Separation issue

1. The flight crew and air traffic control staff did not brief effectively before the calibration flight.
2. The Astra crew was unaware that Brisbane Centre was responsible for management of a portion of R577.
3. The Astra crew was unable to maintain continuous two-way communications with flight service international.
4. The provision of radar coverage and use of secondary surveillance radar transponders in oceanic airspace assisted in the resolution of the conflict.

The briefing planned between the Astra crew and Williamtown Air Traffic Control staff did not occur and as a result, the air traffic control staff were not fully conversant with the Astra crew's intentions. Also, the Astra crew were unaware of the unique delineation in airspace management responsibilities for the area where they intended to operate. These aspects made it difficult for the crew to adequately prepare for contingencies and for air traffic controllers to understand the complex nature of the calibration task.

The Astra crew complied with the clearance issued by Williamtown Air Traffic Control. However, the clearance was issued without air traffic control being fully conversant with the flight profile. The misunderstanding was compounded by the Astra crew believing that their flight would be managed by Williamtown Air Traffic Control. If the crew had been aware of the ramifications of the Jordy Release, it was likely that they would have appreciated that the initial stages of the flight would pass back and forth between R577 and non-controlled airspace. Had they been aware of this fact and that consequently, they were responsible for their own traffic avoidance, the Astra crew may have ensured that air traffic services were advised of the intention to change level well in advance of commencing the climb to FL200. If the crew had been able notify flight service international of their intention to climb from 10,000 ft to FL200 it was probable that they would have been issued with traffic information on the Dash 8 and the conflict may not have occurred.

The operation of the Astra's transponder on the assigned code assisted in the resolution of the situation. An alternative for the crew was to select the transponder to the radio failure code of 7600, which was part of the aviation safety net. Given the complex nature of the task and the communication difficulties being experienced by them at the time, this action may have provided a more timely alert to air traffic agencies. Had air traffic agencies been alerted by the change in code to 7600, it is likely that air traffic control or flight service international officers would have either provided traffic information or initiated action to separate other aircraft in the vicinity of the Astra.

The Brisbane Ocean sector controller saw on radar an unidentified aircraft climbing through flight level (FL)180. The controller issued traffic information to the crew of a Dash 8 operating an instrument flight rules (IFR) flight from Lord Howe Island to Sydney at FL200. The crew of the Dash 8 sighted the unidentified aircraft at an approximate distance of 5 NM and 1,500 ft below them. The two aircraft passed with 3 NM lateral separation at the same level. The unidentified aircraft was subsequently identified as an Astra conducting an IFR calibration flight. It appeared that the crew of the Astra had not complied with Aeronautical Information Publication (AIP) procedures for changing levels in non-controlled airspace. These procedures require pilots of IFR category aircraft operating in non-controlled airspace to notify the relevant air traffic agencies of any changes in altitude/level. Investigation of the occurrence revealed that the Astra crew were not notified of a change in airspace management for their area of operations.

The Astra crew was conducting a calibration of a new radar that had been installed at Williamtown aerodrome. This required the aircraft to track outbound from Williamtown at 1,500 ft, in a direction clear of local operating areas and air routes, until radar coverage was lost and then return on the reciprocal track while climbing. Once the next altitude or level was reached the crew would immediately turn the aircraft to track outbound again. The manoeuvre was to be repeated at 10,000 ft, FL200, FL300 and FL400 and would conclude with the aircraft completing the task at approximately 220 NM from Williamtown.

The Astra crew had prepared and dispatched written briefs on the task to Williamtown and Brisbane Air Traffic Control (ATC) agencies. However, the calibration task schedule was amended and subsequently conducted 24 hours earlier than originally notified. The brief depicted the calibration flight as being a series of steps with an increase in altitude/level as the distance from Williamtown increased but did not depict that the aircraft would fly the reciprocal track at any stage. After the Astra had departed Williamtown to commence the calibration, the Williamtown approach coordinator briefed the Brisbane Flight Service International officer on the task, based on the briefing and other details as subsequently advised by the crew.

The general practice of the Astra crew was to conduct personal briefings with the responsible air traffic control staff on the day of a calibration task and this was their intention on the day of the occurrence. However, when the Astra arrived at Williamtown, the senior air traffic control officer who had planned to attend the briefing was not available. Consequently, the crew only briefed a technical ground party.

The Astra crew was issued a clearance by Williamtown Air Traffic Control to track via the 100 tactical air navigation aid radial on climb to FL210. On this track the Astra would transit Williamtown restricted area R577 which was promulgated as continuously active, 24 hours a day. The intended track of the Astra entered and exited R577 respectively at 25 NM and 130 NM east of Williamtown. The vertical limits for the area are from sea level to FL600. When not required for military operations a notice to airman (NOTAM) deactivating R577 for specific periods was normally issued by Williamtown Air Traffic Control. A deactivation NOTAM was not issued for the period of the calibration flight.

