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An Embraer E120 (Brasilia) aircraft had departed from Cairns for Bamaga. The crew were maintaining flight level (FL) 220 in controlled airspace under the control of Brisbane Sector 10. They had reported their position as over Lockhart River at 1211 EST with an estimate for Bamaga of 1236. At about 50NM from Bamaga, the crew requested clearance to descend.

A de Havilland Dash 8 aircraft had departed Horn Island for Cairns at 1207 and the crew had reported to flight service (FIS 7) their intention to climb to FL210. The flight service officer instructed the crew to contact the Sector 10 controller when approaching controlled airspace for an airways clearance.

At 1220, the flight service officer passed traffic information regarding the Brasilia to the crew of the Dash 8.

At 1226, the controller instructed the crew of the Brasilia to leave the control area on descent and to report passing FL200, which was the base of controlled airspace in that vicinity. The crew commenced descent at that time and set up a rate of descent of 2,000 ft/min. Shortly after, the controller advised the crew that the Dash 8 aircraft was conflicting traffic for their descent and authorised them to transfer to the FIS 7 frequency.

The pilot in command was the handling pilot of the Brasilia while the co-pilot was performing the radio tasks. When they received a clearance to leave controlled airspace they commenced descent immediately and this action was followed by the receipt of traffic information regarding the Dash 8. The pilot in command quickly assessed that this traffic was in direct conflict and instructed the co-pilot to make a specific call to the crew of the Dash 8 as soon as he had transferred communications to the flight service frequency, rather than the general traffic call normally transmitted.

The co-pilot was waiting to report leaving FL200 as instructed by the sector controller. As a result of the instruction by the pilot in command, he made this report early using the phrase "approaching FL200" and selected the radio to the FIS 7 frequency as the aircraft was passing FL205.

Further delay was then experienced as the FIS frequency was congested. The aircraft was passing the base of controlled airspace and, as the weather was consistent with visual meteorological conditions (VMC), the pilot in command elected to continue visually rather than to level off in the control area. He was confident of seeing the Dash 8 and, in fact, saw the aircraft before the co-pilot could make his initial radio transmission on the FIS frequency.

At 1228, the crew of the Dash 8 contacted sector control for an airways clearance while passing FL188. The controller was unable to issue a clearance at that time because the altitude of the Brasilia was unknown and it was not clear if that aircraft had passed the Dash 8.

At the same time, the co-pilot of the Brasilia was attempting to contact the Dash 8 crew on the FIS 7 frequency to establish their respective altitudes. The crews eventually determined that the Dash 8 was climbing through FL190 while the Brasilia was descending through FL180. The Dash 8 crew had not seen the Brasilia but had seen a shadow pass over their aircraft. Having established that the aircraft had passed, the crew of the Dash 8 returned to the sector control frequency where they were cleared to enter controlled airspace on climb to FL210.

The point of conflict was outside of radar coverage but calculation indicated that the aircraft passed with no more than 2,000 ft vertical separation and no discernible horizontal separation.

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