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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 1st May 2019, 22:41
  #4721 (permalink)  
 
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Originally Posted by wonkazoo View Post
Can anyone here definitively state what Reg MCAS was intended to provide compliance with??
I have never seen any official reference to the actual FAA requirement.

This is the most detailed explanation I have seen: 737 MAX Maneuvering Characteristics Augmentation System (MCAS)

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR ß25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.


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Old 1st May 2019, 23:00
  #4722 (permalink)  
 
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Yo gums,
We need some test community inputs to this discussion.’

Not sure how any further input from the test community or any other could contribute.
A distant, philosophical view is that with hindsight people view situations as they wish, based on knowledge (often inaccurate) and (irrelevant) past experiences, then fit these to interpretations of incomplete information - a narrow window from FDR and CVR, together with the hazard of ‘internet’ belief (social media).
We rarely question our own thoughts.
This might be the greater threat to the industry than any technical malfunction if these aspects relate to all flight crews (no reason why not - just being human).

Whilst it is unreasonable to expect everyone to have deep knowledge of CS/FAR 25 etc, there should be general confidence that most, if not all aspects of design, certification, and testing have been considered. Rarely and unfortunately in this instance, these processes are not without mistake; similar to the ‘mistakes’ observed in operation. These do not warrant blame, and rarely can ‘cause’ be identified, which would be meaningless anyway - a social construct.

Experiences from investigation of serious incidents (non fatal), with the benefit of pilot interview, conclude that humans behave rationally according to how they saw the situation at that time (irrational with hindsight to an external observer).
Subsequent review of the FDR enable crews to re-evaluate their understanding, not changing what they did, nor providing understanding of why they acted as they did (they don’t know - don’t recall why), but significantly they are able to realign time frames (wildly misjudged), and the event order according to individual viewpoint (no such thing as a shared mental model).
Thus after the event, there is no way of being sure that any discussion represents anything relevant to crew thought, analysis, belief, and action; even accident reports.

A way forward is to consider what can be learnt from these accidents; of course including the comments above, but where speculation - what if - is a basis for safety improvement. Irrespective of any relevance to these accidents, because they will not occur in exactly the same way again.

Test community input; might readjust the views on the crew’s contribution in minimising the effects of malfunction, particularly with ‘grandfather rights’ aircraft. We are not as good as we think we are.
For the future, be very concerned about the balance between new design (or modification) and pilot ability; technology advances faster than crew training / adaptation.
Human performance will limit the advance of technology; but technology will further erode the human role because of lower cost.

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Old 1st May 2019, 23:27
  #4723 (permalink)  
 
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Agreed! and they think the MAX will fly anytime soon? if their are bookies taking bets let me know how to contact them.
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Old 1st May 2019, 23:58
  #4724 (permalink)  
 
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Salute!

Thanks, PEI. I was and am still looking for the aero explanation that is more relevant than basic control column feel.

Unless you are flying a Mk 1, Mod 1. version 737, you need to try the new suckers. The plane is not the one your grandfather flew.

"Lost" quoted the British dot org explanation, and that that is one that I support most of all.

Gums sends...



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Old 2nd May 2019, 00:42
  #4725 (permalink)  
 
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As far as I know it has not been conclusively reported, but it appears that based on the trim traces that MCAS is in fact capable of overriding thumb trim and not the other way around, despite claims to the contrary.
Completely wrong.
See the Boeing bulletin posted above, trim use will always override MCAS.
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Old 2nd May 2019, 00:59
  #4726 (permalink)  
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Lost in Saigon #4673

It has been said time and time again: MCAS is not for stall protection.
Yes, thank you. After reading what is probably nearer 6,000 posts, and many other articles, I think I've got the hang of what MCAS is, or is supposed to be.

*********

Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.

I also mentioned one of my main bee's in bonnets. The second (underfloor) column switch - which may or may not be there, and if it is, allowed to function at all times. In the same question, they made an eye-opening statement about the switch function. It's going to take time to absorb the circuitry at the bottom of the article, but again, it's not as straight forward as I'd thought.