As an aid to airspace management, Brisbane and Williamtown Air Traffic Control agencies had agreed to automatically release portions of R577 and other restricted areas to Brisbane when the restricted areas were active. This agreement was published in the Northern New South Wales Manual of Air Traffic Services Supplement (MATS SUPP) which in turn was part of both Williamtown and Brisbane operating procedures. The agreement effectively transferred airspace management responsibility for the south-east portion of R577, known as the Jordy Release, from Williamtown Air Traffic Control to Brisbane Centre. The planned track of the Dash 8 passed through the Jordy Release.

The lateral and vertical limits for Australian airspace were published in the Airservices Australia Designated Airspace Handbook (DAH) and depicted on AIP charts. The DAH, charts or a NOTAM can be used for flight crew pre-flight briefing. None of the documents included the MATS SUPP provisions.

A letter of agreement between Brisbane Ocean sector, Brisbane Flight Service International and Sydney Flight Service 4 detailed the airspace management responsibilities for the Jordy Release when transferred to Brisbane Centre. The division of responsibilities was:

• Ocean sector
  That portion of the release above FL200 between 90 NM and 150 NM from Sydney and above FL245 outside of 150 NM Sydney,
• Flight Service International
  That portion of the release at FL200 and below that level outside of 90 NM from Sydney, and
• Sydney Flight Service 4
  That portion of the release below the control area steps inside 90 NM from Sydney.

The Ocean sector controller was required to separate IFR aircraft as if they were in controlled airspace. The officers of both flight service units were required to issue traffic information to pilots of IFR category flights, just as they would have for pilots of IFR flights in non-controlled airspace, in accordance with the Manual of Air Traffic Services (MATS) criteria. Part of the MATS criteria required traffic information to be issued when aircraft were climbing or descending through the level of another aircraft when these aircraft were within 15 NM or 10 minutes longitudinally or laterally. Ocean sector uses radar and procedural control methods to separate aircraft. Radar coverage extends to beyond the eastern limit of R577. Neither of the flight service units had access to radar services.

Because of the flexible airspace management arrangements for the Jordy Release, flight service international officers usually pre-empted requests for confirmation of the availability of planned routes for aircraft inbound to Sydney. Normal practice was to advise crews as early as possible when the area was active with military operations to enable them to commence tracking via alternative routes. The crew of the Dash 8 was not advised that their planned route was unavailable. The crew was subsequently advised to contact the Ocean controller at 140 NM from Sydney for a clearance.

The Astra crew reported that due to R577 being active, they believed that, their flight would be managed by Williamtown air traffic control. The Williamtown coordinator advised flight service international that the crew had been cleared to FL210 but that initially they would maintain 1,500 ft. The coordinator subsequently advised flight service international that the Astra had climbed to 10,000 ft. The Astra crew was instructed that they were leaving the Williamtown restricted area and to transfer to flight service international on high frequency (HF) radio. This was required as they were operating in that portion of R577 being managed by flight service international.

After several attempts, the Astra crew contacted flight service international on HF radio and was advised of another HF frequency to use if they were having communication difficulties. There was no further contact with the crew on HF radio. The flight service international officer attempted unsuccessfully to contact the crew. The problems with the HF radio communication were believed to be the result of propagation difficulties. Once it was apparent to the crew that HF communication was not viable they tried to establish communications with Brisbane Centre via VHF. The crew reported after the occurrence that because they were unable to advise any air traffic service agency of the next climb, from 10,000 ft to FL200, they broadcasted their intention on the VHF emergency frequency, 121.5 MHz. The investigation could not establish whether other crews heard the transmission.

The crew contacted the Nambucca Sector Controller and reported that their position was 120 NM east of Williamtown on the 100 radial and that the aircraft had left 10,000 ft on climb to FL200. The Nambucca controller confirmed that the Astra was tracking west and that at 110 NM from Williamtown the crew would commence a turn to track outbound. The Nambucca controller contacted the Ocean controller to report the Astra's position. At approximately the same time the Ocean controller was issuing a clearance to the crew of EA261 and shortly after noticed an unidentified aircraft approaching the Dash 8. The Nambucca controller issued traffic information to the crew of the Astra, who arrested the climb of their aircraft and then descended to FL185.

The Astra crew did not select the radio failure code of the aircraft's secondary surveillance radar transponder. The investigation did not establish why the crew did not change the transponder code.

