On 17 January 2018, an Airbus A320 (PK-GLH) being operated by Citilink Indonesia on a scheduled domestic passenger flight from Pontianak to Surabaya as CTV678 and an Airbus A320 (PK-GTA) being operated by Citilink Indonesia on a scheduled domestic passenger flight from Balikpapan to Surabaya as CTV635 lost separation in night VMC whilst in the same holding pattern under radar control but coordinated TCAS RA responses resolved the conflict. The normally available ATC conflict alerting system was temporarily out of service.
An Investigation was carried out by the Indonesian National Transportation Safety Committee (NTSC) - the Komite Nasional Keselamatan Transportasi (KNKT) using recorded ATC data. Both two-person flight crews involved held valid licences and endorsements and the approach radar controller involved had 15 years experience.
Both were in the Surabaya TMA East and under radar control at a time when several inbound flights were holding at different positions. Flight CTV878 was instructed to descend to 19,000 feet QNH and enter the hold at waypoint EMARA to expect an eventual approach to runway 28. About two minutes later, this clearance was changed to descend and maintain FL200.
Two minutes later, flight CTV635 made initial contact with the frequency and was instructed to maintain FL210 and join the hold at waypoint EMARA and the controller then instructed another Citilink flight (CTV661) which was also tracking towards EMARA to take up the hold to descend to FL140. Flight CTV878 then advised that it was approaching EMARA and requested confirmation that a hold (as previously cleared at FL 200) was still required but the controller responded using the callsign CTV661 affirming that it was and that FL140 should be maintained to which flight CTV878 replied advising that the request had come from CTV878. The controller then responded with confirming that CTV878 should maintain FL200 and hold at EMARA. This was read back correctly and a request as to whether a left or right hand hold was required received the response that a right hand pattern inbound on 190° was required and this was correctly read back. It was noted that the recording of this exchange did not indicate any reducing audio communication quality.
Two minutes after this, flight CTV635, which was about 23 nm northwest of EMARA, advised approaching FL210 and was instructed to descend to FL200 and take up the hold on reaching. At this time, the controller was handling five aircraft including CTV878 which was maintaining FL200 in the EMARA hold. A little over a minute later, the horizontal separation between CTV878 and CTV635 had reduced below the applicable minimum of 5 nm as CTV635 reached FL200 and half a minute later, flight CTV878 queried the appearance of traffic on their right hand side and received the response “sorry, Citilink 635 immediate left turn, sorry right turn heading to descend to 180 right turn heading 250”. Flight CTV878 then reported having received a TCAS RA (to descend to FL180) and the controller responded with an instruction to “turn right and maintain FL200”. CTV878 then again advised the controller that a TCAS RA had been received and this time it was acknowledged. Both horizontal and vertical separation between CTV878 and CTV635 began to increase.
Less than half a minute later, having not received any TCAS RA notification from CTV635, the controller instructed it to descend to FL180 and received a corresponding readback. He then queried whether the flight was still heading towards EMARA receiving the response that it was turning right and on heading 250°. Immediately after this, flight CTV878 advised that it had followed its TCAS RA “by descending the aircraft to FL180” and following the controller’s acknowledgement asked if the other traffic had also descended. The controller responded by asking CTV878 to standby and then instructed CTV635 to turn right on heading 250° and maintain FL200. The subsequent readback from CTV635 included the instruction and late notification of the fact that it had received a TCAS RA and in a follow-on communication then advised that a “CLEAR OF CONFLICT’ annunciation had been received and that FL200 was being maintained. Forty seconds later, flight CTV878 confirmed that they had also received a TCAS ‘CLEAR OF CONFLICT’ message and maintaining FL180 in the EMARA hold. Both the previously conflicting flights were then completed without further event and both aircraft landed on runway 18.
It was noted that the applicable traffic separation standard when radar control service was being provided was 5nm horizontally and 1000 feet vertically.
Why It Happened
It was noted that the Radar Unit involved was supervised by a Watch Supervisor and provided service within the Surabaya CTR (one sector) and within the Surabaya TMA (two sectors). The investigated loss of separation occurred in the TMA East sector. The Watch Supervisor was provided with a dedicated controller working position in order to monitor the actions of the three radar sector controllers. The controller in the TMA East position was supported by an Assistant Controller who had been assigned to assist the controller because that sector often had more traffic than the TMA West sector.
At the time of the conflict, it was noted that the controller involved had been in position for almost two hours during which he had provided service to up to ten aircraft at a time. Most of this traffic was flights inbound to Surabaya where the prevailing weather conditions had resulted in the use of non-standard holding patterns.
Prior to the conflict neither the Assistant Controller nor the Watch Supervisor had recognised the potential conflict between flights CTV878 and CTV635 but the Supervisor was aware that prior to it, the controller involved had “several times provided a response to the wrong pilot” but had taken no action. The first awareness of the conflict which then triggered TCAS RAs as resolution was the sighting of one aircraft by the other.
The normally available STCA system was not available because it had been de activated pending repair after it had been observed to be duplicating some aircraft targets because of a misalignment of one of the radar heads. The Investigation noted that in the absence of STCA, it was considered that the availability of other means of increasing controller alertness during duty should have been considered. Although no conclusive evidence that the experienced controller’s errors could be attributed to fatigue was found, it was assessed to be of potential relevance. The Investigation therefore referred to both the ICAO Doc 9966 ‘Manual for the Oversight of Fatigue Management Approaches’ and to the ICAO/IFATCA/CANSO publication ‘Fatigue Management Guide for Air Traffic Service Providers’. It was noted that the latter publication highlights the fact that fatigue management relies on the identification of fatigue hazards and effective safety reporting and that the issues associated with fatigue will be difficult to detect unless people are willing and able to report them. It was therefore emphasised that “it must be acceptable to raise legitimate issues about fatigue without fear of retribution or punishment either within the ANSP or any outside organisation”.
The overall conclusions of the Investigation were that the radar controller involved had experienced a failure of alertness and that the other controllers present had not recognised the consequences of this with all involved appearing not to recognise the additional consequences of the temporary absence of STCA and any mitigation to address this.
Safety Action taken by AirNav Indonesia as a result of the Investigation was noted as including the following:
- optimised the function of the Watch Supervisor to assist controller(s) especially during high workload situations.
- ensured that the STCA will be provided during the provision of Air Traffic Service and mitigation put in place if STCA is temporarily unavailable.
The Final Report of the Investigation was approved for publication on 22 March 2022 and subsequently released. In view of the reported Safety Action, no Safety Recommendations were made.