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Indonesian aircraft missing off Jakarta

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Indonesian aircraft missing off Jakarta

Old 10th Nov 2018, 17:28
  #961 (permalink)  
 
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According to the FAA Emergency AD, an erroneous Angle of Attack (AOA) input, among other symptoms, can result in indications of:

IAS DISAGREE alert.

ALT DISAGREE alert.

I'm wondering why an erroneous Angle of Attack (AOA) input would cause those alerts. Is it because the pitot-static position error corrections require an AoA input?

Secondly, I wonder what really occured on previous flights. Was it really an UAS situation or were above alerts incorrectly diagnosed as a pitot-static system problem?
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Old 10th Nov 2018, 17:55
  #962 (permalink)  
 
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Originally Posted by mross View Post
If only the warning to the pilots had been "AoA disagree - A/P off, STS off" or something precise and useful. And Feel Diff Press should NOT alarm.
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Old 10th Nov 2018, 17:55
  #963 (permalink)  
 
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Originally Posted by Gysbreght View Post
According to the FAA Emergency AD, an erroneous Angle of Attack (AOA) input, among other symptoms, can result in indications of:

IAS DISAGREE alert.

ALT DISAGREE alert.

I'm wondering why an erroneous Angle of Attack (AOA) input would cause those alerts. Is it because the pitot-static position error corrections require an AoA input?
If you read the thread carefully you will come across this post:

Originally Posted by Denti View Post
From my own experience on the NG a wrong AoA input will result in UAS, unreliable altitude, vertical speed, wind information and ground speed display. It is a basic correction factor into the ADIRU that does effect all resulting air data related information and surprisingly the non air data related information of ground speed as well. It might be different in the MAX, but somehow i doubt it. As different airflow over the fuselage results in huge position error values for static and pitot tube values, the AoA vane corrects those very different position errors, therefore a wrong AoA indication will result in a completely unreliable air data set.
Then there is also this comment.

Flutter speed
8th Nov 2018, 15:20
"So, for a 737, what would be the largest difference between IAS, uncorrected for AoA, and CAS, within the range of AoA that could be reasonably expected in flight? The large airplanes I've flown didn't correct IAS for AoA, and without looking them up, I don't recall that the airspeed calibration charts having a correction larger than 5 ish knots. Seems if it came down to a choice between an IAS that differed from CAS about 5 kt at low airspeeds/high AoA and no usable airspeed indication at all if the AoA fails, I know what I'd prefer. Especially if my takeoff and landing charts were referenced to IAS, whcih makes CAS a relatively uninteresting number. (except in cruise, where IAS and CAS are usually fairly close anyway) "

Just ran some numbers, in very general terms, depending on sensor position and factors like Mach number, in extreme cases... expect up to 5kts difference. Probably enough to trigger an IAS and ALT disagree on a modern airliner. For a plane flying at moderate speed the difference will be more towards 2kts.

The archive version of this forum can be searched a little bit better with CTRL-F
https://www.pprune.org/archive/index...14857-p-4.html

In case you want to dig around the thread some more.
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Old 10th Nov 2018, 18:07
  #964 (permalink)  
 
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Ref: above post by Garage Years: “...All I can say is it was a good job the FDR was found, because that was what let the cat out the bag with respect to this being AOA sensor related and linked to the STS....”

Perrhaps not the time or place. Didn’t flight safety used to be pro-active? The fact that Mx didn’t suss the issue four times might not reflect on the shop.

Again, any “new system” makes it incumbent upon the airframer to fully test and fully disclose performance of the added function, No?

if the AoA issue re: intermittent trim input was unknown to Boeing, fair enough. I fail to see how it is possible that such an issue could have remained a mystery in a fully comprehensive testing program.

It takes an emergency bulletin to apprise Mx of the issue? Given enough time, I promise an aircrew could have Sussed and rectified the abnormal. Unfortunate that given unlimited time, the airframe builder remained in the dark? This aircraft should not have been released to flight. Even given the wrong problem was addressed in the shop, it remained unrepaired. Inexcusable.

