Description

On 29 April 2023, a Boeing 737-800 (VH-VZW) being operated by Qantas on a scheduled passenger service from Auckland to Sydney as QF146 was instructed to go around when on short final because another Boeing 737-800 (VH-VZM), also being operated by Qantas on a scheduled passenger service from Sydney to Brisbane as QF540, had yet to get airborne following a delayed landing aircraft clearance. The subsequent minimum separation was found to have been 0.8nm laterally and 330 feet vertically. The airport was operating two parallel runways in mixed mode.

Investigation

An Investigation into the event was carried out by the Australian Transport Safety Bureau (ATSB). It was noted that the Captain of the inbound 737 had a total of 13,220 hours flying experience which included 9,430 hours on type and their First Officer had a total of 8,100 hours flying experience which included 2,650 hours on type. The Captain of the departing 737 had a total of 12,413 hours flying experience which included 2,301 hours on type. The First Officer had a total of 11,700 hours flying experience which included 7,800 hours on type. The Controller involved had “about 14 years experience” as a military TWR and APP controller and then with civil ANSP Airservices Australia. The Tower Shift Manager had “about 10 years experience” as an En-route and Tower controller with Airservices Australia and had been at Sydney Airport TWR for the most recent five of those years. They held current endorsements for the TWR, GND, Delivery and TSM positions and had been a TSM for about a year.

After reviewing the available evidence, it was concluded that none of the pilots or controllers involved were likely to have been experiencing a level of fatigue which would have had an adverse effect on their performance.  

What Happened

The prevailing weather conditions were benign, the wind was light and no low cloud was present. As a landing VLJ passed the runway 16L threshold, the TWR controller cleared the departing 737 to line up and wait on runway 16L at the full length and advised that the heading in departure would be 140°. Having acknowledged the instruction and taxied into position, the crew awaited takeoff clearance. When the controller saw that the VLJ was taking longer than anticipated to vacate the runway, he instructed it to expedite the exit. The Investigation found that this aircraft had taken well over a minute to vacate the runway - over 20 seconds more than the controller had anticipated. Once it was clear, the 737 was cleared to take off with the inbound 737 at about 2.4 nm from the runway threshold.

The departing 737 crew advanced the thrust levers to the stabilisation position and waited for both engines to stabilise which took a few seconds longer than normal because of a related ADD but TO/GA thrust was then selected and takeoff commenced. As the takeoff roll began, the inbound 737 was about 1.7 nm from the runway 16L threshold. Its PM Captain reported having assessed that the spacing between the two aircraft had reduced to a distance that would probably lead to a go-around and advised the First Officer to “mentally prepare for a possible go-around”.

The Controller, aware of the reduced spacing between the two aircraft and the potential for compromised on-runway separation, decided to instruct the inbound aircraft to go-around and alerted the TWR Shift Manager (TSM) to the situation which prompted him to look at the runway and begin assessing the traffic scenario. The Controller then began to issue the go-around instruction but as he was speaking, the TSM said ‘wait’ and having hesitated for a moment, he then completed his transmission by instructing the inbound aircraft to continue the approach. The TSM later recalled having “inadvertently spoken” but that his intention had been for the controller to continue with the go-around instruction. The controller then waited for further input from the TSM but there was none (although the TSM subsequently recalled gesturing for the controller to continue with a go-around) and so the controller advised his intention to order a go around and then did so. This instruction was acknowledged with the inbound 737 at just over a mile from the threshold and initiated immediately from about 450 feet agl. As it did so, the controller added that the flight should turn left onto radar heading 090° on reaching 2,100 feet.

Although the flight crew heard this instruction, it was not acknowledged immediately due to the workload associated with initiating a go-around but they reported having interpreted it as an “amended missed approach instruction” and therefore continued to track the extended runway centreline instead of following the SID left turn onto 125° at 600 feet agl. 

Meanwhile, the departing 737 was still accelerating and had passed a recorded 87 KIAS and had heard the go-around instruction issued to the other 737 and were expecting the aircraft to fly the published missed approach procedure which would provide sufficient spacing between the two aircraft although the Captain then began to monitor the TCAS display which showed a ‘proximate traffic’ indication for the other 737. Twenty seconds later, the controller asked if the going around aircraft crew had received the instruction to turn left onto 090° on passing 2,100 feet which received an affirmative response. The controller then visually monitored the two aircraft as they both climbed and also had available a surveillance display which showed the location of aircraft in the vicinity of the airport.

At this point, the TSM stated that he had no longer been ‘comfortable’ with the separation between the two aircraft and had advised the controller that they should issue a safety alert to the going around aircraft and turn it away from the departing one. The controller then did this transmitting “safety alert traffic 12 o'clock low heading 140 if you're visual turn left now” and the crew responded by initiating a left turn whilst climbing through a recorded 1,533 feet. When the TSM subsequently observed that the going around aircraft was not climbing as fast as expected and suggested a further turn onto a heading of 060°, the TWR controller acted accordingly on the basis that to do so “carried less risk than permitting the existing heading to continue until the aircraft reached the MSA” and the crew acknowledged. The controller stated that although the aircraft was still below the applicable MSA when he issued the instruction to continue the turn, he had been “confident that the aircraft was above the highest obstacles in the vicinity”. This instruction was acknowledged by the flight crew and the aircraft began the turn onto the new heading.  

Six seconds later, the closest slant range between the two aircraft occurred as separation reduced to 0.8 nm laterally and 330 feet vertically (see the illustration below). Neither crew had visual contact with the other aircraft and no TCAS RA was issued but the controller and the TSM stated that they had both had the two aircraft in sight throughout the episode. As the going around aircraft continued its turn, the two aircraft began to diverge and shortly afterwards, the inbound (going around) aircraft was transferred to Departures for repositioning to a subsequent landing on runway 16R and the outbound aircraft continued on track to Brisbane. 

