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Summary

Summary

A member of the public reported seeing a single engine aircraft manouevre suddenly to avoid another aircraft, on an intersecting track, while the aircraft were over Brisbane.

An investigation reviewed radar data and air traffic control automatic voice recordings to establish the sequence of events. The investigation found that VH-OXF, a Beech 300, was tracking for a left base to runway 01 at Brisbane Airport at 2,500 ft, while a Cessna 172, VH-IGA, was tracking north over the suburbs at 1,500 ft. The Brisbane departures controller established that the pilot of the Beech could see and was able to avoid the Cessna before reducing the vertical spacing between the aircraft to less than the vertical separation standard of 1,000 ft. The Beech pilot reported seeing and passing over the top of the Cessna and ready for further descent. The controller issued a clearance for a visual approach. The recorded radar data indicated that the Beech began a steady descent from about the intersection of the aircraft tracks.

The controller's options in relation to ensuring separation between the aircraft were either to:

  1. maintain the Beech at 2,500 ft until there was more than 3 NM lateral separation with the Cessna; or
  2. use visual separation procedures by having a pilot report seeing the other aircraft and then instructing that pilot to avoid the sighted aircraft.

To enable the Beech to descend in preparation for landing, the controller used the second option. Examination of the radar data indicated there was no infringement of separation standards.

The recorded radar data indicated that during the period when the Beech was assigned 2,500 ft, the Mode C altitude intermittently indicated 2,300 ft and 2,400 ft. Mode C altitude has a tolerance of plus or minus 200 ft. The pilot was therefore complying with the air traffic control clearance.

 
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