Indonesian aircraft missing off Jakarta

[KenV]

Originally Posted by FIRESYSOK

Autopilot may not have been available; the bulletin saying STS may continue to trim nose-down to prevent a stall condition during manual flight.

No. STS is not an auto trim system. Indeed it normally functions opposite to trim. And no it is not designed to "prevent a stall condition."

[LaissezPasser]

Originally Posted by Mike Flynn

All the SE Asian TV stations now blaming a faulty AOA sensor for the accident.

Just to clarify, SE Asian TV stations are reporting that both Boeing and Indonesia's National Transportation Safety Committee (KNKT) are saying that faulty AoA signals were involved in JT610. The media aren't coming up with this on their own. From Boeing:

The Indonesian National Transportation Safety Committee has indicated that Lion Air flight 610 experienced erroneous input from one of its AOA (Angle of Attack) sensors.

[Lonewolf_50]

The bulletin refers to an already existent procedure.
Company Training: questions need to be answered
Company Maintenance: why was this gripe not fault isolated and fixed?
The description of how to deal with trim versus STS, and the need to turn off a particular system using the cut out switch looks like something that should have been practiced in the sim ... but how often?
From Boeing:

In the event an uncommanded nose down stabilizer trim is experienced on the 737-8 /-9, in conjunction with one or more of the above indications or effects, do the Runaway Stabilizer NNC ensuring that the STAB TRIM CUTOUT switches are set to CUTOUT and stay in the CUTOUT position for the remainder of the flight.

From this, I understand that one can turn it off. If one does not turn it off, one will be fighting it all the way to the next landing. (Do I understand that correctly?)

[phil gollin]

.

The Boeing Bulletin speaks of the problem when there is ONE AOA sensor fault, I was under the impression that there were two sensors ?

IF SO, surely the system should flag up the different readings and not just choose one to follow right or wrong ?

.

[lomapaseo]

Originally Posted by Airbubba

There is definitely some gamesmanship in the terminology in these technical issues. I remember years ago the company claimed it had a flameout on an engine and the manufacturer said that the engine made an uncommanded transition to a low RPM sub-idle condition. I presume a lot of this is from the legal department.

Not at all

It is simply engineering terms which define the actual problem in terms that support corrective actions.

Too often slang definitions creep into the news to facilitate baseline communication with the public. In my view the engineer who designed and certifies the product owns the definitions against the design assumptions

Examples of areas that most confuse the issue are Buzz words , like fire, thermal damage, rapid oxidation.and where they apply as well as flame-out, surge, stall, seizure and where they apply.

of course anything that is uncommanded worries me from the get-go

[KenV]

Originally Posted by Lonewolf_50

From Boeing: From this, I understand that one can turn it off. If one does not turn it off, one will be fighting it all the way to the next landing. (Do I understand that correctly?)

That is correct. If for some reason the electric trim system runs away, it can be cut off. Once cut off one can use the manual trim wheels to manually trim the aircraft. The aircraft remains perfectly controllable. Further, even with a trim that has runaway to the mechanical limit of the system, the aircraft remains perfectly controllable. It's just that control column forces might be high and you might want the other pilot to assist you in overcoming the adverse column forces to lessen fatigue.

[KenV]

Originally Posted by Airbubba

There is definitely some gamesmanship in the terminology in these technical issues. I remember years ago the company claimed it had a flameout on an engine and the manufacturer said that the engine made an uncommanded transition to a low RPM sub-idle condition. I presume a lot of this is from the legal department.

Sorry, no. There is a vast difference technically between a flame out and uncommanded low RPM condition. Vast. To the pilot the results are largely the same, but to the engineer trying to understand the problem and develop a fix, the difference is not only vast, it is critical. It has less than nothing to do with gamesmanship and nothing to do with lawyers.

[Smott999]

Re STS inputs

...up thread we had a great review of STS which indicated that it had many of its “own” inputs, including pitots on the stabilizer leading edge, hydraulic pressure A v B comparisons, also required AP OFF, low weight, aft C/G, 10secs post liftoff, 5 secs after release of trim switches....virtually a separate system.

Does this Boeing bulletin show that the AoA and ADR are in fact also inputs to the STS?

