On 17 August 2016, a Fokker F50 (VH-FKV) being operated by Alliance Airlines on a scheduled passenger flight from Olympic Dam to Adelaide as 'Unity 3201' crossed an active runway in normal day visibility when taxiing at Adelaide after landing, whilst an Airbus A320 (VJ-VGI) being operated by Jetstar Airways was about to touchdown on the same runway. After initially instructing the F50 to 'hold short', the controller responsible for the taxiing F50 realised that this was no longer appropriate and changed the instruction to expedite the crossing. By this time the A320 had already commenced a go around and passed overhead.
An Investigation was carried out by the Australian Transport Safety Bureau (ATSB). It was noted that the F50 was being operated by a second-in command Captain designated as PF occupying the left hand seat with a Check Captain in command and occupying the right hand seat. The nature of the check/training being performed on the F50 and the experience of the four pilots and the controller who was responsible for the F50 as it taxied to its parking gate was not recorded.
It was established that, after landing on runway 12, the F50 had rolled through the intersection with runway 23 (see the illustration below) before turning left onto taxiway D2 and changing from the TWR to the GND frequency. Once the F50 had cleared the runway intersection, the TWR controller had cleared an A320 to take off from runway 23. Once this aircraft was airborne, TWR cleared another A320 which was on approach to runway 23 and about 3nm from touchdown, to land.
As the F50 taxied along D2, the Check Captain in the F50 saw both the departing and approaching A320s but was unable to make an initial contact call to GND because the frequency was continuously in use for other transmissions. He reported that having initially assessed that they should be able to expect a clearance to cross runway 23, the inability to get the call in led him to instead anticipate having to stop at the 23 holding point. As the F50 approached this holding point and the crew were still unable to get a transmission in, the GND controller, who had seen the aircraft taxiing towards runway 23 on D2 transmitted, without a break from their previous transmission "Unity 3201 hold short of runway 23, I’ve got you going to 50 Golf".
The Check Captain of the F50 subsequently advised that the start of the GND controller's transmission appeared to have been missed and that what they had actually heard was "runway 23 and I’ve got you for bay 50 golf". He further stated that "as the instruction included the parking bay", he had assumed that the instruction had been to cross the runway and had therefore read back "cross runway 23 to 50 golf, Unity 3201". The GND controller reported believing that the pilot had read back "short runway 23..." and had assumed that the word ‘hold’ had been ‘clipped’. Both F50 pilots reported that they had "thought they had received a clearance to cross runway 23".
The ground tracks of the two aircraft involved up to the point of conflict superimposed on the aerodrome chart [Reproduced from the Official Report]
The TWR controller saw the F50 on D2 and heard the GND controller say ‘hold short’, but did not hear the crew readback. They reported having then "scanned runway 23 to check it was still clear for the landing A320, which was then over buildings and less than 30 seconds from touchdown and then commenced a handover of the TWR position to another controller".
The F50 PF then checked to their left, called "clear left" and began to taxi the aircraft across runway 23. The Check Captain then reported having looked to their right, sighted the A320 and concluded that "it was a lot closer than they had expected". The GND controller saw the F50 crossing the holding point and responded with "hold short" and then, having immediately realised that this was not an appropriate instruction, followed it with "Unity expedite expedite Unity". The GND controller saw that the A320 had begun a go-around from a low height and was at about 100 feet above the runway. With the go around already initiated, the TWR controller issued the same instruction. The F50 completed the crossing of runway 23 onto taxiway D1 without its PF seeing the A320, which subsequently made an uneventful second approach to a landing.
The Investigation documented a number of specific points considered to be of potential relevance to the investigated event including:
- The controller’s addition of bay information to the hold short instruction was not consistent with the requirements in the MATS which expressly stated "Do not include positions beyond required intermediate holding points in taxi instructions". Its addition to the end of the hold short instruction misled the pilots.
- The F50 PF was normally based in New Zealand where any crossing of an active runway requires pilots to obtain clearance on the TWR frequency.
- Controllers at Adelaide are required to use headsets which means that although they can hear colleagues’ transmissions, they can only hear responses on their own frequency. The ANSP Air Services Australia commented that "although some controllers prefer speakers to increase their situational awareness, it may also result in considerable noise when all three positions are open during periods of increased traffic" and that "such noise is particularly distracting for controllers that have transitioned from an en-route environment where headsets are used and speakers are not permitted." The ANSP also stated that "ATC procedures are designed to ensure controllers can perform their duties safely without reliance on speakers (and) the use of speakers does not always increase situational awareness and should not be relied upon as an effective threat barrier."
- It was noted that applicable ATC procedures were unequivocal and "require the controller to obtain a correct readback of instructions" and that in its absence "the controller must challenge the readback until they are satisfied it is correct".
- The off-going TWR controller "commented that since the incident, in a similar situation, they would wait for the aircraft to land before commencing a handover".
- Controllers involved expressed the view that had controllable stop bars been installed, the event would more than likely not have occurred.
In respect of the clipping of controller transmissions, the ANSP reported to the Investigation that after the event under investigation had occurred, they had reviewed the radio technique of "a controller in the tower" who was using a foot-operated PTT mechanism and had found that "there was a significant (0.5-1 second) delay from the end of the delivery of the instruction by the controller to the time when the controller disengaged the foot PTT". It was observed that "this anomaly in the controller's technique resulted in a number of clipped pilot transmissions due to them starting the readback whilst the foot PTT was still engaged". This controller's error was reported to have been "rectified by using the in-line PTT which decreased the delay of the controller releasing the foot PTT".
The Investigation commented that:
- The A320 crew were "to be commended for their actions in preventing a potentially more serious incident occurring".
- The F50 crew probably thought they were cleared to cross the runway because of the bay allocation added at the end of the hold short instruction - although an effective sighting of the approach A320 "may have led them to query their understood instruction to cross the runway".
- The GND controller did not hear the full response of the F50 crew to his initial "Hold Short" instruction and having "mis-heard" the word "short" instead of the word "cross" actually transmitted, he had then "assumed" that the word ‘hold’ had been clipped from the transmission.
- The TWR controller had commenced position handover when the A320 was on very short finals and as a consequence had ceased watching both it and the runway as they "assumed that the F50 would hold short and that the runway was clear".
It was considered that effective communication serves to minimise the risk of runway incursions and that the investigated event had highlighted the importance of:
- controllers and flight crews using correct phraseology
- controllers and pilots challenging instructions which they have not heard or understood fully
- pilots looking carefully for aircraft or other hazards before entering an active runway.
The Final Report was published on 22 November 2016. No Safety Recommendations were made.