What happened
On 9 January 2021, an aircraft operated by PT. Sriwijaya Air, a Boeing 737-500 with registration PK-CLC, departed from Soekarno-Hatta International Airport for a scheduled service to Pontianak. The flight, designated SJY182, carried 56 passengers and six crew members. During the initial climb phase, the aircraft was cleared by Jakarta controllers to ascend via a standard instrument departure.
Flight data indicates that while climbing through approximately 8,150 feet, a significant discrepancy occurred between the engine thrust levers. The left engine thrust lever began reducing its position, whereas the right engine lever remained stationary. Shortly after, the pilot requested a heading change to avoid weather, which was granted. Following instructions to level off at 11,000 feet to maintain separation from another departing aircraft, the flight's last radio communication occurred at 14:39:59 LT.
Data recorded by the Flight Data Recorder shows that after reaching a peak altitude of roughly 10,900 feet, the autopilot disengaged. The aircraft experienced a severe left roll exceeding 45 degrees and a nose-down pitch of more than 10 degrees. Shortly thereafter, the FDR stopped recording data. Air traffic controllers lost radar contact with the aircraft at 14:40:48 LT. Despite repeated attempts to contact the crew on both standard and emergency frequencies, no response was received. The uncertainty and distress phases were eventually declared by authorities.
Findings
Investigations into the accident identified several critical contributing factors:
- Mechanical friction or binding within the thrust lever mechanism prevented the right lever from adjusting as required by the autopilot, causing the left lever to move backward and create thrust asymmetry.
- Maintenance procedures failed to detect the mechanical binding within the thrust lever system.
- A delay in the activation of the autothrottle disengagement, caused by undervalued spoiler angle inputs, exacerbated the power imbalance.
- Crew members may have experienced automation complacency and confirmation bias, leading to insufficient monitoring of the flight path and thrust discrepancies.
- Inadequate monitoring of the Electronic Attitude Direction Indicator (EADI) likely led to improper control inputs during the roll event.
- A lack of standardized national guidance for Upset Prevention and Recovery Training (UPRT) may have hindered the crew's ability to effectively recover from the upset condition.