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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

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MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Old 30th Jun 2019, 12:44
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Yet, the 160 hour pilot suggested going to "manual" one minute after stab trim cutout while the many-hour pilot was still wondering why the electric trim switches were not working after cutout

As I read it, he gave up after eight seconds of trying "manual." Whether it was too late at this point to hand crank is, of course, the question.
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Old 30th Jun 2019, 13:48
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Originally Posted by fdr

Pilot performance is the result not the cause of the system.
I don't think I'm saying anything different here. There are many important lessons that have come out of these tragic accidents if we choose to look at the entire confluence of events, yet around these parts there seems to be a myopic focus on just a few areas.

Part and parcel with the development of transport aircraft is the creation of training, certification, and operational guidelines for all the people who touch that airframe. I would submit that just as Boeing cut corners with the physical design of the MAX, they and their willing partners in the form of the airline operators have conspired to cut corners in other areas as well. It would clearly be a crime to let someone to either perform maintenance or operate the flight controls on an aircraft for which they were not qualified. So how should we evaluate a case in which those people were given something short of adequate training to perform their duties? We fix the planes but we don't fix the process surround the operation and maintenance of those aircraft?

I think it would be uncontroversial to say that pilot training and certification standards have been lowered over the past few decades. And yet when there is evidence that an operator such as Ethiopian is not even living up to those lower standards, is there not cause for concern? We know problems like this also occur on the maintenance side as airlines continually seek to lower their labor costs, so when it becomes apparent that someone at Lion Air failed to adequately diagnose and repair a faulty AOA sensor, do we just shrug our collective shoulders? Or do we have a peek under the hood there as well?

As I have stated multiple times before, the MAX will eventually be fixed or grounded permanently (the former more likely than the latter), but if all the other issues that these accidents have brought to the fore are not addressed as well, then we are just setting the stage for the next tragedy. That strike me as both a terrible waste and a great disservice to those who perished.
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Old 30th Jun 2019, 14:09
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Originally Posted by fotoguzzi
Yet, the 160 hour pilot suggested going to "manual" one minute after stab trim cutout while the many-hour pilot was still wondering why the electric trim switches were not working after cutout

As a clarification, the FO actually had around 360 hours of total time and about three months of line experience at the time of the accident. The 160-hour reference is to the amount of time he had when he was handed his pilot certificate to sit in the right seat of a 737.

The actual text from the CVR transcript was something along the lines of "Stab Trim Cutout!" repeated twice, after which someone reflexively cutout the power to the stab trim motor (not following any procedure) while the aircraft was grossly out of trim which then set the stage for the final dive. Now if this young FO had called for the "Airspeed Unreliable" checklist early in the game, or the "Runaway Stabilizer Trim" checklist after MCAS kicked in, or advised the Captain to not retract the flaps in the first place, or any number of other deviation callouts as the Captain was mishandling the ship, then I would have less concern about the FO's level of training or experience. But he did not either because of a lack of awareness of what was happening or a lack of assertiveness to correct the Captain. In the end, the result was the same.

That being said, I can't really blame the FO for pursuing his dream of becoming a pilot. If someone let me get a job flying heavy iron while I was that young and with that little amount of experience, I would have jumped at the chance too. And I'm sure the airline was glad to have access to a young, enthusiastic pool of pilot candidates who quite naturally would not demand nearly as much compensation as pilots who had many years and thousands of hours of time under their belt.

It all worked out very well for all concerned..... until it didn't.

Last edited by yoko1; 30th Jun 2019 at 14:37.
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Old 30th Jun 2019, 14:29
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Originally Posted by BDAttitude
From names of signals you should not deduce how they were captured and processed. I am not saying that they are not analogue signals digitized by FDR directly but I just would like to know fore sure.
How would expect those signals to be called anyhow if they were captured by e.g. the FCC and passed to the FDR by some a digital bus?
I would be fascinated to know what goes on under the hood as well. As you may have guessed, I like to know how things work. But frankly, right now we are all just bystanders reading tea leaves in an attempt to make educated guesses.

