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No recording was available of the Common Traffic Advisory Frequency. As both instructor pilots reported either hearing other aircraft completing circuits at Lilydale or of hearing the MAYDAY broadcast of the other aircraft, the investigation determined that both crews were most likely monitoring the correct radio frequency. The reason that radio broadcasts made by the crews of both aircraft were not heard by the other could not be determined. There should have been an opportunity for the crew of BZA to hear the downwind broadcast from the pilot of UMB, and for the crew of UMB to hear the inbound broadcast from the pilot of BZA when that aircraft was 5 NM east of Lilydale, noting that that position would equate to the aircraft being approximately overhead a late downwind position for runway 17 at Coldstream.

The instructor on board UMB was preoccupied with assisting the student to maintain the correct spacing on the downwind leg. The circuit area at Coldstream presented a number of challenges to the novice pilot, including powerlines located a short distance to the south of the aerodrome which necessitated an early left turn, and the downwind leg being displaced further east than normal in order to maintain the aircraft clear of noise sensitive areas. The instructor's focus during the circuit would have been directed towards the aerodrome and providing the student with visual pointers to maintain consistent circuit spacing. As a result, the instructor's attention to altitude keeping and maintaining a lookout for conflicting traffic may have been affected.

The instructor and the student pilot on board BZA reported that their aircraft maintained the appropriate height specified in the Coldstream local instructions, that is, they overflew the circuit area not below 2,000 ft. The altitude reportedly maintained by the student pilot of BZA when passing over the Coldstream circuit area should have been sufficient to provide the appropriate level of vertical separation from the Coldstream circuit traffic. The crew of BZA also had a responsibility to see-and-avoid other traffic during their overflight of the Coldstream Aerodrome. As the PA-28-161 Warrior is a low-wing aircraft, their possible view of the circuit area at Coldstream may have been slightly obscured.

Although altitudes specified in the Coldstream Aerodrome local instruction should have provided sufficient vertical spacing between the aircraft, receipt of radio broadcasts may have increased the likelihood of the crews being alerted to the potential conflict.

While recognising the limitations inherent in the see-and-avoid principle of collision avoidance, this accident serves as a reminder to all pilots to:

  • maintain an effective lookout at all times, being particularly vigilant in areas of high traffic density and while overflying aerodromes
  • maintain an effective listening watch
  • ensure that appropriate and timely radio broadcasts are made
  • maintain accurate altitude keeping.
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