 reported that they were aware of the unidentified aircraft on the TCAS as their aircraft turned left to intercept the outbound track. They lowered the nose of the 717 to assist them in seeing the other aircraft and received the RA at about the same time. Analysis of the 717 flight data recorder information showed that immediately prior to the RA the aircraft's rate of climb decreased in conjunction with an increase in indicated airspeed from 230 kts to 250 kts.

The Brisbane Centre controller was required to provide traffic information to pilots of IFR category flights on other IFR category flights in non-controlled Class G airspace below 8,500 ft around Williamtown Airport. There is no requirement to provide traffic information on VFR category flights. When within radar coverage the traffic information service can be based on radar data.

A radar information service (RIS) is available on request to pilots of VFR category flights in non-controlled airspace, subject to air traffic control workload. The service is available to improve a pilot's situational awareness and to assist in avoiding other aircraft. To receive a RIS, a pilot of a VFR category flight must be in direct very high frequency radio communication with air traffic control and the aircraft has to have a serviceable SSR transponder.

Normally, TAAATS receives SSR data from the Williamtown Air Traffic Control radar located near the airport. That data enables near ground level coverage for flights at the airport. During November, there were problems with the Williamtown radar data received by Brisbane Centre and while the situation was being investigated, that data was not used. A notice to airmen (NOTAM) reviewing and re-issuing a previous NOTAM advised that Brisbane Centre radar information services and radar-based traffic information services below 6,000 ft in the vicinity of Williamtown Airport were unreliable due to limited radar coverage. That NOTAM was issued 26 November 2005.

1. The 24-hour clock is used in this report to describe the local time of day, Eastern Daylight-saving Time (EDT), as particular events occurred. Eastern Daylight-saving Time was Coordinated Universal Time (UTC) + 11 hours.
2. 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.
3. Aircraft are operating is in the vicinity of a non-towered airport if it is within a horizontal distance of 10 NM and within a height above the airport reference point that could result in conflict with operations at the airport.
4. Visibility greater than 5 km and aircraft are able to be flown 1,500 m horizontally and 1,000 ft vertically clear of cloud.
5. Operations are conducted in accordance with Civil Aviation Safety Authority, Civil Aviation Regulation 262AM, Limited category aircraft - operating limitations.
6. Aeronautical Information Publication (Supplement) H51/05 issued 24 November 2005.
7. A means to relate the direction of an aircraft relative to another aircraft - 12 o'clock is in front of an aircraft, 9 o'clock is to the left and 3 o'clock is to the right.
8. An aircraft's SSR Mode C readout has to be confirmed to be within 200 ft of a pilot reported level to be deemed to be verified.
9. The orange text box lists the distance between the aircraft plots (3.1 NM) and the bearing from the unidentified aircraft to the 717 (311 degrees M).

On 18 December 2005 at 0954 Eastern Daylight-saving Time, shortly after departure from Williamtown (Newcastle) Airport, NSW while operating under common traffic advisory frequency (radio) (CTAF (R)) procedures, the pilot of a Boeing Company 717-200 (717) aircraft reported receiving a traffic collision and avoidance system (TCAS) resolution advisory (RA). The other aircraft was subsequently identified as a British Aircraft Corp 167 Strikemaster aircraft.

A review of recorded radar data, after the occurrence, showed that the RA activated when the 717 was 9 km south of the airport. The radar data in conjunction with TCAS data from the 717 showed that the minimum horizontal distance between the aircraft was 1,500 m with the Strikemaster about 250 ft above the level of the 717. There had been an AIRPROX.

The investigation found that the Strikemaster pilot's intention had been to avoid routes likely to be used by other aircraft. However, on the day, the pilot misperceived the 717's destination and the Strikemaster had radio problems that led to a reduction in the pilot's situational awareness. A near collision was prevented by the combined use of radar-based traffic information and TCAS.

Following the occurrence, the operator of the Strikemaster reviewed and amended procedures, for flights conducted near Lismore and Williamtown Airports to enhance pilots' situational awareness. That review included the standardisation of aircraft radio operating procedures.

Brisbane Centre issued an operational note to controllers advising that the pilots of Strikemaster flights may request a radar information service and controllers are to be prepared to provide a discrete secondary surveillance code.

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.