Loss of separation

The Boeing 727 VH-ANA was operating on a scheduled flight from Brisbane to Cairns. The aircraft was third in the arrival sequence and had been positioned on left downwind for runway 15 by the Approach One (APP1) controller. The other two aircraft, a Britten Norman Islander VH-INO, and a Boeing 737 VH-TJU, had been sighted by the crew of ANA who were then instructed to make a visual approach and to follow TJU. The co-pilots of ANA and TJU were the pilots flying, and the pilots-in-command were carrying out the non-flying pilot support duties in each aircraft.

The crew of ANA, having been instructed to sight and follow TJU, extended downwind for about 2 NM to ensure adequate separation from TJU. As ANA was turning onto the base leg of the circuit, the Aerodrome controller (ADC) instructed the crew of ANA to continue the approach.

Meanwhile, the pilot of a Cessna 182, VH-DAL, who had been conducting parachuting operations 3 NM west of the aerodrome, was returning for a landing. DAL was being controlled by the Approach Two (APP2) controller. The APP2 controller noted the position of the other arriving aircraft on the radar display. He judged that there would be sufficient time to land DAL between TJU and ANA if the pilot was assigned runway 12, the non-duty runway.

The ADC was the arbiter for the use of the non-duty runway for 'one-off' landings and the APP2 controller co-ordinated the use of runway 12 with the ADC. The ADC concurred with DAL being processed for landing on runway 12, between the landings of TJU and ANA on runway 15.

As TJU was landing, the ADC requested the pilot of that aircraft to hold short of the Bravo 4 taxiway or roll through to Bravo 5 taxiway and to advise his preference. This was to allow an aircraft stopped on Bravo 4 to cross runway 15. The crew of TJU were unable to acknowledge the request immediately, as the aircraft was still decelerating with reverse thrust, and their priority was to complete the landing roll safely. The pilot in command of TJU had not completely understood the instruction and told the co-pilot to disregard it until they had slowed to a safe speed. The aircraft was stopped short of Bravo 4 taxiway, and the crew then advised the ADC that they would hold in their present position. The ADC's intention was to taxi an aircraft across the runway in front of TJU. The ADC advised the crew that he would 'get the jet away' in front of them and, that once it had passed, they were clear to taxi via Bravo 4.

The pilot of DAL had been instructed by the APP2 controller to make a straight-in visual approach for runway 12. The APP2 controller then confirmed with the APP1 controller that he was aware that DAL was being sequenced for runway 12. The APP1 controller acknowledged the advice of the use of runway 12 for DAL. The APP2 controller observed on the radar display that separation between DAL and ANA was reducing. He then contacted the ADC and offered to take DAL out of the arrival sequence and to re-establish the aircraft behind ANA. The ADC declined the offer and stated that if there was insufficient separation with ANA he would instruct the pilot of DAL to go around. The APP2 controller then instructed the pilot of DAL to contact the ADC. The pilot of DAL acknowledged and changed to the aerodrome control frequency.

When the pilot of DAL contacted the ADC and reported on final he did not state which runway he was making the approach for, nor was he required to do so. Also, the ADC did not provide traffic information to either the pilot of DAL or the crew of ANA about the other aircraft, or that both runway 15 and 12 were in use. The ADC instructed the pilot of DAL to continue approach and to expect a go around due to traffic on runway 15. The ADC then requested the crew of TJU to expedite vacating runway 15. ANA was now established on final approach and the ADC instructed the crew to continue approach and to expect a late landing clearance. The crew of ANA were watching TJU closely and the pilot in command assessed that he would have to go around if TJU remained on the runway for much longer. The crew of ANA then reviewed the missed approach procedure. After ANA had passed through 300 ft, the pilot in command decided to go around and was about to instruct the co-pilot to do so when the ADC cleared ANA to land.

As the co-pilot began the landing flare, the crew were surprised to see DAL passing from right to left in front of them. DAL crossed the runway in front of ANA and was cleared to land when at or near the threshold of runway 12. The crew of ANA were of the opinion that a mid-air collision may have occurred had the go-around been executed.

The incident was a result of inadequate management of the arrival sequence, and inappropriate decisions made by the aerodrome controller.

The investigation revealed that there was a need to evaluate the application of separation standards for all controlled aerodromes with intersecting approach and departure paths and runways. In response to the BASI recommendation R970067, Airservices Australia and the Civil Aviation Safety Authority conducted a review of the applicable standards and procedures.

