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The crew of British Aerospace Jetstream 31, VH-TQJ, had flight-planned from Williamtown to Sydney via Mt McQuoid at flight level (FL) 120. The crew of Beechcraft Super King Air B200, VH-KCH, had flight-planned from East Sale, to Williamtown via Mt McQuoid at FL 250. Both aircraft were equipped with dual very high frequency (VHF) radios and Mode C transponders, which were operating at the time. A transponder is a radio device which, when triggered by a secondary surveillance radar signal, transmits a response that provides, when selected to mode C, altitude and positional data on a radar display for air traffic controller reference.

The routes flown by the two aircraft were within the Class G demonstration airspace detailed in the Aeronautical Information Publication (AIP) Supplement H66/98 of 5 November 1998. The route segment from Williamtown to Mt McQuoid included airspace inside the mandatory broadcast zone for the Williamtown control zone up to and including 5,000 ft. It also included the mandatory transponder area from 5,000 ft to 8,500 ft for a radius of 30 NM centred on Williamtown, and Class C airspace above 8,500 ft to the south-west. The crew of the King Air were descending on air traffic services route Whisky 170 (W170) on track from Mt McQuoid to Williamtown. W170 was a low-level two-way route. The crew of the Jetstream tracked to intercept W170 to Mt McQuoid after departing from Williamtown and were on climb.

Williamtown was listed in paragraph 4.3 of AIP Supplement H66/98, which required aircraft not receiving a radar information service to "monitor and, when required, use the frequency specified", which was 118.3 Megahertz (MHz), the Williamtown mandatory broadcast zone frequency when inside the Williamtown mandatory transponder area. The crews of both aircraft attempted to monitor and use the mandatory broadcast zone frequency while they were inside the mandatory transponder area.

The meteorological area forecast for Williamtown issued at 1800 Eastern Summer Time (ESuT) was for 3 octas of stratus at 1,000 ft, 6 octas of cumulus at 1,800 ft and 6 octas of stratocumulus at 2,500 ft. The trend type forecast included moderate turbulence below 5,000 ft from 1800 to 1900 ESuT, and from 1900 to 2100 ESuT the visibility was expected to reduce to 4,000 m in drizzle with broken cloud at 1,000 ft. The reported weather was a varied cloud base from 1,200 ft to 1,600 ft, with the tops 4,000 ft to 5,000 ft. A higher level cumulo-nimbus cloud cell was reported to the west of Williamtown.

While taxiing at Williamtown, the crew of the Jetstream made "all stations" broadcasts on the national advisory frequency and the mandatory broadcast zone frequency. The crew of the King Air did not hear these transmissions because they had not selected or transferred to those frequencies at that time, nor were they required to. The King Air was established in Class C airspace and the crew was communicating with the air traffic controller located in the Brisbane Area Control Centre.

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. AIP Supplement H66/98 required crews to monitor the mandatory broadcast zone frequency when below 8,500 ft, the upper limit of the mandatory transponder area, except when receiving a radar information service.

At 18:45.41, the crew of the King Air was cleared by Brisbane Centre to leave control area on descent, was given the area QNH, and informed that control services would terminate passing 8,500 ft. They were also advised that Williamtown was operating on mandatory broadcast zone procedures. The crew acknowledged this transmission. During this period, the crew of the Jetstream broadcast their departure on the mandatory broadcast zone frequency on their VHF 1 radio. On their VHF 2 radio, they had the Brisbane Control frequency of 126.9 MHz, selected, which they had been monitoring, together with the MBZ frequency, for four minutes prior to departure. The crew of the King Air reported that they did not hear the Jetstream's departure broadcast.

At 18:46.41, the controller asked if the crew of the King Air was still on the control frequency, and then provided the crew with traffic information regarding a Beech 1900 inbound to Williamtown from the north that was descending through 6,000 ft. The crew of the King Air acknowledged the traffic information. The King Air was passing 8,700 ft on descent when the traffic information was passed.