Last edited by Concours77; 10th Nov 2018 at 19:08. Reason: include Garage Years text...
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Old 10th Nov 2018, 18:15
  #965 (permalink)  
 
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Originally Posted by hum View Post


A pilot who understands the relationship between AoA, Speed and G will quickly recognise a faulty AoA indication. This is not something you can teach in a simulator and in my opinion must form the basis of all UPRT.
Best thing I have heard for a while. 100% agree
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Old 10th Nov 2018, 18:25
  #966 (permalink)  
 
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Originally Posted by rideforever View Post
When pilots go to "manual" do they routinely also disable the STS, and if not why not ?
That depends on what you mean by "go to manual". If you mean, just disengaging autopilot and hand flying in a normal situation, No. As I understand it (not a 737 pilot) the STS is there specifically for when the airplane is being hand flown, than is has no function when the autopilot is engaged.
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Old 10th Nov 2018, 18:38
  #967 (permalink)  
 
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Originally Posted by Gysbreght View Post
Secondly, I wonder what really occured on previous flights. Was it really an UAS situation or were above alerts incorrectly diagnosed as a pitot-static system problem?
Looking at the altitude profile the accident and previous flight there is this sudden descent.
That fits with a high reading AoA sensor leading to stick shaker activation followed by a pitch down by the pilot.
Also the automatic trim down would have activated.

Now i've been told before on this thread only the AoA can trigger the stick shaker which makes not much sense to me (the posts were deleted anyway).
My understanding is the combination of AoA and airspeed leads to stick shaker activation.
If someone wants to explain the exact stick shaker logic on the 737-8 please feel free i would be curious

Anyway the plane displayed IAS disagree with the stick shaker activating on the captains side. They continued the previous flight using the instrumentation on the right hand side.

The AoA sensor was replaced according to KNKT, see this quote from avherald:
On Nov 8th 2018 the KNKT reported an angle of attack sensor had been replaced on Oct 28th 2018 following the flight JT-775 from Manado to Denpasar (the aircraft completed the subsequent flight JT-43 to Jakarta and suffered the crash the next flight JT-610). The aircraft subsequently flew to Jakarta, the crew however reported there were still problems. The search for the CVR is hampered by thick mud.
So maybe they introduced the AoA problem by replacing the sensor or they made it worse, the sensor that came off the airplane is at Boeing being tested.
It's also possible that the plane already had an underlying pitot-static issue that was not fixed.
Or somehow the input logic receiving the AoA data was flawed and replacing the sensor did nothing.

Without more facts the answer to your question is mostly just: Outside the investigation no one can say for sure at the moment.
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Old 10th Nov 2018, 20:21
  #968 (permalink)  
 
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Putting an engineer on board the final flight means .... that they may have been accustomed to fixing / testing serious issues on passenger full planes, and his presence may have complicated the situation by either the pilots trying to activate the fault in order that the engineer can see the problem, or that the engineer used any arising faults as an opportunity to do some diagnosis rather than let the pilots recover.
Well it's 100% insane. Once you "put the engineer on board", your intentions are clear.
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Old 10th Nov 2018, 20:31
  #969 (permalink)  
 
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Originally Posted by rideforever View Post
Putting an engineer on board the final flight means .... that they may have been accustomed to fixing / testing serious issues on passenger full planes, and his presence may have complicated the situation by either the pilots trying to activate the fault in order that the engineer can see the problem, or that the engineer used any arising faults as an opportunity to do some diagnosis rather than let the pilots recover.
Well it's 100% insane. Once you "put the engineer on board", your intentions are clear.
Withoit question. Trouble shooting a serious problem is the domain of flight test, NOT commercial carriage. This is not on Boeing. Except to the extent that it was initiated by the “mystery”, which is on Boeing.
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Old 10th Nov 2018, 23:00
  #970 (permalink)  
 
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10

737 MAX8 Emergency Airworthiness Directive


posted on November 10, 2018 09:22

737 MAX8 Emergency Airworthiness Directive

The recently released Emergency Airworthiness Directive directs pilots how to deal with a known issue, but it does nothing to address the systems issues with the AOA system, which may be the causal system in the Lion Air accident. This is significant. The positive takeaway is that we are advised, as pilots, that once we recognize the issue, we can stop the negative impacts by taking the trim system out of the loop.

