Here's a better plot with infrared satellite overlay. While they almost certainly were in IMC when the problem occurred, it doesn't look like they came close to significant CBs.

https://cimg4.ibsrv.net/gimg/pprune....bc5aa064b1.png

My dad is an LAE. From the flight pattern recorded, he suspect that the check valve is corroded. Due to the fact that most of the aircraft are grounded due to COVID-19, virtually no maintenance was done on them to save cost.

Regardless of what has happened here, one thing is for sure. There will be many rusty pilots, engineers and aircraft flying around soon if the world ever gets back to normal flying. And in a lean financial market. Be prepared for a few incidents.

katekebo

Far from me to give any conclusion but: this is rainy season and also season of mud dauber wasps which are active in Indonesia. I have been working in JKT for some years. There are airlines who never put pitot probes covers for night stops.
And some parking stations are close to ditches where wasps can collect mud.
I have pictures of aircraft's fuselage fully covered by mud dauber wasps (not a sriwijaya aircraft), taken some years ago. Not only maintenance staff is not trained to recognize them and possible consequences but also they do not take any action for inspection - at least - when there is presence of mud dauber wasps in vicinity of an aircraft not having been protected.
All my attempts to convince maintenance staff installing pitot probes covers were useless, even after I contacted safety Dept. (not sriwijaya, again)

Once again, this is a general situation, I do not blame particularly Sriwijaya and do not say that this is what happened.

On top of it, an obstructed pitot (obstructed by wasp's nest) should be at least detected by the crew during take off run when comparing speed between CAPT and FO, unless both of them are obstructed.
Obstructed Pitot by wasp's nest is not necessarily something you can notice from the ground during the turn around of the aircraft.

My bet is on a ripped off vertical tail due to gusts / fatigue / rudder movement, as descending and decellerating like that indicates a very high drag (parachute like) configuration. Check AA 587. Maybe I'll try to calculate later what the drag coefficient must have been...

Flexible response, no, groundspeed on FR24 doesn't depend on the actual aircraft's pitot static system. Groundspeed is groundspeed, not airspeed anyway.

For the PP's on here, how does this accident profile fit a deployed reverser in flight?

If the flight data is correct, Reverser Unlocked or severe damage to the left engine could be a possibility. The aircraft appeared to yaw to the left at an increasing rate until it entered the sea. Looks like when the autopilot finally gave up the aircraft yawed left and the nose pointed down. Airspeed must have increased, increasing the drag on the left engine causing yaw to accelerate. The engine being slightly below the C of G pulled the nose further down until the aircraft was inverted and then continued to pull the nose up relative to the horizon while inverted, causing the increase of lateral speed (GS). The yaw to the left continued while inverted causing the increasing heading until it entered the sea. This is just a guess based on the data.

Let’s face it, being in Indonesia it could be anything, maintenance, training, pilot error are all strong trends in previous accidents. We won’t know until the investigation is done so let’s wait for that.

waito


Nicely graphed, though the rounded time values on the X-axis are a bit irregular.

At the risk of stating the obvious, while the ROD (fpm) remained more-or-less constant during the descent, the flight path angle wrt the ground must have become shallower towards the end of the dive (i.e. the aircraft was beginning to pull out), since FPA is simply the resultant of horizontal and vertical velocity.

For example:
At 4000' (07:40:22) V/S 40,000 fpm (395 kts); GS 120 kts -> FPA -73°
At 2000' (07:40:25) V/S 40,000 fpm (395 kts); GS 210 kts -> FPA -62°

All values approximate, but the difference is marked enough to accommodate any slight variation.

The captain's wife was interviewed by the news website and said that he unusually apologized to her and their children. Coincidence or something siginficant related to the accident?

WatchTheSkies

I don't see how the data supports much, if any, of that scenario.

For a start, the aircraft descended in a RH spiral, not a LH one.

Yes, but the 737 also had at least one high-speed rudder hard-over event in the cruise.
Due to their speed this crew were able to recover, but it was a major event that nearly turned them upside-down. But if the crew were less able, or disorientated due IMC, or if there was an airframe failure due to these stresses, then a high-speed rudder hard-over could result in a plunge into the ocean.