On the other hand, there are people who actually do know, and these folks are advising the various investigative teams that have been looking at these accidents for many months now. These teams are working with information and resources that dwarf anything any of us have to draw on, and we can make the observation that not a single authoritative source has expressed any concern that the Main Electric Trim was not functioning correctly in the accident aircraft. Lots of other problems have come to light, but this is not one of them. While this may not be as definitive as some would like, it is strongly suggestive that this is not an area of concern.
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Old 30th Jun 2019, 14:42
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Originally Posted by yoko1
On the other hand, there are people who actually do know, and these folks are advising the various investigative teams that have been looking at these accidents for many months now. These teams are working with information and resources that dwarf anything any of us have to draw on, and we can make the observation that not a single authoritative source has expressed any concern that the Main Electric Trim was not functioning correctly in the accident aircraft. Lots of other problems have come to light, but this is not one of them. While this may not be as definitive as some would like, it is strongly suggestive that this is not an area of concern.
Many of the things that were to concern of the amateurs here, were first brushed aside only to be confirmed later in the investigations.
I am confident that there will be an explanation for the blips and a confirmation of the motor overload and insufficiency of the cable drum reversion and we will finally learn how much stability margin without MCAS remains.
Until it's there I will be happy speculating about the failure modes, making sure ​they are not forgotten despite the efforts to keep low profile.
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Old 30th Jun 2019, 14:55
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Originally Posted by yoko1
I would be fascinated to know what goes on under the hood as well. As you may have guessed, I like to know how things work. But frankly, right now we are all just bystanders reading tea leaves in an attempt to make educated guesses.

…………. and we can make the observation that not a single authoritative source has expressed any concern that the Main Electric Trim was not functioning correctly in the accident aircraft. Lots of other problems have come to light, but this is not one of them. While this may not be as definitive as some would like, it is strongly suggestive that this is not an area of concern.
I thought quite a few fairly authorative sources had conjectured that the forces on the stabiliser in the out of trim situation they found themselves in were such that it couldn't be counter-acted by the electric trim or the manual wheel?

Alchad
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Old 30th Jun 2019, 15:50
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Originally Posted by Alchad
I thought quite a few fairly authorative sources had conjectured that the forces on the stabiliser in the out of trim situation they found themselves in were such that it couldn't be counter-acted by the electric trim or the manual wheel?

Alchad
Correct on the difficulty of using manual trim to alleviate a severe out of trim condition. However, there is nothing in the entire history of the 737 trim system since the introduction of the NG that indicates that the Manual Electric Trim would stall out under a heavy aerodynamic load (well, at least as long as it stays within the certified flight envelope). There have been other various issues over the years as one might expect from any aircraft system, however this is not one of them. If this were the case, it would already be out there by now.
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Old 30th Jun 2019, 16:28
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yoko1,
Apparently you either have not read the EASA query of the NG trim system (previously referenced), or you choose to ignore what was written.
Similarly you overlook that ease of trimming in normal flight conditions might not be the same as in abnormal conditions - extreme tail angles, nor that trimming with an alleviating tail load (nose down) involves different forces against the load in failed conditions (nose up).

Conversely to your previous assertion; there is nothing in the entire history of the 737 trim system since the introduction of the NG (larger tail) that indicates that the Manual Wheel or Electric Trim would be able to recover from high aerodynamic load; excepting what has been deduced from two accidents.

Whereas you fail to qualify your assertion, the opposing argument is that where an aircraft is deliberately mis trimmed and the resultant force held on the control column, it is probable that a limiting force or ability to apply more trim is reached before the physical stops (flight test techniques AC25-7, within the aircraft flight envelope not necessarily at the extremes of physical tail angle).
But if the aircraft is already in a extreme condition, greater that previously identified during flight test to due to a malfunction, then there is nothing to say either way that the aircraft can be recovered - except accidents.


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Old 30th Jun 2019, 16:30
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I just noticed one more thing about the FDR traces of the trim commands. MCAS trim commands show up in the trace even when the cutout switches are used. But that command doesn't reach the motor, and the stabilizer angle remains the same. This suggests the automatic trim commands are sent to the FDR to be logged directly by the FCC, and are not measured somewhere between the cutout switches and the motor, because if they did they wouldn't show in the FDR trace.