A Boeing 747 was being radar vectored for Melbourne runway 34 when radar controllers noticed an unidentified aircraft at 2,400 ft in controlled airspace (CTA) where it should not have been above 1,500 ft outside controlled airspace. The departures south controller immediately instructed the Boeing 747 pilot to turn right 60 degrees due to conflicting traffic. Melbourne radar advisory service (RAS) gave a general broadcast to any aircraft heading up the western lane to descend immediately to 1,500 ft, but there was no response. Both aircraft passed within approximately 1 mile and 700 ft.

With advice from Essendon Tower, RAS identified the offending aircraft near Brooklyn as VH-LOR and instructed the pilot to descend to 1,500 ft. About 5 minutes later, 2 miles west of Mount Cottrell, LOR climbed above 2,500 ft into CTA without a clearance and was again corrected by RAS.

A student pilot was flying LOR on his second solo navigational exercise. He subsequently advised that he must have made errors in navigation because each time he inadvertently penetrated CTA he thought he was much further west where the base of the CTA was higher.

The student pilot was required to undergo more dual instruction before the next solo navigational flight.

A Boeing 737, enroute to Brisbane and an A300 enroute to Sydney, both departed from RWY 27 at Tullamarine via a DOSEL 4 standard instrument departure (SID) with a requirement to maintain 5,000 ft.  This was to accommodate an AC50 which had departed Essendon on track to Horsham via overhead Melbourne.  After vertical separation was achieved between the AC50 and the two jets, the pilot of the AC50 was instructed to turn onto a heading of 280 degrees and the crew of the Boeing 737 instructed to initially climb to 6,000 ft and then 7,000 ft as the levels became available.  When these two aircraft were laterally clear of one another, the Boeing 737 was cleared to climb to FL200. The A300 was maintained at 6,000 ft.

Shortly after, the controller’s attention was drawn to a potential conflict between the Boeing 737 and an inbound SA227 from Mildura.  The trainee turned his attention to an inbound A320 tracking via a 27 ARBEY STAR assigned 6,000 ft.  This aircraft was now in conflict with the A300 which was maintaining 6,000 ft and was turning right in compliance with the DOSEL 4 SID.   The training officer recognised the conflict and told the trainee to instruct the crew of the A300 to climb.  For reasons unknown, the trainee did not instruct the A300 to climb - nor did the training officer choose to use the 'B' System handset, which would have enabled him to override the trainee's communications and climb the A300.

The trainee then instructed the A300 to turn left onto 340 degrees.  Both the trainee and the training officer agreed that the aircraft seemed to be slow in commencing the turn.  Because of this, the crew was further instructed to turn immediately onto a heading of 310 degrees and the crew of the A320 to turn left immediately heading 090 degrees. The pilot of the A320 reported sighting the A300 and the two aircraft passed within 2 NM at the same level.  There was a breakdown of separation.

ANALYSIS

The 27 DOSEL 4 SID does not provide separation assurance with the 27 ARBEY STAR once the departing aircraft climbs above 5,000 ft.  In such instances, controllers are required to ensure that the vertical separation is maintained until lateral separation is established.

Neither the trainee nor the training officer applied separation assurance techniques.  The controllers relied upon the performance of the A300 to climb above incoming aircraft.  However, because of the AC50 from Essendon, which was tracking overhead Melbourne, the A300 was held at 6,000 ft, resulting in a lower altitude than normal at the position at which the DOSEL 4 SID requires a turn.   In resolving the conflict between the AC50 and the A300, the controllers failed to recognise in time the conflict between the A300 and the A320.

SIGNIFICANT FACTORS

1. The training officer was inexperienced in the approach departures training role.
2. The training officer chose not to use an override system for radio transmissions.
3. The training officer failed to adequately monitor the trainee and to correct the situation as it developed.
4. Neither controller applied adequate separation assurance techniques.

The SAAB 340B aircraft was conducting a regular public transport flight from Wagga to Sydney via the Bindook VOR navigation aid. The crew had been told by ATC to expect an OAKDALE 2 arrival into Sydney. Approaching Bindook the SAAB was instructed by ATC to enter the holding pattern at FL170.

The captain removed his headset to advise the passengers on the public address (PA) system of the delay, whilst the first officer handled the aircraft. Shortly after, as the captain was addressing the passengers, ATC cancelled the holding and instructed the SAAB to turn onto a heading of 180 degrees, on descent to FL160. The first officer acknowledged the cancellation of the holding, and the descent clearance, but did not readback the assigned heading.

When the captain had completed the PA announcement the first officer advised him that they had been cleared to leave the holding pattern and track direct to the Bindook VOR for an OAKDALE 2 arrival. The captain was unaware that the aircraft had been assigned a heading.

ATC noticed the SAAB turning left through its assigned heading and queried the crew, reiterating the required heading of 180. As the SAAB turned right to 180 degrees, the separation between the SAAB and a British Aerospace 146 (BAe 146) was reduced to 3.5 NM laterally and 500 ft vertically. The SAAB passed behind the BAe 146 as ATC issued instructions to the aircraft to increase their vertical separation. The minimum required separation standard was 5 NM horizontal, or 1,000 ft vertical.

The expectation of an OAKDALE 2 arrival may have caused the SAAB's first officer to anticipate a standard intercept from the Bindook aid. This error could have been averted if ATC had queried the lack of the assigned heading readback by the first officer, and if the captain's attention had not been diverted by making a PA announcement. The captain reported that PA announcements are usually made at the commencement of holding, to minimise any conflict with normal ATC communications. However, he had not anticipated that the holding would be cancelled so quickly, and consequently had not heard the ATC instructions.

The Inner North sector controller became distracted with a number of tasks at the position and with other aircraft in the sector and forgot to pass the radar identification of the B737 to the next control position; Departures South. The transfer of aircraft radar identification between the two positions is normally conducted between 35 NM and 40 NM from the aerodrome. This ensures that the next controller is aware of the position of inbound aircraft and can plan the separation with traffic in his sector prior to the aircraft entering the area.

At the time, the Flow controller had not annotated the flight progress strip (FPS) for the inbound B737 with a landing time. The FPS was located adjacent to the Departures South position to enable controllers to appreciate the sequence of arriving aircraft. The Flow controller handed over responsibility to another controller. The new Flow controller observed the inbound B737 on the radar display but did not annotate the FPS with a landing time.

The inbound B737 was maintaining 8,000 ft and was 20 NM from the aerodrome. The Inner North controller noticed that he had not transferred the inbound B737 and instructed the crew to call Departures South. The Inner North controller then electronically transferred the identification of the aircraft to the Departures South position. This was contrary to air traffic control procedures. The controller transferring responsibility for an aircraft must receive notification of acceptance of the aircraft, either by voice or electronically, from the controller receiving the aircraft prior to instructing the aircraft to call. The Inner North controller did not advise the Departures South controller of the late transfer of the radar identification and transfer of the inbound B737.

The Departures South controller was busy with other aircraft in the sector and monitoring the departure of a B737 on a Cowes standard instrument departure (SID) from runway 27. This aircraft had been initially assigned climb to 5,000 ft. The Departures South controller did not observe the inbound B737 within his area of responsibility and approved the outbound B737 to climb to flight level (FL) 200. When the crew of the inbound B737 contacted Departures South, the controller recognised the potential conflict and instructed the outbound B737 to maintain 7,000 ft. The crew of the outbound B737 were unable to arrest the rate of ascent until the aircraft had reached 7,600 ft. The Departures South controller issued traffic information to the crew of the inbound B737 about the other aircraft. The crew were able to observe the outbound B737 and maintain visual separation until radar separation was regained.

The two aircraft passed with vertical separation of 400 ft and horizontal separation of 2 NM. The separation required was 1,000 ft vertically or 3 NM horizontally. There was a breakdown in separation.

ANALYSIS

The Inner North controller became distracted with other aircraft in his sector and did not transfer the identification of the inbound B737 to the Departures South controller as required by local instructions. Consequently, the Departures South controller was not aware of the inbound B737 as the aircraft entered his sector.

The lack of a landing time on the flight progress strip was a missing cue that may have alerted the Departures South controller to the inbound B737. The reason for the landing time not being annotated on the flight progress strip could not be ascertained.

The Departures South controller did not observe the inbound B737 enter his airspace prior to instructing the outbound B737 to climb to FL200. Had he maintained a more regular scan of the display it is possible that he would have observed the inbound B737 entering his sector.

When the Inner North controller became aware that the inbound B737 had entered the Departures South sector without a transfer of the aircraft's identification, he should have contacted the controller on the intercom. This would have alerted the Departures South controller to the presence of the inbound B737 and may have enabled separation to be maintained between the two B737s

SIGNIFICANT FACTORS

1. The Inner North controller did not pass the radar identification to the Departures South controller prior to the inbound B737 entering the latter's airspace.
2. There was no landing time annotated on the flight progress strip for the inbound B737 for the Departures South controller.
3. The Departures South controller did not observe the inbound B737 entering his sector.
4. The Inner North controller transferred the inbound B737 to the Departures South controller prior to conducting radar co-ordination.

