MIAMI – This week, the Komite Nasional Keselamatan Transportasi (KNKT) released the preliminary report for the Sriwijaya Air (SJ) 182 accident that occurred last month.

As per ICAO Annex 13 regulations, a preliminary report is released by the investigative authorities one month after the date of occurrence. These Preliminary Reports contain factual information only, summarizing what is currently known about the accident.

No factual analysis is contained nor expected in a preliminary report. As always, aircraft accident investigations under ICAO Annex 13 are conducted to further aviation safety, and not to point fingers or assign blame.

Sriwijaya Air Boeing 737-524(WL); @CGK2017 – 39283339542. Photo: CC BY-SA 2.0, via Wikimedia Commons

The Flight – As Known so Far

On January 9, 2021, Sriwijaya Air 182, a Boeing 737-500 with registration PK-CLC, scheduled to fly from Jakarta to Pontianak. At 14:36 local time, the aircraft took off from Soekarno-Hatta International Airport with 62 persons on board, 2 Pilots, 4 Cabin Crew, and 56 Passengers.

The aircraft had clearance from ATC to FL290, and the Pilots engaged the autopilot at 1,980 feet. Less than two minutes later, the left thrust lever reduced, with the left engine thrust also decreasing, as recorded by the Flight Data Recorder (FDR). The right thrust lever and thrust of the right engine stayed constant.

Due to weather conditions in the flight path, the crew asked ATC for a route deviation, which was approved. However, with this new route also came an instruction to stop the climb at 11,000 feet due to traffic.

Before reaching 11,000 feet, the thrust lever and thrust in the left engine continued to decrease, putting the aircraft even more into a left hand turn. Air Traffic Control contacted the aircraft with a new flight level of 13,000 feet, which the crew read back. However, this was the transmission from the aircraft.

According to the FDR, the aircraft only reached 10,900 feet, where the aircraft was in a roll of 45°. The left throttle lever and thrust continued to decrease. Twenty seconds before the end of the FDR recording, the aircraft was in a 10° nose down pitch and the Auto Pilot disconnected.

Sriwijaya Air Flight 182 Speed and Altitude. Phoenix7777, CC BY-SA 4.0, via Wikimedia Commons

Loss of Contact

After losing contact with SJY182, the controller tried to reestablish contact, including using 121.5 MHz. Fifteen minutes after loss of contact, the Indonesian Search and Rescue Agency was alerted.

The wreckage of SJY182 was found underwater at a depth of 16 m, 80 m from the last ADS-B recorded position. At the time of this report, only the Flight Data Recorder has been found. The housing of the Cockpit Voice Recorder has been located, but the Crash Survivable Memory Unit has not been found.

The flight data recorder, submerged in fresh water for preservation reasons. Photo by BASARNAS (Indonesian National Search and Rescue Agency) – Public Domain

Deferred Maintenance Items

The Investigation Team noted that in the days leading up to the accident, two Deferred Maintenance Items were recorded in the Aircraft Maintenance Log.

On December 25, it was reported that the First Officer’s Airspeed Indicator was not functioning properly. However, the part was replaced on January 4th and passed the test.

On January 3, the autothrottle was reported as not serviceable. The autothrottle computer’s electrical connector was cleaned, and the BITE test was passed. However the problem appeared again the following day, and this time, cleaning the connector did not fix the problem. On January 5, the Take Off/Go Around switch was cleaned, another BITE test was performed, this time successfully, and the DMI was closed.

No further DMI items were noted between January 5 and the date of the accident.

Top: Route of Flight 182 Bottom: Altitude-speed graph of Flight 182. Image: Phoenix7777 – Own work Data source: Route: Sriwijaya Air flight 182 crashes near Jakarta, Flightradar24 KML file Map: Open Street Map

Next Steps

The Investigation Team also laid out their next steps. A top priority is the recovery of the Crash Survivable Memory Unit of the Cockpit Voice Recorder.

As well, the investigators wish to research why split thrust levers occurs. A review into the specific autothrottle system history and records will also be performed. Pilot training with regards to upset prevention and recovery will also be looked at.

Flight operations with respect to Human Factors as well as organizational factors will also be taken into account.

Featured image: PK-CLC, the aircraft involved in the accident, at Soekarno-Hatta International Airport in 2017, in an earlier livery. – CC BY-SA 2.0, Wiki Commons