FNB should have been reported as potential traffic to the crew of FNA since its track was planned to be within 15 NM laterally of FNA's route and because it was climbing through FNA's level.

The flight service officer reported that he would have normally passed traffic information to the pilots when FNB taxied at Carnarvon. However, on this occasion, he did not recognise that the two aircraft were potentially conflicting traffic. Having calculated a release time of 0225 when he expected FNB to be above FL200, in controlled airspace and therefore clear of FNA, he may have assumed that the traffic had no potential for conflict. An indication of his assumption was that he filed the flight strip for FNB soon after the release time; therefore, the potential for conflict of these two aircraft was never recognised by the flight service officer.

The flight service officer did not have a history of inadequately passing appropriate traffic information although he had, on occasion, used inappropriate air traffic control procedures.

The incident probably occurred as a result of a combination of factors. Firstly, the flight service officer reported that he was pre-occupied with his personal situation and was tired. As a result, he probably did not adequately monitor the progress of his routine actions. Secondly, he reported that he might have used a presumed rate of climb for the Fokker 50 as a basis for determining whether the traffic was potentially conflicting. Such a calculation was erroneous and irrelevant because the traffic assessment criteria in the Manual of Air Traffic Services required the traffic information to be passed. The use of this calculation may have been a manifestation of the flight service officer's occasional use of inappropriate procedures. His pre-occupation, tiredness and deviation from standard operating procedures may have contributed to the flight service officer losing situational awareness and as a result, he did not recognise that the flight paths of FNB and FNA were potentially conflicting.

Once the flight service officer had committed to not advising the crews of FNB and FNA of each other's presence, there were no organisational defences available, such as active supervision, to preclude the mistake going unnoticed. As a result, once the flight service officer had made the error, there was no backup.

Because the pilot in command of FNB did not need to take any avoiding action, it is unlikely that the provision of TCAS stopped an accident from occurring. However, the fact that TCAS had alerted the crew of FNB to the close proximity of traffic unknown to them indicated that it would have been the only defence available had the two aircraft been on a collision course. The effectiveness of TCAS in alerting and directing the crew of FNB to proximal traffic also illustrated the weakness of the principle of see-and-avoid.

CONCLUSION

The flight service officer was fatigued and distracted, probably as a result of stress and inadequate rest before commencing his shift. Consequently, he did not adequately monitor the progress of his routine actions and he did not notice that the two aircraft were potentially conflicting traffic. He did not provide traffic information to the pilots of the two aircraft as required by Manual of Air Traffic Services.

Sequence of events

At 0208 coordinated universal time (UTC) VH-FNB, a Fokker 50, departed Carnarvon for Perth southbound via air route B469 and was climbing to FL210. The two-way air route was a direct track between Carnarvon and Geraldton . VH-FNA, also a Fokker 50, had departed Geraldton for Learmonth and was tracking via the B469 northbound. The crew of FNA had reported to Perth Flight Service at 0201 that the aircraft was on descent to FL180 from FL200. At 0213, they reported at position HAMEL and were estimating overhead Carnarvon at 0246. HAMEL was a reporting point located 120 NM south-south-east of Carnarvon. FNB's estimated time of arrival at HAMEL was 0242.

About 65 NM south-south-east of Carnarvon and approaching FL180, the co-pilot of FNB glanced down at the traffic alert and collision avoidance system (TCAS) display and noticed a return in the 11-o'clock position at about 7 NM at the same level. The pilot in command of FNB, aware that FNA was likely to be in the area, communicated with the crew of FNA and requested their position. At 0226, as the pilot in command of FNA replied, the pilot in command of FNB saw FNA pass approximately 400 ft below his aircraft on a reciprocal track. FNA was not fitted with a TCAS and the crew of FNA did not see FNB. Flight service had not directed traffic information to either crew.

Flight Service procedures

Both aircraft were outside controlled airspace and operating in a directed traffic information environment in which air traffic control does not provide positive separation between air traffic. However, flight service was required to provide aircraft operating under instrument flight rules information on other possibly conflicting military or instrument flight rule traffic. The requirements and parameters for issuing traffic advice by flight service are in the Manual of Air Traffic Services. The procedures in the manual required the flight service officer to provide traffic information to the crews of both FNA and FNB about each other's flight routes because both aircraft were going to be within 15 NM of each other laterally and FNB was climbing through FNA's level.

When aircraft are planned to depart non-controlled airspace and enter controlled airspace, the flight service officer calculates a release time, which is the time at which the flight service officer expects the pilot of the aircraft to be communicating with air traffic control. When air traffic control receives notification from flight service that an aircraft will be entering controlled airspace, the controller calculates a time of acceptance when the aircraft is expected to be on frequency. Neither the release nor acceptance times are coordinated between flight service and air traffic control.