Apologies if I (mere SLF) am mis-interpreting....but it sounds as if once the AP is disengaged, the STS was then available to act on the stabilizer. And the STS was taking ADR info sent from (apparently faulty ) AoA data. Thus pushing nose-down. And perhaps loading the yoke make it harder to pull up.

From what I can find on prior posts (and online Boeing tech info) it did not seem that STS took AoA data as an input. But does it?

All info appreciated! Thanks for a great forum.

[wiedehopf]

Direct link for larger picture: https://pbs.twimg.com/media/DrbMs-VVsAA9NgB.jpg:large
Bulletin in picture form on twitter:

Twitter

[Lonewolf_50]

To emphasize the procedures in the Runaway Stabilizer Non-Normal Checklist.

The bulletin establishes what looks like a very clear point: a Stabilizer can runaway, and there is a checklist for this malfunction.

Is the case of this aircraft (with AoA malfunction and the Runaway Stab malfunction) a compound emergency (by definition) or is this (Runaway Stabilizer) a predictable malfunction if an AoA sensor fails?
If the latter, how is this taught and practiced in the sim?
My motive for asking the question in this way is my experience in pilot and crew training, emergency procedures training and with that latter bit how one best uses simulator training to teach and train pilots and crews.

[Ian W]

Originally Posted by Lonewolf_50

The bulletin establishes what looks like a very clear point: a Stabilizer can runaway, and there is a checklist for this malfunction.

Is the case of this aircraft (with AoA malfunction and the Runaway Stab malfunction) a compound emergency (by definition) or is this (Runaway Stabilizer) a predictable malfunction if an AoA sensor fails?
If the latter, how is this taught and practiced in the sim?
My motive for asking the question in this way is my experience in pilot and crew training, emergency procedures training and with that latter bit how one best uses simulator training to teach and train pilots and crews.

Perhaps it is being picky, but a trim that operates until overridden by pilot input, waits a few seconds and does it again is not 'runaway' as it can be prevented and most pilots would operate the STAB TRIM CUTOUTs after the second or third time.

A rather confusing issue is that while the other erroneous outputs and stick shaking seem to be limited to 'the affected side' - but the stab trim obviously is for both pilots. Therefore, the previous flights must have operated the STAB TRIM CUTOUTs as the FO on the previous flight flew the aircraft manually for the remainder of the flight. None of this was passed on to the subsequent crew as 'we had an AOA problem?'

[Clandestino]

Originally Posted by Lonewolf_50

The description of how to deal with trim versus STS, and the need to turn off a particular system using the cut out switch looks like something that should have been practiced in the sim ... but how often?

Probably depending on the local CAA and company; personally I've done it on initial and then at least once every three years.

Originally Posted by Lonewolf_50

From Boeing: From this, I understand that one can turn it off. If one does not turn it off, one will be fighting it all the way to the next landing. (Do I understand that correctly?)

Nope. As it was mentioned already a few times here: after disabling electrical trim, manual trim handle(s) can be pulled out of trim wheel and stab handcranked into desired position. Not overly difficult, at least in sim.

[A Squared]

Originally Posted by Smott999

From what I can find on prior posts (and online Boeing tech info) it did not seem that STS took AoA data as an input. But does it?

Well ... I don't know, but Boeing seems to believe that faulty AoA data can cause the STS to behave erratically, so it kind of seems that AOA data, or a derivative thereof, must be input somehow.

[Airbubba]

Originally Posted by KenV

To the pilot the results are largely the same, but to the engineer trying to understand the problem and develop a fix, the difference is not only vast, it is critical.

Thanks, I'm obviously just a pilot and yes, when the motor quits, it's about the same to me. I'll figure out how to get it back on the runway and let you geniuses on the ground figure out how to fix it.

Kinda like 'that feller isn't embezzling, he's merely repurposing funds without proper authorization' I suppose.

The runway stab trim checklist on the Boeings has been about the same for decades now and it does have you disconnect the autopilot, turn off the stab trim switches and grab the trim wheel in that order until you find something that works to stop the runaway. Is that still the drill on the 737-8?

[b1lanc]

Originally Posted by A Squared

It was earlier reported that Lion had identified potentially similar defects on other 737 max aircraft in their fleet. in addition to the other occurrences of UAS problems on the accident airplane.