Crash: Ethiopian B38M near Bishoftu on Mar 10th 2019, impacted terrain after departure

The posting, again, of the old out-sourced fabrication issue a few posts back. Could this be connected, though some time after the associated whistle-blowing.
14 - Russia's MAK revoked the certificate of airworthiness for the entire 737 family (from 737-100 to 737-900) three years ago claiming they found an issue in the pitch/altitude control system of the aircraft (suggesting that at least the Tatarstan crash in Kazan as well as the Flydubai crash in Rostov may have been the result of that weakness) but did not receive a satisfactory response by the FAA and Boeing, also see News: Russia suspends airworthiness certification for Boeing 737s, but does not prohibit operation of 737s. What was the issue they found?
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Old 2nd May 2019, 01:29
  #4727 (permalink)  
 
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Originally Posted by Loose rivets View Post

Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.

.
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.


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Old 2nd May 2019, 01:43
  #4728 (permalink)  
 
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Originally Posted by sadtraveller View Post
Conclusion:

In my opinion, Boeing has acted disgracefully in this situation and should be prosecuted criminally for manslaughter (perhaps this is an opportunity for Barr to prove that he is not a Trump stooge). Meanwhile the FAA has been completely compromised and corrupted by the kleptocracy that is taking over America. These are systemic failings rather than a one-off incident, and they raise the question of how many other similar failures remain lurking in the shadows due to negligent management practices and oversight in a country that is rapidly losing any respect for the rule of law. No outcome short of a complete (and transparent) overhaul of Boeing's safety culture, prosecution and incarceration of senior management, and possibly even a break-up of the company (e.g. splitting off commercial aviation from defense) will make me comfortable flying on any recently-produced Boeing metal. I'll be putting my money where my mouth is by exclusively booking Airbus until these changes are made. I'm not holding my breath, so it looks like I'll be flying Airbus for some time to come.
sadtraveller Thank you for this neat summation of the facts. As a fellow frequent flyer, I'm with you in wanting a safe flight despite non-optimal staffing in the front seats.

I'd also question the focus on MCAS alone. Boeing did not react to Lion Air as if a known risky design choice had been exposed. My own impression was that MCAS hadn't been on their minds, and they were surprised and slightly irritated that it had caused problems. Ergo, I assume that the MCAS design was of a piece with their general design standard. If this is the case, it's reasonable to think that there may be more potential bear-traps in the MAX design. Are there any other systems dependent on a single sensor? Are there any other silently disabled features that Boeing haven't mentioned? Can any other flight characteristics be affected by features undisclosed to the pilot?

If I were an aviation authority, I'd want those questions answered. It's a shame the FAA isn't leading the way.
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Old 2nd May 2019, 02:28
  #4729 (permalink)  
 
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Originally Posted by 737 Driver View Post
Originally Posted by Loose rivets
Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.
.
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots donít need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.
Unfortunately the AV herald "question" referenced above commingled the under floor cutout switches with the 2 center 'stab cutout' switches. If carefully parsed it might be correct but very hard to read.
Here are the relevant facts on the cutout switches:

The 737 NG the cutout switches:
left cutout kills all electric trim (protects against stuck pilot switches as well as some faults in motor module)
right disable automatic trim only.

On the 737 MAX either cutout switch disables all electric trim.
The 28V power to the main trim motor 115V 3 phase relay is in series through both switches. The av herald shows some function not identified secondary contacts that are probably status inputs to other systems.

The 737 MAX labels were changed to 'primary and 'backup', the changed functionality was not mentioned in at least some version of the 'i-pad conversion course.

One theory is that the ET pilots used the right switch and then discovered they had no manual electric trim, which they would have had on NG.

Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.
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Old 2nd May 2019, 02:55
  #4730 (permalink)  
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Originally Posted by Loose rivets
Having just read the AV H article in the parallel thread, I posted on some of the issues, and implied the impossibility of average crews comprehending the BackUp switch circuitry after a quick read in. If the AV H's questioner's knowledge is correct, things are very, very different to the concept of two switches simply being in series.
.
737 Driver #4736
I have no doubt that this is true, but it is also largely irrelevant from a procedural viewpoint. The pilots don’t need to be able to read a wiring diagram and tell you all the things that happens when they throw the cutout switches. They just need to know when they need to throw the cutout switches - as in the case of the runaway stab trim procedure.