At the heart of this investigation is the MCAS system (description from Boeing):

MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

This is the first description you, as 737 pilots, have seen. It is not in the AA 737 Flight Manual Part 2, nor is there a description in the Boeing FCOM. It will be soon.

APA Safety recommends that you familiarize yourselves thoroughly with the information provided by CA XXXXXXX, 737 Fleet Captain, and the AA 737 fleet team. We have been working closely with CA XXXXXXX to get you accurate information as quickly as it becomes available. The AA 737 fleet team has placed this information in CCIs to 737 pilots, in bulletins, and in changes to flight documents.

At the present time, we have found no instances of AOA anomalies with our 737 MAX8 aircraft. That is positive news, but it is no assurance that the system will not fail. It is mechanical and software-driven. That is why pilots are at the controls.

Awareness is the key with all safety issues. You are aware this anomaly may occur and there is a mitigation procedure. No different than should you experience an engine failure.

As we continue to receive details, we will provide them in emails only to the 737 group. We chose to send this initial email to all pilots because it is a subject that is generating a great deal of interest.

Should you have questions, please do not hesitate to email or call us here at APA Safety: XXX-XXX-XXXX.

Captain XXXXXXX
DFW 737I
APA Safety Committee Chairman
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Old 11th Nov 2018, 00:57
  #971 (permalink)  
 
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Originally Posted by Halfnut View Post
10
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
Wow, thanks for posting. This appears to be the missing puzzle piece, a new MAX system apparently unknown outside of Boeing which directly links AoA to the stab for improved stall protection (when sensors are working). I can't see how MCAS can be retained in the present form, with a single high alpha AoA sensor failure now capable of failing the MCAS to inappropriate downtrim as well as providing a set of additional cascading alarming failures (primarily UAS) to reduce the pilot's chances of recognising the problem in time and deactivating trim. I do still wonder whether the STS did not contribute to the accident at all, or whether in UAS it made things worse by adding inappropriate trim input to the MCAS's.
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Old 11th Nov 2018, 01:04
  #972 (permalink)  
 
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Thanks Halfnut for sharing that email. As the email states: '... it is a subject that is generating a great deal of interest ...'. By sharing outside the email's target audience, you have helped immensely with our understanding of what may have happened and more importantly you have allowed us to be better prepared if something similar happens again in the future. Thank you.
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Old 11th Nov 2018, 02:34
  #973 (permalink)  
 
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Rideforever & Concours777: engineers are carried for a number of reasons the main one being in case of a lack of engineering support at the outstation in this case a small regional airport which probably didn't have a line maintenance agreement in place. This is very common especially for rapidly growing LCC's that haven't had time to get it done. Not a sign that they wanted to troubleshoot something in the air at all.
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Old 11th Nov 2018, 03:20
  #974 (permalink)  
 
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hum posted this A pilot who understands the relationship between AoA, Speed and G will quickly recognise a faulty AoA indication. This is not something you can teach in a simulator and in my opinion must form the basis of all UPRT.
to which you responded
Originally Posted by speed2height View Post
Best thing I have heard for a while. 100% agree
Where there is an AoA indication to see, that point being made is well made. This thread has been moving with good pace, so I may have missed the post that shows the Lion Air 737's being equipped with an AoA gage for the pilot to see, and thus recognise a faulty AoA indication. I'll check the tech log thread as well
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Old 11th Nov 2018, 03:29
  #975 (permalink)  
 
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Originally Posted by Lonewolf_50 View Post
to which you responded Where there is an AoA indication to see, that point being made is well made. This thread has been moving with good pace, so I may have missed the post that shows the Lion Air 737's being equipped with an AoA gage for the pilot to see, and thus recognise a faulty AoA indication. I'll check the tech log thread as well
Read somewhere that it was not so-equipped.
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Old 11th Nov 2018, 05:14
  #976 (permalink)  
 
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One line of thinking is that the crew did end up using the cutout switches at some point, however, by this point it had been trimmed nose down (due AoA fault operating the MCAS). Now the electric trim is unavailable with significant nose down trim, the only option left is manual trim (very slow) or to reduce IAS the air-load Speed increased to the point where the elevator could not overcome the trim.

