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.