B735, en-route, north northwest of Jakarta Indonesia, 2021
B735, en-route, north northwest of Jakarta Indonesia, 2021
On 9 January 2021, a Boeing 737-500 was climbing though 10,700 feet less than five minutes after departing Jakarta in daylight when it began to descend at an increasing rate from which no recovery occurred and 23 seconds later was destroyed by sea surface impact killing all 62 occupants. The Investigation concluded that the departure from controlled flight was unintentional and the result of the pilots’ inattention to their primary flight instruments when, during a turn with the autopilot engaged, an autothrottle malfunction created apparently unrecognised thrust asymmetry which culminated in a wing drop and a consequent loss of control.
On 9 January 2021, a Boeing 737-500 (PK-CLC) being operated by Indonesian carrier Sriwijaya Air on a scheduled domestic passenger flight from Jakarta Soekarno-Hatta to Pontianak as SJ182 ceased responding to ATC calls as it climbed towards FL130 and subsequently descended rapidly and impacted the sea north northwest of Jakarta at high speed which resulted in the destruction of the aircraft as a result of which none of the 62 occupants survived.
The aircraft ground track annotated with times (the green area represents light rain). [Reproduced from the Official Report]
An Investigation into the Accident was carried out by the Indonesian Komite Nasional Keselamatan Transportasi (KNKT) otherwise known as the National Transportation Safety Committee (NTSC). A Preliminary Report of the initial progress of the Investigation was released on 10 February 2021.
After a three day search, the FDR SCMU was recovered along with CVR ULB but the latter was not attached to the CVR CSMU which was not recovered until 30 March. Relevant data from both recorders was successfully downloaded and enabled an effective investigation to be conducted. All relevant recorded ATC and aircraft maintenance data were also available.
It was noted that the 54 year-old Captain, who was acting as PF, had a total of 17,904 hours flying experience including 9,023 hours on type and that the 34 year-old First Officer had a total of 5,107 hours flying experience all but 150 hours was on type. Both pilots had been flying rather less frequently than normal but had still respectively recorded 142 / 113 hours over the previous 90 days and 53 / 30 hours over the previous 30 days.
It has been established that the flight took off from runway 25R at Jakarta on an ABASA 2D SID and, after changing frequency to the Terminal East frequency soon after getting airborne was re-cleared to continue climb to FL290.
Recorded flight data showed that the AP was engaged as the aircraft passed 1,980 feet QNH with LNAV and MCP SPD/LVL CHG modes engaged. LNAV mode was then changed to HDG SEL just over a minute later when climbing though 5,400 feet QNH at about 220 knots. Half a minute after this, approaching 8000 feet QNH, the AP vertical mode was changed to V/S and the A/T mode from N1 to MCP SPD. FDR data showed that the left engine thrust and Thrust Lever Angle (TLA) then decreased very slightly to initially match those for the right engine - 91.8% N1 and 46° respectively.
At this time, convective cloud avoidance in otherwise clear sky conditions led to the First Officer asking the Captain whether they wanted to deviate from the SID by turning right onto heading 070° and received the response to request 075° which was done and approved. The controller then asked the flight to stop climb at FL110 for traffic separation which was acknowledged as the aircraft passed 8,800 feet by which time FDR data showed that the left engine thrust had continued to reduce and was down to 86.4% N1 with a 39° TLA.
Half a minute later, the altitude alert sounded with 1,000 feet to go to the selected altitude. At this point, whilst the right engine thrust and TLA were unchanged, the left engine thrust had continued to decrease and was now recorded as 72.7° N1 with a 25° TLA. The next four seconds saw it drop further to 67.5% with a 22% TLA with the control column simultaneously recording a 15° right roll. At first, the aircraft continued to turn right to capture the selected heading of 075° but after a further 8 seconds and passing 10,450 feet, the bank angle had returned to zero although the heading had only reached a maximum of 046° towards the selected heading. As the left engine thrust continued to decrease - having reached an N1 of 59.3% with a TLA of 17.6° - and the right engine N1 and TLA remained unchanged - the aircraft began a turn to the left.
Six seconds later, with the rate of climb continuing to reduce and now less than 1000 fpm, the First Officer called “set standard” (i.e. barometric subscale to 1013 hPa). As the left engine N1 and TLA continued to decrease to 49% and 13°, the left turn continued and the angle of bank continued to increase. A re-clearance to FL130 was given and acknowledged and the latter was the last recorded radio call from the flight. A few seconds later, the climb ceased at a maximum of around 10,700 feet and the aircraft began to descend. As the Captain was recorded saying “130” followed by the First Officer repeating this, an EGPWS ‘BANK ANGLE’ Alert was annunciated - the recorded bank angle had reached 37° to the left and the left engine N1 and TLA were continuing to decrease - through 35.1% and 9.3° respectively - whilst the right engine N1 and TLA remained unchanged at climb thrust.
