Maintenance and pilot actions are the current focus of the Sriwijaya Air crash according to a preliminary report from the Indonesian National Transportation Safety Committee (NTSC).
A faulty autothrottle system that had failed twice before has emerged as a key factor in the loss of the 737-500 that crashed on January 9, killing all 62 onboard.
Indonesia’s NTSC is yet to determine the exact cause of the disaster and is hampered by the lack of the Cockpit Voice Recorder.
According to NTSC investigator Nurcahyo Utomo the left engine throttle lever was trimming back the power while the right lever remained constant.
“Both autothrottles were experiencing anomalies,” Mr. Utomo told media Bloomberg reported.
“The left one rolled back too far while the right one was not moving at all and appeared to be stuck. We don’t know which one is malfunctioning, we are still trying to find out.”
What is puzzling industry experts is why the pilots did not simply disengage the autothrottle system and control the engines manually.
“Were they trying to overthink the problem? It is tragically simple. Disengage and fly the plane,” said one check and training captain.
Aircraft can operate with an inoperative autothrottle and the 737 – like all twin-jets – can fly perfectly well on one engine and only require a little rudder trimming.
The NTSC preliminary report also found that the Aircraft Maintenance Log (AML) recorded that the aircraft had two Deferred Maintenance Items (DMIs) related to the first officer’s Mach/Airspeed Indicator and the other to the auto-throttle system.
With regard to the auto-throttle system it noted that:
On January 3, 2021, the pilot reported that the auto-throttle was unserviceable. The engineer rectified the problem by cleaning the auto-throttle computer’s electrical connector. After re-installation, the Built-in Test Equipment (BITE) test result was good.
On January 4, 2021, the pilot reported that the auto-throttle was unserviceable. The engineer tried cleaning the auto-throttle computer’s electrical connector but the problem remained and it was transferred to DMI number list 07958.
On January 5, 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 auto-throttle computer. The BITE test result was good and the DMI was then closed.
The NTSC in its Safety Actions made the following findings:
The Quality Maintenance Division of Sriwijaya Air issued a quality notice on January 18, 2021, to the maintenance control center and engineers for ensuring:
- the repetitive defect handling must be conducted in accordance with the Safety Circular from the DGCA and Company Maintenance Manual;
- to follow the procedure described on the Aircraft Maintenance Manual (AMM), Fault Isolation Manual (FIM), and Illustrated Part Catalog (IPC) for troubleshooting;
- to fill the Aircraft Maintenance Log in accordance with the Quality Procedure Manual (QPM);
- to follow part robbing procedure as described in the QPM and Aircraft Maintenance Procedure Manual (AMPM).