Flight Service Officer actions

The flight service officer was managing the combined sectors of Flight Service 1 and 8 at the time of the incident. Flight service team leaders were responsible for the administration of shifts. They did not provide active supervision of flight service officers at their work positions.

At the time of the incident, the flight service officer was responsible for monitoring 13 VHF and 8 HF frequencies, and 15 aircraft. Of the 15 aircraft, approximately 10 were active and the remainder were pending. Consequently, the flight service officer and his supervisor reported that the workload was considered light to moderate.

The flight service officer reported that he had assessed that after departure it would take FNB 15 minutes to climb through FL200 into controlled airspace. He therefore calculated that FNB would be in controlled airspace at about 0223 and he rounded up his expected release time to 0225. Air traffic control reported that they added a standard 15 minutes to the departure time advised unless the aircraft was known to have a low performance climb, such as the Fokker 50. In this case, 20 minutes was added. Therefore, the expected on-frequency time for FNB would have been 0228 and in accordance with standard practice, communications checks would have commenced by 0231.

The flight service officer reported that he would have normally passed traffic information to the pilots of both aircraft when FNB taxied at Carnarvon. The flight service officer was unable to explain why he did not believe the two aircraft were potentially conflicting. Following the incident, the flight service officer reported that he thought the Fokker 50 would have climbed at about 1,500 ft/min, a figure he reportedly derived from a previous conversation with one of the operator's pilots. Advice from the operator indicated that it was highly unlikely that a Fokker 50 could achieve such a rate-of-climb, particularly at the flight levels at which FNB was operating. The local flight service management reported that flight service did not advocate calculations based on rate of climb as a traffic assessment method.

The flight service officer reported that he filed the flight strip for FNB soon after the release time of 0225, believing the aircraft to be clear of non-controlled airspace.

Human factors

The flight service officer had received some air traffic control training and reported that he had previously used air traffic control techniques to determine whether traffic was in potential conflict. The use of air traffic control techniques was not required or advocated in flight service procedures. The flight service officer had passed a performance check the day before, when the checking officer noted him using some air traffic control procedures to decide what traffic information should be provided. The supervisor counselled him at the time not to use such procedures. The flight service officer reported that he had been using air traffic control techniques for some time to determine what information needed to be passed to aircraft operating outside controlled airspace in an attempt to reduce the amount of, what he considered, unnecessary radio calls. Although the flight service officer had used unapproved procedures in the past, he did not have any history of providing inadequate or inappropriate traffic information. During the investigation, it was reported that other flight service officers who had undergone some air traffic control training were also known to have applied air traffic control procedures in the flight service environment.

The flight service officer reported that Airservices Australia had managed his career within the previous 2 years in such a manner that caused him concern because there was substantial uncertainty related to his future employment. The flight service officer also reported that as a result of this uncertainty, he was facing significant personal issues and was probably pre-occupied by these issues when he started the shift on the day of the incident. He had approached his local management prior to the incident for leave and the request was being processed at the time of the incident.

The flight service officer reported that he was uncertain about his future employment and as a result of this uncertainty he had only a small amount of sleep the night before his shift and was feeling tired at the time of the incident. The flight service officer's immediate supervisor reported that he was unaware that the flight service officer may have been experiencing significant personal stress or that the flight service officer was fatigued when he commenced duty.

Traffic alert and collision avoidance system

Both aircraft belonged to the same operator. The operator was introducing TCAS as each aircraft underwent a major servicing. FNB was the only aircraft in the operator's fleet that had been fitted with TCAS and not all of the operator's pilots had been trained in the use of TCAS. The operator's policy was that unless both pilots in a crew were qualified to use TCAS, then the equipment was only to be used in the traffic advisory (TA) mode. A TA is indicated on the equipment display to the crew when the aircraft are within about 48 seconds of their closest passing, based on projections derived from current flight path and speed.

The pilot of the Chipmunk did not see the Metroliner. It is likely that the pilot commenced his take-off roll when the Metroliner was at least 3 NM from touchdown. At that distance, the Metroliner may not have been readily detectable by the Chipmunk pilot. His lookout for possible traffic would most likely have been directed towards the circuit and approach of runway 03 rather than towards runway 21 and he would not have been expecting to see traffic approaching from the opposite direction. With the aircraft in the level attitude during take-off, he would have been looking along the runway and his attention would have been focused on maintaining directional control. The Metroliner would most likely have been obscured from the Chipmunk pilot's view by the nose of his aircraft when he subsequently rotated to the climb attitude.