At 18:47.01, the controller attempted to provide the crew of the King Air with additional information about unidentified traffic (the Jetstream), 3 NM south-west of Williamtown. The transmission was over-transmitted by another aircraft and was unreadable. The crew of the King Air heard only the last few words of the transmission, and did not consider it relevant to their flight. The King Air was passing 8,500ft, which meant that it was leaving Class C controlled airspace and entering the Williamtown mandatory transponder area in the Class G demonstration airspace. During the high workload at that period of the flight, the crew was preparing for an instrument arrival into Williamtown due to the weather conditions. The controller did not follow up this "unacknowledged" transmission, because the crew of the Jetstream reported on the control frequency, its reported position identifying it as the previously unidentified traffic.

The mandatory broadcast zone at Williamtown was established in Class G (uncontrolled airspace). While within the mandatory broadcast zone, pilots were required to maintain a continuous listening watch on the mandatory broadcast zone frequency. They could arrange mutual separation within the mandatory broadcast zone. The frequency was not normally monitored by air traffic services. Pilots of IFR aircraft were required to report "Changing to Williamtown mandatory broadcast zone 118.3 MHz" when they were inbound to the Williamtown mandatory broadcast zone area.

At 18:47.24, the crew of the Jetstream reported their position to Brisbane Centre and that they were passing 4,000 ft on climb. The controller instructed the crew to squawk (transponder) ident, to squawk code 1201, and to maintain 5,000 ft due to inbound traffic. The squawk code and altitude restriction were read back correctly by the Jetstream crew, although the crew then inadvertently selected the wrong code of 1207. During the period of the Jetstream's transmission, the crew of the King Air was making an "all stations" broadcast on the Williamtown mandatory broadcast zone frequency. The Jetstream crew reported that they did not hear the broadcast.

At 18:47.56, the controller again attempted to provide traffic information about the Jetstream to the crew of the King Air, together with an instruction to maintain 6,000 ft. That transmission was over-transmitted by another aircraft and was not heard by the King Air crew. The over-transmission was confirmed by the crew of the Jetstream, who transmitted "two in together". The controller reported that he was "95% sure" that the King Air crew had read back "maintain 6,000 ft", which was not supported by analysis of recorded audio data. The transmission "two in together" was recorded. At the time of the controller's transmission, the Beech 1900 crew initiated communication with the King Air crew on the mandatory broadcast zone frequency and between them they subsequently arranged mutual separation.

 

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. 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 others' 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.
 

As a result of this incident and other occurrences, the Bureau of Air Safety Investigation undertook a systemic investigation of the Class G airspace demonstration. That investigation was part of the Bureau's normal systems safety investigation role. The Bureau of Air Safety Investigation released interim recommendation IR980253 on 9 December 1998, which recommended that the Civil Aviation Safety Authority (CASA) terminate the demonstration. The demonstration was terminated on 13 December 1998 by CASA. This investigation supported the decision to terminate the demonstration.

 
General details
Date: 16 November 1998 Investigation status: Completed 
Time: 1850 hours ESuT Investigation type: Occurrence Investigation 
Location   (show map):11 km SW Williamtown, Aero. Occurrence type:Separation issue 
State: New South Wales Occurrence class: Airspace 
Release date: 13 December 1999 Occurrence category: Incident 
Report status: Final Highest injury level: None 
 
Aircraft 1 details
Aircraft manufacturer: British Aerospace PLC 
Aircraft model: 3100 
Aircraft registration: VH-TQJ 
Serial number: 703 
Type of operation: Air Transport Low Capacity 
Damage to aircraft: Nil 
Departure point:Williamtown, NSW
Departure time:1848 hours ESuT
Destination:Sydney, NSW
Aircraft 2 details
Aircraft manufacturer: Beech Aircraft Corp 
Aircraft model: 200 
Aircraft registration: VH-KCH 
Serial number: BB-1125 
Type of operation: Military 
Damage to aircraft: Nil 
Departure point:East Sale, VIC
Destination:Williamtown, NSW
 
 
 
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Last update 13 May 2014