What happened
On 17 January 2018, two Airbus A320-200 aircraft, registered PK-GLH and PK-GTA, were operating scheduled passenger flights toward Juanda International Airport in Surabaya. Both flights, operated by Citilink, were navigating through the Surabaya East Terminal Control Area (TMA East) under radar surveillance.
Due to heavy rain and a recent runway closure for maintenance, several aircraft were directed to hold at Waypoint EMARA. During the management of these holding patterns, the horizontal separation between the two aircraft reduced to approximately 5 nm. The air traffic controller had instructed both aircraft to maintain an altitude of 20,000 feet.
The potential conflict remained undetected by the ground controller until the pilot of PK-GLH reported seeing traffic on their right side. This prompted the activation of the Traffic Collision Avoidance System (TCAS) Resolution Alert (RA) on both aircraft. Following the TCAS instructions, the pilots performed necessary maneuvers to increase separation. Both aircraft eventually landed safely at Juanda International Airport without damage or injury.
The investigation
The KNKT investigation focused on the air traffic control environment and the surveillance systems in use. Investigators found that the controller was managing a high volume of traffic, having handled 10 aircraft in the 30 minutes preceding the event, many of which were in non-standard holding patterns due to weather.
Crucially, the investigation established that the Short-Term Conflict Alert (STCA) safety net was deactivated at the time of the incident. This deactivation was due to a technical issue where a radar head misalignment caused duplicate aircraft targets, rendering the STCA unreliable. While an assistant controller and a watch supervisor were present, neither identified the developing conflict between the two Citilink flights.
Findings
- The high workload of the air traffic controller likely impaired the ability to maintain necessary alertness.
- The deactivation of the Short-Term Conflict Alert (STCA) removed a critical safety net that could have alerted the controller to the closing distance between the aircraft.
- The controller had previously issued instructions to the wrong pilot, a mistake that went unaddressed by the watch supervisor.
- Heavy rain and recent runway maintenance had created a complex traffic situation with multiple aircraft in non-standard holding patterns.