The crew of the Metroliner did not see the Chipmunk until that aircraft was almost overhead theirs. As they had not received any response to their broadcasts on the CTAF, they were relying on visual acquisition of any unalerted traffic as their only defence against a conflict. However, the Chipmunk was a small visual target with little relative movement. In addition, it had no anti-collision lighting and would initially have been below the Metroliner crew's horizon. The contrast between the background and the colour of the Chipmunk was minimal and would have made the aircraft difficult to discern.

The straight-in approach procedure at CTAF aerodromes did not appear to adequately address the limitations of unalerted "see and avoid" principles. The assumption of two pairs of eyes being more likely to detect unalerted aircraft than one pair of eyes, did not prove to be an adequate defence in this incident.

In this instance, the crew of the Metroliner elected to make a straight-in approach in wind conditions that most probably favoured a reciprocal runway direction. Without any response to their broadcast intentions, the crew probably assumed that there was no traffic and believed it was safe to use runway 21. As regulations in force at the time of the occurrence did not include a requirement for an airline operator to assess the circumstances and the likelihood of encountering non-radio traffic at individual locations, it was likely that many crews were conducting straight-in approaches as an expected routine. Had the crew been aware, for example, of the potential for greater numbers of recreational movements at weekends and on public holidays, they may have decided not to conduct the straight-in approach.

Although the company had provided a radiocommunication service to comply with the requirements for this procedure, its effectiveness to alert the crew to other aircraft was restricted by company procedures and by the physical location of the radio operator. The radio operator was not familiar with aircraft movements and had never been instructed to provide runway-in-use information. Had the radio operator been afforded a full view of the entire runway and the approach path of the Metroliner and permitted to issue traffic information, a timely warning may have been broadcast to the crew of the Metroliner about the presence of the Chipmunk.

The pilot of the Chipmunk chose to continue his flight without radio communication and without knowledge of a procedure that could place him in potential conflict with a passenger-carrying aircraft. Knowledge of scheduled aircraft movements at that location may have influenced the pilot to avoid commencing a non-radio flight when the arrival of a scheduled service was imminent.

Sequence of events

The crew of a Metroliner, conducting a scheduled passenger service from Sydney, reported that at 30 NM from Latrobe Valley, they obtained weather information from their ground agent on the common traffic advisory frequency (CTAF). They recorded the wind as light and variable. At approximately 17 NM, the co-pilot broadcast on the CTAF advising their intention to join for a 5 NM straight-in approach to runway 21. Two similar broadcasts were made, one at 10 NM and the other at 5 NM. At approximately 1 NM from touchdown, the crew saw a Chipmunk aircraft a short distance to their left and about 400 ft above them, travelling in the opposite direction. The crew continued their approach and landing as they considered that there was no further risk of collision.

The Chipmunk pilot was conducting a private flight. He reported that the aircraft radio had undergone maintenance prior to the incident flight and that he was unsure if a functional check of the radio had been made. He determined that the wind was a light north-easterly and elected to use runway 03. He broadcast on the CTAF that he was taxiing, and made a further transmission advising that he was backtracking along runway 03. He did not hear any response to either broadcast and had not heard the aerodrome frequency response unit. He assumed that his aircraft radio was not operating and proceeded with the flight without making any further radio broadcasts.

The pilot of the Chipmunk reported that he did not see any aircraft either when entering the runway or when lining up for takeoff. He departed and climbed away, unaware that the landing Metroliner had passed below him. Although he was aware that scheduled flights operated into the aerodrome, he was not familiar with those schedules. He was also unaware that such aircraft could conduct straight-in approaches to CTAF aerodromes. He reported that although his aircraft was equipped with a landing light, he was not in the habit of using it in daylight conditions.

Witness information

An instructor at the aerodrome reported that, at the time of the occurrence, the Chipmunk and the Metroliner were the only aircraft in the circuit. He added that other aircraft movements that day had been made from runway 03.

Following this incident, another instructor observed the straight-in approach procedure under similar conditions. He stationed himself at a position similar to that of the Chipmunk pilot at takeoff in order to determine the visibility of the approaching Metroliner. He reported that when the crew broadcast their 5 NM position, he was only able to see the aircraft after several seconds of looking. However, at an estimated 3 NM, with the landing gear extended and the taxi light illuminated, it was much easier to see the aircraft.

Weather

Weather conditions at the time of the occurrence were described as fine and clear with good visibility. Witnesses on the ground reported the wind as 5-8 kt from the north-east. Data recorded from the automatic weather station (AWS), indicated that a north-east wind had prevailed from 1100 Eastern Standard Time (EST) onwards. The Latrobe Valley METAR (meteorological observation) at 1304 EST indicated that the wind direction was 070 degrees magnetic and the windspeed was 7-10 kt. Those conditions favoured the use of runway 03.

