The two gliders were taking part in the Australian National Gliding Championships at Narromine NSW. One, a German registered Rolladen Schneider LS8A glider, registered D-1003 was flown by a British national, while the other, an Australian registered Rolladen Schneider LS6B glider, VH-HDT, was flown by a Japanese national. Both pilots complied with the Gliding Federation of Australia (GFA) requirements for foreign nationals operating in Australia.

Although some gliders in the competition had self-launch capability, both gliders involved in the accident were launched by aero-tow. The 52 gliders competing on the day were launched in approximately 45 minutes. Launching commenced at mid-day with D-1003 being launched at 1225. After release at 2,000 ft AGL the pilot manoeuvred to take up a position near start point "Charlie" from where he would start the day's task. Shortly before the accident D-1003 was observed to be steadily banking to the right in a thermal at approximately 6,000 ft AMSL, in the company of at least six other gliders.

VH-HDT was launched at 1230 and at approximately 1315 was observed to be approaching from the south-west, flying fast, and manoeuvring to join those gliders circling near start point "Charlie". HDT was seen to bank to the right to conform to the established circling direction within the thermal. Concurrent with banking to the right HDT pulled up sharply, directly below D-1003. Immediately prior to the collision, HDT's angle of bank was observed to rapidly reduce. The cabin section of HDT was observed to collide with the lower centre and forward fuselage of D-1003. The structural integrity of the forward fuselage of HDT was destroyed and the forward fuselage was observed hanging approximately 20 degrees below the normal fuselage line. The glider descended vertically, rotating to the right, before impacting the ground and coming to rest inverted. The pilot did not survive the collision.

The post mortem examination did not disclose any abnormalities that may have contributed to the accident.

The pilot of D-1003 advised that he had not sighted HDT approaching the thermal. He felt and heard a huge impact concurrent with D-1003 pitching violently nose down. The pilot immediately released the canopy and seat harness before parachuting clear. D-1003 descended in a flat spin to impact inverted 200 metres from HDT. The pilot landed a further 200 metres away, sustaining back injuries during the landing.

The wreckage of both gliders was examined and no pre-impact fault was found in either glider. The examination confirmed that HDT had impacted the lower fuselage of D-1003. There was some evidence of intermeshing of the wings during the collision sequence, however the structural integrity of the wings was probably not greatly compromised by the collision.

The on-board data recorders were recovered and examined. Because of impact damage to security switches within the units, all recorded data had been lost.

Both pilots involved in the accident had been assigned to start the task at start point "Charlie". Start point Charlie was approximately 15 km north of the town of Narromine and had been allocated to 13 of the competing gliders on the day of the accident. A start point is a GPS position based on a geographical feature that the pilot must transit in order to register a start for the day's task. After passing though the start point the pilot reports the start time and allocated point to the competition organisers by radio. An on-board recorder that is interrogated for verification by the competition organisers also records the start time and position.

The organisers had nominated four start areas for the day. The four start areas were dispersed at locations up to 20 km from the airfield. Within each start area were five geographical start points. Each pilot was issued with a randomly selected list of four valid start points, one from each area. The pilot could use any one, but only one, of these four start points. This created a double dispersion: a scatter of start areas, and a scatter of start points within those areas. The use of multiple start areas and start points was introduced to eliminate the overcrowding of airspace that occurred when just one start point was used.

Approximately 20 minutes after the last glider is launched the organisers declare the points open which signals to the pilot's that they can, in their own time, commence the day's task. The start points were not open at the time of the accident. The system of multiple start points has been deemed successful in reducing the incidence of congestion within the start area and on the competition task.

The pilot of HDT was wearing sunglasses and was looking through four facets when observing features and events outside of the canopy. At the time of the accident the sun was essentially overhead, the sky was clear and visibility was unlimited. There would not have been an impediment to the vision of the pilot of HDT as he approached the thermal from the south-west. Neither the pilot's visual acuity nor the weather were considered to be factors in the accident.

The pull up when entering a thermal is a manoeuvre used by glider pilots to convert forward speed to height. There are certain etiquette's surrounding the approach to an occupied thermal and these form part of the training undertaken by all glider pilots. The pilot of HDT was highly experienced; he was a gliding instructor in Australia and Japan, and he had taken part in many national and international gliding competitions. Accordingly, he would have been aware of the factors to be considered when joining the thermal. It is not known why the pilot of HDT did not observe the position and flight path of D-1003 while approaching the thermal. D-1003 was essentially on a consistent flight path within the thermal and should have been visible to the pilot of HDT.

