On 17 January 2018, two Airbus A320-200 aircraft registered PK-GLH and PK-GTA were being operated by PT. Citilink Indonesia (Citilink) as a scheduled passenger flight to Juanda International Airport (WARR), Surabaya. The PK-GLH departed from Supadio International Airport, Pontianak with flight number CTV878 and the PK-GTA departed from Sultan Aji Muhammad Sulaiman International Airport (WALL), Balikpapan with flight number CTV635. On board on both flights were the same composition of two pilots, four flight attendants and 180 passengers.

The flights from departure until commenced approach to Surabaya were uneventful and there were no record or report of aircraft system malfunction during the flight.

Both aircraft entered Surabaya East Terminal Control Area (TMA East) and the provision of air traffic services in TMA East was utilizing surveillance system (radar service). Both aircraft were instructed to hold over Waypoint EMARA at same altitude of 20,000 feet.

At 18:31:49 LT, the horizontal separation of both aircraft reduced from 5 nm while the vertical separation was about 200 Nm and continued reducing. The Traffic Collision Avoidance System Resolution Alert (TCAS RA) of both aircraft were active, and thereafter the horizontal and vertical separation was increasing. The rest of flights were uneventful and both aircraft landed using runway 18. The aircraft was undamaged and there was no injury to person.

The investigation highlights safety issue of workload of the air traffic controller that can impair alertness and ability to perform safety-related duties. The ATS Provider had provided means to help controller increasing their alertness during duty, including the safety net of the surveillance system, the availability of assistant and watch supervisor for the controller. However, the potential conflict between the aircraft was undetermined until one of the pilots confirmed a traffic on their right side.

 At the time of issuing this report, the Komite Nasional Keselamatan Transportasi had been informed of safety actions taken by the Directorate General of Civil Aviation and AirNav Indonesia.