Thread by @737handbook

KNKT (Indonesian NTSB) issued final Aircraft Accident Investigation Report on #Sriwijaya Air (#SJY182, 737-500, #PKCLC) crash into Java Sea dated 9th of January 2021

Source: knkt.go.id/Repo/Files/Lap…

At 0736 UTC (1436 LT) in daylight conditions, flight SJY182 departed from Runway 25R of Jakarta, and the FDR data recorded that the A/T was engaged in N1 mode after airborne and the A/P system engaged at altitude of 1,800 feet.

The SJY182 pilot requested to the Terminal East controller (Air Traffic Controller/ATC) for a heading change to 075deg to avoid weather conditions and was approved. The ATC instructed the SJY182 pilot to stop climbing at 11,000 feet.

At 07:38:00 UTC, the A/P lateral control changed from LNAV to HDG SEL and five seconds after, the A/P vertical control changed to Pitch V/S and A/T changed from N1 to MCP SPD. The change in A/P mode required less engine thrust.

Left thrust lever and the N1 speed of the left engine started continuously reducing (as it should to maintain MCP SPD), while the right thrust lever and N1 speed of the right engine remained fixed until the aircraft entered an upset condition.

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

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.

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.

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.

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.

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.

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.

The pilots did not identify the flight anomaly before it developed into an upset condition

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.

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.

The aircraft entered an over speed situation and which subsequently, entered developed into an accelerated stall.

Prior to the accident flight 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.

The maintenance actions were stopped after the BITE test resulted “no faults”. 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.

The A/T problem was repeatedly deferred and rectified. 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.