KNKT, Indonesia’s air accident investigating agency, released its final report on the Sriwijaya Air Boeing 737 crash that killed 62 people in January 2021. Investigators found that the fatal incident was due to a faulty automatic thrust system, which was not properly monitored and corrected by the pilots.
The aircraft, a Boeing 737-500 registered PK-CLC, made its last flight on 9 January 2021 when it took off from Jakarta’s Soekarno-Hatta International Airport (CGK) to Pontianak Supadio International Airport (PNK). The aircraft had 62 people on board: two pilots, four flight attendants, 50 passengers, and six crew members traveling as passengers.
As the aircraft flew over the Java Sea near the island of Laki, it plunged 3,000 meters in less than a minute. There was no emergency declaration by the flight crew. There were no survivors either.
Accident sequence
At 07:36 UTC the aircraft took off from Jakarta as Sriwijaya Air flight 182. At that time, the autothrottle (A/T) was in TO/GA mode.
Two minutes later and at about 7,800 ft, the autopilot (A/P) switched from SPD to V/S. In this mode, the A/T would regulate the aircraft’s speed and rate of climb according to the pilot’s selection on the mode control panel (MCP). When the aircraft has reached the selected climb and speed targets, the automation varies the thrust of the engines.
The right thrust lever did not move and the N1 assembly continued to operate at 91.8% power. Consequently, the A/T started to move back the left thrust lever and the N1 speed of the left engine started to decrease from 92.3%. As the thrust required during the initial climb is greater than that needed for the V/S mode, the thrust started to decrease. Because the right thrust lever did not move, the left engine thrust decreased more than normal to compensate and allow the selected speed and climb rate to be captured.
Related content: Boeing set to pay two hundred million dollars penalty for 737 MAX «misleading statements»
At 07:38:50 UTC, the crew requested a right turn to heading 075 to avoid weather en route, which was approved by the ATC. The pilot then selected heading 075 on the MCP and the A/P responded by initiating a right turn. At 07:39:19 UTC, when the altitude was about 9,800 feet, the rate of climb was reduced to 2,000 feet per minute, as the aircraft was reaching its assigned altitude of 11,000 feet. Speed indicated 230 knots, and the aircraft continued to roll to the right at an angle of about 15°.
The left thrust lever position continued to retract, and the left engine N1 speed was at 81.5% and continuing to decrease. Meanwhile, the right thrust lever and right engine N1 set remained at 91.8% power.
At 07:39:59 UTC, the aircraft was crossing 10,550 ft with a climb rate of about 950 ft/minute, the aircraft heading was 036°. The roll to the left was 24°. At this time the pilot selected 13,000 ft on the MCP, as instructed by ATC.
The aircraft’s A/T has a function called the cruise thrust divergence monitor (CTSM), which monitors the aircraft’s engine thrust and aileron and disengages the autothrottle before the aircraft reaches a critical asymmetric thrust condition. At 07:40:10 UTC, the A/T was disconnected. The flight data recorder indicates that the crew did not command this disconnection and that only 9 seconds after the disconnection the thrust levers were manually moved.
At 07:40:05 UTC and at 10,700 ft (the maximum altitude reached during the flight) the A/P was disconnected by the activation of the stab trim switch, which commands the position of the horizontal stabilizer. As the asymmetrical power condition had been being counteracted by the A/P, the disconnection of the A/P removed the only resistance the aircraft had to pitch left.
At that instant, when the aircraft was at 37° left bank, the GPWS bank angle warning («bank angle, bank angle») sounded. The aircraft continued to bank to the left past 45° with more than 10° nose down.
At 07:40:28, less than 30 seconds later, the FDR stopped recording. The last recorded aircraft coordinate was in the vicinity of the crash site. The left thrust lever position had decreased to 8° and the left engine N1 speed to 34% while the right thrust lever and its N1 remained unchanged.
Causes of the accident
The primary cause of the crash of Sriwijaya Air flight 182, according to the KNKT report (which can be accessed here), was a problem with the A/T system that was not detected and properly corrected by the flight crew. «The investigation concludes that the pilots had sufficient time to monitor the thrust lever asymmetry. However, they did not identify the flight anomaly before it became a hazardous condition,» the report said.
In addition, the KNKT said 65 A/T issues were reported in the aircraft’s maintenance log, the latest on 21 August 2019.
