Qantas B738 and Qantas B738 at Sydney on Apr 29th 2023, loss of separation between go around and takeoff

Last Update: January 31, 2024 / 10:16:15 GMT/Zulu time

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Photo of Qantas VH-VZW, Boeing 737-800 — Qantas VH-VZW, Boeing 737-800 (Photo credit: Ev Brown / Flickr / License: CC by)

Incident Facts

Date of incident
Apr 29, 2023

Classification
Report

Airline
Qantas

Flight number
QF-146

Departure
Auckland, New Zealand

Destination
Sydney, Australia

Aircraft Registration
VH-VZW

Aircraft Type
Boeing 737-800

ICAO Type Designator
B738

A Qantas Boeing 737-800, registration VH-VZW performing flight QF-146 from Auckland (New Zealand) to Sydney,NS (Australia) with 100 passengers and 7 crew, was on final approach to Sydney's runway 16L.

A Qantas Boeing 737-800, registration VH-VZM performing flight QF-540 from Sydney,NS to Brisbane,QL (Australia) with 52 passengers and 6 crew, was instructed to line up and wait immediately after a private jet landed on runway 16L. The private jet took 23 seconds longer to vacate the runway than the controller anticipated. VH-VZM was cleared for takeoff when VH-VZW was 2.4nm before the runway threshold, however, their engine acceleration and stabilization took 4 seconds longer than normal. When the crew of VH-VZM pressed their TOGA button, VH-VZW was 1.7nm from the runway threshold and tower intended to instruct VH-VZW to go around.

However, the tower supervisor instructed the controller, involuntarily, to wait. The instruction to go around was therefore transmitted 12 seconds after the controller had decided to issue the go around instruction. In the meantime the captain of VH-VZW had already assessed that they would likely need to go around and instructed the first officer, pilot flying, to mentally prepare for a go around. The crew acknowledged the go around instruction, initiated the go around at about 450 feet AGL about 1.1nm before the runway threshold.

The controller, although being aware of turn instructions not to be issued below MSA of 2100 feet, instructed the crew to turn left onto heading 125 degrees as per the published missed approach procedure. The crew understood this turn instruction as an amended missed approach, due to work load did not immediately read the instruction back and thus kept runway heading.

The separation between the two aircraft thus reduced to about 330 feet vertical and 0.8nm horizontal before VH-VZW began their turn left and the aircraft settled on diverging trajectories.

Australia's ATSB reported that no TCAS advisory had occurred during the conflict.

The ATSB released their final report concluding the probable causes of the incident were:

Contributing factors

- 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.

- The instruction issued to the arriving 737 flight crew 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.

A number of safety actions were taken as result of the occurrence and investigation into it.

The ATSB analysed:

The aerodrome controller east (ADCE) instructed the crew of VH-VZM to line up and wait on runway 16L. A take-off clearance could not be issued until a landing aircraft had vacated the runway. This aircraft took about 23 seconds longer to exit the runway than the ADCE had originally anticipated. This extra time meant VH-VZW, which was on final approach to land, was about 2.4 NM (4.4 km) from the threshold when the crew of VH-VZM was issued clearance to take off.

On receipt of the take-off clearance, VH-VZM took about 14 seconds to commence the take-off roll due, in part, to a permissible unserviceability affecting one engine’s performance. This delay further compressed the spacing between the 2 aircraft. Shortly after VH-VZM commenced the take-off roll, the ADCE identified the spacing between the 2 aircraft had reduced to a distance that meant VH-VZW needed to go around to maintain the runway separation standard. The ADCE also had the option to not issue take-off clearance to the crew of VH-VZM or cancel it during the 14 seconds it took for the aircraft to commence the take-off roll, and this would have prevented the complication of having one aircraft taking off while another was going around. Having decided to issue a go-around instruction to the crew of VH-VZW, the ADCE’s initial communication was inadvertently interrupted by the tower shift manager (TSM) who said ‘wait’.

The TSM later stated that they did not intend to verbalise anything, and they meant for the ADCE to issue the go-around instruction. This verbal slip likely occurred at a time of high workload as the TSM attempted to assimilate the information associated with the traffic scenario. The verbalisation of ‘wait’ by the TSM interrupted the ADCE’s management of the traffic scenario.

While the TSM did not have the authority to direct the ADCE to issue a control instruction, the TSM was able to provide advice to the ADCE. Consequently, the ADCE deferred the go-around instruction for 12 seconds while they waited for the TSM to provide further guidance, which did not eventuate. This 12-second delay meant VH-VZW was 160 ft lower and 0.5 NM (0.9 km) closer to the threshold (and to VH-VZM) by the time the go-around instruction was finally issued.

Four seconds after instructing the crew of VH-VZW to go-around, the ADCE issued further instructions to the crew that required them to turn onto a heading of 090° at 2,100 ft. These instructions were provided to the crew at a time of high workload when VH-VZW had not yet reached 600 ft when they were meant to turn left onto a heading of 125° in accordance with the missed approach procedure. Consequently, the crew misinterpreted the instruction as cancellation of the missed approach procedure and they did not turn at this altitude. The timing of the additional instruction likely added workload compared with letting the crew undertake the go-around manoeuvre, and the turn at 600 ft, before issuing any further turn instructions.

The TSM and ADCE monitored the 2 aircraft as they climbed away from the runway. During that time the ADCE issued a safety alert to the crew of VH-VZW to advise of the traffic ahead and then vectored the aircraft while it was below the minimum sector altitude. Although contrary to the Airservices Australia standardisation directive, the issuance of this instruction likely reduced the risk associated with the separation occurrence.