On 2 July 2004, at about 1805 Eastern Standard Time, a de Havilland DHC-8-315 (Dash 8) was taxiing at Sydney Airport for a night departure on a scheduled fare-paying passenger flight to Williamtown, NSW. The Sydney Tower Surface Movement Controller (SMC) had issued a clearance for the crew of the Dash 8 to taxi from Domestic Terminal 1 to the holding point for runway 16 Left. The clearance was via taxiway Charlie, across runway 25 to Bravo 10.
Figure 1: Sydney aerodrome chart
A Saab 340F (Saab) had landed on runway 16 Right and taxied from that runway via runway 25 then turned left onto taxiway Bravo. The SMC issued a clearance for the crew to taxi via taxiway Golf to the apron. The SMC had instructed the crew of the Saab to expedite. The dictionary definition of expedite means to 'speed up the progress of, or to hasten'. The Saab pilot in command (PIC) interpreted this instruction as to not waste any time and to keep the aircraft moving but at a safe speed.
As both aircraft were about to enter the intersection of taxiways Golf and Charlie (see figure 1), the copilot of the Dash 8, seated on the right of the cockpit, saw the Saab. He called for the PIC to stop. The PIC brought the Dash 8 to an abrupt stop as the Saab taxied through the intersection at a constant speed. Neither of the PICs saw the other aircraft until they had crossed at the intersection. The copilot of the Saab was busy with after-landing duties during the initial taxi period after vacating the runway. As the Saab taxied through the intersection he was occupied with a company frequency transmission on the radio and did not see the Dash 8 at any stage. The SMC was unaware of the situation until queried by the PIC of the Dash 8 regarding which aircraft had right of way.
A replay of the surface movement radar (SMR) of the occurrence showed that the taxi speed of the Saab was 26 knots. At 1800:08, the taxi speed of the Dash 8 was 21 knots. It then decelerated and stopped at 1800:11 with the Saab passing directly in front, moving from right to left. The SMR displayed the distance between the aircraft as 42 m. The impression of both PICs was that the aircraft passed in very close proximity.
Normally, pilots use a technique of scanning left and right to check for other aircraft that may conflict with them at intersections. Generally pilots on the left observe the left side and those on the right observe the right side. The standard procedure for the Saab operator was for pilots to survey the area to both the left and the right of the aircraft before entering or crossing any taxiway.
Neither operator had a policy or general guidance material in relation to aircraft taxi speeds, although the Saab operator did require turns in the aircraft to be 'at a speed below 20 kts'. Consequently, the speed at which aircraft were taxied varied between pilots.
Pilots recalled taxi speed limits from their training as 'fast walking pace' but agreed that this was impractical over the long distances involved at Sydney Airport. Neither the Civil Aviation Regulations (CARs) nor the Aeronautical Information Publication (AIP) nominate any specific aircraft taxi speed limits.
A groundspeed readout (utilising a Global Positioning System (GPS)) is available in the Dash 8, but its use is limited by its position on the lower pedestal in the cockpit. This makes it difficult for pilots to cross refer to it while taxiing and looking out for other taxiing aircraft. Similarly, a groundspeed readout is available in the Saab from either the lower pedestal or from the electronic horizontal situation indicator. In addition, a pilot's ability to estimate speed is more difficult at night due to the lack of visual cues used in judging the relative motion of the aircraft with other objects.
The Saab taxi light was unserviceable. The pilots' recollection of the event indicates that the Dash 8 taxi light was not illuminated. Some pilots turn taxi lights off while holding or passing close to other aircraft, to prevent a dazzling effect. The navigation lights and rotating beacons of both aircraft were operating. Both aircraft were backlit to some degree by the domestic terminal and suburban lighting on the far side of the airport.
The SMC was operating the SMC east and SMC west positions on combine. Immediately prior to the occurrence, the controller was endeavouring to manage the arrival and departure from the apron of four aircraft while updating radar system data. System updating included the assigning of labels to radar returns on the SMR for aircraft taxiing for departure. The controller was aware of the disposition of aircraft but his impression, at the time he issued the clearance to the crew of the Dash 8, was that the Saab would pass through the intersection before the Dash 8 would be near that intersection. Consequently, he did not provide traffic information on the other aircraft to either crew, nor did he assess that there was a need to use a segmented or a conditional clearance to either crew. A segmented clearance enables a crew to taxi and to stop at a possible point of conflict, while a conditional clearance enables a crew to taxi subject to specified requirements. In this situation, the Dash 8 crew could have been instructed to taxi and to hold short of the intersection of taxiways Charlie and Golf; or alternatively, to taxi to the runway, with a requirement to pass behind the Saab on taxiway Golf.
Even though crews receive and acknowledge clearances to taxi on the aerodrome manoeuvring areas, the CARs and AIP put the onus on the PIC to maintain a good lookout and observe other traffic to avoid collision.
Both airlines have a procedure where one or both of the pilots listen (and transmit as required) to the company frequency from taxi to just before take off and, from after landing till after shutdown at the parking bay. The Saab copilot was the only pilot reported to be operating a radio at the time of the occurrence.