On 6 January 2018, a Boeing 737-900 (PK-LPF) being operated by Lion Air on a scheduled domestic passenger flight from Sentani to Surabaya as LNI3795 and an Airbus A320 (PK-LUI) being operated by Batik Air on a scheduled domestic passenger flight from Makassar to Surabaya as BTK6137 lost separation in the TIRUS hold coming to within 1.9nm laterally when approximately 600 feet vertically apart. An ATC attempt to intervene after an STCA annunciation had occurred as the A320 descended onto the 737 was unsuccessful as both calls were ignored, but soon afterwards coordinated TCAS RAs were generated and followed and a closer conflict thereby averted. The two aircraft then completed their respective flights in sequence.
An Investigation was carried out by the Indonesian National Transportation Safety Committee (NTSC) - the Komite Nasional Keselamatan Transportasi (KNKT). Relevant data was recovered from the FDRs of both aircraft but corresponding data from both CVRs had been overwritten. Relevant ATC voice and radar recordings were also available.
It was found that the 48 year-old Batik Air Captain was a Malaysian national who had been employed by the company and based in Jakarta for almost two years but whose permanent residence and family was in Malaysia. He had and had a total of 15,869 hours flying experience of which 6,750 hours were on type and had recently returned to Indonesia after a short trip home to resolve a family medical issue. His 44 year-old First Officer had a total of 2,812 hours flying experience, of which all but 150 hours were on type. It was noted that the Lion Air Captain had a total of 6,603 hours flying experience and their First Officer had a total of 2,502 hours flying experience. The approach radar controller involved had over 20 years experience.
The Batik Air A320 Captain was acting as PM and correctly read back a series of descent clearances including the one to descend to 11,000 feet QNH. Eight seconds after the correct readback had been given, 11,000 feet was set on the altitude selector. Then, one second later, the selected altitude was changed from 11,000 feet to 10,000 feet. At this point, the Batik A320 was descending through 13,200 feet QNH with a 1,050 fpm rate of descent and the Lion Air 737 was passing 10,300 feet QNH and about to level at 10,000 feet as cleared.
As it then did so, the Batik Air A320 had just passed 11,000 feet and was continuing to descend. Six seconds later, the Batik Air A320 received a TCAS TA on the 737 below but it continued to descend albeit at a reduced rate. After a further 8 seconds, an STCA alert resulting from the conflict led the radar controller to instruct the A320 to “maintain 11,000 feet” to which there was no response. As this instruction was repeated - and again ignored - the A320 received another TA as it passed 10,600 feet and at the same time the 737, still descending FL100 as cleared, received a TCAS RA ‘DON’T CLIMB’. At this point, separation was 2.8nm and 400 feet. One second later, the A320 received a TCAS RA ‘DON’T DESCEND’ and levelled off at 10,500 feet. The RAs continued for 20 seconds until ‘Clear of Conflict’ was annunciated when the minimum lateral separation (5nm) was reached.
For the next ten seconds, the PM Batik Air Captain then tried to convince the controller that he had been cleared to descend to 10,000 feet but this was correctly refuted by the controller who then instructed the A320 to climb back to its cleared altitude of 11,000 feet. The remainder of the two aircraft’s flights were without further event and they later landed at Surabaya four minutes apart.
The roster of the Batik Air A320 Captain and their First Officer prior to the investigated event was reviewed. Two days earlier, they had operated a 4½ hour flight from Jakarta Soekarno-Hatta to Manokwari which involved a night flight and then a 24 hour period free of duty before reporting at 0605 local time for a two sector duty from Manokwari to Surabaya via Makassar. It was noted that the Batik Air OM Part ‘A’ contained an entry on the potential hazards of fatigue such as it was speculated may have been a factor in the clearance setting error which led to this investigated conflict.
It was discovered during the Investigation that both A320 pilots had seen the 737 below them on their TCAS traffic display but they “did not pay attention to its altitude - the PM Captain was aware that the aircraft was under radar surveillance control and believed that all aircraft would be controlled and separated by the controller”. This attitude served to reduce the PMs interest in monitoring other traffic communications hence the altitude clearance for the 737 was not monitored. Radio communication with the A320 was minimal and with the AP engaged and the hold set up, it was considered that minimal activity on the flight deck “might have made the PF First Officer feel sleepy” and led to the PM Captain being reminded of his “family medical issue” which would have “reduced both pilots’ awareness.”
Two Contributory Factors were identified in respect of the occurrence as:
- The vertical separation reduced to below the requirement due to the target altitude of the Batik Air A320 being set to 10,000 feet, which caused the aircraft to pass through its assigned altitude.
- Unrecovered fatigue and family issues reduced pilot awareness during less busy flight deck activity and resulted in reduced ability to monitor other traffic and misunderstanding of assigned altitude.
Two Safety Recommendations were made as a result of the Investigation as follows:
- that Batik Air establish a fatigue risk management system which may refer to ICAO Annex 6 and ICAO Document 9966. [04.O-2018-01.1]
- that the Directorate General of Civil Aviation (DGCA) review the regulation covering fatigue risk management. [04.O-2018-01.2]]
The Final Report of the Investigation was approved in October 2018 and subsequently released.