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The crew of a Dash 8 reported on the mandatory broadcast zone frequency that they were inbound to Dubbo, at a position 40 NM south-east, and on descent from FL180. The only response to this transmission was from the pilot of a Piper Cherokee who advised that he was approximately 15 NM east of Dubbo at 4,000 ft and inbound. Approaching 4,000 ft the crew of the Dash 8 requested the position of the Cherokee. The pilot advised that he was now 8 NM from Dubbo and descending to 2,400 ft. They then asked the pilot if he was south of "the highway".

The pilot of the Cherokee confirmed that he was south of "the highway". The crew of the Dash 8 indicated that they would remain north of the highway and join a 5 NM final approach to runway 23, and requested that he remain south.

When the crew subsequently reported that they were 5.5 NM from Dubbo and about to turn final for a straight-in approach to Runway 23, they observed a Piper Cherokee pass from their right to left at an estimated distance of 400 m and 200 ft below.

Weather conditions at the time were reported to be CAVOK.

"The highway" to which the crew was referring was the Mitchell Highway that runs south-east from Dubbo to Wellington, almost directly beneath their track. The pilot of the Cherokee, who held a private licence, was undertaking a solo navigation exercise as part of the training for upgrading to a commercial licence. He reported that he was just north of his planned track from Gulgong to Dubbo. When asked by the crew of the Dash 8 if he was south of the highway he assumed that they were referring to the Dubbo to Dunedoo road, aligned east-north-east from Dubbo and that he could see to his north. He was not familiar with the Dubbo area and was not aware of the existence of the Mitchell Highway although this road was annotated as a highway on his Visual Navigation Chart. Additionally, he thought that the Dash 8 would pass behind him and join for a 5 NM final north of this road.

The use of a line feature to assure separation between aircraft is an accepted and generally sound technique. However, the use of the generic term "the highway" by the crew of the Dash 8 introduced an ambiguity that neither the crew of the Dash 8 nor the pilot of the Cherokee was aware of at the time. Specifying the road by name or description should have removed this ambiguity.

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