If true begs the question - only Lion? Seems to me this could be a fleet-wide issue. A lot of max delivered, how are others addressing the potential?
The occurrence aircraft suffered an uncommanded nose down event on the prior flight but the crew was able to recover and continue the flight at a lower altitude (source AVH).

[Denti]

Originally Posted by KenV

No. STS is not an auto trim system. Indeed it normally functions opposite to trim. And no it is not designed to "prevent a stall condition."

I do agree on the first part, but even on the NG the second part is simply wrong, as Boeing themselves write in their FCOM that approaching a stall the STS will trim forward to make it harder to get into the stall. It cannot prevent a stall if the pilot really wants to reach that of course, it just makes it a lot harder.

Try to fly it slowly into a stall and don't manually trim, and you will notice the activity of the STS trimming nearly continuously forward in the approach.

[Rananim]

The STS(which btw I agree is not technically an anti-stall device but we can view it as such as it will indeed trim down if you do approach a real stall in manual flight) would be disabled by selecting only the AP STAB trim cutout switch to CUTOUT.Boeing use bulletins to get vital information out immediately.The reminder to do the RUNAWAY STABILIZER NNC covers this runaway STS..They may refine procedures checklists later...or may not.This is probably what the commander of the previous flight did...he saw unwanted trim and he disabled both trims..fair enough.That wasnt the time or place to work out why/how he was getting unwanted trim...he just needed to disable all trim.He can trim the stabilizer manually once the trouble-shooting has ended.
RUNAWAY STABILIZER NNC...in the Boeing,unlike the Airbus where it is quiet,the trim wheel makes a lot of noise.That noise(if its continuous and not momentary) is actually the first and best indication of a runaway stabilizer,well before the out of trim condition develops.The pilot disengages automation and if it still continues he puts both trim switches to cutout.If that doesnt do it,the other pilot grasps and holds the flying pilots trim wheel.The point being is the pilot has good aural and tactile warnings of a runaway.However,in a UAS with stick shaker operating,those warnings may be diluted...and lets not forget this was an AP trim motor "runaway"(slower,quieter.more insidious)
Re manual vs auto flight...I think this is just a case of lost in translation.Boeing confirm that STS trim occurs in manual flight only and that it will trim to the stops(as the AMM quite clearly says it will) unless disabled.The AMM also says very clearly that the AUTOPILOT will trim it to the stops regardless of which AP you engage providing the FCC controlling the stab trim signal is being fed corrupt ADIRU data ...so theyre saying use manual flight which is what the UAS checklist calls for anyway.

[JPJP]

Originally Posted by KenV

No. STS is not an auto trim system. Indeed it normally functions opposite to trim. And no it is not designed to "prevent a stall condition."

Hey Ken,

You're correct that the STS is not associated with the Stab Trim Autoflight circuit of the Stabilizer trim system. STS is only a function of the Stab Trim Main circuit (the pilots manual flight side of the stab trim) The other side is the autopilots (Stab Trim Autoflight). Hence, the reason why there are two cutout switches under Stab Trim Cutout.

However; The STS is designed to prevent a stall condition. Three components work together as the aircraft approach the stall - The STS, The SMYD (Stall Management Yaw Damper) and the EFS (Elevator Feel Shift). As the aircraft reaches high angles of attack they work together.

As you said, the STS is only active in manual flight, and in this case may have worked against them.

Cheers

[Longtimer]

FAA Emergency Directive

Can be viewed at: http://rgl.faa.gov/Regulatory_and_Gu..._Emergency.pdf

[Concours77]

Originally Posted by Longtimer

Can be viewed at: http://rgl.faa.gov/Regulatory_and_Gu..._Emergency.pdf

It is inconceivable this system was not challenged during test with loss of/compromised AoA, to elicit this very behaviour.

Inconceivable, but clearly they did not know. Because if they knew, and the system was approved, somebody got some splainin’ to do.

FAA:
This AD was prompted by analysis performed by the manufacturer showing that if an erroneously high single angle of attack (AOA) sensor input is received by the flight control system, there is a potential for repeated nose-down trim commands of the horizontal stabilizer. We are issuing this AD to address this potential resulting nose-down trim, which could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain....

Erm, other than that, what’s for supper...?