What happened
On 17 January 2026, an ATR 42-500, registration PK-THT, was conducting an unscheduled aerial surveillance mission from Yogyakarta to Maros, Indonesia. The flight, operated by PT Indonesia Air Transport, carried two pilots, two flight attendants, and six passengers. During the approach to Sultan Hasanuddin International Airport, the aircraft encountered severe weather involving convective cumulonimbus clouds.
As the aircraft descended, the flight crew believed they were following instructions to proceed to Waypoint KABIB. However, the aircraft was actually significantly off-course. At approximately 04:22 UTC, the aircraft's terrain warning system issued multiple "TERRAIN" and "PULL UP" alerts. Shortly thereafter, the aircraft struck the slope of Mount Bulusaraung at an elevation of approximately 4,300 feet. The impact resulted in all 10 persons on board being fatally injured, and the aircraft was destroyed.
The investigation
The KNKT investigation focused on the discrepancy between the aircraft's perceived position and its actual location. Data from the Flight Data Recorder (FDR) revealed that the Global Navigation Satellite System (GNSS) was operating in a degraded mode for much of the flight. This caused the aircraft's internal navigation display (EHSI) to show a position that differed from the actual coordinates tracked by the ground-based ADS-B surveillance system.
Investigators also examined the cockpit environment and air traffic control (ATC) operations. It was noted that the flight crew was not using headsets or boom microphones, which hindered clear communication. Furthermore, the investigation looked into why the ATC surveillance system's Minimum Safe Altitude Warning (MSAW) failed to alert controllers when the aircraft descended below the required safety altitude.
Findings
- The primary cause of the accident was a navigation system degradation that created a discrepancy between the aircraft's displayed position and its true location.
- The aircraft's GNSS was in a degraded mode, causing the cockpit instruments to show an incorrect position.
- The ATC surveillance system failed to trigger a Minimum Safe Altitude Warning (MSAW) despite the aircraft flying below the prescribed minimum altitude.
- The flight crew was not utilizing headsets or boom microphones, which impacted communication effectiveness.
- Air traffic controllers were engaged in non-duty-related discussions during the period of the incident.
Safety action
Following the accident, the Directorate General of Civil Aviation issued directives for pilots to verify positions using conventional means when navigation degradation is detected. AirNav Indonesia updated its MSAW datasets and revised instrument approach procedures for Runway 21. Additionally, the operator, PT Indonesia Air Transport, issued safety notices mandating the use of headsets and microphones during critical phases of flight.