Straight-in approach procedures

On 26 March 1997, the Civil Aviation Safety Authority permitted multi-crew regular public transport aircraft to conduct straight-in approaches to non-controlled aerodromes other than those within a mandatory broadcast zone. Effective from 1 May 1999, amendment 35 to the Civil Aviation Orders (CAO) part 82.3 incorporated a new sub-section 5A that required the provision of a ground-based radiocommunication service at aerodromes where straight-in approach procedures were conducted. However, following industry concerns about the operator's liability in relation to the provision of traffic information and the cost to airlines of providing such a service, that section was amended. CAO amendment 41 was issued with a new sub-section 5B that restricted the information to be given by the radiocommunication service to wind direction and runways in use at the aerodrome.

The company operating the Metroliner contracted the services of a ground-handling and booking agent to perform the required radiocommunication service, and personnel were trained and approved to provide that service. The procedure required the agent's staff to obtain weather information from the AWS and, when requested, broadcast this information to company pilots on the CTAF. Information about other traffic or the runway in use was not transmitted.

The agent advised that crews could obtain traffic and runway information from other aircraft on the CTAF. Other duties performed by the agent's staff precluded them from continuously monitoring the CTAF for information about other traffic. Additionally, the company radio was situated in the airport terminal and only a limited view of the airfield was afforded through the windows. Visual observation of aerodrome traffic from this location was not possible. The Chipmunk would not have been visible from this vantage point at any time it was taxiing or airborne.

There were no specific company instructions for crews making straight-in approaches at CTAF aerodromes. Company management personnel were satisfied that the procedures contained in the Aeronautical Information Publication were adequate. Company standard operating procedures required the landing and recognition lights to be turned on as aircraft were approaching 10,000 ft when transition checks were being completed. The taxi light was not to be turned on until after the landing gear was extended. The crew reported that that procedure had been followed.

The pilot in command advised that the crew's preferred choice of runway direction at Latrobe Valley on the flight from Sydney was runway 21. This not only reduced flying time but also allowed crews the 3-minute cooldown period for the engines while backtracking to the terminal. It also avoided running the engines near the terminal and creating unnecessary noise.

Determination of the runway in use, as required by the Aeronautical Information Publication (AIP) procedure, was assumed by crews to be either the reported runway being used by other traffic in the circuit or, in the absence of other traffic, runway 21, provided that wind conditions were favourable. The view expressed by the pilot in command was that the straight-in approach procedure offered a better level of safety than that provided by the normal circuit entry and that it was more expedient. He believed that the procedure resulted in less circuit manoeuvring and a reduced risk of traffic conflicts.

Aerodrome traffic

This incident occurred on a Sunday afternoon. The airline operated six services per week into Latrobe Valley but only one of these flights was conducted on a weekend. The pattern of activity at the aerodrome was not recorded however, staff at the Aero Club were able to confirm that most recreational flying activity took place at weekends and on public holidays. Only a small number of aircraft operating from the aerodrome were not radio-equipped. The scheduled movements at Latrobe Valley were not published in any of the aeronautical publications.

1. The crew of the Jetstream did not hear the King Air crew's inbound broadcast on the mandatory broadcast zone frequency.
2. The crew of the King Air did not hear the Jetstream crew's taxi broadcast on the mandatory broadcast zone frequency; nor did they hear the transmissions made on the Brisbane control frequency by the air traffic controller that provided essential traffic information regarding the Jetstream, and instructed them to maintain 6,000 ft.
3. The air traffic controller had insufficient time to establish communications with both crews and provide them with sufficient information to enable them to take action to prevent a near collision.
4. The crews had not been alerted to the presence of each other's aircraft.
5. The procedures used in the Demonstration Class G Airspace Trial, which encompassed the Williamtown MBZ and MTA, did not fully consider the impact of radio congestion.

The incident occurred because the crew of the King Air had not been alerted to the possibility of conflicting traffic and continued their descent for Williamtown in the Class G demonstration airspace. The crew had been alerted about another aircraft, a Beech 1900, which was to the north of Williamtown. The air traffic controller believed that the crew of the King Air had been alerted regarding the Jetstream. The controller assumed that he had heard a read back of the level requirement, when in all probability, the controller heard the "two in together" transmission. He also assumed that the King Air would stop descending and maintain 6,000ft. These assumptions were unfounded.

Analysis of the incident highlights problems involving radio communications, attentional focus, workload, and Class G Airspace training and procedures.

