Sriwijaya Crash Report Points to Engine Thrust Imbalance

 - February 10, 2021, 12:25 PM

A preliminary report issued Wednesday by Indonesia’s National Transportation Safety Committee (KNKT) into the January 9 crash of a Sriwijaya Air Boeing 737-500 indicates the airplane experienced an imbalance between the left and right thrust levers as the aircraft climbed past 8,150 feet, a minute and 28 seconds before the start of its fateful dive.

After taking off from Runway 25R, the pilots asked for a heading change to 075 degrees to allow them to avoid weather. After air traffic control cleared the change, the airplane began a right turn. ATC then told the pilots to stop climbing at 11,000 feet due to conflicting departure traffic from Runway 25L.

At about 10,600 feet, the aircraft started turning left. About five seconds after the 737 reached its highest altitude of 10,900 feet, the airplane’s autopilot system disengaged at a recorded heading of 016 degrees and a pitch angle of 4.5 degrees nose up. The aircraft then continued to roll left to more than 45 degrees as the thrust lever position of the left engine continued decreasing while the right engine thrust lever remained static and the airplane pitched more than 10 degrees downward. The FDR data also recorded the left engine N1 decreasing while the right engine N1 remained unchanged. ATC lost radar contact with the airplane 43 seconds after the FDR recorded the airplane’s 10,900-foot altitude. All 56 passengers and six crewmembers died in the crash.  

More than a month after the accident, authorities still haven’t recovered the cockpit voice recorder.

The preliminary report did not list any suspected contributory factors, and infrared satellite images showed “no significant development of clouds” in the aircraft’s flight path.

However, on January 20 Sriwijaya Air’s chief pilot issued a notice to pilots to follow operating experience guidance, review training aids, ensure awareness of aircraft position, attitude, and aircraft systems in every phase of flight.

Eight days later Sriwijaya Air’s standards, quality, and training division included upset recovery training as part of the training syllabus in the next Line Oriented Flight Training (LOFT) pilot proficiency check (PPC).

In the preliminary report, the KNKT characterized safety actions by the airline and the Directorate General of Civil Aviation (DGCA) as “relevant to improve safety,” but that other safety issues remained. Namely, it said the investigation could not find guidance from the DGCA to operators or training organizations on upset prevention training, but only upset recovery training. As a result, the KNKT recommended that the DCGA include guidance for an upset prevention and recovery training (UPRT) program in the Civil Aviation Safety Regulations (CASR).