ISA+18C... 11,000' 250CAS ~..... Thats a normal day in JKT, which gives a FZL around... ~FL170... The OAT at 11K is around +11C, and the 250kts adds around 18C ruffly. The swept wing of the 737 doesn't pick up much at all in almost all cases other than SCLD. Trying to get it to ice up takes effort. With convective weather, entry increases temp.

That will surprise the boys... and everyone else. it wouldn't give a turn, or the recorded ADSB data. if the tail fethers separated in flight, they would be found early on floating in almost all cases, and would have low levels of damage relative to the catastrophic fragmentation of a high speed impact. If bits separated in flight they should be already in plain sight.

dP is not large at 11,000', and so a collapse would give... floor collapse? Post THY the floor venting got better, to stop the collapse of floors in decomp, and the control cables were separated. Have to open up an IPC or AMM Ch 27 to work out where they do go on the B737, but elevator cables are duplexed. The rudder is single control run. Aileron runs go to the PCU's which are in the gear wells.

[img]https://cimg0.ibsrv.net/gimg/pprune.org-vbulletin/1760x1148/screen_shot_2021_02_10_at_10_44_28_am_31ebf9def28778ee21401e 04e2a90385828ff059.png







https://cimg1.ibsrv.net/gimg/pprune....dddbe005c4.png



reversal? unlikely for the conditions needed to give the historical freeze of the inner sleeve of the valve.

fdr

interesting

“Our environment is stochastic, with a bucket of inputs and factors that impact every moment of our interface with the job at hand. from noise of heavy rain impact, accelerations at the seat from turbulence, ATC interactions, cockpit coordination, cabin needs, flight path management and systems.”

Quite true. But I always thought that that was the job.

I have 20,000 hours flying Boeing aircraft with most being routine interspersed with moments , uuhm , not routine.
In the not routine moments my crew has responded as trained. Professionally and effectively.
Maybe I have just been lucky.
I have also operated in Indonesia.

It wont be the aircraft.

Educated guess - the speculation of a thrust lever clutch failure will be correct. One engine throttle failed to move in response to the autothrottle command at level off (bad clutch, or high downstream forces due to a seized pulley or kinked push-pull cable). Crew was rusty, possible distracted by something else, and failed to notice the throttle split. Aircraft rolled due to the thrust asymmetry, crew caught off-guard by sudden AP disconnect, lost SA, and did all the wrong things. Specifics of why the crew failed to react appropriately to a relatively benign failure will remain a mystery unless the CVR memory module is located.
Critics and lawyers will try to blame Boeing (deep pockets - unlike the operator), ignoring that the autothrottle defect had existed and gone uncorrected for some time, and SOP for the pilots is to monitor T/L position and engine parameters.

Preliminary KNKT Accident Report is published: http://knkt.dephub.go.id/knkt/ntsc_a...y%20Report.pdf

"The thrust lever position of the left engine continued decreasing while the right engine thrust lever remained."

Aircraft rolled to the left...

Left engine thrust lever and corresponding left engine N1 decreasing. Pilots preocupied with ATC communication and bad weather, requesting heading change due to weather and then ATC ordering leveling off at 11.000 feet due to traffic. Further reduction in left engine thrust. AP disconnection followed by rapid left roll past 45 degrees. End of recording only 20 sec later.

No FDR readout provided in the report. Problems with the AT in the days before the accident.

The virtual absence of FDR data, apart from a handful of snapshots, is unusual compared to recent KNKT preliminary reports.

There is no indication of how, during the last 20 seconds of flight, the aircraft progressed from an "over 45°" left bank (QRH criterion for an upset condition) to a RH spiral descent.

I suspect, David, it is called "Passing through the inverted"? Done any aerobatics?

Maybe. Maybe not.

It just seems rather strange to describe a 180° roll (if indeed that's what occurred) simply as "over 45°". There are no references to "inverted" anywhere in the preliminary report, which would seem a curious omission if the case.

And the problems were "fixed" by cleaning the contacts. This is reminiscent of the "fixes" done to the Inertial Reference Systems before the AdamAir accident in 2007. From the Final Report (ASN link):

Didn't work then, and probably didn't work now.

If this is what happened then sadly it would have all been displayed in front of them - a big thrust lever and N1 split with one thrust lever stuck at climb thrust while the other reduced to a low setting for levelling off. No autopilot control of the rudder on B737 to compensate for a large engine thrust difference, so the ailerons would have been hard over - again plain for the pilots to see on the yokes.

(When I flew B737, we were told to have hand on thrust levers whenever they were expected to move, e.g. levelling off from a climb.)

