Breakdown of separation, 7 km south-west of Sydney Airport, New South Wales, on 21 September 2007, VH-EKX, Saab SF-340B, VH-CKO, Eurocopter EC120

On 21 September 2007, a SAAB Aircraft AB, SF-340B (SAAB) registered VH-EKX was inbound to Sydney, NSW on a scheduled passenger service from Wagga Wagga, NSW. The crew was cleared by air traffic control to track direct to short final runway 16R at Sydney Airport. The last assigned altitude issued to the SAAB crew was 2,500 ft. A Eurocopter EC120 helicopter registered VH-CKO had departed Sydney Airport on a Georges River 1 departure and, when identified on radar by air traffic control, was cleared to track direct to Kingsgrove at 1,500 ft for aerial work. Air traffic control had coordinated the positions and intended tracks of both aircraft and established a 1,000 ft vertical separation standard.

At 1058.08 Eastern Standard Time, the short-term conflict alert activated on the display consoles of the air traffic controllers. Two seconds later, the copilot of the SAAB reported that they were responding to a resolution advisory from the aircraft's traffic advisory and collision avoidance system (TCAS). Radar analysis indicated that the separation between the two aircraft reduced to 200 ft vertically and 0.5 NM horizontally. There was an infringement of the vertical separation standard of 1,000 ft.

The helicopter pilot had climbed to 2,000 ft and later reported that he had been distracted from monitoring his assigned altitude by a focus on meeting the client's photographic requirements. The pilot in command and the copilot of the SAAB both reported that they believed that they had been cleared by air traffic control to make a visual approach and had descended their aircraft to 2,000 ft.

The investigation established that there was a safety issue in that there is no requirement for a controller to confirm that descent below a previously assigned altitude has not been given in a clearance to a circuit position that does not include a visual approach clearance. Airservices Australia is examining options to address this safety issue.