Nov. 10—The final investigation report into the crash of an older model Boeing 737-500 in Indonesia that killed all 62 people onboard in January 2021 blames a fault in the jet's autothrottle system — which had repeatedly malfunctioned on the aircraft before the crash — along with an inadequate response from the pilots.
Overreliance by pilots on automated systems that can fail has become an increasingly worrying thread in aviation accidents. In modern airliners, the flying is automated much of the time.
The report released Thursday by Indonesia's national transportation safety committee, known by its Indonesian acronym KNKT, found that the 737's autothrottle system that automatically adjusts the power to the jet's two engines became stuck on the right engine after takeoff "as a result of friction or binding within the mechanical system."
So as the jet climbed away from the runway in Jakarta and the pilots adjusted the autopilot mode to reduce thrust, the autothrottle duly eased back power to the left engine but the right engine continued at full power.
The resultant asymmetric thrust caused the plane to turn to the left even as the pilots steered the control wheel to the right and the autopilot followed by moving control surfaces on the wing to roll right.
Another system on the plane designed to monitor for asymmetric thrust also malfunctioned and delayed disengaging the autothrottle as it should have.
But as this was happening, the pilots were unaware of it.
The pilots should have seen from the instrument panel attitude display that the plane was deviating from its flight path to the left. And they should have noted the right thrust lever not having moved backward like the left lever, alerting them to the asymmetric thrust. They apparently missed both clues.
Just under 5 minutes after takeoff, as the jet banked steeply left, a warning alert sounded in the cockpit: "BANK ANGLE."
Two seconds after the alert sounded, at an altitude of 10,700 feet, the pilot in command disengaged the autopilot system to take manual control.
This pilot, 54 years old with almost 18,000 hours of flight time, half of that in a 737, clearly didn't realize that the autopilot had been compensating and masking the effect of the asymmetric thrust in the engines.
With the autopilot gone, the countering forces from the control surfaces on the wings were removed and "the yaw and roll forces of the asymmetric power rolled the aircraft to the left," the investigation report states.
The pilot was so unaware of what was happening that he steered the control wheel further left instead of right, which "increased the roll tendency of the aircraft to the left."
The plane rolled more than 45 degrees left and went nose down.
At that moment, the autothrottle finally disengaged. But it was too late to recover. The flight data stopped recording as the plane crashed into the sea.
System faults and complacency
The report faults the pilots for their lack of recognition of the situation.
It blames "pilot automation complacency" (overreliance on the automated system) and "confirmation bias" (believing that the plane was steering right as commanded, when in fact it was rolling left).
The Indonesian safety authority found that Sriwijaya Air provided "inadequate" training for its pilots in upset recovery, which means righting an airplane if it inadvertently stalls, rolls or pitches to deviate from the intended flight position.
Indonesia now mandates detailed upset recovery training for all airline pilots.
The KNKT report also states that the system that was supposed to monitor the 737's autothrottle for asymmetric thrust and disengage it — the Cruise Thrust Split Monitor — may have been misrigged by maintenance personnel, or its failure may have been due to a sensor fault providing an incorrect value for the positions of the control surfaces on the wings to the autothrottle computer.
The report notes that Boeing is issuing a bulletin to all 737 operators requiring repetitive inspections of the control surface sensors. An Airworthiness Directive that will make this mandatory is pending from the Federal Aviation Administration.
Unresolved autothrottle issues
The plane was a 737 "classic" model, built in Renton in 1994.
Sriwijaya had grounded the jet during the COVID-19 pandemic from March 2020 until a return to service on Dec. 19, 2020, just three weeks before the crash.
After Sriwijaya acquired the plane in 2012, its maintenance log recorded 65 pilot reports relating to autothrottle malfunction, including 32 pilot reports of the autothrottle disengaging.
And in the year before the crash, maintenance data showed records of seven events involving asymmetric thrust.
In response to more autothrottle malfunction reports in early January 2021, just days before the crash, mechanics cleaned the electrical connectors.
They then suspended further maintenance efforts when test equipment reported "no faults."
The report says one factor that contributed to the crash was the inability of Sriwijaya's maintenance processes to identify the mechanical issue with the right thrust lever, which left the problem "prolonged and unresolved."
Anthony Marsh, an attorney with Seattle's Herrman Law Group, which has filed suit against Boeing over the Sriwijaya crash, said via email that the repeated issues with the autothrottle system are "troubling."
"Though Boeing has been dealing with this issue on its aircraft for some time, it looks like it still hasn't found a sustainable solution that keeps the autothrottle system from causing problems," Marsh wrote.
Boeing declined to comment, deferring to the KNKT final report.
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