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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.

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