09 Jan 2021 - 737-500, PK-CLC (27323/2616), FF 13/5/1994 (27 Years old), operated by Sriwijaya Air on flight SJ-182 from Jakarta to Pontianak crashed into the Java Sea 5 minutes after take off. No Mayday call was made. All 56 passengers and 6 crew on board died.
On 10 Nov 2022 the KNKT issued their final report. The synopsis is as follows:
Synopsis
On 9 January 2021, a Boeing 737-500 aircraft, registration PK-CLC, was on a scheduled domestic flight, from Soekarno-Hatta International Airport (WIII), Jakarta, to Supadio International Airport (WIOO), Pontianak, and departed at 0736 UTC (1436 LT).
During climbing, the autopilot (A/P) directional control was changed from LNAV to HDG SEL and subsequently the vertical control changed to Pitch V/S and MCP SPD. These changes required less engine thrust therefore the engine power reduced. The FDR recorded that left thrust lever moved backward and the left engine thrust decreased, however the right engine remained at its climb power setting, resulting in an asymmetric thrust condition. The investigation concluded that the autothrottle (A/T) system command being unable to move right thrust lever was a result of friction or binding within the mechanical system except the torque switch mechanism. The maintenance record showed that the A/T problem was reported 65 times since 2013 and the problem was unsolved and still exist on the accident flight.
The Cruise Thrust Split Monitor (CTSM) system delayed to disengage the A/T and the thrust asymmetry continued to increase. The investigation believed that the delay of CTSM was due to an error in the spoiler signal value.
As the thrust asymmetry became greater, the aircraft turned to the left instead of to the right as intended. The aircraft entered an upset condition, and the pilot was unable to recover the situation. Inadequate of upset prevention and recovery training contributed to the inability of the pilot to prevent and recover from the upset condition.
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The report contain 64 findings and 6 contributing factors, of which these are the most significant:
Findings
2. PK-CLC aircraft complied with the FAA Airworthiness Directive (AD) 2000-23-34 to address asymmetric thrust events, by the incorporation of the Cruise Thrust Split Monitor (CTSM), prior to delivery to Sriwijaya Air.
3. The CTSM monitors flight spoiler position and the net thrust difference between the two engines based on their N1 values. When the difference in the output thrusts exceeds a calculated limit for the flight conditions present, and the amount of flight spoiler deflection is greater than 2.5° for 1.5 seconds, the A/T will be disengaged.
4. Since 2013, the AML recorded 65 pilot reports 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.
5. The AML record showed that 48% the A/T system maintenance actions taken involved cleaning of the electrical connectors. Additional maintenance actions taken involved the replacement of the suspected faulty components of the A/T system. The investigation conducted examination of the previously installed components and did not find any abnormality of the component examined.
7. Since 24 March 2020 until 19 December 2020, Sriwijaya Air grounded the PK-CLC aircraft for several maintenance performances. Since the aircraft was released to service until the accident flight, the AML recorded 43 pilot reports, including 3 pilot reports of A/T problem. Among the pilot reports, 6 were entered as DMI of which 2 were related to A/T problem.
8. The Quick Access Recorder (QAR) data recorded 7 asymmetric thrust lever events between 2020 and 2021. There was no pilot report of these occurrences in the AML. Most of the pilots stated that they did not recall the occurrences.
9. One of the asymmetry thrust lever events occurred on 15 March 2020, which resulted in the aircraft rolled to the left up to 41⁰. The A/T disengaged when the calculated flight spoiler deflection was 10.8°. It was most likely that the A/T disengaged by the activation of CTSM. The accident flight PF was the PIC on this flight.
20. The weather radar provided by the Badan Meteorologi Klimatologi dan Geofisika (BMKG – Bureau of Meteorology, Climatology and Geophysics), did not indicate any significant development of clouds along the accident flight path.
21. The change in A/P mode required less engine thrust and the FDR data recorded when the left thrust lever and the N1 speed of the left engine started continuously reducing, while the right thrust lever and N1 speed of the right engine remained fixed until the aircraft entered an upset condition.
22. The investigation assumed that the A/T 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
23. Since the right engine thrust lever position did not move backwards, the left engine thrust lever decreased more than normal to compensate the engine thrust required in capturing the selected speed and rate of climb, and the thrust levers became asymmetry.
24. The design of the CTSM described that the CTSM should disengage the A/T when the flight spoiler deflects greater than 2.5° for a minimum of 1.5 seconds.