The autopilot suddenly dropping out would have been the final straw - the ailerons were being held hard over, but once the autopilot dropped out they would have returned to neutral, and unless correct manual action was taken quickly, the pilots would have been hard pushed to retain control.

Perhaps there should be some sort of alarm that the autopilot is approaching its drop-out point, so crews would know before it actually happens?

So, given what's not in the report ... how would upset recovery with split thrust set look like?

(Pax) Sorry to butt in but can someone explain to me what ' off axis entry into a microburst' means pls as I can't follow the discussion. Does it just mean asymmetric wrt the centreline of the aircraft? Thanks.

And the BITE result - obvious after the first repair attempt - wasn't a good indication if the repair was effective. Nevertheless it was sufficient for sign off after the third try.
How are these situations supposed to be handled by maintenance?

Uplinker, Different causes, I know, but the result not unlike this one:- https://en.wikipedia.org/wiki/China_Airlines_Flight_006

It sounds like an asymmetric situation was badly mishandled. More thrust on one side than the other, anyone with a multi engine rating knows what happens next. The cause could just as easily have been a real engine failure, the result was the same.

Six years ago TransAsia flight 235, an ATR72 crashed in Taiwan after the crew proved unable to control a twin on one engine. The regulator required all the operators pilots to undertake a test in the simulator in handling an engine failure, not all passed.

Why would the thrust lever of the left engine reduce?
 

The preliminary report says that the "thrust lever of the left engine started reducing, while the thrust lever position of the right engine remained." Assuming the RH remained at a high power setting, I cannot see whey the LH thrust lever would start coming back.

The A/T drives both levers together, if one jams, the other lever still moves. If the motor fails, the levers will stay in the last position. Both levers run off the same air data source so one would not be commanded to drive back.

I can see no reason why the A/T would drive one lever back, or if the clutch slips the lever would fall back itself.

With nose low:
1. BOTH thrust levers to idle.
2. Unload the wings (ie push to less than 1g).
3. Roll wings level (nearest horizon).
4. Pull nose up towards horizon.
5. Set an appropriate attitude and smoothly increase thrust (5 degrees & climb thrust are good ballpark settings).
6. Take a few slow deep breathes.

Note: If at recovery step 5 and you still have asymmetric thrust, increasing thrust will need to be combined with coordinated rudder to prevent yaw.

@FiveGirlKit:
The auto-thrust computer will change from commanding climb thrust to looking at IAS when levelling off from a climb. The IAS will increase, so auto-thrust starts pulling the levers back. It does this via motors* which engage to the levers via clutches which are designed to slip above a certain torque.

One thrust lever jams with its engine at climb thrust but the auto-thrust does not know this, it only knows the IAS is still increasing, so it keeps driving the motors to pull both levers back. One lever is still stuck on climb thrust, so the auto-thrust keeps driving the motors to pull both levers back - but only one lever moves; the other lever is jammed and its clutch slips.

In this way you could end up with one engine at, say, 87% N1 and the other at say 35% N1 - a big split.

*Edit: Sorry, I originally claimed one motor for both levers, which is not the case. My bad.
.

knkt.go.id/public/files/uploads/posts/posts/postbody/ip.201-1-13-knkt-2021.pdf

Preliminary report of Sriwijaya Air crash.. (sorry in Indonesian languange. Use google translate copy paste to translate).

From Report..
1. Auto Throttle broken. Repaired twice.
2. During take off AT pulls back left engine. AP engaged around 2000 feet.
3. At around 8000 feet, AT again pulls left engine down. Right Engine stays. Continue climb.
4. At 10600 feet, AT again pulls left engine. Right engine stays.
5. Altitude drop. Pitch nose up. Roll started (to left). Then AP disengage. Then nose pitch down.
6. FDR stops recording after 20 seconds. No detail data released.

What can cause AT to pulls 1 engine? Too much engine's vibrations? Faulty sensor data, computer error? Or, (God forbids) thrust reverser deployed during flights?

The preliminary report doesn't explain what pilot do next after 3rd AT pulls down left engine. But i find it hard to believe, even for (i apologize to use the word) un-intelligence pilot NOT to noticed that 1 of the engine start pulling down power, and creates rolls. OR am i wrong? I'm not pilot btw. I just curious (my background is electronics).

That's the latest news from Sriwijaya accident report (preliminary)..

Uplinker

That's not true. 737 A/T has two servomechanisms, one per each lever. Have a look at , particularly around 0:45, where you will see that A/T can move each thrust lever separately.

This functionality is required to synchronize the engine RPM, as the same thrust lever angle might not result in exactly the same N1, depending on many factors, particularly with these hydromechanically-controlled engines.