Boeing 737-377, VH-CZC

VH-CZC, a Boeing 737-300, had taxied at Cairns for departure from runway 15 bound for Sydney.

VH-TJW, a Boeing 737-400, had taxied after CZC, also for a departure from runway 15 bound for Brisbane.

Both crews had been cleared via the runway 15 SWIFT 2 standard instrument departure. That procedure required crews to turn their aircraft at the earlier of 400 ft or the departure end of the runway and then track to 030 degrees M until climbing through 4,000 ft. At that point the aircraft should be turned right onto a track of 170 degrees M to intercept the 139 degrees radial of the Cairns VOR (VHF navigation aid). The SWIFT 2 standard instrument departure was designed to counter the limitations of high terrain surrounding Cairns aerodrome and the tracking requirements of inbound aircraft from the south and east.

At 0604:15 Eastern Standard Time, the aerodrome controller coordinated the two planned departures with the approach/departures controller using the phrase "next CZC followed by TJW". The reply was "CZC unrestricted, TJW unrestricted". That instruction authorised the aerodrome controller to allow the takeoffs in accordance with Cairns Local Instruction TOWER-20, which stated that the aerodrome controller must ensure that 3 NM separation between aircraft would exist prior to the second aircraft passing the upwind end of the runway.

At 0604:48, the crew of CZC was cleared for take-off and at 0606:43, the crew of TJW was cleared for take-off. Radar analysis showed that both aircraft commenced a left turn at approximately 400 ft at, or about, the departure end of the runway and that the required 3 NM spacing was achieved.

The approach/departures controller had approved a request for a change of level from a pilot of an aircraft that had departed Cairns approximately 7 minutes earlier. After issuing the departure clearances, the controller commenced the process of making the change in the air traffic computer; an action that required nine clicks of the mouse. In order to make this change, the controller looked away from the air situation display (which was on the main screen) and used the auxiliary screen to observe the flight plan window while using the keyboard to input the data.

While the controller was performing the information change task, the crews of the departing aircraft contacted him as required. The controller acknowledged the radio broadcasts then returned to the data input task. He did not continue to check the positional information on the air situation display. A few moments later, he glanced at the display and realised that TJW had turned earlier than CZC and was also out-climbing that aircraft. As he was about to take corrective action, the controller saw that TJW had climbed through 4,000 ft and had commenced the right turn onto a heading of 170 degrees M. The manoeuvre had placed TJW on a track inside that of CZC and led to a rapid reduction of horizontal separation to less than the required standard of 3 NM. The vertical separation standard of 1,000 ft had not been achieved at the time. Radar analysis showed that the aircraft passed 1.7 NM horizontally and approximately 400 ft vertically apart.

The controller immediately cancelled the standard instrument departure for TJW and issued the crew with a radar heading of 100 degrees M to ensure that, although separation standards would be breached, TJW would pass behind CZC. The crew of TJW reported that they had the other aircraft in sight and monitored their flight path to ensure that they remained clear of that aircraft. Both crews subsequently reported that they received a traffic alert and collision avoidance system (TCAS) advice.

CZC, the B737-300 series aircraft, had taken 1 minute and 56 seconds to reach 4,000 ft whereas TJW, the B737-400 series aircraft, took only 1 minute and 27 seconds to pass the same altitude. In addition, the initial opening speed created by the departure spacing off the runway had reduced to zero within 1 minute and 30 seconds of the departure of TJW.

Controllers at Cairns considered that the aircraft were "like types" for the purposes of departure standards and neither the aerodrome controller nor the approach/departures controller considered increasing the separation requirements specified in Local Instructions. However, Boeing 737-400 series aircraft are known to normally out-perform the Boeing 737-300 series.

Separation standards used for consecutive departures allowed controllers to increase the minimum distance at their discretion. On this occasion it was not increased and, although the runway departure standard was not breached, the decision did not allow for any unexpected manoeuvre by either aircraft. Both crews acted in accordance with the standard instrument departure and the closure was the effect of aircraft performance.

The approach/departures controller elected to input data to the air traffic computer during the departure sequence. Although these actions were necessary, they were labour intensive and diverted his attention from the air situation display. They were not urgent and the decision resulted in the controller not having his full attention on the relative positions of the aircraft when they first appeared on the display.

The design of the SWIFT 2 standard instrument departure did not guarantee separation assurance. Whenever the second aircraft reached 4,000 ft prior to the first aircraft (whatever the reason) a reduction in horizontal separation was likely.

The runway 15 SWIFT standard instrument departure allowed the possibility of a following aircraft turning inside a preceding aircraft.
The performance of the B737-400 series aircraft was superior to that of the B737-300 series aircraft.
The use of minimum departure separation standards was inappropriate.
The decision of the approach/departures controller to concentrate on data entry tasks during the departure sequence was not sound practice.

As a result of the investigation, Airservices Australia have: Designed a new runway 15 SWIFT standard instrument departure to minimise the possibility of conflict in similar circumstances. This procedure is scheduled for publication on 24 February 2000; and Initiated regular discussion sessions amongst Cairns air traffic controllers to, among other things, review techniques used with successive departures. The first of these meetings took place on 24 June 1999.