25. At 07:39:40 UTC, the aircraft climbed passed an altitude of about 10,250 feet and was turning to the right at a roll angle of 15⁰, with the control wheels deflected to the right about 19°, the left aileron deflection down 3.3° and the right aileron deflected up 5.8°, the calculated spoiler deflection was 3.7°. These conditions met the requirement of the CTSM activation to disengage the A/T however it was delayed. Should the CTSM activated timely, the further thrust asymmetry could be prevented.
26. The QAR data of a flight on 15 March 2020 showed that the CTSM disengaged the A/T when the control wheel deflection was between 15° and 36° to the right, which corresponds to a calculated spoiler deflection of between 2.15° and 10.88° and on the accident flight the CTSM disengaged the A/T when control wheel deflection was between 20° and 33°, which corresponds to a calculated spoiler deflection of between 4.14° and 9.47°. In both events the A/T disengaged at the time when the control wheel moved beyond the A/P saturation position.
27. The investigation believes that the delay of CTSM activation was possibly due to the right flight spoiler position signal value read by the A/T Computer was too low to activate the CTSM. The investigation could not determine the cause of the flight spoiler signal value being too low. The multiple sources that possibly caused a too low of the flight spoiler signal value, including a mis-rigged or erroneous spoiler sensor, mis-rigged spoiler actuator, or a sheared or damaged spoiler linkage.
28. The Sriwijaya Air advised to the investigation that flight spoiler sensor rigging had never been performed on PK-CLC aircraft while being operated by Sriwijaya Air as it never met the requirement to do so.
29. At 07:39:48 UTC, the FDR data recorded that when the aircraft’s altitude was about 10,450 feet and the heading was 046⁰. The aircraft began turning to the left instead of to the right as a result of the thrust lever asymmetry.
30. At 07:39:54 UTC, the ATC instructed SJY182 to climb to an altitude of 13,000 feet, and the instruction was read back by the SIC at 07:39:59 UTC. This was the last known recorded radio transmission by the flight.
31. At 07:40:05 UTC, the A/P disengaged when the aircraft altitude was about 10,700 feet due to the pilot’s activation of the stabilizer trim switch. Thereafter the aircraft continued to descend until the end of FDR recording.
32. There were several indications available that the pilots could have checked to identify the aircraft anomalies, such as engine parameters, thrust levers position, and roll angle displayed on the EADI. The FCOM stated that the pilot must always monitor aircraft course, vertical path and speed.
33. The aircraft’s wings level and began turning to the left is considered a deviation from its intended heading and was an indication of an aircraft upset condition. There was no pilot action taken in response to this aircraft upset condition. The absence of pilot action suggested that both PF and PM were not adequately performing their duties in monitoring the aircraft in a proper flight path.
34. The condition of engine asymmetric power that led to the yaw and roll to the left was countered by the aileron and flight spoiler that were commanded by the A/P to turn the aircraft to the right. When the A/P was disengaged, the aileron and flight spoiler forces that countered the asymmetric power were removed and as a result, the yaw and roll forces of the asymmetric power rolled the aircraft to the left. Four seconds of pilot action to turn the control wheel to the left increased the roll tendency of the aircraft to the left.
35. The EGPWS Bank Angle warning activation was triggered by the aircraft roll angle of 37° to the left. The deflection of control wheel to the right and inadequate monitoring of the EADI might have made the pilot assumed that the aircraft was rolling excessively to the right and deflected the control wheel to the left to recover it. The control wheel activation to the left created more roll tendency to the left which was counter to restoring the aircraft to safe flight parameters.
36. The investigation concluded that during the accident flight, the pilots should have enough time to monitor the thrust lever asymmetry and able to recognize deviation of the flight path. However, the pilots did not identify the flight anomaly before it developed into an upset condition. This lapse of not identifying the anomaly could be due to reduced active monitoring because of pilot automation complacency and confirmation bias that aircraft was performing the right turn as commanded. Without using the EADI as primary reference in assessing the aircraft attitude, the pilot was not able to apply correct recovery inputs.
40. Sriwijaya Air provided upset recovery training to its pilots which consisted of simulator training to perform an upset recovery maneuver every 24 calendar months during Pilot Proficiency Check. However, there was no detailed training program nor guidelines to conduct the upset recovery training.
41. The lack of detail of the upset recovery training program of Sriwijaya Air indicates the implementation of the upset recovery training inadequate in ensuring that pilots have enough knowledge to prevent and recover an upset condition effectively and timely.
