What happened
On 6 January 2018, two passenger aircraft were performing approach maneuvers near Waypoint TIRUS, east of Surabaya, Indonesia, when they entered a state of reduced separation. The first aircraft, a Boeing 737-900ER registered PK-LPF operated by Lion Air, was maintaining an altitude of 10,000 feet. Simultaneously, an Airbus A320-200 registered PK-LUI operated by Batik Air was cleared to descend to 11,000 feet.
While the air traffic controller had issued a descent clearance of 11,000 feet to the Batik Air crew, the pilot monitoring mistakenly communicated a target altitude of 10,000 feet to the pilot flying. This error led the crew to set the aircraft's target altitude to 10,000 feet, causing the aircraft to descend through its assigned level. As the Airbus A320-200 passed 11,000 feet, the controller's Short-Term Conflict Alert (STCA) was triggered. The situation escalated when the aircraft came within approximately 600 feet of vertical separation and 1.9 nautical miles of lateral separation. The conflict was resolved only after the Traffic Collision Avoidance System (TCAS) issued Resolution Advisories (RA) to both crews, prompting the Boeing 737-900ER to descend and the Airbus A320-200 to halt its descent.
The investigation
The KNKT investigation focused on aircraft separation and human performance, as both the radar systems and the aircraft were found to be fully operational. Investigators examined the flight records and cockpit communications, revealing that the Batik Air crew had correctly read back the 11,000-foot clearance to the controller, but the pilot monitoring had incorrectly advised the pilot flying to descend to 10,000 feet.
Further examination of the Batik Air crew's status revealed that the pilot in command was dealing with significant personal stressors, including a family medical issue, and had not fully recovered from a recent overnight flight. The investigation also noted that the cockpit environment was relatively low-activity during the descent, which may have contributed to a lapse in situational awareness.
Findings
- The vertical separation between the two aircraft dropped below required standards because the Airbus A300-200 was set to a target altitude of 10,000 feet, causing it to descend below its assigned 11,000-foot level.
- Unrecovered fatigue and personal family issues significantly reduced the pilot's awareness and ability to monitor traffic during a period of low cockpit activity.
- A communication slip occurred where the pilot monitoring incorrectly relayed the altitude clearance to the pilot flying, despite a correct readback to Air Traffic Control.
Safety action
Following the incident, the KNKT issued recommendations to Batik Air to implement a more robust fatigue risk management system. Additionally, the KNKT recommended that the Directorate General of Civil Aviation (DGCA) review national regulations to provide more detailed guidelines for fatigue management in the industry.