When was the last time this plane actually flew in the air, before this?

brika

about 140 flights since its 6 month grounding.

Indonesia locates black boxes after deadly plane crashIndonesia locates black boxes after deadly plane crash (bangkokpost.com)

"We have located the position of the black boxes, both of them," said Soerjanto Tjahjanto, head of Indonesia's transport safety agency.
"Divers will start looking for them now and hopefully it won't be long before we get them." A military vessel picked up the plane's signal, and divers recovered wreckage from around 23 metres below the water's surface, the transport ministry said Sunday, citing Indonesia's military chief Hadi Tjahjanto.

It did not specify if the signal was from the downed plane's voice and flight data recorder.

I have a genuine question about pitch control on 737s, maybe you would know?

The original 60s version had the low, thin, P&W engines.

The MAX has these large, forward CFM motors. These cause pitch-up under thrust and so necessitated the introduction of MCAS to fix it.

This model is the 2nd gen 500 from the 1980s, so it's too early for MCAS, I understand.

However, the 500 does have rather large CFM motors. It looks a lot like the MAX. So, presumably, it also had the pitch-up issue (though perhaps to a lesser extent than the MAX).

If so, my question is, what was done in the late 80s to mitigate the pitch-up tendency on the 500 series when it rolled out?

Unlikely
 

procede

the 500 has the shortest body, it’s tough as old boots, highly unlikely, the short body with the highest thrust engine options does make it a little more demanding engine out, but only at LOW SPEED at altitude this wouldn’t be an issue, this appears to be very rapid loss of control.

with regards to so called explosions reported by fishermen, could be thunder or sonic boom if it went down under power ballistically.

Only 15m of water so the front will be buried in the mud FDR should be on top

Not a 737 expert so will leave the rest of your question for someone else to answer, but I do know enough to correct the misunderstanding above. MCAS has nothing to do with thrust-pitch coupling: it was introduced to address aerodynamic effects of the MAX’s LEAP engine pods, which are larger and mounted further forward than the CFMs. At high angle of attack, the airflow impinging on the under-surface of the pods induces a pitch-up moment which must be counteracted to meet certification requirements relating to stick force gradient. No doubt the CFMs are worse than the P&W in that regard: just not bad enough to need counteraction in the way the LEAPs do.

The dauber wasp theory is unlikely as the aircraft was on a 1:30hr turnround prior to the accident flight.

A downdraft of the magnitude required to get the vertical speeds recorded from the FR24 data is most unlikely as how would they dissipate before the surface. The fishermen nearby didn’t report hurricane force winds.

EIFFS mentions fishermen hearing ‘explosions’ that he purports may have been a sonic boom. After COPA 201 had a loss of control due to VG failure at FL250, it exceeded the speed of sound before breaking up at FL100. With the Vertical speed assumed from the FR24 data and the ground speed also recorded from the same source at the later stages of the dive, it can’t have been far off.

Not sure which data you are referring to but i was using this one below

Analysis
042 to 030 in 6s = -2°/s
030 to 023 in 3s = -3.5°/s
023 to 006 in 3s = approx -6°/s ( approx -60ft/s)
006 to 339 in 6s = approx -5°/s ( approx -296ft/s)
339 to 338 in 2s = -0.5 °/s ( approx -413ft/s)
338 to 011 in 4s = approx +8°/s ( approx -681ft/s)
011 to 093 in 7s = approx +12°/s ( approx -736ft/s)

( when upright, heading reducing means left turn. Increasing means a right turn. Opposite direction when inverted, of course)
Initially there was a left yaw for 9 sec that became a nose dive. From the data you can see, when the aircraft pitched down, there was a huge forward acceleration vector which temporarily reduced the yaw vectors effect on the heading change. But as drag increases exponentially with velocity, the yaw vector caught up and increased the heading change again. The increase in lateral speed and direction of heading change is IMO best explained by the aircraft going inverted and the nose moving upwards relative to the horizon due to the exponentially increasing drag from the left engine

Who says it was at stalling speed at 4000’?