VH-ANP, inbound from the north was being processed for a runway 26 ILS approach to Essendon. VH-EAJ, also inbound from the north, was being processed for a runway 27 ILS approach to Melbourne. While tracking in an easterly direction, on a right downwind leg for the ILS approach, the pilot of ANP reported he had Essendon in sight. He then requested priority as a Med 1 flight for clearance direct to Essendon.

To facilitate separation with, and priority for, ANP, EAJ was instructed to climb to and maintain 4,000 ft. Shortly afterwards, when in the Epping area, and maintaining 4,000 ft, EAJ was instructed to turn right heading 310 deg for a vectored right orbit. At this time, ANP was to the east of EAJ, on a southerly heading, on descent to 2,500 ft, on a wide right base for runway 26 and with the mode C level indicating 3200 ft. There was a strong westerly wind blowing above 2,000 ft and this resulted EAJ making good a track of approximately 335 deg from a heading of 310 deg.

The two aircraft passed with less than three miles horizontal separation when vertical separation was 800 ft. The pilot of ANP had EAJ in sight.

An Australian-registered B747 was maintaining flight level (FL) 350 on a flight from Sydney to Hong Kong and was approaching the Zamboanga very high frequency omni-directional radio range beacon (VOR). The crew received a traffic alert and collision avoidance system (TCAS) traffic information advice on an unidentified aircraft at the same level, crossing from right to left and also converging on the Zamboanga VOR. The crew of the B747 initiated a right turn that ensured that their aircraft would pass behind the other aircraft. There was a breakdown of separation.

Investigation by the Bureau in conjunction with the Air Transportation Office of the Republic of the Philippines, established that the other aircraft was foreign-registered and that the crew of that aircraft had been instructed by Manila air traffic control to climb to FL 370 for separation with the Australian aircraft. The instruction had been correctly acknowledged by the crew at the second attempt. The instruction included an authorisation to descend back to FL 350 when 5 minutes past the Zamboanga VOR.

Manila air traffic control had taken all necessary action to ensure positive separation of the two aircraft. The reason why the foreign-registered aircraft was not at FL 370 when passing the Zamboanga VOR could not to be determined.

The crossing point of the two air routes at Zamboanga was not under radar surveillance at the time of the occurrence. However, as part of the improvement to the air traffic system by the Government of the Philippines, a new radar head was commissioned late in 1996 and now provides coverage in this area.

FACTUAL INFORMATION

A Piper PA23 aircraft, VH-MBU, on a right base leg for runway 15 conflicted with a Boeing 737 on left base leg as a result of a co-ordination breakdown.

VH-MBU was inbound from Stoney Creek and the Approach 2 controller co-ordinated with the Tower Controller to process the aircraft via right base leg behind a Boeing 747 on final approach for runway 15. The pilot was asked to sight the B747, which he did. The pilot was then told to contact the Tower. There was no co-ordination with Approach 1. The Approach 1 controller had transferred a Boeing 737 VH-TAK, to the Tower and asked Approach 2 what he was doing with MBU and should TAK be following MBU, to which he replied in the affirmative. TAK was turning left base on Tower frequency by this time. Co-ordination with the Tower Controller alerted him to the confliction. The Tower Controller advised MBU to orbit right base and TAK was allowed to continue with the visual approach.

Radar data confirmed that the two aircraft conflicted with a separation of 1.5 NM at the closest point of approach.

ANALYSIS

The Approach 2 controller failed to co-ordinate the landing sequence with the Approach 1 controller. The reason for this lapse in performance was not positively established but the controller's supervisor thought that a desire to provide an expeditious service may have been a contributing factor.

FACTUAL INFORMATION

The controller started acting as the Aerodrome Controller (ADC) at 1100 local time. Up until 1120, the workload was described as being light. At 1120 the ADC was responsible for one helicopter under the final approach path of runway 32, at three miles from the field and operating not above 500 ft. Another helicopter was flying from the north of the aerodrome towards the same area and had been directed to track to the west of the aerodrome. A third helicopter was waiting at Burleigh for a clearance to enter the zone. A Piper PA28 was about two miles from the runway, on final for runway 32. Two jet aircraft were also approaching final for runway 32, and were about 15 and 22 NM from the airport. An Airbus A320 reported ready at the holding point for runway 32. That aircraft was given a clearance to line up.