Given the undocumented MCAS system and an unannunciated AoA fault, whilst dealing with an unreliable airspeed, it is highly unlikely that many pilots would have the insight or knowledge to understand the need to reduce speed (and how do you know what to set, the only guidance in the checklist is two attitude / thrust combinations depending upon the flap configuration), or recover with manual pitch trim from 5000'.

A very interesting simulator experiment would be to do a series of progressively more nose down trims, setting the UA QRH attitude & thrust and seeing if there is a speed or nose trim position that becomes unrecoverable using the elevator alone with the stab trim switches in cutout.

The more official and semi-official documentation and information that is starting to come out (ie the APA MCAS) suggests it's not looking good for Boeing.
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Old 11th Nov 2018, 05:23
  #977 (permalink)  
 
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Originally Posted by LEOCh View Post
I can't see how MCAS can be retained in the present form
Hilarious. Not kidding - that made me laugh. Like the batteries that burned uncontrollably ? Then they put a box around them.

Originally Posted by LEOCh View Post
I do still wonder whether the STS did not contribute to the accident at all, or whether in UAS it made things worse by adding inappropriate trim input to the MCAS's.
Since at least one very large pilot group (+ 12,000) had no idea that MCAS even existed; one wonders if it’s a bastardized STS that Boeing augmented and renamed, then wasn’t ‘effectively emphasized’.
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Old 11th Nov 2018, 07:14
  #978 (permalink)  
hum
 
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Seat of the pants

Originally Posted by Lonewolf_50 View Post
to which you responded Where there is an AoA indication to see, that point being made is well made. This thread has been moving with good pace, so I may have missed the post that shows the Lion Air 737's being equipped with an AoA gage for the pilot to see, and thus recognise a faulty AoA indication. I'll check the tech log thread as well
Strangely my original posts have been deleted... I cannot imagine what the PPRuNe police objected to!

I was making a general point, AoA is fundamental to everything we do as pilots, We must insist that - especially in aircraft where it is measured and used to influence systems - it is displayed to pilots. We must also fundamentally change our philosophy whereby there is an obsession with speed alone from day 1 of flight training, Lift (=G) varies with both speed AND AoA.

When, as it seems in this tragic case, an important sensor gives an erroneous value, and assuming that value is displayed (which it was not apparently) a trained pilot will quickly separate truth from lies.





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Old 11th Nov 2018, 08:12
  #979 (permalink)  
 
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Originally Posted by shamrock7seal View Post
Rideforever & Concours777: engineers are carried for a number of reasons the main one being in case of a lack of engineering support at the outstation in this case a small regional airport which probably didn't have a line maintenance agreement in place. This is very common especially for rapidly growing LCC's that haven't had time to get it done. Not a sign that they wanted to troubleshoot something in the air at all.
Lion Air did indeed state that the presence of the flying spanner on the flight was not connected with the recurring technical issues (though they didn't admit at the time that there had in fact been a sequence of events).

That statement may or may not be true.

The CVR, if and when it's found, may shed some light on what role, if any, the engineer had on the flight.
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Old 11th Nov 2018, 09:51
  #980 (permalink)  
 
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Maybe now is the time to repost one of mine that was deleted. A Flight Critical System was not referenced in the AFM, and the system turns out to be reliant on one only AoA vane. On vane failure, PF side is affected, PNF’s is not. Upon failure, one pilot has an emergency, PNF’s “does not”. Creates confusion to say the least: “What are you doing!”

Without AoA display there is no ability to cross reference. Without knowledge of the system, aircrew is challenged by similarity to UAS.

There has been one fatal accident seemingly related to this lack of included description in the Flight Manual.

FAA has issued an ESB AD to mitigate this lack of system inclusion in the Flight manual. On its face, it appears this will solve the ongoing threat to flight safety.

Will it? There is no reason to believe it will not. Whatever reliabilitty issues there were, (sole source of data driving activity of system), the resolution seems appropriate, and sufficient. Reliability aside, The system can now be mitigated on failure due this add to the AFM.

Questions. How can a flight critical system be fitted to an aircraft such that the operator is not completely informed of its existence and functionality? Can the design be improved and the system modified? To be determined, and Probably not...



Last edited by Concours77; 11th Nov 2018 at 10:19.
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