The weather at Cairns was visual meteorological conditions but
with passing showers, especially over the western escarpment. The
cloud base and visibility associated with the showers restricted
visual flight in or near the showers. Tower control had advised the
terminal area controllers that conditions were marginal for visual
approaches from the north-west.
The base of controlled airspace in the north-western sector was
3,500 ft reducing to 2,500 ft at 12 NM from the Cairns airport, and
ground level at about 6 NM west of the airport. In the
south-western sector, the base remained at 3,500 ft until reducing
to ground level at about 5 NM west. The terrain was mountainous,
with average heights of more than 2,000 ft to within 5 NM west of
the airport. The higher peaks in this general sector of airspace
were over 3,000 ft, with some over 4,000 ft to the south-west.
An Embraer E110 (Bandeirante) was operating under the instrument
flight rules and on descent to 5,500 ft on an arrival to Cairns
from the north-west. Air traffic control approved the crew to
divert right and left of track to avoid showers and to facilitate
visual contact with the ground. On first contact, approach control
gave the crew a radar vector of 060 degrees and descent to 4,000
ft, the lowest altitude the controller could assign with reference
to the published radar lowest safe altitudes. Shortly after, the
crew reported "visual" and were cleared to descend to 2,000 ft
visually and to track direct to Cairns under their own navigation.
This clearance required the crew to maintain their current track
and continuous visual reference with the ground until within 5 NM
of the aerodrome. It allowed flight below the lowest safe altitude
and transferred the responsibility for terrain clearance to the
crew.
A Piper PA24 (Comanche) was operating under the visual flight
rules and maintaining 5,500 ft on approach to Cairns from the
south-west. The pilot encountered showers in the area and requested
descent to remain visual. The controller issued the pilot with a
clearance limit of Stoney Creek (a visual reporting point
approximately 6 NM due west of Cairns airport where the terrain
contours form a "saddle" feature) and a maximum altitude of 5,500
ft. This clearance allowed the pilot to proceed both inside and
outside controlled airspace and to track as required to Stoney
Creek. The position of Stoney Creek was at the common boundary of
the 3,500 ft, 2,500 ft and ground level control area steps. About 1
minute later, the controller observed that the Bandeirante had
deviated left of track and, when this was confirmed by the crew,
issued instructions for a visual approach to a right base to runway
15 at maximum speed. The instructions included a frequency change
to aerodrome control at 5 NM. The crew were still required to
maintain their track direct to the airport until within 5 NM.
The pilot of the Comanche reported at 3,000 ft and 5 NM
south-west of Stoney Creek. Although this position was outside
controlled airspace, the controller cleared the pilot to descend to
1,000 ft and asked him to report if any diversions were required to
remain in visual conditions. No amendment to the clearance limit of
Stoney Creek was issued.
Shortly after, the controller realised that the Bandeirante had
diverted right of track and was converging with the Comanche to the
extent that radar separation standards would not be maintained. The
controller attempted to again radar vector the Bandeirante but the
crew had radio difficulties and did not hear the instruction. After
a brief check of the radio, they transferred to the aerodrome
control frquency when about 7 NM from Cairns. The radar vector was
for the crew to turn onto a heading of 060 degrees. The instruction
was issued as the aircraft was descending through the base of
controlled airspace and was below the radar lowest safe
altitude.
Radar analysis subsequently indicated that, at that point, the
Bandeirante was about 2 NM north of the Comanche, on a converging
track and with both aircraft at approximately 2,300 ft. These
positions placed both aircraft outside controlled airspace and
below surrounding terrain.
The controller observed the altitude of the Bandeirante on the
radar display screen but did not advise the crew that they had
descended outside controlled airspace. MATS required a radar
controller to advise a pilot when the aircraft was observed to
deviate significantly from its cleared route or if the controller
believed it would deviate from controlled airspace.
The controller then asked the pilot of the Comanche if he could
see the Bandeirante. When the pilot answered "negative", the
controller instructed him to make a right orbit. The pilot began
the turn as directed but realised that the turn would place the
aircraft close to terrain that was above his altitude. He requested
a left turn and, although this direction initially placed the
aircraft closer to the Bandeirante, he then saw that aircraft. The
controller then formally instructed the pilot of the Comanche to
maintain separation with the Bandeirante. No traffic information on
the Comanche was passed to the crew of the Bandeirante.
As both aircraft were outside controlled airspace, there was no
technical infringement of separation standards. Radar analysis
indicated that the closest point was 1.3 NM when the aircraft were
approximately 100 ft vertically apart.
The descent procedure used by the controller after the crew of
the Bandeirante reported "visual", was allowed under the terms of
visual approach procedures in both the Australian Manual of Air
Traffic Services (MATS) and the Australian Aeronautical Information
Publication (AIP). However, the AIP also required crews to maintain
an altitude of not less than 500 ft above the lower limit of
controlled airspace. MATS stated that a controller must provide
this same buffer except at pilot request or when visual approach
procedures applied.
Although cleared to track direct to the airport, the crew of the
Bandeirante diverted initially 1 - 2 NM left of track. About 1
minute later, they diverted right of track and entered into
conflict with the Comanche. Both turns were to avoid showers and to
maintain visual contact with the ground but no request for track
diversion was made to air traffic control. The AIP stated that a
pilot must maintain track while on a visual approach and should not
report visual until certain that such tracking can be achieved. If
a pilot on a visual approach finds that such tracking is no longer
appropriate, air traffic control must be informed immediately.
When the controller issued the second radar vector, the crew did
not hear the instruction but thought the transmission was for their
aircraft. They made an attempt to acknowledge but when they again
could not hear the approach controller, and considering that they
were only 2 - 3 NM from the nominated frequency change point, they
elected to make an early change to the airport control frequency.
The crew remained on that frequency and had no further
communication problems. Why there was a period of poor air-ground
communications could not be determined. The attempt to radar vector
the aircraft onto a heading of 060 degrees occurred when the
aircraft was descending through 2,500 ft (the base of controlled
airspace) and in an area where the terrain rose above 2,000 ft.
During the descent from 2,500 ft to about 2,200 ft to track
through the Stoney Creek gorge, the Bandeirante passed within 1,000
m of terrain spot heights of 2,139 ft and 2,119 ft.
As the controller realised that the aircraft were coming into
conflict and that the crew of the Bandeirante were not receiving
radio broadcasts, the pilot of the Comanche was instructed to make
a right orbit. The intention of this instruction was to provide
adequate separation between the aircraft, however, it would have
resulted in the pilot flying in close proximity to terrain that was
higher than his current altitude. Spot heights on the Cairns Visual
Terminal Chart indicated heights of between 3,070 ft and 3,477 ft
within 3 NM to the right of the track, with general terrain above
2,000 ft within 1 NM. Had the pilot not countered the instruction
by changing direction to a left turn, the aircraft would have been
placed in a potentially unsafe situation.
