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 (Aerocommander) 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 Aerocommanders. The Aerocommanders 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 Aerocommanders more than
the Dash 8. Consequently, the controller re-ordered the landing
sequence to place the Dash 8 ahead of the Aerocommanders 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
Aerocommander 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 Aerocommander 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 Aerocommanders 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 Aerocommander turned right
to track behind the Dash 8 on the final approach to runway 15. The
Aerocommanders 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 familarisation 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.