Boeing Co 737-376, VH-TAX

The crew of a departing Boeing 737 (B737) reported that as they
were climbing to 5,000 ft on a standard instrument departure from
runway 27 at Melbourne, they saw an arriving B737 on descent in
their 12 o'clock position, passing from right to left. The
departing aircraft's traffic alert and collision-avoidance system
indicated that there might have been less than the required radar
separation standard of 3 NM between the two aircraft. The crew was
not alerted by air traffic control regarding the arriving B737.

Radar data and air traffic control automatic voice recordings
were reviewed to establish the sequence of events. The
investigation found that the Departures controller had assigned
responsibility for separation to the pilot of the arriving B737,
which was tracking for left downwind runway 34. The transmission
from air traffic control to the crew of the descending B737
included the statement "...clear of the 737 out to your left,
descend to 3000".

Airservices Australia advised that the procedure was used when
separating runway 27 departing aircraft from arriving aircraft
tracking on left downwind for runway 34. The aim was to release the
climb and descent restrictions of both aircraft as soon, and as
efficiently, as possible. The procedure of one aircraft sighting
and confirmed past the other aircraft was considered to be a
legitimate technique in such situations.

Visual separation of air traffic may have been a valid method to
use in those circumstances. However, the criteria for its
application 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 departing B737 was an IFR aircraft but was not
provided with the required traffic information.