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A de Havilland Canada DHC-8-314 (Dash 8) was on initial descent to flight level (FL) 140 and was being radar vectored for a right circuit to land on runway 35. At the same time a Boeing 737-376 (B737) had departed from runway 35 and had made a right turn to avoid weather on climb to FL200. The approach controller had issued radar heading instructions to the crew of the Dash 8 to provide track shortening and to maintain separation with the B737.

The approach controller expected that the radar heading issued to the crew of the Dash 8 would have ensured that a radar separation standard of 3 NM would continue to exist between the B737 and the Dash 8 while the vertical separation standard of 1,000 ft did not exist between the two aircraft. During the occurrence, radar separation reduced to 1.9 NM when there was 600 ft vertical separation between the two aircraft. There was an infringement of separation standards.

At the time of the infringement of separation standards, both aircraft were above `approach' airspace, within the vertical limits of an overlying sector, but under the control of the approach controller. Transfer of control of aircraft in these circumstances was permitted in accordance with the provisions detailed in the Manual of Air Traffic Services. The minimum radar separation standard in that overlying sector of airspace was 5 NM. The minimum radar separation standard within the airspace under the control of the approach controller was 3 NM.

The radar heading issued to the crew of the Dash 8 resulted in that aircraft taking up a track that was approximately 15 degrees different from that which the approach controller had expected. He reported that the radar heading assigned to the crew of the Dash 8 may not have correctly accounted for the prevailing wind speed and direction, and also that the B737 did not track as he anticipated. The crew of the B737 later reported that they had proceeded in accordance with their airways clearance and that the time taken to execute the turn onto their track was normal.

During the occurrence the approach controller had developed an incorrect mindset that the required radar separation standard was 3 NM instead of 5 NM. He later reported that the need to apply a 5 NM radar separation standard in that overlying airspace was rare due to the natural disposition of traffic.

A combination of the approach controller's incorrect assessment of the effects of the prevailing weather conditions on the radar heading of the Dash 8, the expectation of the B737 track, and the use of an inappropriate radar standard compromised the planned horizontal separation standard while a vertical separation standard did not exist.

Had the approach controller assessed the effectiveness of the assigned heading before issuing instructions to the crews that cancelled the provision of a vertical separation standard, it is likely that the infringement would not have occurred. Application of an effective separation assurance strategy in accordance with the Manual of Air Traffic Services would have ensured that the prescribed radar separation standard was maintained.

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