Going a bit further in analysing the FR24 data, leading to a possible cause of this accident. I say again, possible
Nothing out of usual until reaching FL100 on a 047 heading. Then at 07:39:50 a left-hand turn is initiated, rather shallow.
It takes 17 seconds (time to reach the moment they start losing altitude) to achieve a change of heading of 24 degrees. This gives an average rate of turn of less than 1.5 deg/s. A Standard Rate Turn, i.e. to make a 180 in one minute, is 3°/s.
The graph below shows this change of heading and altitude.
Left : 737-700 Right : 737-500
The Rate of Turn is calculated from the FR24 data but they report heading as an integer. Therefore, the first change is given to be from 047° to 046° in half a second, a RoT of 2°/second. This is purely theoretical as a change of heading is not sudden and the first two seconds of the graph may be disregarded.
Considering an initial RoT of 0.4°/s, and a speed of 287 kts (OK, FR24 reports ground speed but the TAS shouldn’t be much different) the resulting bank angle is about 6 degrees only, probably unnoticeable (horizon not visible in the climb) if the change of heading was unintentional.
The RoT creeps up to about 1°/s ten seconds later, bank angle 15°, and around 1.8°/s when hell broke lose and they start losing altitude with a bank angle of 25°.
But what causes this change of heading? Is it part of the chain of events that lead to the catastrophic outcome?
Maybe a serious incident that occurred 10 years ago can shed some light.
An Air Nippon (now ANA) 737-700 while cruising at FL410 suddenly nose-dived with a roll angle reaching 130° and ND pitch of -35°. They lost 6000ft before recovering.
The root cause of the severe incident is reported as : The pilot inadvertently operated the rudder trim switch while intending to unlock the flight deck door
Link to the Japan Transport Safety Board : https://www.mlit.go.jp/jtsb/eng-air_report/JA16AN.pdf
Also reported here : https://www.aviation-accidents.net/air- ... ght-nh140/
In 2014, the FAA has issued a Special Airworthiness Information Bulletin accessible via EASA on : https://ad.easa.europa.eu/ad/NM-15-03
Boeing issued Service Letter 737-SL-27-238, “Inadvertent Activation of Rudder Trim
,” dated September 19, 2012, to provide operators with additional information.
In the case of Air Nippon, the FO had transitioned from 737-500 to 737-700. The pedestal layout is significantly different between these two aircraft.
On the 735, the Rudder Trim button is further aft than the Door Lock Selector. This is the opposite on the 737-700.
This difference between the Classic and the NG is explained by the fact that the Door Lock Button has been installed after the “9/11” under a Supplemental Type Certification and commonality between the two types was not deemed important. The resulting location of the Door Lock may also differ between companies.
Sriwijaya operates both the B737-500 and the B737-800.
It would be interesting to read the KNKT accident report on the SJA182.