Because of the short distance from the parking area to the holding point, the controller was aware that airline aircraft sometimes take a long time to actually line up at the threshold. The cabin crews on the aircraft often need a considerable period of time to complete their briefings and the pilots taxi slowly to avoid stopping the aircraft. The taxiway for runway 32 was situated about 400 m from the threshold of runway 32, with the result that the time taken for aircraft to be ready for take-off could vary. On this occasion the Airbus was reported to have been slow to line up.

The ADC asked the pilot of the Piper if he would accept a landing on runway 35, a 552 m long strip which crossed the main runway about 850 m from the threshold of runway 32. The change was accepted and the pilot was asked to report on final approach. The helicopter under the final approach path was asked how long he wanted to remain there, and was then told to report on completion of his activities. The ADC then cleared the Piper to land on runway 35. Another aircraft reported ready to line up, and the ADC instructed that pilot to hold at the holding point. A helicopter previously cleared to track northbound east of the coast from Point Danger to Burleigh was asked to make an orbit or hold due to a jet aircraft about to take off and turn right.

Departure instructions for the Airbus were obtained from the Approach Controller and the ADC cleared the aircraft for take-off, followed by a right turn to a heading of 120 degrees. During this process he reported checking the radar screen and runway 32. He did not check the flight strips. The controller then devoted his attention to the helicopter under the final approach path for runway 32. He initially asked the pilot to fly to the west of the final path for about three miles. In response the pilot informed the ADC that he was finished in the area and requested a clearance to track to Surfers Gardens.

When a break in this interchange occurred, the pilot of the Airbus informed the controller that the other aircraft was now clear of his path, and asked to confirm whether he was clear to take off. This was re-affirmed. The pilots of the Piper had heard the Airbus being given a take-off clearance and had been prepared to go around if that aircraft began to roll. The pilots of the Airbus had seen the Piper during their pre-take-off scan and had waited until the aircraft had crossed their runway.

As the Piper was originally to land on runway 32, the ADC had that runway marked on the flight progress strip. When the runway was changed the new runway should have been marked. This was not done. In addition, when the second runway was activated he should have notified the Surface Movement Controller. This was not done, reportedly because the aircraft was close to landing and the second runway would only have been active for a short period of time. The controller workload at Coolangatta was described as becoming busy irregularly. During a normal working week controllers were often on duty in the various positions when activity was low.

The controller estimated that he was only able to achieve about two hours per week when the activity was moderate to busy. As a result, he found it difficult to maintain a system which catered for the busy periods. Most of the time he was able to remember the complete aircraft activity picture without reference to the various systems to aid memory, such as the flight progress strips. The investigation was also informed that tower controllers often did not maintain current information on the flight strips when activity became high. There was a need to retain the activity picture in their mind as well as maintaining the information on flight strips, and this latter aspect often lost priority.

Prior to issuing a take-off clearance the controller reported that he should have scanned the runway, the circuit area, the apron and taxiway, the flight progress strips, the active bay, and the radar. On this occasion he did not do all of these actions. He attributed this to the irregular activity pattern which was not conducive to a regular scan.

No deficiencies in workstation layout or visibility from the control tower were found. Ranges to the west of Coolangatta could have made the Piper difficult to see when it was on final approach and close to the runway. The radar display was "gated" so that aircraft within one mile of the runway were not displayed. This was done to avoid the display becoming crowded with returns of aircraft on the ground. Depending on the position of the Piper aircraft, it may have not been displayed on the radar.

During the four days prior to this incident the controller had been suffering from the effects of a virus. Although he had initially started the day not feeling like going to work, by the time he had arrived he was keen to be working.

ANALYSIS

The term 'mental workload' refers to the difference between the amount of information processing resources required by a situation and the amount of such resources available to the person at that time. The controller was probably experiencing a significant mental workload around the time of the incident. The control situation at the time involved a significant number of different aircraft, producing an unusual task load. Although the controller rated the workload as moderate, there were periods of radio communication without significant breaks. The controller's ability to cope with the situation may have been degraded by a lack of proficiency and his recent health, although the influence of these factors could not be established.

