Jump to Content

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.

General details
Date: 17 March 1999 Investigation status: Completed 
Time: 0839 hours ESuT  
Location   (show map):28 km WSW Sydney, (VOR) Investigation type: Occurrence Investigation 
State: New South Wales Occurrence type: Loss of separation 
Release date: 27 April 1999 Occurrence class: Airspace 
Report status: Final Occurrence category: Incident 
 Highest injury level: None 
Aircraft 1 details
Aircraft manufacturer: S.A.A.B. Aircraft Co 
Aircraft model: 340 
Aircraft registration: VH-KDQ 
Serial number: 340B-325 
Type of operation: Air Transport Low Capacity 
Damage to aircraft: Nil 
Departure point:Canberra,, ACT
Destination:Sydney, NSW
Aircraft 2 details
Aircraft manufacturer: Piper Aircraft Corp 
Aircraft model: PA-31 
Aircraft registration: VH-JVD 
Serial number: 31-7852041 
Type of operation: Air Transport Low Capacity 
Damage to aircraft: Nil 
Departure point:Shepparton, VIC
Destination:Sydney, NSW
Share this page Provide feedback on this investigation
Last update 13 May 2014