B738 / B738, Perth Australia, 2018
B738 / B738, Perth Australia, 2018
On 28 April 2018, a Boeing 737-800 exited the landing runway at Perth and without clearance crossed a lit red stop bar protecting the other active runway as another 737 was accelerating for takeoff. This aircraft was instructed to stop due to a runway incursion ahead and passed 15 metres clear of the incursion aircraft which by then had also stopped. The Investigation concluded that, after failing to refer to the aerodrome chart, the Captain had mixed up two landing runway exits of which only one involved subsequently crossing the other active runway and decided the stop bar was inapplicable.
On 28 April 2018, a Boeing 737-800 (VH-XZM) being operated by Qantas on a scheduled domestic passenger flight from Sydney to Perth which had just landed then continued over an illuminated stop bar protecting a different runway on which another Boeing 737-800 (VH-VZL) being operated by Qantas on a scheduled domestic passenger flight from Perth to Sydney was accelerating for takeoff. The TWR controller observed this and instructed the departing aircraft to stop due to an incursion ahead and as it did so, its right wingtip passed 15 metres from the nose of the now stopped inbound aircraft.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). Relevant FDR data from both aircraft was successfully downloaded but that from the inbound aircraft was overwritten after it was not stopped following the incursion.
The people involved
It was noted that the crew of the inbound aircraft were both based at Brisbane and were on the second day of a three day duty sequence together which had been preceded by three days off. This second day’s duty had commenced after a 16 hour rest period and the incursion occurred on their second sector of the day. Although the Captain had operated into Perth “many times, including three to four times in the last 4 to 5 months” he stated that he had only rarely landed on runway 03 and when having done so had always exited onto taxiway ‘D’. He also stated that he was aware that controllable illuminated stop bars had recently been installed at Perth but had not previously encountered them there. The First Officer had recently completed their training to qualify as a 737 First Officer after previously being an Airbus A330 Second Officer. He stated that he had landed at Perth on five earlier occasions of which two, including two landings on runway 03, both of which had been followed by exits onto taxiway ‘D’. No other details of the flying experience of either pilot were provided.
It was noted that the controller operating the TWR position had in excess of 30 years’ experience and that the controller operating the GND (Surface Movement Control) position had three years’ experience at Perth which had followed prior experience as a controller overseas. Both controllers were qualified in all three positions - Runway Control, Surface Movement Control and Clearance Delivery. At the time of the incursion, the GND controller was also operating the Clearance Delivery position.
It was established that prior to their flight to Perth, for which the First Officer was PF, the crew had reviewed relevant NOTAMs and had noted the recent installation of stop bars at Perth. Prior to beginning descent, the flight had been cleared for an RNAV-X approach to runway 03. When briefing for this approach, the First Officer had noted that taxiway ‘J2’ (see the illustration below) was the preferred exit from this runway given their expectation of a Terminal 4 gate and that this would involve crossing runway 06. The First Officer subsequently stated that although he had been aware that the taxiway ‘J2’ intersection with runway 06/24 was a designated ‘hot spot’, he had not mentioned this in the briefing, and had suggested requesting a taxiway ‘D’ exit since this would provide a more direct routing to the terminal. The Captain “recalled the briefing as professional and very thorough” but stated that he had not fully absorbed all of it “due to the level of detail”. He admitted overlooking the significant difference between the two potential landing runway exits, namely that a taxiway ‘D’ exit would occur after crossing intersecting runway 06 whereas the preferred taxiway ‘J2’ exit would occur before crossing it. The failure to appreciate this was in part considered to be due to the fact that during the briefing, he had been referring to the Apron Chart (see the second illustration below), which showed only a small part of runway 06 and none of runway 03 rather than the Aerodrome Chart, given that he subsequently stated that he had “interpreted the First Officer’s point about a requirement to cross runway 06 as being during the landing roll on runway 03”.
The Captain recalled that his initial radio call to Perth TWR had been made “during a period of high workload while closely monitoring the First Officer, the aircraft speed and the approach profile and that the aircraft was not slowing down as quickly as desired”, a matter about which he had previously cautioned the First Officer. As this “high workload” continued, “the request to take taxiway ‘D’ after landing was forgotten” as was presetting the Perth GND (Surface Movement Control) frequency as the COM 1 standby frequency, a routine task since once clear of the landing runway, aircraft were required to automatically change to from the TWR frequency to the applicable GND frequency.
