On 25 June 2005, a Bombardier Aerospace Dash 8-315 (Dash 8) aircraft was being operated on a scheduled passenger service from Weipa to Cairns, Qld, while three Aero Commander Div Shrike Commander aircraft (Aero commander) were tracking to Cairns from Cooktown, Qld. The four aircraft were in an arrival sequence of seven aircraft tracking to runway 15. Visual meteorological conditions existed during the period of the aircrafts' arrivals.
Based on the aircrafts' estimated arrival time, the Dash 8 was 3 minutes behind the three Aero commanders. The Aero commanders were maintaining 1,000 ft above mean sea level, and the Dash 8 was on descent from 5,000 ft. The Cairns approach controller estimated that low level headwinds would delay the Aero commanders more than the Dash 8. Consequently, the controller re-ordered the landing sequence to place the Dash 8 ahead of the Aero commanders for landing. The approach controller notified the aerodrome controller of the change in the landing sequence.
At 1705 Eastern Standard Time, when the Dash 8 was on final approach at about 6 nautical miles1 (NM) from the airport, the crew received a traffic alert and collision avoidance system (TCAS) traffic advisory. The crew saw an Aero commander to the left at about 3 NM, and observed on the TCAS display another aircraft in close proximity. Very shortly after, they received a TCAS resolution advisory to climb their aircraft, which they followed. The crew reported that as they were climbing, they saw an Aero commander pass to the left of the Dash 8.
The four aircraft were being operated on instrument flight rules category flights that required separation by air traffic control. Controllers could use a 3 NM radar separation standard, a vertical separation standard of 1,000 ft, visual separation by the aerodrome controller, or require a pilot to sight and follow another aircraft. The approach controller's intention was to separate the aircraft using sight and follow procedures.
As the aircraft approached the airport the aerodrome controller used non-standard coordination phraseology that was misunderstood by the approach controller. The approach controller subsequently instructed the crew of the Dash 8 and the three Aero commanders to transfer to the aerodrome controller's radio frequency. The resultant traffic situation presented to the aerodrome controller was difficult to resolve using visual separation or sight and follow procedures.
The TCAS events occurred as the first Aero commander turned right to track behind the Dash 8 on the final approach to runway 15. The Aero commanders were from the same operator and often used sight and follow procedures between company aircraft on arrival.
An Airservices Australia (Airservices) investigation found that the occurrence was due to the following factors:
- the approach controller made an error of judgement in assessing the new traffic sequence
- the aerodrome controller used non-standard coordination phraseology that was misinterpreted by the approach controller
- the use of sight and follow procedures for the number of aircraft involved increased controller workload
- the approach controller's lack of recent familiarisation with tower visual separation procedures.
The Airservices report made six recommendations, and the following action has been taken in regard to the recommendations:
- tower and approach/departure controllers were reminded to use standard coordination phraseology to reduce the possibility of ambiguity
- a tower and approach/departure area familiarisation program has been implemented that requires participating controllers to complete a project paper
- letters of agreement regarding sight and follow procedures have been finalised with two local operators.
- A nautical mile is equal to 1852 metres.