42. The absence of the guidance of the national standard for the UPRT, might have made the Sriwijaya Air was unable to have adequate implementation of the UPRT.
52. Since 2013 until the accident flight, the AML data recorded 65 pilot reports related to the A/T system and 61 problems related to the differences in engine parameters. The AML record showed that 48% of the A/T system maintenance actions involved cleaning of the electrical connectors.
53. The maintenance actions were stopped after the BITE test resulted “no faults”. Additional actions, based on pilot write ups, were available, including the subsequent FMC CDU test as described in the AMM 22-04-10 or 22-31-00, such as the guidance in the INTERACTIVE TEST page in the FMC CDU. The subsequent trouble shooting steps would use procedure contained in the AMM chapter 71-00-49 (Power Plant–- Trouble Shooting (Engine Controls)).
54. The troubleshooting step for pilot report of thrust lever split event, should also be in accordance with the procedure in AMM chapter 71-00-49, which contained maintenance steps to check the friction of the engine control cable.
55. The termination of the trouble shooting after the BITE test result of “no faults” and without the pilot report of thrust lever split, resulted in the engineers stopped the trouble shooting steps and not proceed to examine the engine thrust control as required in AMM chapter 71-00-49. This is likely the reason why the defect prolonged.
56. The reported A/T problem was repeatedly deferred as DMI from 20 December 2020 to 4 January 2021. The DMI was first raised on 20 December 2020 and was rectified and closed on 30 December 2020. But the defect was reported again on 3 and 4 January 2020 and DMI raised again which indicated that the A/T problem was not properly rectified.
57. The Sriwijaya Air maintenance management established the MCC which has responsibilities including monitoring the defect and DMI rectification. The progress of DMI rectification was recorded and monitored through DMI control/summary which review by the MCC on daily and weekly basis.
58. The monitoring efforts by MCC did not appear to have raised awareness amongst the line maintenance engineers of the recurring A/T defect and the additional trouble shooting steps in the “INTERACTIVE TEST” function in the FMC CDU menu.
59. It is evident that the recurring defect monitoring efforts under the maintenance management program has not been implemented effectively given the prolonged unsolved A/T defect on the accident aircraft.
60. The Flight Data Analysis Program (FDAP) established by the Sriwijaya Air retrieved an average data of 28.1%. The low rate of FDAP data analysis reduced the ability of the program in monitoring flight operation safety performance.
61. The QAR data of PK-CLC was downloaded, however Sriwijaya Air was unable to analyze the data as it did not have the correct data frame file to process the PK-CLC QAR data. As a result, the excessive bank angle event due to thrust asymmetry during the flight on 15 March 2020 was not detected. For subsequent thrust asymmetry events, the event conditions did not meet the triggering value set in the FDAP and were not captured.
62. The samples of the hazard reporting of the Sriwijaya Air on the period of 2020 showed that majority of the hazard were reported by ground personnel. Few hazards were reported by pilots and maintenance personnel and there was no hazard report by dispatchers. This unbalance composition of the hazard reporters is likely an indication that the hazard reporting program has not been emphasized to all employees which could result in hazards not identified and properly mitigated.
63. The list of hazards in the HIRA that was made during the termination of the joint cooperation, did not include the ability to maintain FDAP, that was performed by another approved maintenance organization. The FDAP data retrieval showed there were no QAR data retrieved during the period of October to December 2019 which was due to lack of human resources and equipment. The investigation noted that the HIRA did not describe the details of the hazards that may have exist.
64. The evidence of low rate of FDAP data analysis, unbalance composition of hazard reporters, and the lack of detail in the hazard identification suggested that Sriwijaya Air safety management system (SMS) has not been implemented effectively.
The KNKT concluded the contributing factors as follows:
• The corrective maintenance processes of the A/T problem were unable to identify the friction or binding within the mechanical system of the thrust lever and resulted in the prolonged and unresolved of the A/T problem.
• The right thrust lever did not reduce when required by the A/P to obtain selected rate of climb and aircraft speed due to the friction or binding within the mechanical system, as a result, the left thrust lever compensated by moving further backward which resulted in thrust asymmetry.
• The delayed CTSM activation to disengage the A/T system during the thrust asymmetry event due to the undervalued spoiler angle position input resulted in greater power asymmetry.
• The automation complacency and confirmation bias might have led to a decrease in active monitoring which resulted in the thrust lever asymmetry and deviation of the flight path were not being monitored.