The Captain briefly activated the control column stabiliser trim switch which disconnected the AP and the aircraft rolled left to an angle of bank of “about 49°” in response to a four second deflection of the control column of up to a recorded 18°. A couple of seconds later, this was rapidly reversed to a recorded control wheel deflection 33° to the right with corresponding aileron deflection recorded. A/T disconnection followed as the left TLA reached the flight idle position.
Ten seconds later, the right thrust lever was moved to flight idle and 8 seconds after this, the FDR recording ceased. The ATC radar target of the aircraft disappeared from the controller’s display and the controller reported the last radar position and the non-response of the flight to calls made during the observed descent to their supervisor. The response to this report was slow. It was 15 minutes after the aircraft had disappeared from the radar display before ATC informed the State SAR organisation, an hour after target disappearance before the INCERFA phase was declared and two hours after target disappearance before the DETRESFA phase was declared.
The wreckage of the aircraft was subsequently found on the seabed at a depth of approximately 16 metres about 80 metres southeast of the aircraft’s last known (ADS-B) position. It was spread over an area of about 80 by 110 metres.
An examination of the weather conditions in the area where it appeared control had been lost found that there had been “no significant development of cloud” along the aircraft flight path.
Why It Happened
- The Operation of the Aircraft
The available evidence indicated that the upset caused by uneven thrust with the AP and A/T engaged had not been recognised by the pilots despite clear evidence of progressive reduction in aircraft pitch attitude and rate of climb, the heading reversal away from the initial turn towards the approved and selected weather avoidance heading and the especially the changing roll attitude. These changes would have been clear from their Electronic Attitude Direction Indicators (EADI) but there was no reference to these by either pilot and it seems they were not referred to as the flight continued in automated flight in VMC. Additional evidence of abnormality would also have been available from the increasing disparity in engine thrust shown by both the indicated engine thrust difference and the corresponding TLA positions.
Despite these cues, it seems that the EGPWS BANK ANGLE Alert was the first time the potentially hazardous situation was recognised. It was considered possible that at this point the Captain may have “assumed that the aircraft was rolling excessively to the right and deflected the control wheel to the left to recover it (but) 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”.
Overall, it was concluded that the pilots should have had enough time to recognise the thrust lever asymmetry and deviation from the intended climbing right but had failed to identify the flight anomaly before it developed into an upset condition. It was suggested that not identifying the anomaly “could have been due to reduced active monitoring because of pilot automation complacency and confirmation bias that aircraft was performing the right turn as commanded”. Non-recognition of the upset condition meant that correct recovery inputs were not applied.
It was noted that Sriwijaya Air was exclusively a Boeing 737 operator with a fleet consisting of two 737-900ERs, thirteen 737-800s and six 737-500s including the accident aircraft. The Company OM (D) included a requirement for Upset Recovery Training to be included as part of recurrent pilot training every two years. The FCTM was found to refer pilots to the corresponding ICAO publications “for detailed information on the AUPRTA”.
- Operational Oversight
It was found that the Sriwijaya hazard reporting system was only really used by some ground handling personnel and contained few reports from pilots or maintenance personnel and no hazard reports by dispatchers. It was concluded that the situation was likely to indicate that the importance of the system “had not been emphasised to all employees (resulting in) hazards not being identified and properly mitigated”. It was explicitly noted that the list of hazards in the Hazard Identification Risk Analysis System did not include the ability to maintain the Flight Data Analysis Programme which was sub-contracted to another AMO. A three month period during which no data were downloaded had been attributed to “lack of human resources and equipment” with no documented resolution and this period excepted, the proportion of flights for which data was analysed was generally low. Acquisition and analysis of safety data was found to be generally poor which it was concluded indicated that “the Sriwijaya Air Safety Management System (SMS) has not been implemented effectively”.
- The Immediate Response to the Accident
The applicable emergency response procedures once the aircraft was no longer in contact with ATC were found to have been in accordance with the State Civil Aviation Safety Regulations Part 170. However, whilst this regulation was “adopted from ICAO Annex 11 subchapter 5.2” it was found that this had been done without use of the alternative word ‘or’ against each of the alternative triggers to initiate the three successive stages of an emergency (Uncertainty, Alert and Distress) which was contrary to the Standard described in Annex 11.
- The Airworthiness Context
A very detailed airworthiness investigation found that the 27 year-old aircraft had experienced various intermittently recurring A/T system faults since its entry into service at the operator in 2012. It was found that almost half these faults had been followed by release to service after cleaning of the electrical connectors. Other actions had involved the replacement of the suspected faulty A/T components but when the Investigation examined the previously installed components no fault was found with them. A replacement of the right hand engine occurred during this time but reports of A/T problems continued after this. Some involved the A/T system being unable to move right thrust lever because of friction or binding within the mechanical system.