However this doesn't seem to be the case for the control column trim switches. In the FDR trace from 05:40:40 to 05:43:05 there are no recorded pilot trim commands. However, in this interval there was this discussion between the pilots:

"At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working."

This suggests the pilots tried to use the trim switches, but it didn't work and the command doesn't show in the FDR trace. If that's the case, it means the information about the thumb switch commands is recorded by the FDR from somewhere between the cutout switches and the motor.

Unfortunately I wasn't able a lot of information about that motor, except this document, previously referenced in the initial Ethiopian thread:

http://www.eaton.eu/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectio nMethod=LatestReleased&noSaveAs=0&Rendition=Primary&dDocName =PCT_3401314

It has information about some circuit board fixes for the two versions of the motor used on the NG. That issue didn't affect the MAX version of the motor according to the document. According to that document the part number for the motor used on the MAX is 6355D0001-01, but unfortunately I haven't been able to find any schematics for it.

In any case, that "motor" is not just some dumb piece of hardware, since the issue fixed for the two NG versions is "Solder joint fatigue of the memory chip". A dumb motor wouldn't have a memory chip. It's quite likely the motor is designed to communicate with the FDR, so it's possible the pilot trim commands are logged to the FDR by the motor.

Later edit: actually I see yoko1 found some schematics two days ago on post number #782: MAX’s Return Delayed by FAA Reevaluation of 737 Safety Procedures

Unfortunately, as he concluded, the interesting stuff happens inside the "controller" part of the "motor", for which we don't have a schematic:

Last edited by MemberBerry; 30th Jun 2019 at 17:24.
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Old 30th Jun 2019, 16:39
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Originally Posted by BDAttitude
---and we will finally learn how much stability margin without MCAS remains.
This is something I too am VERY curious about.

I have tried to make some thought experiments.
We know MCAS at low speed was supposed to administer up to 2.5 units AND trim.
We dont know from which value of AOA that AND trim was supposed to be started.
We know stall begins at approximately 14 degrees AOA and is well developed at 15 degrees AOA.
We know Boeing have said MCAS was only supposed to be activated at very unusual flight conditions, so therefore one can deduce that MCAS only should become active at high AOA.
We dont know how many units of trim the elevator can fully compensate starting from neutral elevator position, but Mentour Pilot says that the elevator can compensate for full nose down trim in a NG simulator at low to normal speeds.

Now, on to my thought experiment.
Lets say MCAS was supposed to kick in at 10 degrees AOA.
Lets say that the elevator can compensate for 3.5 units of (out of) trim.
Lets say that 70 percent of elevator authority/travel is used to reach the stall AOA on a NG starting at level flight and in trim.
Lets say that the FCC starts MCAS trim input at 10 degrees AOA and have put in full 2.5 units AND trim at 14 degrees AOA.
Lets say the NG have a linear stick force and travel all the way from level and neutral and up to stall at 14 degrees AOA.
Lets say the MAX with active MCAS have the same linear stick force and travel as the NG.
Lets say we start at 3 degrees AOA at level and straigt in this thought experiment

Using these numbers we see the following;
We need to put in 6.4 percent of available elevator travel for each degree increase in AOA. (70/(14-3))
At 10 degrees AOA we have put in 44.8 percent of the available elevator travel (6.4*(10-3)
Each unit of stabilizer trim (out of trim) needs 28.4 percent of available elevator travel (100 percent/3.5 units trim)
From 10 to 14 degrees AOA we need to put in an additional 25.6 percent of available elevator travel (6.4*4)
But at the same time (10 to 14 degrees AOA) MCAS has put in 2.5 units of nose AND trim.
That MCAS AND is worth 71 percent of available elevator travel (28.4*2.5)

So, we see, with these numbers, that we put in a positive 25.6 percent elevator travel going from 10 degrees to 14 degrees AOA, but at the same time MCAS puts in AND negative 71 percent worth of elevator travel.
The sum of these two values is 45.4 percent of nose down elevator travel, in other words, when going from 10 degrees AOA to 14 degrees AOA in a MAX WITHOUT active MCAS we need to relax the stick from a position corresponding to 44.8 percent aft position at 10 degrees AOA and push it forward to a position of 0.6 percent forward position when we reach stall onset at 14 degrees AOA.