HDT was equipped with reasonably sophisticated navigation and communications systems. It may be that the pilot was attending to this equipment while approaching the thermal and did not notice D-1003 until after initiating the pull up. The rapid change to the angle of bank observed just before impact was probably the initiation of an unsuccessful avoidance manoeuvre by the pilot of HDT.

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 aerodrome circuit was active with a number of aircraft conducting circuit training, or inbound for landing. Both runways 35 and 30 were in use. Runway 30 intersected runway 35 approximately 750 m from its threshold. The tower was staffed with three controllers operating the aerodrome control (ADC), coordination and surface movement control positions.

The ADC had instructed the crew of a De Havilland Dash 8 (Dash 8) to line up for a departure from runway 35. The pilot of an American Aircraft Corporation Cheetah (Cheetah) had been instructed to continue approach for runway 30. The ADC cleared the Dash 8 crew for take-off and then looked towards the final approach path for runway 30 to monitor the approach of the Cheetah. As the ADC completed the transmission to the Dash 8 crew, he saw that the Cheetah was on short final and immediately transmitted an instruction to the Dash 8 crew to hold their position. This instruction was over-transmitted by the acknowledgment of the take-off clearance by the pilot in command of the Dash 8. The ADC commenced to issue a land-and-hold-short instruction to the pilot of the Cheetah; however, seeing that the Dash 8 was starting its take-off roll, he ceased the transmission and issued a further instruction to the Dash 8 crew to hold position. This instruction was over-transmitted by the Cheetah pilot reporting that he was initiating a go-around. The Dash 8 became airborne prior to the intersection of the runways, however, the crew held their aircraft at approximately 50 ft above ground level while the Cheetah passed approximately 100 ft above and slightly behind the Dash 8.

The automatic voice recording of the radio transmissions from the ADC during this period indicated that there were few or no intervals between any of his transmissions. The pilot of another aircraft that was operating in the circuit heard the hold-short instruction. The crew of the Dash 8 reported that they did not hear either of the hold-short instructions.

The ADC had formulated a traffic plan but this changed when a turbojet aircraft he thought was going to conduct a full-stop landing on runway 35, conducted a touch-and-go. The ADC had instructed the crew of the Dash 8 to line up on runway 35 after the approach of the turbojet aircraft, and had amended the approach of the Cheetah to runway 30 to facilitate the departure of the Dash 8. The ADC did not instruct the pilot of the Cheetah to report at a position that would have enabled him to adjust the traffic sequence if required. There was also a delay between the time the ADC approved the Dash 8 crew to line up, and the eventual issue of the departure instructions and a clearance. This delay was due to radio transmissions from the pilot of the turbojet aircraft, as well as the coordination of inbound traffic with the approach controller. During this period, the Cheetah pilot had continued his approach to runway 30. The ADC did not check the position of the Cheetah in relation to the intersection of the runways prior to issuing the take-off clearance to the Dash 8 crew. That was done as he was completing the transmission of the take-off clearance to the crew. As a final action to resolve the conflict between the aircraft, the ADC proposed to issue a land-and-hold-short instruction to the pilot of the Cheetah; however, the landing distance available (590 m) to the pilot did not meet the distance required (900 m) for land-and-hold-short operations.

The Manual of Air Traffic Services details the radiotelephony phraseology (RTF) to be used by controllers to cancel take-off clearances or to stop a take-off in emergency conditions. The ADC used only part of the required RTF for the cancellation of the take-off clearance. He did not use the RTF for an emergency situation.

A Boeing 747 (B747) was en route between Sydney and Perth, tracking via air route Q158. The crew contacted Perth air traffic control at waypoint BIDDY, and were issued with a standard terminal arrival route (STAR) to Perth. The controller and the crew entered into a discussion regarding Perth STARs. The crew were asked if the aircraft was Area Navigation (RNAV) equipped and replied that the aircraft was not so equipped. Aircraft that are not RNAV equipped are not permitted to track on air route Q158 from Sydney to Perth.

The investigation subsequently revealed that the aircraft was RNAV equipped and therefore, permitted to use Q158. There was no infringement of separation standards.