The controller was in the habit of retaining all the relevant information and the operational picture in his memory. This meant that he was not in the habit of conducting a full sequence of checks prior to issuing a take-off or landing clearance. One of these checks involved a scan of the flight progress strips. When his workload became high, he was in the habit of allowing activity on the flight progress strips to lapse, in favour of retaining his mental picture. That was the situation on this occasion. He probably developed a "loop", whereby he did not scan the strips because he knew they were not accurate.

When the pilot of the Piper was asked to use runway 35, the aircraft was not far from landing. As far as the controller was concerned it would not be long before that runway was no longer in use. He did not comply with the procedure to notify the SMC of the runway's activation because of the short time the runway would be in use. This action created another gap in the separation assurance of the two aircraft involved. It also excluded the other controller from any cross-checking capability.

The ADC's workload had become high very quickly and he had some problems, other than the aircraft landing on runway 35, that needed his attention promptly. The evidence indicates that he omitted the Piper from his thinking as soon as it had been cleared to land. When runway 32 was scanned prior to giving the Airbus a take-off clearance, the Piper was not seen as it was not near this runway. When the radar was scanned, the Piper was probably inside the area which was not displayed on the screen. The only other prompts which could have reminded the controller about the Piper were a scan of the strips and a strip indicating that runway 35 was active. It is likely that he would have been reminded about the presence of the Piper if he had scanned the strips, even though he had not kept them up to date.

The potential for a more serious incident was averted by the crews of the two aircraft. Both crews were aware of the other aircraft and were prepared to take avoiding action. The crew of the Airbus did not commence take-off until the other aircraft was clear of their runway. They also checked again with the controller to ensure that everything was in order for their departure. The crew of the Piper were prepared to go around and take other avoiding action if the Airbus began its take-off roll. For these reasons, a serious incident involving an inadvertent breakdown of separation was not likely.

FACTORS

1. The controller was experiencing a high level of mental workload.

2. The controller was not maintaining accurate flight progress strip information.

3. The controller did not take the appropriate steps to activate runway 35.

4. The controller forgot about the Piper after issuing a landing clearance.

5. The controller did not scan all the available information prior to issuing the Airbus a take-off clearance.

6. The pilots of both aircraft maintained sufficient situational awareness to avoid each other.

SAFETY ACTION

The use of the team concept by Airservices Australia is being examined by the Bureau as a result of previous incidents. The details of this incident will also be considered during that research.

The incident occurred on the first day of a two-day pageant to celebrate the 75th anniversary of Essendon airport. A flying display was programmed to commence at 1300 eastern standard time, the first event being a multiple parachute drop from VH-KTD. Air traffic services planned to give priority to programmed pageant events. Before and between events, several local operators took the opportunity to conduct joy flights. Joy flight traffic was heavier than anticipated by air traffic services.

Because the wind was a moderate south westerly, air traffic services established a left-hand racetrack pattern using runway 17 for most departures and runway 26 for arrivals.

Shortly before the incident, joy flight aircraft were being recovered to clear the airspace for the paradrop aircraft awaiting clearance for take-off from runway 17. The aerodrome controller decided to require the pilot of VH-NAY, a joy flight aircraft on approach to runway 26, to hold short of the runway 17 strip after landing so he could expedite the take-off of the paradrop aircraft. As NAY was already on final for runway 26 when the controller made this decision, he decided to wait until NAY had landed and slowed to taxying speed before issuing the hold short instruction. However, in the busy traffic situation the controller forgot to issue the hold short instruction to NAY and cleared the paradrop aircraft for take-off. NAY entered the runway 17 strip (gable marker line) as KTD became airborne north of the runway 17/26 intersection.

The incident would not have occurred if operations had been confined to a single runway. However, the use of runway 17 for departures and 26 for arrivals was a standard operating procedure in suitable weather conditions. The procedure increases traffic throughput and reduces aerodrome controller loads. Use of the "hold short" requirement, once the landing aircraft has reduced to taxying speed, was common.

The Essendon tower team leader was rostered on as an extra staff member in view of the expected heavy traffic. However, because he had attended the briefing for participating pilots, and was still relaying the details of the briefing to other tower staff, he had not taken up the position of assisting the aerodrome controller before the incident occurred.

Significant factors

The following factors were considered relevant to the development of the incident:

1. Joy flight traffic was heavier than anticipated.

2. The extra tower controller was still briefing other tower staff and was not assisting the aerodrome controller when the incident occurred.

3. There was a degree of pressure on the aerodrome controller to ensure that joy flight aircraft were on the ground prior to the pageant commencing.