In the accident flight, the right thrust lever did not move to in accordance with the targets selected on the MCP and so “the left thrust lever continued move backwards to compensate the engine thrust required to capture the selected speed and rate of climb” which resulted in the recorded thrust lever asymmetry. The failure of the right thrust lever to retard was eventually attributed to friction or binding within the mechanical elements of the system. However, the delayed disengagement of the A/T as the thrust lever asymmetry increased was found to be the result of an incorrect secondary flight control position input.
Given the contribution of the delay in A/T disconnection to the undetected development of the thrust symmetry, this aspect was further considered. It was found that the aircraft had not flown between March and December 2020 and that during this time various maintenance work had been performed. Between release to service after this work and the departure of the accident flight, there had been only three pilot reports of an A/T problem although seven asymmetric thrust lever events were recorded on the QAR, none of which had been the subject of a pilot defect entry in the Technical Log. It was considered noteworthy that one of these asymmetric thrust lever events which had occurred on 15 March 2020 and had resulted in the aircraft rolling up to 41⁰ to the left was likely to have been the result of a delay in the triggering of automatic A/T disconnection and that “the accident flight Captain was in command of this flight”.
The component of the A/T Computer which activates A/T disconnection was noted to be the ‘Cruise Thrust Split Monitor’ (CTSM) which monitors engine thrust and aircraft roll input for thrust asymmetry. Examination of QAR data for both the accident flight and the 15 March 2020 flight and the accident flight showed that a malfunction in CTSM activation had in both cases been “possibly due to the right flight spoiler position signal value read by the A/T Computer (being) too low to activate the CTSM”. It was not possible to determine why this would have occurred but multiple sources that could have been the cause were identified including “a mis-rigged or erroneous spoiler sensor, a mis-rigged spoiler actuator or a sheared/damaged spoiler linkage”. The aircraft operator stated that flight spoiler sensor rigging had never been performed on the aircraft while being operated by them since “it never met the requirement to do so”.
Six Contributing Factors to the Accident were identified 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 AP to obtain selected rate of climb and aircraft speed due to the friction or binding within the mechanical system, and 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 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 from an upset condition in an effective and timely manner.
Two Safety Recommendations were issued in the 10 February 2021 Preliminary Report of the Investigation as follows:
- that the Directorate General of Civil Aviation includes a requirement for UPRT in the Civil Aviation Safety Regulation (CASR) and develops guidance to increase the effectiveness of UPRT. [04.R-2021-01.01]
- that the Directorate General of Civil Aviation reviews the requirements for notification of the rescue coordination centre in the CASR 170 to ensure that the requirement is in accordance with the standards contained in ICAO Annex 11. [04.R-2021-01.02]
- The Directorate General of Civil Aviation responded to the above two Safety Recommendations made in the Preliminary Report by taking action to develop CASR Part 121 regulations and guidance material for implementation of a URPT programme in Indonesia and amending the CASR Part 170 reference to ICAO Annex 11 subchapter 5.2 in respect of the error in triggers for emergency response phases.
- Boeing response following the accident included the following:
- issued a Flight Operation Technical Bulletin (FOTB) on Airplane Upset Prevention and Recovery.
- issued an updated Multi Operator Message (MOM) on the potential for latent flap indication system wiring failure and its impact on the A/T function with advising operators to perform an inspection within 250 flight hours of its date of reissue.
- began work on an SB to provide additional guidance on the flap synchro inspection.
- issued a revision to the 737-300/-400/-500 Maintenance Planning Document (MPD) requiring repetitive inspections of spoiler and aileron deployment and associated position sensors.
- Began work on an SB to address a pending FAA AD which subsequently introduced a requirement for an initial inspection of the spoiler deployment and aileron position sensing system within 250 flight hours of issue and further tests of this system at intervals not exceeding 2,000 flight hours.
The Federal Aviation Administration (FAA) responded to the issue of the updated Boeing MOM recommending 250 hour one-off inspection for flap indication system wiring failure which, if present, could cause A/T malfunction by issuing AD-2021-08-14 which mandated both initial and recurrent inspections to confirm system integrity.
Three new Safety Recommendations were issued based on the findings of the completed Investigation as follows:
- that Sriwijaya Air consult the DGCA prior to modifying any flight operation procedure and also obtain an NTO from the aircraft manufacturer prior to modifying any existing aircraft manufacturer’s flight operation procedure. [04.O-2021-01.03]
- that Sriwijaya Air increase the amount of data retrieval in the Flight Data Analysis Programme so as to improve the ability to monitor flight operations safety performance. [04.O-2021-01.04]
- that Sriwijaya Air emphasise the hazard reporting program to all employees to encourage hazard reporting. [04.O-2021-01.05]
The 202 page Final Report of the Investigation was released on 10 November 2022.
- Loss of Control
- Recovery from Unusual Aircraft Attitudes
- Flight Path Monitoring
- Cross-checking Process
- ICAO Emergency Phases
- Aircraft Maintenance
- Aircraft Technical Log
- Safety Occurrence Reporting
- Upset Prevention and Recovery Training (UPRT) - Updated Perspectives