I think it is fair to say that an aircraft exhibiting a pitch stability similar to the values in my thought experiment would be a VERY awkward plane to fly manually in a high AOA flight region.

Of cource, theese numbers is more or less pulled out of my ass. And I've made many simplifications. As I said, this is just a thought experiment trying to understand the effect on a MAX with a non-operable MCAS system.

Feel free to correct my thought experiment and/or try to come up with better values.
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Old 30th Jun 2019, 17:29
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Originally Posted by MemberBerry
However this doesn't seem to be the case for the control column trim switches. In the FDR trace from 05:40:40 to 05:43:05 there are no recorded pilot trim commands. However, in this interval there was this discussion between the pilots:

"At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working."

This suggests the pilots tried to use the trim switches, but it didn't work and the command doesn't show in the FDR trace. If that's the case, it means the information about the thumb switch commands is recorded by the FDR from somewhere between the cutout switches and the motor.
You are assuming that "The First-Officer has replied that the trim was not working" means that he attempted unsuccessfully to use the electric trim switches.

It could equally have been him just confirming that just over a minute previously both pilots had agreed to disable the stab trim (05:40:35). So the electric trim switches obviously weren't going to work, and there wouldn't have been any point in trying them.
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Old 30th Jun 2019, 17:54
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I agree that's one way to interpret that conversation. Unfortunately it seems the preliminary report included just some parts of the CVR transcript. In particular I doubt that when they reenabled electric trim towards the end they didn't say anything about that.
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Old 30th Jun 2019, 17:56
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Originally Posted by Alchad
...I wish the Mods would use the stick whenever "pilot training" (or lack of it) gets mentioned and cull the post.
Here's the opposing view. The thread title refers to possible reasons why the MAX's return is being delayed. You couldn't rule out that Boeing is being challenged on the core premise of the entire MAX project, that the MAX is "just another 737", and that no special training or simulators are needed. The justification for including thoughts about pilot training is that a requirement for new pilot training may indeed be one of the reasons for delayed return to service.

Originally Posted by WHBM
I think the balloon will burst when the production line has to be stopped.
Agreed. My best guess is that Boeing have been told to fix not just the software but the whole system that compensates for the aerodynamic effects of those big new engines, AND they're a bit short of good grey-haired engineers who might be able to figure out how to do this, AND they've been told that when they've fixed it they've then got to re-certify the thing, AND they've also been told to provide MAX simulators and a training programme for pilots, AND the most senior management are completely and utterly out of their depth because they can't solve this by outsourcing, or re-branding, or tweaking spreadsheets or buying back shares, or whatever the hell it is that non-engineers do when they are running an engineering company.
If they do stop the line it will be hell getting it started again, probably with new people who haven't really got a clue.
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Old 30th Jun 2019, 17:59
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[QUOTE=MemberBerry;10506346]I just noticed one more thing about the FDR traces of the trim commands. MCAS trim commands show up in the trace even when the cutout switches are used. But that command doesn't reach the motor, and the stabilizer angle remains the same. This suggests the automatic trim commands are sent to the FDR to be logged directly by the FCC, and are not measured somewhere between the cutout switches and the motor, because if they did they wouldn't show in the FDR trace.

​[/QUOTE​​​​​​]
This raises another important question...why the MCAS command is active with a cutout? The cutoff signal is transmitted to the FCC (labelled B) on drawing 27-41. The only answer that makes sense is a programming error that does not disable FCC commands when in cutout, it is 101 coding to interlock the code with the cutputs, including disabling interrupts relevant to such function, and we go back to a other question that is still unanswered, why there is the of PRI and B/U cutout? a back of a switch, that is already available as a CB, and if a back up was needed why not on the NG?