Radio communications

The crew of the King Air did not hear the taxi transmission broadcast by the Jetstream crew on the mandatory broadcast zone frequency; nor did they hear the transmissions made on the Brisbane control frequency by the air traffic controller that provided essential traffic information regarding the Jetstream and instructed them to maintain 6,000 ft.

When the crew of the Jetstream made their first "all stations" broadcasts on the national advisory frequency and the mandatory broadcast zone frequency, the King Air was in Class C airspace and its crew were communicating with air traffic control. The crew of the King Air had no reason to be on the national advisory frequency or the mandatory broadcast zone frequency and could not hear the Jetstream's transmissions.

The crew of the Jetstream reported that they were not aware that communication between themselves and the air traffic control agency could be established while on the ground at Williamtown. The establishment of such communication may have alerted the controller to the potential conflict between the two aircraft much earlier than was the case.

Attentional focus

Throughout the sequence of events for this particular occurrence, when monitoring more than one frequency, the crews had to decide upon which frequency to maintain their primary focus in the face of competing cognitive demands.

The crew of the King Air selected and monitored the mandatory broadcast zone frequency at approximately 31 NM south-west of Williamtown, when the aircraft was passing approximately FL 150. This was entirely reasonable, and accorded with the requirements of AIP. (Annex B to AIP Supplement H48/98, paragraph 5.1, and AIP Supplement H66/98 required crews to monitor the mandatory broadcast zone frequency when below the 8,500 ft upper limit of the mandatory transponder area, except when receiving a radar information service.) However, the focus of the crew's attention would have been centred on the transmissions made on the Brisbane Control frequency.

When the crew of the King Air were cleared by Brisbane to leave the control area on descent, their attention would have been focussed on receiving their clearance and providing an accurate readback to air traffic control. Their focus would not have been on monitoring the mandatory broadcast zone frequency. Accordingly, the crew of the King Air did not hear the Jetstream's departure report, which was being broadcast on the mandatory broadcast zone frequency at that time. Moreover, the Brisbane controller's transmission on the control frequency of 126.9 MHz was the only transmission that linked the King Air with the Williamtown mandatory broadcast zone.

The crew of the Jetstream were broadcasting their departure report on the mandatory broadcast zone frequency using their VHF 1 radio. Their attention would have been focussed on their transmission and on handling the aircraft. They had the Brisbane Control frequency of 126.9 MHz selected on their VHF 2 radio, but their focus would have been on their transmission on the mandatory broadcast zone frequency on VHF 1. The crew of the Jetstream did not hear the transmission that "linked" the King Air with the Williamtown mandatory broadcast zone. Such a link may have assisted the crew of the Jetstream to develop a better mental model of the air traffic situation.

Although the crew of the Jetstream were monitoring the Brisbane Control frequency of 126.9 MHz for over 4 minutes prior to their departure from Williamtown, insufficient information was broadcast on that frequency for the crew to develop an accurate picture of the air traffic situation. Notwithstanding the monitoring of the control frequency, the crew's attentional focus during their taxi and departure would have been on any transmissions made on the mandatory broadcast zone frequency.

When the King Air crew confirmed that they were still on the control frequency, the controller gave them traffic information on a Beech 1900. The King Air was passing 8,700 ft on descent when this traffic information was received and focus of their attention was shifting towards the establishment of mutual separation with other aircraft in the mandatory broadcast zone.

The crew of the Jetstream reported their position to Brisbane Centre passing 4,000 ft on climb. During the period of the Jetstream's transmission, the crew of the King Air was making an "all stations" broadcast on the Williamtown MBZ frequency. The crew of the Jetstream did not hear this broadcast because their attention was focussed upon making their report, and reading back the clearance. The crew of the King Air did not hear the Jetstream's transmission because they were concentrating on arranging mutual separation in the mandatory broadcast zone with the Beech 1900.

Weather

The meteorological area forecast for Williamtown and the trend type forecast were not favourable and indicated that an instrument approach would be necessary for aircraft inbound to Williamtown. The actual weather was consistent with the forecast.

Class G airspace

Contributing to the incident was the confusion on the part of aircrew concerning the requirements of the Demonstration Class G Airspace trial. Also contributing, was the fact that the design of the Class G procedures encompassing the Williamtown mandatory broadcast zone and mandatory transponder area did not fully consider the impact of radio congestion.

During the Class G airspace trial, there was some confusion within the aviation community regarding the termination of radar services. The Civil Aviation Safety Authority reviewed the first fortnight of the trial and as a result clarified some of the procedures in "Aiming Higher", vol. 1 no. 8, October-November 1998. In the section "Termination of radar services by Air Traffic Control (ATC)", the article explained that when an aircraft receiving a radar control service leaves controlled airspace and the radar service continued as a radar information service, ATC would advise "Control service terminated". Further, ATC would advise "Radar service terminated" when a radar information service was terminated.