I know, and I concede I've always been a 'know every nut and relay' kind of bloke. But this, if true, means the function of the BU switch is much more subtle than I'd realised.

My first reaction was of curiosity, wondering where that would take us. It seemed to imply only throwing the RH switch might be significant.

However, it'll be 03 Sparrows again before I get my head down - about par for the course since November. I'll look at the AV H's list and circuits again tomorrow as I usually have a photographic memory for systems, but these logic changes are leaving me in a fog. Don't like it. Hope it's not age related!!
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Old 2nd May 2019, 03:19
  #4731 (permalink)  
 
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Originally Posted by MurphyWasRight View Post

The 737 NG the cutout switches:
left cutout kills all electric trim (protects against stuck pilot switches as well as some faults in motor module)
right disable automatic trim only.

<snip.

Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.
Small correction. On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.

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Old 2nd May 2019, 03:20
  #4732 (permalink)  
 
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Originally Posted by sadtraveller View Post
[snip]

18. (Opinion) If there are regions of the 737 flight envelope (e.g. extreme trim, opposing elevator) under which pilots lose vertical authority, the whole 737 fleet, both maxs and NGs (assuming that they are also affected by the same issue), should be grounded until such a time as trim deflection is mechanically limited to prevent entry into these uncontrollable regions of the flight envelope.

[snip]
This
My reading of this whole catastrophe is that the certification of the B737MAX and the B737NG may now be thrown into question.

The FAA will undoubtedly approve "MCAS 2.0" in as quick a time as they consider respectable, but the evidence shows that there is such a degree of regulator capture that this will be hard to see as an objective evidence-based process.
Others, particularly CAAC and EASA, may not be so easily convinced. The Russian regulator would probably be onside with that judging by their actions a couple of years ago to attempt remove certification from the B737NG.

That would be catastrophic for Boeing and for the air transport industry world-wide: half the global narrow-body fleet grounded indefinitely hardly bears thinking about.
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Old 2nd May 2019, 04:19
  #4733 (permalink)  
 
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My reading of this whole catastrophe is that the certification of the B737MAX and the B737NG may now be thrown into question.

The FAA will undoubtedly approve "MCAS 2.0" in as quick a time as they consider respectable, but the evidence shows that there is such a degree of regulator capture that this will be hard to see as an objective evidence-based process.
Others, particularly CAAC and EASA, may not be so easily convinced. The Russian regulator would probably be onside with that judging by their actions a couple of years ago to attempt remove certification from the B737NG.
I think the Whistle blower testimony may impact what decision that the FAA is even able to make. I'll bet that both Boeing and the FAA are keen to just patch MCAS and move on.
However if they start getting expert whistle blower testimony that brings in to question the whole certification process of MCAS and the reasons for its existence the FAA will be out of options other than some sort of re certification process which is a whole new can of worms.
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Old 2nd May 2019, 08:11
  #4734 (permalink)  
 
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To be honest I would be fairly confident that given the amount of scrutiny that the redesigned MCAS has, it would be "safe enough".

What concerns me would be any other, as yet unknown/undisclosed "features" that are lurking in the background that haven't yet been disclosed because they haven't yet contributed to a smoking hole in the ground - essentially have any other corners been cut or sub-optimal design or engineering decisions been made. I would hope that this is what the FAA and other regulators are scrutinising right now.

I would also be fascinated if a gap analysis has been done between the MAX and what would need to change, were it a new aircraft type being certified today - several people have commented that "it would never be certified today" and "it relies on grandfather certification" - so in what ways does the MAX come up short to today's expected regulations?
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Old 2nd May 2019, 10:05
  #4735 (permalink)  
 
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What a truly superb analysis. I suspect this could very well be plaigerised in its entirety and feature in various court cases in the future.

