Asymmetric engine thrust may have lead to the crash of Sriwijaya Air Boeing 737
The Sriwijaya Boeing 737-500, registration PK-CLC performing flight SJ-182 from Jakarta to Pontianak (Indonesia), was climbing out of Jakarta about 11nm north of Jakarta's Sokarno International Airport over the Java Sea when radar and radio contact was lost with the aircraft.
Indonesian KNKT has released a preliminary report on the B737-500 crash base on reports from the FDR. The CVR is still missing.
Here are some key findings:
Autopilot (AP) system engaged at an altitude of 1,980 feet.
After the aircraft climbed past 8,150 feet, the thrust lever position of the left engine started reducing, while the thrust lever position of the right engine remained. The FDR data also recorded the left engine (N1) was decreasing, whereas the right engine N1 remained.
At 10,900 feet, the AP system then disengaged with a recorded heading of 016°, the pitch angle was 4.5° nose up, and the aircraft continued to roll to the left to more than 45°. The thrust lever position of the left engine continued decreasing while the right engine thrust lever remained.
Maintenance action on the Autothrottle:
On 3 January 2021, the pilot reported that the autothrottle was unserviceable. The engineer rectified the problem by cleaning the autothrottle computer’s electrical connector. After re-installation, the Built-in Test Equipment (BITE) test result was good.
On 4 January 2021, the pilot reported that the autothrottle was unserviceable. The engineer tried cleaning the autothrottle computer’s electrical connector but the problem remained and it was transferred to DMI number list 07958.
On 5 January 2021, the engineer rectified the problem as stated in the DMI number 07958 by cleaning the autothrottle Takeoff and Go Around (TOGA) switch and conducted a BITE test on the autothrottle computer. The BITE test result was good and the DMI was then closed.
Aircraft upset is a condition where the aircraft attitude or airspeed is outside the normal bounds of operation for which it is designed and it can lead to loss of control.
NTSB data for 1994–2003 count 32 accidents and more than 2,100 lives lost worldwide due to accidents from loss of aircraft control.
While the investigation is ongoing, the possible cause of the upset could be A/T malfunction causing asymmetric thrust. This, in turn, would explain AP disengagement and sudden left roll.
Boeing 737NGs and Boeing MAX’s come equipped with RCAS system which provides roll/yaw asymmetry visual warnings when the AP is 75% saturated and Roll Authority visual and aural alerts when the AP is 100% saturated and is about to disengage.
KNKT said that following the crash Sriwijaya had taken safety actions including adding upset recovery training in its next pilot proficiency check and reminding engineers that repetitive defects must be handled in accordance with safety manuals.
The Sriwijaya accident is Indonesia's third major airline crash in just over six years.
Read earlier reports from the crash: