An Aero Commander Division 500-S (Shrike) had departed runway 35
at Essendon aerodrome. The pilot had been issued a clearance to
overfly Melbourne airport and then track to Fentons Hill Very High
Frequency Omni-direction Radio Range navigation (VOR) beacon, 10 NM
north of Melbourne, and to climb to 3,000 ft. A Boeing 767-336
(B767) was on final approach to runway 27 at Melbourne and passing
1,400 ft on descent, when the crew received a Traffic Alert and
Collision Avoidance System (TCAS) traffic advisory (TA) with an
aircraft 600 ft below. The TCAS then issued a resolution advisory
(RA) indicating an aircraft 300 ft below. The B767 crew initiated a
TCAS climb and conducted a go around in accordance with company
operating procedures.
The clearance issued to the Shrike pilot was in accordance with
an overhead Melbourne departure procedure. That procedure had been
implemented to facilitate departures from either runway 26 or 35 at
Essendon aerodrome when there were arrivals to runway 27 or 34 at
Melbourne. A letter of agreement between Melbourne tower and
terminal, and Essendon tower detailed the procedure, including
coordination requirements and separation responsibilities. The
procedure relied upon the application of either radar, visual, or
procedural separation being applied by the Melbourne or Essendon
Aerodrome Controller subject to a number of conditions. The
procedure was limited to a few approved Essendon operators.
The Shrike pilot had planned a flight from Essendon to
Shepparton, about 80 NM north-northwest of Melbourne. The pilot
reported that he had selected the transponder code and switched the
unit to "On" prior to departing. Once airborne, and after
transferring to the Melbourne Aerodrome Control frequency, he was
advised that the transponder was not operating. The pilot was sure
that the indicator light on the transponder was operating. He
recycled the transponder.
The B767 crew first saw the Shrike on their aircraft's TCAS as
they passed 3,000 ft on final approach. After the RA, it
disappeared from the TCAS as the B767 passed 2,000 ft on climb. The
crew was subsequently vectored for a landing on runway 34 via a
left base. During the go around, cabin crew in the B767 saw the
Shrike pass below and make a right climbing turn.
After the B767 pilot reported going around, the Melbourne
Aerodrome Controller instructed the Shrike pilot to turn right and
to pass behind the B767. The Melbourne Aerodrome Controller was not
required to issue the B767 crew with traffic information on the
Shrike.
The Melbourne Aerodrome Controller was using visual separation
procedures to separate the aircraft. Visual separation enabled
controllers to provide less vertical or lateral displacement
between the aircraft than what would be required using a radar or
vertical separation standard. Aerodrome controllers regularly used
visual separation procedures to separate aircraft in the vicinity
of aerodromes.
The TCAS is an independent on-board collision avoidance system
that continually surveys the airspace around an aircraft, seeking
replies from other aircraft in the vicinity via their transponders.
The system determines the range, relative bearing, and relative
altitude of other aircraft and uses that information to predict
flight paths. If a flight path is predicted to penetrate the
collision area surrounding the TCAS fitted aircraft, the system
informs the crew by visual and aural annunciations. Depending on
the closure speed TCAS issued either a RA or a TA alert.
Analysis of recorded radar data during the investigation
revealed an inconsistency in the altitude readout from the Shrike.
As the B767 descended between 1,500 ft and 1,400 ft, the Shrike's
altitude changed from 1,250 ft to 736 ft and then to 1,650 ft over
a 35 second period. A second altitude inconsistency was recorded
after the RA alert. The investigation did not establish the reason
for the inconsistent altitude readout. The operator of the Shrike
reported that transponders could require up to five minutes warm-up
prior to operation.
The recorded radar data showed that, at the closest point of
approach, the Shrike was about 1.5 NM to the south and 300 ft below
the B767. That was just as the B767 commenced the go around. At
about the same time, the track of the Shrike changed from
north-westerly to northerly.