Regards

I think Alchad that you would have to say that only a few of the Comments 1-33 are accurate and even then, were not the cause of the crashes. Only factors leading to the events. I will look at the 33 comments as will others and I am sure they will be examined one by one for accuracy, relevancy and bias. I look forward to that input from others wiser than I.
Happy flying
ps - would fly on a Max today if I could fine one going somewhere, provided the pilot had been trained in multiple error warnings and knew his pitch and power settings for each phase of flight. The airlines I fly on do that. Someone said today "are we going back to the Cessna 152?" Well, the 737 is a large 152 and that is its beauty in my view. It flies with very little working except it needs the engines, and even without them, has made many safe landings. But, the pilots HAVE to know how to fly. The 737 is not an Xbox. But then nor is an A330 as we saw in the AF 447 crash where the pilots had no idea what was happening to them.
We are not going to get away from the issue of world wide movement towards minimalist training supplemented by smarter and smarter avionics which can save the pilots from their own errors. An old but not bold pilot.
Y
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Old 2nd May 2019, 12:20
  #4736 (permalink)  
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737 Driver

. . . On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.
This is a very revealing quote. I can see how the procedures evolve, or de-evolve. But I can also see how it could lead even attentive crews down a very dark garden path.

#845 on the parallel thread

we have promise of a further probing into the AV H's list and circuits. I haven't woken up yet.

737 Driver's comments about the need to know levels I concede completely though as I say, if I were young . . .


FrequentSLF #843 on parallel thread.
Quote:
I am not a pilot, but I have eletrical and automation background.
Based on the wirings that are available on the net the systems are as stated.
MCAS is not stopped by column switches!
MCAS cannot be disabled without cutting off manual electrical trim, which means only wheel cranking can be used if CUT OFF switches are used.
CUT OFF switches are connected in series, and renamed PRI and B/U, either one will CUTOFF all electrical controls (manual thumb on control column, autopilot, STS, MCAS), while on NG one switch will cut off automatic trim, while the other whole cut off the electrics.
Well, I found it hard to reconcile the MAX system as described in the AV H's question list - with the outline wiring we'd all been lead to believe was simply a series BackUp configuration. It was too late last night to trace circuits, but 737 Driver did protest it doesn't matter about the details, as long as the crew take the right actions. I have never thought like that and if I'd been on type, (and 40 years younger) I would have known exactly what that circuit did. At least, I hope I would, given I'd got the first clue that something like MCAS was lurking in the background. But then, I'd spent sproghood on the electronics workbench.

Now to another issue in the Herald's list. They go back to the black box taking in good AoA data and corrupting it - in both if not all, cases. Certainly, the chance failure of three vanes stretches the old credulity, though the balance weight hypothesis in the ET flight is very compelling. But along a pure logic line, I would have looked very carefully at the prior reports - in addition to the three major vane issues. The more I read the more I'm not satisfied the Herald's suggestion is not correct. It leaves me with a deep concern about what I've described as a ghost in the machine.

Apart from the digital errors fitting nicely with the three vane positions - they take the very rapid change of angle as being more electronic than a pendulous swing. But as I say, the latter is good fault modelling. It's just that slight angle change from a long steady error state, to a slightly different long steady state. That's odd.
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Old 2nd May 2019, 12:45
  #4737 (permalink)  
 
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Originally Posted by Loose rivets View Post

Now to another issue in the Herald's list. They go back to the black box taking in good AoA data and corrupting it - in both if not all, cases. Certainly, the chance failure of three vanes stretches the old credulity, though the balance weight hypothesis in the ET flight is very compelling. But along a pure logic line, I would have looked very carefully at the prior reports - in addition to the three major vane issues. The more I read the more I'm not satisfied the Herald's suggestion is not correct. It leaves me with a deep concern about what I've described as a ghost in the machine.