The Perth Aerodrome Chart with relevant locations highlighted. [Reproduced from the Official Report]
The Apron Chart showing the Captain’s erroneous perception of an exit onto ‘J1’ from runway 06 (orange) commenced after crossing runway 06 during landing and the ‘D’ exit route (blue). [Reproduced from the Official Report]
The final approach met the aircraft operator’s stabilised approach criteria and during its final stages, the departing company 737 checked in with TWR and was cleared to line up and wait on runway 06. Once the inbound aircraft had touched down on runway 03, idle reverse thrust was selected by the First Officer and the Captain realised that an exit onto taxiway ‘D’ had not been requested and “believed they were now committed to vacate via taxiway J2”. Since the aircraft was not fitted with a steering tiller for use by the right seat pilot, the Captain routinely took over control at a speed of approximately 60 knots and increased braking to enable a ‘J2’ exit. This left turn onto ‘J2’ commenced soon after the transfer of control at just over 50 knots and as the aircraft passed over the left edge of the runway, the speed was down to 35 knots.
Once the TWR controller was certain that the landed aircraft would vacate onto taxiway J2 they issued the 737 waiting to depart from runway 06 with a takeoff clearance. The inbound aircraft Captain stated that, whilst he remembered hearing the controller issuing a takeoff clearance to another Company aircraft, he did not remember hearing the words ‘runway 06’ in relation to the issue of this clearance.
The GND controller stated that they had been aware that the arriving aircraft had exited runway 03 onto taxiway ‘J2’ and that its crew would soon be calling on their frequency to report that they were at the holding point. However, once the TWR controller had issued a takeoff clearance to the departing 737, they had switched their attention to other aircraft on the apron knowing that the arriving aircraft would not be able to cross runway 06 from ‘J1’ to ‘J2’ for some time and that access to the runway was stop bar protected.
Still not in contact with the GND controller, the inbound aircraft crossed the holding point for runway 03 with a groundspeed of a little over 20 knots. As this happened the First Officer, who was aware of the need to get a runway crossing clearance from the GND controller at about this time, assessed that the Captain was still slowing the aircraft down at a rate compatible with a stop at the runway 06 holding point and assumed that he would stop. The First Officer had then gone ‘head down’ and discovered that the Perth GND frequency had not been pre-set as the COM 1 standby frequency and stated that the time spent referring to the aerodrome chart to find this frequency delayed their initial contact with the GND controller.
The Captain stated that he remembered “briefly looking down to understand the reason for the delay and telling the First Officer what the required frequency was". He also said that although aware that controllable lit stop bars had recently been installed at Perth, he had been “surprised to see one ahead” and, since he believed that the aircraft was on a taxiway which would lead directly to the apron without crossing runway 06, had “rationalised that the stop bar had been mistakenly constructed with omni-directional lighting” and was meant for aircraft taxiing in the opposite direction towards runway 03. The Captain also thought that, given their current taxi speed and how quickly this stop bar had appeared after exiting the runway, it “could not be meant for their aircraft” and having “not recalled noticing any markings that identified runway 06” had intentionally “continued taxiing over the red illuminated stop bar and through the runway 06 holding point” at a recorded groundspeed of 17 knots.
Three seconds later the ATC ‘Integrated Tower Automation Suite’ (INTAS) annunciated the first of two Alerts at the TWR controller’s position which included a synthetic voice stating ‘warning runway zero six stop bar violation’ as the departing aircraft was beginning a rolling takeoff. It was noted that this initial alert was triggered whenever an aircraft was “registered to be 8 metres past the (lit) stop bar” but because the INTAS was being used with default settings, this alert was not duplicated at the GND controller’s position. However, this controller “later reported hearing the aural alert through a speaker at the TWR controller’s workstation” and being thus prompted to look up and see the inbound aircraft entering runway 06 without clearance and the departing aircraft rolling for takeoff on the same runway. At this time they were part way through issuing a pushback clearance to another aircraft whose flight crew had then correctly read this back.
A call from another aircraft then prevented the TWR controller from transmitting but as soon as this stopped, and 7 seconds after the first INTAS Alert, they instructed the aircraft taking off to “stop immediately, stop immediately, runway incursion ahead”. Its Captain subsequently stated that they had been aware of the other aircraft on ‘J2’ and that when the stop instruction was given, they had been “just starting to have an element of doubt as to whether the other aircraft would stop”. The rejected takeoff was initiated at a groundspeed of 58 knots. As the inbound aircraft had by now finally changed to the GND frequency, its crew did not hear the stop instruction and “were unaware of that aircraft’s presence and proximity” so had continued taxiing towards runway 06 at about 10 knots. The inbound aircraft’s Captain then reported having seen that another departing aircraft on the apron had pushed back and was obstructing his expected taxi track and that a turn to the right would therefore shortly be required.