The ground tracks of the two aircraft involved. [Reproduced from the Official Report]

Discussion

The responsibilities of the runway controller and the TSM

Given the tactical interaction between the runway controller and the TSM during the loss of separation under investigation, the authority of each under Airservices Australia SOPs was examined. These stated that the purpose of supervision by the TSM was “to provide tactical management of risks while maintaining efficient air traffic operations” and involved the “observation of air traffic service delivery and, where necessary, supporting, intervening, or directing activities within the area of responsibility”.

However, the procedures relating to the TSM’s operational command authority (OCA) responsibilities were noted to state the following:

“OCA does not give the holder the authority to instruct an operational controller to take certain actions such as directing a controller to issue an operational control instruction. The operational controller is always responsible for traffic separation but may accept advice from the OCA holder.”

This OCA held by the TSM therefore gave him ‘authority to make decisions on behalf of a unit’ (in this instance the Sydney Tower) which they described as “involving a ‘duty of care’ for the overall safety of aircraft traffic managed”.  

The Airservices Australia SOPs therefore required the TSM to “initiate and manage necessary short-term mitigation actions” if a situation occurs that has or may cause a risk to continued safe operations. These mitigations were described as including “workload management, reduction in task complexity and additional support, observation or supervision”. Both the controllers involved in the event under investigation stated that “if the situation necessitated it, it was acceptable for a TSM to issue an instruction to a controller to de-conflict an impending or actual unsafe scenario”.

Visual Separation

The separation provided during the potential conflict between the two 737s was visual which required the controller to visually observe the two aircraft and apply an azimuth (horizontal) spacing between them. The MATS advised that such a task should consider:

• aircraft performance characteristics particularly in relation to any faster following aircraft and closure rates
• the position of the aircraft relative to each other
• the projected flight paths of the aircraft  
• the possibility of a TCAS RA 
• the weather conditions  
• the possibility of visual errors

It further cautioned that visual separation by judgement of relative distance or height should only be used when there are “wide margins and there is no possibility of the aircraft being in close proximity” and noted that “visual determination of the relative distance of aircraft in close proximity can be in error or affected by optical illusion”.  

Any visual traffic separation provided by Sydney TWR controllers at night (or during IMC) was also locally required not to involve the issue of turn instructions prior to an aircraft having reached the 2,100 feet MSA. The Controller stated that they had been aware of this restriction.

It was noted that the Investigation had concluded that “the visual and surveillance information available to the controllers in the tower, accounting for the possibility of visual errors, was sufficient for visual separation".

Runway Traffic Separation Standards

The MATS did not permit a take-off clearance to be issued until the controller has visual contact with an arriving aircraft and is likely to land or separation can be assured if the arriving aircraft makes a missed approach. In the event under investigation, the MATS “did not permit the landing aircraft to cross the runway threshold until the departing aircraft was airborne and had either commenced a turn or was beyond the point on the runway at which the landing aircraft could be expected to complete its landing roll and there was sufficient distance to enable the landing aircraft to manoeuvre safely in the event of a missed approach”.  

Two Contributing Factors were formally documented based on the Findings of the Investigation as follows:

1. The go-around instruction issued by the aerodrome controller was delayed by about 12 seconds due to an inadvertent interjection by the Tower Shift Manager.
2. The instruction issued to the flight crew of the arriving 737 by the aerodrome controller subsequent to the go-around was interpreted by the flight crew as an instruction to cancel the published missed approach procedure and continue on the runway track before turning at 2,100 ft. Consequently, the 737 flight crew did not turn left at 600 ft as required by the procedure.  

Safety Action taken by ANSP Airservices Australia as a result of the event was that it had or would undertake the following: 

• A detailed analysis of landing runway occupancy times at Sydney, and possibly other major aerodromes, to determine expected runway occupancy times for different types of aircraft (and operators) and conditions (runway direction/wind/time of day/surface condition). This data will be disseminated to ATC to assist when managing runway separation standards.
• The addition at Sydney of defensive controlling techniques and minimum assignable altitudes applicable to go around scenarios, in particular at night or in IMC.
• A safety assurance review to examine go-arounds at Sydney involving a second aircraft and requiring controller intervention. Specifically, this will analyse how separation and terrain clearance is being managed and possible reasons behind any unfavourable trending.
• A ‘Standardisation Directive’ has been disseminated to Sydney TWR controllers referring to MATS 9.4.1.2.1 and outlining the importance of using the (aerodrome) MSA of 2,100 feet as the standard at night or in IMC as applicable during go around scenarios.
• Night-time go around scenarios (have or will be) added to compromised separation training.
• A Standardisation Directive (ATS_DIR_23_0037) has been disseminated to Sydney Tower referring to the National Air Traffic Services Administrative Manual initial occurrence response requirements to be followed and the importance of taking steps to assess the risk associated with the potential safety occurrence.

In Conclusion, a Safety Message based on the Investigation Findings was included in the Report as follows:

Aerodrome Controllers are required to maintain an orderly flow of air traffic, with minimal delays, while ensuring safe separation between arriving and departing aircraft. This complex operation requires controllers to exercise their professional judgement when applying visual separation standards in a variety of environmental and procedural scenarios. In such a setting, it is inevitable that errors will be made by controllers and pilots alike. Consequently, the sociotechnical system within which these activities take place should be designed to be resilient of these errors and reduce the impact individual actions can have on the overall safety of operations.

The Final Report was released on 31 January 2024.

Related Articles

• Loss of Separation
• Parallel Runway Operation
• Missed Approach
• Pilot Workload
• ATC Team Coordination