Description

On 16 June 2017 a Bombardier DH8-400 (OE-LGJ) being operated by Austrian Airlines on a scheduled international passenger flight from Prague to Vienna and an Airbus A319 (OE-LDG) being operated by Austrian Airlines on a scheduled international passenger flight from Vienna to Podgorica as OS727E lost prescribed separation in daylight and broken cloud whilst under radar control. Both flights received and responded to TCAS RAs which resolved the collision risk.    

Investigation

A Serious Incident Investigation was carried out by the Federal Safety Investigation Authority (FSIA) of the Austrian Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (BMK). Relevant QAR and ATS recorded data were available.

The 39 year old DHC8 Captain, who was acting PF, had a total of “approximately 9,679 hours” flying experience and the 28 year-old First Officer held an MPL and had a total of “approximately 231 hours” flying experience after gaining his licence almost six months previously. The 45 year-old A319 Captain, who was acting as PF, had a total of “approximately 15,072 hours” flying experience and the 26 year-old First Officer held an MPL and had a total of “approximately 586 hours” flying experience. The aircraft type element of each pilot’s experience was not recorded. The 32 year-old (Executive) radar controller had 7 years experience and the investigated event occurred approximately 30 minutes after he had taken a 60 minute rest period which had ended six hours after his shift had commenced.

What Happened

The inbound DHC8 was approaching from the northwest and working Approach Radar. All the available sectors were combined at the time of the event due to what were assessed by the supervisor at the Approach Control Unit to have been acceptably low traffic conditions. It had originally been intended that the DHC8 would be positioned for a right hand downwind join for runway 34 but it was then decided to slightly reduce its track miles by providing vectors to a left base join for the same runway. Accordingly, it was instructed to fly a radar heading of 145° and descend to 8,000 feet.

Shortly after this, the Airbus A319 which had just taken off from runway 29, called on frequency following the SASAL 2C SID passing 4,000 feet for 5,000 feet and on what was an almost southerly track (see the illustration below). It was re-cleared to 6000 feet and the DHC8 was then re-cleared to 7,000 feet. About half a minute later, the A319 requested permission to deviation slightly to the right to avoid weather which was approved. This approval was followed by what the controller intended to be an instruction to the DHC8 - which was ahead of the A319 - to turn left onto 090° in order to join right hand for runway 34 rather than the more usual left hand join. This intended change of plan resolved two tactical control problems - the fact that the aircraft was assessed to be too high for a left base join and the horizontal separation issue with the departing A319. However, this left turn was unintentionally issued as an instruction to turn right onto 090° - 270° turn. This error was not recognised by the controller despite being queried by the DHC8 crew before they acknowledged it on the grounds that it was ‘unusual’. The flight was then transferred to Wien Director for the provision of further inbound radar vectoring as per standard procedures. The A319 was then cleared to climb to FL230.

Aircraft tracks (A - inbound DHC8, B - departing A319) superimposed on the runway 29 SID chart. [Reproduced from the Official Report]

The DHC8 made its initial call to Wien Director by reporting it was in a right turn onto 090° to which the controller’s response was to request confirmation that a right turn was being made which was given. Seven seconds later, with the A319 passing 6,400 feet, the Wien Radar controller instructed it to “stop climb immediately” and three seconds later, the crew of the DHC8 received a TCAS TA in respect of the A319. At this point, ATS monitoring equipment recorded a separation between the two aircraft of 2.9nm laterally and 500 feet vertically and the A319 received a TA against the DHC8.

Two seconds later, Wien Director advised the DHC8 of ‘essential traffic information’ by transmitting “traffic on your right wing, same altitude, climbing through your level”. Two seconds after this, an STCA activation occurred as a result of separation between the two aircraft being reduced to 2.6nm laterally and 200 feet vertically. After a further two seconds, Wien Radar advised the A319 of ‘essential traffic information’ by transmitting “essential traffic, 12 o'clock, 2.3 nm crossing, left to right, miss-navigating, same altitude” - the vertical separation recorded by ATC at that time was 200 feet with the A319 still climbing and the DHC-8 maintaining 7,000 feet as cleared.

After a further three seconds, the DHC8 reported having sight of the A319 and almost immediately both aircraft received coordinated TCAS RAs, the DHC8 to DESCEND and the A319 to CLIMB with the ATC recorded separation 2.3 nm laterally at the same level. One second after these RAs, the DHC8 was cleared to descend to 6,000 feet and 4/5 seconds after them, both aircraft reported ‘TCAS RA’ to their respective controllers. Neither controller acknowledged these calls and both flights received ‘Clear of Conflict’ annunciations after 24 seconds. Both aircraft then reported the resolution of the conflict but again reference to a completed response to the TCAS RA by the A319 was not acknowledged. The development of the conflict which followed the radar controller’s unintentional right turn instruction to the DHC8 is shown in detail in the illustration below.

An annotated depiction of the loss of separation with the DHC8 in green and the A319 in blue. [Reproduced from the Official Report]

The Investigation compared relevant altitude data from the available QAR and ATS data sources and noted a multi-second delay in the presentation of Mode S sourced altitude data on a controller’s screen and its resolution of 100 feet even though the original Mode C data resolution is 25 feet.

