Loss of separation involving a Cessna 550, VH-KTK and Saab SF-340B, VH-EKX, 35 km south-south-west of Canberra, Australian Capital Territory, on 24 January 1997

FACTUAL INFORMATION

A Cessna 550 aircraft had departed Wagga on a training flight to Canberra and was maintaining flight level (FL) 170.

A Saab SF340B aircraft had departed Sydney on a regular public transport flight to Albury and was maintaining FL 120 on the route section between Canberra and Albury. Both aircraft were under radar control.

At 1218 ESuT, the crew of the C550 requested descent and were cleared to FL 130 by air traffic control. The pilot read back FL 120 but this incorrect level was not detected by the controller. One minute later the controller informed the crew of the C550 that there would be a delay for further descent due to crossing traffic (the SF340B) on the Canberra to Albury track.

At 1222, the controller noticed that the radar return for the C550 was indicating FL 120 and was 3 NM from the SF340B on crossing tracks. There was a breakdown of separation.

Radar analysis indicated that at the time vertical separation broke down, the C550 had passed through the intended track of the SF340B by approximately 1 NM and that the closest point between the aircraft was 3 NM. The required separation standard was 5 NM horizontally or 1,000 ft vertically.

ANALYSIS

Air traffic control

Because the controller believed he had heard the crew of the C550 read back FL 130 he annotated this on the flight progress strip. This meant that, in his mind, he had provided separation for the conflict he was expecting to occur. He then monitored the aircraft as they passed in the belief that vertical separation would be maintained until he could re-establish radar separation. It was only when he observed on radar that the C550 was maintaining FL 120 that he realised that a breakdown in separation had occurred. At that time, the aircraft had already passed each other, and separation was increasing.

Flight crew of the C550

The crew thought they heard FL 120 given as the descent instruction and when this read back was not questioned, they continued a normal descent to FL 120, believing it to be the assigned level. They did not see the other aircraft and considered that, as air traffic control were providing separation, any instructions to the other aircraft would not have been queried, even if they indicated a climb through the level of the C550.

SIGNIFICANT FACTORS

1. The crew of the C550 read back an incorrect assigned flight level.

2. The air traffic controller did not detect the incorrect read back.