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.