Saab SF-340B, VH-SBA, Sydney Aerodrome, New South Wales, on 2 September 1999

The aerodrome controller (ADC) had given a conditional clearance for a Saab 340 to line up on runway 16R behind a landing Boeing 737 (B737). The ADC then gave the surface movement controller (SMC) a conditional clearance for a Boeing 767 (B767) to cross runway 16R at taxiway Lima when clear of the landing B737.

The SMC issued the clearance for the B767 to cross the runway as the B737 vacated the runway at taxiway A4. The ADC observed the B737 vacate the runway at taxiway A4 and cleared the Saab 340 for take-off. The pilot of the Saab 340 rejected the take-off clearance and advised the ADC that there was a B767 crossing the runway.

The Manual of Air Traffic Services (MATS) 6-2-3 paragraph 31 stated:

"Before clearing an aircraft for take-off, and immediately before the take-off is commenced, the tower controller shall make a visual check from the control tower to determine, as far as practicable, that the take-off path is not obstructed." The ADC made a visual check of the runway, however, he only scanned between the runway 16R threshold and the point where the B737 was vacating the runway at taxiway A4. The ADC did not continue the visual check through to the upwind end of the runway. The B767 was crossing the runway at taxiway Lima, which was between taxiway A4 and the upwind end of the runway. The ADC had forgotten about the conditional clearance given to the SMC for the B767 to cross the runway.

There was no standard practice in Australia for the use of "blocking strips" or "memory prompts" by controllers to alert them of the presence of aircraft not under their direct control crossing or entering an active runway. In this particular incident, the ADC did not use, nor was he required to use, a memory prompt to remind him of the conditional clearance given to the SMC for the B767.

In establishing that the take-off path of an aircraft was unobstructed, controllers were required to make two separate visual observations of the take-off path; one before issuing the take-off clearance and another before take-off was commenced. In addition, controllers needed to have a high level of situational awareness about the movement of other aircraft on the airfield, particularly aircraft subject to conditional clearances to cross or enter an active runway given to aircraft under the control of the SMC. In this particular incident, the controller relied on short term memory to maintain awareness of the clearance issued to the B767.

Memory prompts can assist controllers to maintain situational awareness. Items such as pens, blank flight progress strips or the like were commonly used by controllers to act as memory prompts, but their use was inconsistent in application. Use of some form of memory prompt in this particular incident may have helped to maintain the controller's situational awareness and enhanced the effectiveness of the controller's visual scanning of the take-off path.

The ADC did not adequately scan the runway prior to issuing a takeoff clearance to the crew of the Saab 340 or immediately before the takeoff was commenced.
The ADC forgot about a conditional clearance issued to the crew of the B767 to cross the runway at taxiway Lima.

Local action

As a result of its investigation into this incident, Airservices Australia suggested the following safety actions:

"Sydney Tower experiment with the use of blocking strips for aircraft crossing runways to see if a satisfactory method of usage can be found which is beneficial to controllers."

"Team Leaders discuss with their teams the relevance of surface movement controllers taxiing aircraft, which require runway crossings, by taxiways which provide the ADC with the optimum view of the aircraft and present the best opportunities for expeditious runway crossings."

Australian Transport Safety Bureau action

As a result of this and other occurrences the Australian Transport Safety Bureau, formerly the Bureau of Air Safety Investigation, is currently investigating a safety deficiency. The deficiency relates to the use of conditional clearances for runway entry and runway crossings by vehicles and aircraft and procedures used by air traffic controllers to alert themselves that vehicles or aircraft are on an active runway.

Any recommendation issued as a result of this deficiency analysis will be published in the Bureau's Quarterly Safety Deficiency Report.