Five seconds after the departing aircraft had been instructed to stop, (and twelve seconds the first INTAS Alert) a second INTAS activation occurred and “warning runway zero six occupied” was annunciated. This too was not duplicated at the GND controllers position and “as the first INTAS alert was still active, both controllers now had multiple warnings sounding, which increased the noise level”. As this second INTAS alert began the inbound aircraft First Officer made an initial call on the GND frequency and said that they were “taxiing for bay 13”. There was no response and it was not clear whether this transmission had been heard, but the Captain stated that at about this time, he had detected in their peripheral vision “an aircraft going faster than would be expected on a taxiway (and) applying the brakes to stop their aircraft”. The First Officer reported that “probably about the time the Captain began braking” they had looked up and realised that their aircraft was not where they had assumed it was and had consequently called “Stop, Stop, Stop”. The departing aircraft completed its rejected takeoff four seconds after its right wingtip had passed the nose of the inbound aircraft as it came to a stop. Just before this, the GND controller asked if the inbound aircraft was on frequency and, when this was confirmed, advised that it had “crossed a stop bar and had a runway incursion and should hold position”.
The Aerodrome Chart with relevant annotations. [Reproduced from the Official Report]
Why it happened
The Investigation considered five aspects of the incursion in some detail. Three of these related to the poor performance of the inbound aircraft flight crew and the other two to the movement area layout and the associated air traffic service provision. In summary the observations made were as follows:
- Flight Crew situational awareness and mental model - the inbound aircraft Captain based his actions on a flawed mental model of the taxi in route taken. His failure to fully adsorb the First Officer’s approach and landing briefing and add his own contribution for when he would necessarily take over control thereafter saw him referring to an inappropriate chart and failing to intervene in respect of potentially relevant runway incursion ‘hotspots’. This made it possible for him to “rationalise” that the stop bar he subsequently saw had been “installed incorrectly”.
- Flight Crew workload and distraction - although the workload prior to landing was unexceptional, workload and distraction at key points in time combined together to result in a situation where the problem with the Captain’s mental model was not detected and corrected. The Captain’s failure to set the GND frequency as the standby frequency after changing to TWR subsequently led to both pilots independently being distracted instead of making the required frequency change to GND and actively validating the taxi route against the aerodrome layout. Further distraction followed when the Captain focussed on an aircraft on the other side of the runway about to be crossed rather than the fact that it was an active runway and no crossing clearance had been given.
- Approach briefing and approach briefing guidance - the briefing was the primary opportunity to identify risks to the safe completion of the flight, but did not include any discussion about the charted hotspot on taxiway J2 or any meaningful discussion of alternative runway exits and the taxi-in implications of them. It was considered that providing explanations of the reasons why hotspots have been designated would help flight crews to better understand and mitigate the risk(s) they were intended to highlight.
- Taxiway location and design – whilst the exit from runway 03 to taxiway J2 was relatively gentle, it did not (quite) ‘qualify’ as an RET despite having a superficially similar appearance to one. This regularly encouraged some pilots to make relatively high speed exits onto it - in the case of the incursion aircraft, the exit was made at a ground speed which was four knots over the 31 knot design limit, which an examination of Qantas’ OFDM data showed was not unusual. The consequence of such an exit was that flight crews may not have sufficient time during a high workload period after landing to identify and comprehend runway holding point visual cues and to allow ATC to intervene, if required, before a runway incursion risk was created.
- Air traffic control response - The TWR controller assumed that the inbound aircraft would most probably have changed to the GND frequency and so when a collision risk presented itself, the most effective immediate action was to instruct the departing aircraft to reject its takeoff. Only after this action had commenced did the GND controller ask if the inbound aircraft was on his frequency which did not accord with appropriate task prioritisation. Although not directly related to the incursion event, during the investigation of it, a number of “limitations with ATC processes that increased the potential risk of other occurrences” were identified:
- INTAS Alerts were not received at the GND controller position if it was combined with other positions.
- Australian communication practices for active runway crossings differ from those recommended by ICAO which state that active runway crossings should be controlled directly by the runway controller.
