A Saab SF-340B (Saab) was conducting a scheduled passenger
service from Canberra to Sydney and had been assigned descent to
7,000 ft by the Approach South radar controller in order to
maintain the minimum vertical separation standard of 1,000 ft,
between the Saab and a Piper Chieftain. The Chieftain was ahead in
the approach sequence and had been assigned descent to 6,000 ft.
The two aircraft were separated laterally by distances greater than
the minimum radar separation standard of 3 NM, but their respective
tracks were converging.
The controller requested the Saab crew to expedite descent to
7,000 ft. That instruction was read back by the pilot in command,
however, the readback was indistinct. The controller repeated the
request "Expedite descent to 7,000 ft". The pilot in command
acknowledged that transmission with the aircraft's callsign.
Shortly after, the controller observed that the altitude readout
for the Saab was indicating that the aircraft was descending
through 7,000 ft. The controller queried the Saab crew to confirm
that they were maintaining 7,000 ft, then instructed the crew to
turn left onto a heading of 360 degrees due to the traffic ahead.
As the crew responded to that instruction, the lateral and vertical
separation between the Saab and the Chieftain reduced to 1.5 NM and
500 ft respectively.
The traffic sequence had been busy, with the controller
endeavouring to change the landing sequence by placing the Saab
ahead of the Chieftain. The controller was required to monitor the
lateral and vertical separation between a number of aircraft as two
standard arrival routes converged. As a consequence, the controller
was required to establish vertical separation to ensure separation
was maintained between all aircraft as they approached an area of
lateral conflict.
All radio transmissions between the controller and the Saab were
recorded. A review of the recorded information indicated that the
altitudes assigned by the controller, and the readbacks from the
Saab crew, were clear and distinct except for the response to the
controller's initial request to expedite descent. The Saab crew
were unable to explain why they were not alerted to the possibility
of an incorrect altitude following the controller's reiteration of
the request to expedite descent to 7,000 ft.
The Aeronautical Information Publication GEN 3.4-10 paragraph
4.4 details readback requirements for flight crew. For other than a
route clearance, "the key elements of clearances, instructions or
information must be read back ensuring sufficient detail is
included to clearly indicate compliance". A level instruction must
be read back. In this occurrence, the controller did not expect a
readback of the altitude, because the previously assigned altitude
had not changed. The indistinct response from the Saab crew caused
the controller to doubt that his request had been understood, so he
repeated the request. When that transmission was acknowledged by
the pilot in command, the controller believed that the crew now
understood his request to expedite descent to 7,000 ft.
The Saab crew believed that an altitude clearance of 6,000 ft
had been issued between the time the controller assigned them 7,000
ft, and when the controller requested that they expedite descent to
7,000 ft. However, the recorded transmissions did not include any
reference to 6,000 ft directed to, or received from, the Saab crew
during that period.