On 9 January 2021, Sirwijaya Air flight SJ182 took off from Soekarno-Hatta International Airport (WIII), Jakarta with intended destination of Supadio International Airport (WIOO), Pontianak, and operated with a Boeing B737-500 registration PK-CLC.
At 0736 UTC (1436 local time), in daylight conditions, Sirwijaya Air flight SJ182 departed from Runway 25R of Jakarta.
On board the aircraft were two pilots, four flight attendants, and 56 passengers: the Pilot in Command (PIC) was Pilot Flying (PF) while the Second in Command (SIC) acted as Pilot Monitoring (PM).
After takeoff (as detected from FDR – Flight Data Recorder) the AP was was engaged when the aircraft altitude was 1,780 feet.
The A/P mode selected for directional control was selected LNAV and the vertical control was MCP SPD and LVL CHG.
At 07:38:00 UTC, the FDR recorded the A/P mode directional control was changed from LNAV to HDG SEL when the aircraft passed altitude about 5,400 feet and the aircraft speed was about 220 knots.
At 07:38:35 UTC, the A/P vertical control changed to V/S mode and the A/T changed from N1 mode to MCP SPD mode.
At 07:38:40 UTC, the FDR data recorded that as the aircraft climbed to an altitude of about 7,950 feet, the left thrust lever started decreasing from 47.5° thrust lever position and the N14 speed of the left engine decreased from 92.3%, while the right thrust lever position remained at 46° and the N1 speed of the right engine at 91.8%
At this point the crew of Sirwijaya Air flight SJ182 was instructed to level off at 11.000 ft because of traffic separation.
In the meanwhile, the left thrust lever position had decreased to 22° and the N1 speed of the left engine decreased to 67.5%, while the right thrust lever position and the N1 remained unchanged.
The flight crew made an input for a right turn (as cleared by ATC, to avoid weather) but, because of asymmetric thrust the airplane was actually decreasing the bank angle and started to turn to the opposite side, to the left.
When clear of traffic, ATC instructed the crew to climb to 13.000 ft. The crew acknowledged and this was the last transmission by the flight crew.
At the same time the EGPWS ((Enhanced Ground Proximity Warning System) warned the crew with a “BANK ANGLE” alert.
The aircraft was at 10.700 ft with a left roll angle of about 37°.
After reaching the maximum altitude of 10.700 ft, the aircraft started to descend.
The pilot activated stabilizer trim switch which disengaged the A/P system. The aircraft heading was 016°, and the aircraft rolled to the left at an angle of more than 45° with an accompanying by EGPWS bank angle alert: the left roll was further exacerbated by left roll pilot commands.
The airplane impacted the Java sea shortly after.
The safety investigation found that:
- the A/T (auto throttle) system command was unable to move right thrust lever as a result of friction or binding within the mechanical system, except the torque switch mechanism
- Since 2013, the AML (Aircraft Maintenance Log) recorded 65 pilot reports (PIREP) relating to the A/T, including 32 pilot reports of A/T disengagement. In addition to the 65 A/T pilot reports, the AML also recorded 61 pilot reports relating to differences in engine parameters which 53 out of the 61 reports occurred during descent. The AML also recorded 69 pilot reports relating to the problem of A/P