- If the ICAO method were to be adopted, just-landed aircraft could be required to remain on the runway control frequency until any active runway crossing(s) had been completed.
- If the current frequency transfer policy were to remain in use then at least flights landing on runway 03 at Perth and exiting onto taxiway J2 could be proactively alerted to the runway crossing ahead.
The Investigation formally documented 11 Contributing Factors as follows, two of which (in bold below as in the Official Report) were identified as ‘Safety Issues’ (as was the ‘Other Factor’ also identified):
- The Captain of the arriving aircraft developed an incorrect mental model of the exit taxiways off runway 03, believing the aircraft would not have to cross runway 06 after exiting on either of the potential taxiways (J2 or D). As a result, the Captain did not expect to cross a runway holding point or stop bar and, upon seeing the stop bar was illuminated, incorrectly thought that it must only apply to aircraft coming from the other direction.
- During the approach phase, the Captain's workload as Pilot Monitoring was increased due to the First Officer's low level of experience and the air traffic control request for a high-speed descent. This workload contributed to the omission of routine secondary tasks, such as requesting a taxiway D exit from the TWR and pre-setting the GND frequency.
- After vacating runway 03, the First Officer focused inside the flight deck for a period of time to select the surface movement control frequency, instead of performing the more safety critical task of monitoring the aircraft's taxi path as it approached a known runway incursion hot spot.
- When approaching the holding point for runway 06, the Captain briefly focused inside the flight deck to gain an understanding of the delay in changing to the next frequency. This reduced the time available to notice the holding point signs and lights ahead.
- The Captain taxied passed the runway 06 holding point (with an illuminated stop bar) without an air traffic control clearance.
- After taxiing passed the illuminated stop bar, the Captain was distracted by the presence of another aircraft on the apron. This focused the Captain’s attention on the distant apron area, reducing the likelihood of visually identifying the runway immediately in front of the aircraft.
- During the approach briefing, the flight crew discussed taxiway J2 and taxiway D, but they did not discuss the potential threat of the hot spot associated with taxiway J2.
- Although Qantas provided detailed guidance to flight crews about the content of departure and approach briefings, it did not specifically require aerodrome hot spots to be briefed. [Safety Issue]
- Although some aerodrome navigational charts in Australia had identified hot spot locations, they generally provided limited explanatory information to enhance flight crew understanding or awareness of why the hot spot was there and what actions they could take to mitigate the associated risk.
- The location and design of taxiway J2 at Perth Airport significantly increased the risk of a runway incursion on runway 06/24 for aircraft landing on runway 03. Taxiway J2 was published as the preferred exit taxiway for jet aircraft and, although mitigation controls were in place, they were not sufficient to effectively reduce the risk of a runway incursion. [Safety Issue]
- Although the flight crew of the aircraft taking off on runway 06 were provided with an instruction to stop immediately to reject their take-off, no safety alert or instruction was provided to the flight crew of the arriving aircraft during the period between when the controllers received a stop bar violation alert and the Captain applying the brakes 15 seconds later.
One Other Factor that increased risk was also identified:
- Airservices Australia’s configuration of the Integrated Tower Automation Suite (INTAS) at Perth Airport had resulted in a situation where controllers performing some combined roles had the INTAS aural and visual alerts inhibited at their workstation. As a result, controllers performing such combined roles would not receive a stop bar violation alert or runway incursion alert at their workstation. [Safety Issue]
Two Other Findings were also documented:
- The stop bar alert and the TWR controller’s high level of situational awareness led to a timely instruction to the flight crew of the departing aircraft to stop immediately.
- The high level of situational awareness of the departing aircraft flight crew significantly aided their immediate action to reject their take-off on runway 06 following the controller’s instruction.
Safety Action taken as a result of the investigated occurrence whilst the Investigation was in progress was recorded as having included, but not been limited to, the following:
- Qantas added a requirement to flight crew SOPs to brief on relevant charted hot spots in their departure and arrival and approach briefings to mitigate against the possibility of collision or runway incursion.
- Airservices Australia on receiving a recommendation to that effect from the Civil Aviation Safety Authority, issued an instruction that taxiway J2 would no longer be used for any operations and that this would be included in the ATIS broadcast.
- Airservices Australia changed the A-SMGCS alerting input for the combined GND controller position INTAS so that relevant aural and visual alerts would no longer be inhibited.
- Perth Airport subsequently decided that taxiway J2 should be closed permanently and the pavement has since been removed.
The Final Report was released on 1 October 2020. No Safety Recommendations were made.