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The crew of a Metroliner, VH-WBA, had flight-planned from Plutonic Mine in Western Australia to Perth. On first contact with Perth Tower, the crew were instructed to continue the approach for sequencing with VH-EAM, which was to land on the crossing runway 24. After EAM had landed and was clear of runway 03/21, the aerodrome controller noted that his 'runway designator' strip had 'Tractor' written on the runway 03/21 section. He observed the tractor to be clear, scanned the runway and, when WBA was 5-6 NM from touchdown, cleared the aircraft to land on runway 21.

At the time the landing clearance was issued, Car 23 was parked on the runway, approximately 200 m from the southern end. The surface movement controller reacted by instructing Car 23 to 'vacate 21 immediately'. Using binoculars, the controller observed the technician throwing tools and equipment into the back of the vehicle. The controller assessed that Car 23 would not vacate runway 21 prior to WBA crossing the runway threshold, and advised the aerodrome controller. The aerodrome controller then directed that the aircraft go around, advising the crew that a vehicle was on the runway. The pilot in command estimated that WBA was at about 400-500 ft when they were instructed to go around and both pilots looked for, but did not sight, the vehicle on the runway.

The surface movement controller was in radio communication with four vehicles. Car 23, a white vehicle equipped with a flashing yellow light, had been on and off runway 21, conducting instrument landing system checks. Tender 1 was being repositioned across the airfield and two tractors were engaged in mowing near the threshold of runway 21. Shortly before the incident, both tractors were clear of the threshold area and Car 23 requested approval to enter the runway for the fourth time. Once on the runway, Car 23 required a 1-minute recall to allow for the clearing of tools and equipment. The surface movement controller noted that no aircraft were due immediately and checked with the aerodrome controller for a clearance for Car 23 to re-enter runway 21. The aerodrome controller approved the clearance, but did not change the 'runway designator' strip notation to indicate that runway 21 was now obstructed by Car 23. The surface movement controller then cleared Car 23 to enter the runway.

The procedures in use at the time required that an individual clearance be obtained each time a vehicle entered or crossed a runway. As a memory prompt, both the aerodrome controller and the surface movement controller utilised red flight progress strips engraved with the words 'runway obstructed'. The aerodrome controller would place the strip above the 'taxiing aircraft' bay on the console. In addition, the aerodrome controller used a red 'runway designator' strip which was divided into three sections indicating the runways 03/21, 06/24 and 11/29.

When a vehicle was cleared onto the runway, the 'runway designator' strip was moved into the console's 'active runway' bay. The vehicle's callsign would then be written in the section that designated the obstructed runway. The annotation of the 'runway designator' strip with vehicle callsigns was not a requirement of the Local Instructions, but was a normal practice of the aerodrome controller.

The use of the flight progress strip was intended to assist the controller's situational awareness in two ways: (1) the action in placing the strip in the appropriate console bay (setting the memory prompt) should have assisted in consolidating a strong memory trace, and (2) the strip could be checked at any time should the controller be unsure as to the status of the item or condition referred to. The use of the 'runway designator' strip with the vehicle callsign notated provided an additional memory prompt.

Four air traffic controllers were rostered for duty in the control tower at the time of the incident. In addition to the aerodrome controller and the surface movement controller, the team leader was in the Airways Clearance Delivery position and a fourth controller was performing administrative duties. The Coordinator position was not occupied because the team leader considered that the traffic was light. Each controller was appropriately trained and rated in their operating position. All were extensively experienced in tower operations at Perth. The controllers had earlier completed a busy traffic sequence and were in a lull between sequences at the time of the occurrence.

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