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A Boeing 747-200 (B747) was being radar vectored from the west for sequencing to runway 21 at Perth. A de Havilland Canada DHC-8-102 (Dash 8) was being radar vectored from the east for sequencing to land on runway 21 behind the B747. When the crew of the Dash 8 reported that they had sighted the B747, the air traffic controller assigned them the responsibility for separation from the B747. The rate of closure between the two aircraft was high and the crew of the Dash 8 received a traffic advisory from their traffic alerting and collision avoidance system. Although the Dash 8 crew was being issued with radar vectoring instructions by air traffic control, they were obliged to turn their aircraft to the right to avoid the B747.

Radar data and air traffic control automatic voice recordings were reviewed to establish the sequence of events. The investigation found that the approach controller had assigned the responsibility for separation to the pilot of the arriving Dash 8 while the aircraft was being radar controlled. The radar separation standard required 3NM horizontal separation while there was less than 1,000ft of vertical separation. During the occurrence, radar separation reduced to 1.82NM when there was 100ft vertical separation. A radar or vertical separation standard was not required when some other form of separation was being applied. In this situation the controller was relying on visual separation.

Visual separation of air traffic may have been a valid method to use in circumstances where less than the required radar separation is achievable. However, the criteria for the application of the standard were clearly detailed in the Manual of Air Traffic Services (MATS) Part 4 Section 5. In particular, MATS stated:`In circumstances where an aircraft has been instructed to maintain separation from, but not follow, an IFR aircraft, traffic information shall be issued to the IFR aircraft, including advice that responsibility for separation has been assigned to the other aircraft'. The arriving B747 was an IFR aircraft but was not provided with the required traffic information.

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