The air traffic controller's transmissions reflected this aircrew confusion. Indeed, his workload was increased because of deficiencies in the education program provided to the aviation community prior to the introduction of the Demonstration Class G airspace. Also, although the crew of the King Air complied with all of the provisions of the AIP, they did not fully understand the procedures to be used when operating in the Demonstration Class G airspace.

The design of the procedures used in the Demonstration Class G airspace did not fully consider the impact of radio congestion. Even though the controller attempted to separate the two aircraft, he was not required to do so in Class G airspace.

1. A conditional clearance authorising the surface movement controller to clear CZH to cross runway 34 was issued to the surface movement controller by the aerodrome controller.
2. The aerodrome controller's training officer was not aware that a conditional clearance was active when he instructed the aerodrome controller to clear EAL for take off.
3. There was no tactile memory marker alerting the controllers that an aircraft had been cleared to cross an active runway.
4. The aerodrome controller did not scan runway 34 before issuing the take-off clearance.
5. Neither the aerodrome controller nor the training officer cancelled EAL's take-off clearance when they became aware that CZH was crossing runway 34.

The investigation revealed that the local practice for issuing a conditional clearance for a taxiing aircraft to cross an active runway might have been deficient. When a taxiing aircraft needed to cross an active runway, coordination was required between the surface movement controller and the aerodrome controller. If traffic conditions permitted, it may have been possible to issue the taxiing aircraft with an immediate clearance to cross the active runway. However, at other times, a taxiing aircraft may have been issued with a conditional clearance, authorising it, for example to cross the active runway after a landing aircraft had vacated the runway.

When a conditional crossing clearance had been issued, the continued safe operation of the system was dependent on both the surface movement controller and the aerodrome controller remembering that this traffic coordination had been arranged and was still pending. Interviews with controllers and observation of current work practices indicated that the coordination between the surface movement controller and the aerodrome controller for conditional clearances was verbal only. Neither controller used any form of memory aid to record the fact that a conditional clearance had been issued and was still pending. In this occurrence, this led the aerodrome controller, at a time of high workload and possible stress, to forget that a conditional crossing clearance was pending.

In addition, the training officer was not aware that a clearance had been coordinated between the surface movement controller and the aerodrome controller. Because the system did not provide any physical record that a conditional clearance was pending, there was no cue to alert the training officer to the fact that this was the case.

The use of memory markers recording actions by annotation or other means, assists controllers in remembering vital operational information in two ways. Firstly, the associated actions that the controller carries out in setting the memory marker assist in the consolidation of a strong memory trace. Secondly, the marker can be checked at any time by the controllers if they are uncertain of the current status of the item or condition that it refers to.

1. The Beech pilot and the Saab crew did not hear the taxi or departure transmissions from each other on the MBZ frequency.
2. The air traffic control radar was unable to detect aircraft at an altitude that would have enabled the controller to provide pilots with timely traffic information.

The pilots of both aircraft were reliant on mandatory radio broadcasts to gain an awareness of other traffic operating within the confines of the MBZ. It is possible that they had simultaneously broadcast their respective taxi reports, thus blocking each other's transmissions. However, it is more likely that radio transmissions were inhibited by terrain shielding, resulting in neither crew being aware of the other aircraft while they taxied and departed. The demonstration procedures created a situation where pilots had one opportunity to become aware of other traffic. If they missed an MBZ broadcast, there was no other formal procedure to provide that information.

If a directed traffic information service had been in place at the time of this occurrence, a potential confliction would have been recognised by a third party. Traffic information would have been provided to the Beech pilot and the Saab crew, even if the MBZ reporting procedures had failed. In the past, the directed traffic information service provided a level of redundancy to the aviation system that was not available during the Class G airspace demonstration.

The radar information service was limited by the inability of the radar to detect aircraft at relatively low altitudes in that area. Despite attempts to obtain a radar advisory service as soon as possible after departing Lismore, the Beech pilot was unable to obtain traffic information from the radar controller. Supplementation of the radar information service with a directed traffic information service in such an area of limited radar coverage, would have enhanced the situational awareness of the respective flight crews. Alternatively, the provision of radar coverage from ground level to the lower limit of controlled airspace, together with a radar information service for taxiing aircraft, would have also minimised the potential for confliction.

A conflict was ultimately avoided because the pilots were using the air traffic control frequency. This was despite the fact that both aircraft were still operating within the MBZ and monitoring the MBZ frequency.