Pitot blocked by wasp mud dauber happened to an Etihad A300 with only a 2 hours stop in BNE on November 21st 2013 (called Mayday).
But indeed it is far too early to make such conclusion for SJ182
I just hope they could recover pitots and inspect them.
A B737 from an operator based in Jakarta lost controlled some years ago (they recovered hopefully) in high altitude, due to incorrect airspeed indication and incorrect procedure applied (PF remaining on side of incorrect speed).
During investigation they found wasp nest in PF side's pitot.

Let's wait the end of the investigation ; I just hope they will consider this possibility ; there is very small awareness within all operators based in JKT as well as for the airport authorities when it comes to dangers caused by the wasps and corrective actions to be taken, especially during the rainy season.
BNE is typical example of what should be done by airport authorities, as preventive measures.

If it speared vertical into the water with 400ish kn then from most part of the plane, especial the front, only small shrapnel will be left over. The impact with water can be the boom, the fishermen heard, since at those speeds hitting water or concrete makes no difference.

Good that they located the recorders in shallow water, so we will find out what happened.

I have see wasps start building a nest in a pitot tube within 10 minutes of shutdown.

WatchTheSkies

I posted a link earlier to the data I was using (it may have been deleted), but I'll use yours for simplicity.

Your own analysis shows an average track angle rate of approximately +10°/s during the last 11 seconds of flight (from around 8000' until impact).

I agree with that. So I'll ask again - where do you get a "left turn" from ?

Unreliable airspeed would have happened at takeoff, not at 11000 ft. Nothing suggests any issues before they suddenly went straight down.

Exactly that is why it has such a large tail... There are also not that many flying around, especially at that age. So more chance of unnoticed fatique cracks.

It has the same tail as every other classic 737. -300, -400, -500.

Photos
 

https://cimg2.ibsrv.net/gimg/pprune....7054c7039.jpeg
https://cimg3.ibsrv.net/gimg/pprune....03d1e2fd3.jpeg
https://cimg4.ibsrv.net/gimg/pprune....b3d151320.jpeg
https://cimg5.ibsrv.net/gimg/pprune....f6cdd11e0.jpeg
https://cimg6.ibsrv.net/gimg/pprune....ac706554e.jpeg

WatchTheSkies

Drag does not increase exponentially as a function of velocity. Without turbulence the drag is linear in velocity (Stokes' law). At relatively large speed it is quadratic (Rayleigh).

DaveReidUK

I am assuming the aircraft was inverted from 8000' as that explains the lateral speed increase and the revers from the initial 20s of left turn. So when you are inverted. From the perspective in the cockpit you are turning left when the heading change was +10°

Euclideanplane

There are different types of drag. The drag in the case of reverser unlock or severe damage is parasite drag. Refer to
www.skybrary.aero/index.php/Drag

DaveReidUK

The first 20 sec of the data is the left turn. I assume the aircraft became inverted as it explains the lateral speed increase and heading direction changes. When the plane is inverted, it was still turning left relative to itself, right if you are viewing the upside down plane from the ground, producing the +10°/s increase in heading

Look to Silkair in '98 for similarities.

Icing in the climb?

Explosions?
 

Could have been engine surges from a sudden/rapid change in attitude resulting in airflow changes into the inlets...

WatchTheSkies

That is correct. Parasitic drag refers to an additional volume of air stuck to the body of the aircraft and causing additional drag, e.g. such as with a windmilling or structurally damaged engine. It causes more drag than usual, but the dependence on speed is not different, as it says when quoting the page which you referenced: "Parasitic drag increases with the square of the airspeed". That is quadratic, which is bad enough, but not exponential.

Possible, but not likely. SOP is probe heat ON after start. As others have said, low CB activity in the area they passed through. Icing at 10000 ft in a CB is possible, but otherwise you need to get higher than 10-11000 ft before you encounter icing in this area. Besides, unreliable airspeed does not lead to a straight down descent.
Something severe happened very fast with this flight. Even if you lose control you will try to do something to regain control. That would show up on the data, but they went down fast without much input to reverse the situation. It feels like a deliberate action or a situation it was not possible to get out of.

I tend to agree with your guess that the aircrafts initial departure from normal flight was a left bank past 90 degree with an increasing nose down (relative to the horizon) attitude.
We will soon get the answer when preliminary data from the flight data recorder are released.