Why It Happened

It was noted that provision of an expedited inbound routing for the DHC8 was a routine practice although usually descent would have been achieved earlier so that the aircraft altitude would have allowed a left turn to be made directly onto a left base for runway 34 to be given rather than to a facilitate a right hand downwind join.

It was noted that the in-position Radar controller was seated in between their ‘planning controller’ and the Director controller. Having believed that he had instructed the DHC8 to turn left when he had in fact directed it to turn right the long way round, (classified as a ‘slip’) the radar controller then saw to no reason not to clear the A319 to continue climbing and therefore did so.

It was concluded that at the latest when the DHC8 made its initial call to Wien Director, this controller had identified the conflict between the two aircraft. However, whether he had then verbally asked the radar controller about the intentions of this aircraft and thereby made him aware of the conflict or whether the radar controller had first recognised it himself was not established. In any event, 2/3 seconds after the initial DHC8 call to the Director controller, the radar controller had looked at the extended traffic labels on his screen and become aware that the DHC8 was making its instructed head change using a right turn instead of the expected left turn. Having then assessed that the aircraft was likely to cross tracks with the A319 when both aircraft were in the vicinity of 7,000 feet, his almost immediate response was to instruct the A319 to “stop climb immediately”. As he did this based on the situation display available to him (which was delayed not current), both aircraft were on intersecting tracks with approximately 3.2nm lateral separation and received TCAS TAs in respect of each other with their vertical separation decreasing as the A319 climbed through 6,550 feet and the DHC8 descended through approximately 7,100 feet

Given the actual separation, it was noted that the instruction to the A319 to stop climbing was “inappropriate” - had it not been issued, it was calculated that continued climb of the aircraft would have created a 1,200 feet vertical separation. Instead, as both vertical and lateral separation reduced, an STCA activation was triggered at 1000 feet vertical separation and 3 nm lateral separation - the applicable ATC minima - and displayed in red on both controllers’ screens. This resulted in them issuing traffic information to the aircraft under their control and was followed by the DHC8 crew reporting that they had the A319 in sight. This prompted the Director controller to instruct the DHC8 to continue descent to 6,000 feet, “probably to increase vertical separation”. Both aircraft then received coordinated TCAS RAs as the A319 levelled off at approximately 7,050 feet and the DHC8 continued its right turn having reached 7,000 feet as cleared and both aircraft followed them.

The context for the radar controller’s original error in transmitting an instruction to the DHC8 to turn right instead of left which precipitated the subsequent conflict was assessed to have been overload due to an increase in traffic and requests for weather avoidance deviations. His subsequent error in instructing the A319 to stop its climb which had further reduced separation was considered to have been prompted by surprise and not been informed by an assessment of the trajectories of the two aircraft on his situation display. It was therefore only the correct response of the crews of both aircraft to their respective TCAS RAs which increased the achieved separation although this was still less than the prescribed minimum in respect of both vertical and lateral separation in the Class ‘C’ airspace where the conflict occurred.

It was also noted that the radar controller’s ‘instant’ instruction to the A319 to immediately stop the climb they had just commenced occurred was almost immediately followed by the TCAS TA. It was considered that the resulting traffic situation displayed on the crew’s navigation displays (ND) would have represented “an unusual and contradictory situation” since they would have observed vertical separation continuing to reduce to an indicated ‘00’ when they had “most likely expected it to increase”. This was considered likely to have resulted in a significant increase in pilot workload even before the TCAS RA which followed. It was considered that this could have been linked to the inadvertent failure of the crew to switch off the FD when disconnecting the AP to respond manually to the RA which on this aircraft resulted in the FD indicating a command pitch attitude at variance with the RA command. However, it was accepted that this omission had not affected the crew’s response to the RA. It was noted that all Airbus A320 series aircraft delivered after 2017 had been equipped with a new AP/FR TCAS Mode which when selected means that a TCAS RA response is within the capabilities of the AP and so it is no longer necessary to disconnect the FD.

The Probable Causes of the investigated Serious Incident were formally documented as:

• Confusion of ‘right’ with ‘left' by the Radar Executive Controller when instructing the arriving aircraft to change heading.
• A subsequently inappropriate instruction to the departing aircraft to stop its climb to resolve the conflict with the inbound aircraft:

Five Probable Contributory Factors were also identified as:

• Prevailing cold front with thunderstorm activity in the Vienna TMA.
• Lack of listening capability of several aircraft that were on the Wien Approach Radar Control frequency.
• Overload of the Radar Executive controller due to traffic deviation attributable to the prevailing weather conditions which resulted in a complex traffic situation and a busy frequency.
• The update rate and the time-delayed altitude display of the surveillance radar system.
• Insufficient consideration of the weather in ATC sector planning.

Safety Action taken by the ANSP was noted to have consisted of scheduling by default an additional approach radar sector to cover the regularly higher traffic level typical of the period around midday and notifying their controllers of their reasoning that it would relieve the workload of individual controllers at this time.

The Final Report was published in the definitive German language and simultaneously in an English language translation on 6 July 2023. No Safety Recommendations were issued as result of the Investigation “as the air navigation service provider had already taken (relevant) actions” and no explanation was provided as to why over six years had elapsed before the Report had been published.

Related Articles

• Loss of Separation
• ACAS Resolution and Traffic Advisories
• Controller Workload
• ATM Sector Management
• Surprise
• Cumulonimbus (Cb)