• The aircraft rolled to the left instead of to the right as intended while the control wheel deflected to the right and inadequate monitoring of the EADI might have created assumption that the aircraft was rolling excessively to the right which resulted in an action that was contrary in restoring the aircraft to safe flight parameters.
• The absence of the guidance of the national standard for the UPRT, may have contributed to the training program not being adequately implemented to ensure that pilots have enough knowledge to prevent and recover of an upset condition effectively and timely.
On 15 Feb 2021 Boeing issued an FOTB on the prevention of aircraft upsets, details here.
On 10 Feb 2021 the preliminary report was issued.
On 30 Mar 2021 the CVR memory module was recovered about 500 meters off the coast of Pulau Laki (Laki Island) at a depth of 14 meters under 16cm of mud.
On 18 May 2021 the FAA issued AD 2021-08-14 for 737 Classics requiring operators to verify that the flap synchro wire used by the auto-throttle, is securely connected to a safety sensor. A faulty connection could result in the failure of the auto-throttle system to detect the position of the flaps if the engines are operating at different thrust settings due to another malfunction posing a safety risk.
Boeing discovered that the most recent version of the 737 Classic autothrottle computer does not properly account for a possible latent failure of the flap position sensor. This can cause the autothrottle system’s asymmetric cruise thrust monitor to malfunction, creating a thrust imbalance between the aircraft’s two engines.
The FAA said "At this time, the preliminary data of the ongoing accident investigation shows that it is highly unlikely that the accident resulted from the latent failure of the flap synchro wire. However, the FAA has determined that the unsafe condition identified in this AD could exist or develop in Model 737-300, -400, and -500 series airplanes, and that this AD is therefore necessary to address the identified unsafe condition.”
Boeing issued a Multi Operator Message (MOM) on 30 March 2021 to operators directing them to perform electronic checks of the auto-throttle computer to confirm the wire is connected within 250 flight hours. But because of present COVID low utilization rates, the FAA has now insisted on the checks within 250 flight hours or two months from now, whichever occurs first. The FAA is requiring follow-on inspections every 2,000 flight hours after the initial inspection.
Sriwijaya Air Ssafety Record
Sriwijaya Air has a poor safety record. With 5 accidents in 737's recorded since 2008
09 Jan 2021 - 737-500, PK-CLC, 27323/2616, FF 13/5/1994 (age 27 Years), Sriwijaya Air, Jakarta, Indonesia
This accident
31 May 2017 - 737-300, PK-CJC, 24025/1556, FF 17/05/1988, Sriwijaya Air, Manokwari, Indonesia
Overran Runway 35 by 30m whilst landing in heavy rain. The preliminary report states that at approx 550 feet, the PIC instructed the SIC to turn on the wiper and reconfirmed to SIC that the runway was in sight. Between altitude 500 feet to 200 feet, the EGPWS aural warnings “Sink Rate” and “Pull Up” sounded. Aircraft written off but no fatalities.
1 Jun 2012, PK-CJV, 737-400, 24689/1883, FF 19 Jun 90, Sriwijaya Air, Pontianak, Indonesia
The aircraft departed the left hand side of the runway after landing in heavy rain. The nose landing gear dug in soft ground and collapsed. WIOO 010530Z 23022KT 0600 FEW009CB BKN007 29/25 Q1008 RMK CB OVER THE FIELD
20 Dec 2011, PK-CKM, 737-300, 28333/2810, FF 5 Aug 96, Sriwijaya Air; Yogyakarta, Indonesia
The Captain (PF) flew an unstable VOR/DME approach to runway 09. After touchdown, PF activated the thrust reversers but the crew did not feel any deceleration. Prior to the end of the runway, PF believed that the aircraft would not be able to stop on the runway and decided to turn the aircraft to the left. The aircraft stopped at 75m from the end of runway 09 and 54m from the left side of the centre line. The nose and right hand main gear collapsed.
27 Aug 2008; PK-CJG, 737-200Adv, 23320/1120, FF 23 May 85, Sriwijaya Air; Jambi, Indonesia:
The airplane had been landing towards northwest in heavy rain and marginal conditions, when the brakes failed. The airplane went about 250 meters past the runway and 3 meters below runway elevation. The right hand wing received damage, both engines and the main landing gear detached. Initial reports state that the flaps were at 15 and the thrust reversers were stowed. Jambi Sultan Taha has a single 2000 x 30m asphalt runway and no ILS. Passengers reported that the captain made an announcement before landing of a possible problem and "not to worry".
PK-CLC 737-500 Sriwijaya Air LoC into Java Sea
