de Havilland Canada DHC-8-102, VH-XFT

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 4.5.1.11
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.