Apart from the digital errors fitting nicely with the three vane positions - they take the very rapid change of angle as being more electronic than a pendulous swing. But as I say, the latter is good fault modelling. It's just that slight angle change from a long steady error state, to a slightly different long steady state. That's odd.
There were not 3 separate 3 AoA issues, the last 2 Lion Air flights had the same AoA sensor with exact same offset. The AoA sensor had been replaced before as a troubleshooting measure but we still have no facts on the returned units condition.
As to the slight change in steady state in ET trace:
The first significant) downward bump in vertical G was at 05:41:15 coincident with the change. Assuming the AoA (sans vane) sensor was wedged at a limit (bird innards even) it was jostled and settled slightly lower.

I don't really understand the drive to make both instances have the same cause since the traces are totally different.

Both Lion air flights had an active AoA with a fixed offset. This 'might' be electronics but could also be explained by a mechanical issue.

ET had a sudden swing to full scale and then tracked G at the end. The speed of the initial swing is hard to read from the graph but does not look faster than the random variations on the working unit.
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Old 2nd May 2019, 13:13
  #4738 (permalink)  
 
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Originally Posted by Loose rivets View Post
737 Driver

Quote:
. . . On the NG, the left switch disables the main electric (pilot actuated) trim and the right switch disable all automatic inputs (Speed Trim/Mach Trim/Autopilot Trim). We used to try to identify the offending system and actuate these switches separately in the runaway stab trim procedure depending on the situation. Somewhere along the way, Boeing changed their philosophy on the procedure (probably part of a larger trend of moving away from “troubleshooting” type actions) and now we always use both switches when necessary. I have previously speculated that two switches were retained in the MAX for both redundancy against possible relay welding and to simply harmonize the procedural aspects of runaway trim between the NG and the MAX. That is, both aircraft have two switches (for different reasons) and you always use both switches at the same time.
This is a very revealing quote. I can see how the procedures evolve, or de-evolve. But I can also see how it could lead even attentive crews down a very dark garden path.
This could be a case of compounded unexamined assumptions along the way:

The initial 737NG safety analysis used the availability of separate cutouts (automation and all) to justify smaller wheel/harder to move under high loading since in most cases pilot electrical trim would still be available.
This may also have been a factor in dropping training on the 'unloading' technique.

Over time training was simplified, not by itself a bad thing, but did not take the above into account.

When MAX was designed the fact that training always used both switches was used to justify removing the seperate functions for whatever reason.

Some have suggested that the training was changed to accomodate the upcoming MAX, I highly doubt this was true since there was no technical need to change the functionality to add MCAS.

Last edited by MurphyWasRight; 2nd May 2019 at 13:25. Reason: 787 > 737
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Old 2nd May 2019, 13:16
  #4739 (permalink)  
 
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Originally Posted by MurphyWasRight View Post

Others have posted that the runaway trim procedure was changed from first using the right switch only to using both a few years ago for unknown reasons.
No one has posted a rational sounding reason why the switch functions were changed.
I did my 737-3/400 initial course in 1998 and have no recollection at all of ever using just one of STAB TRIM CUTOUT switches. They were always, unless seniity has completely overtaken me, used together. Seems rather more than 'a few years ago' to me.
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Old 2nd May 2019, 13:34
  #4740 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
:



Alll 3 of the FDR traces clearly show MCAS kicking in ~5 seconds after the last manual electric trim.
It is not clear what the current published FDR traces are actually reporting... I am not saying that your interpretation is wrong but it is not absolutely certain. Does the thumb switch have absolute authority in all circumstances (save for when electric trim is disabled)? The wiring schematics available suggests otherwise and the detailed software logic is not publicly known. The FDR data is not clear whether it is reporting inputs or final outputs or something in between. I have read what is available in this thread and still have doubts that there is sufficient information to be sure. What is certain is that the MCAS system was not properly or adequately designed. It is inherently dangerous and should never have been approved. Whether the pilots should or could be expected to resolve the defective system is a secondary issue. It is simply unacceptable for a critical system to continue to rely on plainly unreliable sensor information. The common factor is unreliable AoA that is easily detected but the system ignores. Autopilot is disabled but why on earth is MCAS allowed to continue to operate?
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