[QUOTE=MemberBerry;10506346]I]
Unfortunately I wasn't able a lot of information about that motor, except this document, previously referenced in the initial Ethiopian thread:

http://www.eaton.eu/ecm/idcplg?IdcSe...me=PCT_3401314

It has information about some circuit board fixes for the two versions of the motor used on the NG. That issue didn't affect the MAX version of the motor according to the document. According to that document the part number for the motor used on the MAX is 6355D0001-01, but unfortunately I haven't been able to find any schematics for it.

In any case, that "motor" is not just some dumb piece of hardware, since the issue fixed for the two NG versions is "Solder joint fatigue of the memory chip". A dumb motor wouldn't have a memory chip. It's quite likely the motor is designed to communicate with the FDR, so it's possible the pilot trim commands are logged to the FDR by the motor.
/[QUOTE]
The controller drawings are basically useless, is that the quality of documentation provided to maintenace?

BTW does anybody knows what type of signals are transmitted to the controller? My guess that are analogue signals elaborated by the main trim enable speed and direction and AP trim enable speed and direction boxes in 27-41
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Old 30th Jun 2019, 18:03
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I can't help thinking about the 2 vs 3 AoA sensor thing. Strikes me that EASA/ChinaAA could well say, if MCAS is required to make the aircraft certifiable, and AoA sensors (evidently) have a high failure rate, then you need 3 of them, otherwise you have a single point of failure. I guess it's POSSIBLE to retrofit a third sensor, but not something you do in a couple of days per aircraft.

I also suspect that EASA could be persuaded to roll over on this one, but what about China? In the middle of a trade dispute and only too happy to rub egg into Trump/USA's face...? I even wonder whether Trump's sudden reversal on Huawei this week might not be connected.
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Old 30th Jun 2019, 18:40
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Originally Posted by n5296s
I can't help thinking about the 2 vs 3 AoA sensor thing. Strikes me that EASA/ChinaAA could well say, if MCAS is required to make the aircraft certifiable, and AoA sensors (evidently) have a high failure rate, then you need 3 of them, otherwise you have a single point of failure. I guess it's POSSIBLE to retrofit a third sensor, but not something you do in a couple of days per aircraft.

I also suspect that EASA could be persuaded to roll over on this one, but what about China? In the middle of a trade dispute and only too happy to rub egg into Trump/USA's face...? I even wonder whether Trump's sudden reversal on Huawei this week might not be connected.
If and when the FAA and EASA lift the restriction on the Max, China will inevitably follow suit. To do otherwise would be counter-productive. Trump has been hinting for the last month that the Huawei ban could be lifted in return for progress on trade talks, so nothing to do with the 737.

And it's inconceivable that the fix will involve retrofitting a third AoA sensor on 400+ aircraft, particularly when only two are required in order to identify that one is sending bad data and trigger the "AoA Disagree" warning that the Max already has provision for.
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Old 30th Jun 2019, 18:45
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Originally Posted by Eesh
I have a stupid question, may be better to get it discounted early.

When MCAS is active, could there be any difference in the control of electric trim from the trim switches at the Captains side and the FO side?
The Lion air Captain was winding back the trim wheel for seconds. The FO who was watching this all the time suddenly decided to use only blips, looks strange.

-Sanish
My guess is that the Captain may have told the FO that he was 'cancelling' the nose down trim inputs with the trim switch. To a low hours FO brought up on playstations and mobile 'phones, the use of a button to cancel is just a simple momentary press. If The Captain had said 'until the ND trim is removed' he might have understood the need to sustain it.
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Old 30th Jun 2019, 19:11
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Originally Posted by safetypee
yoko1,
Apparently you either have not read the EASA query of the NG trim system (previously referenced), or you choose to ignore what was written.


Yes, I'm aware of their comments, but they have nothing to do with the matter at hand. This issue has been addressed numerous times already by others, and repeating the question doesn't change the answer. I suggest you go back and review those responses.

Similarly you overlook that ease of trimming in normal flight conditions might not be the same as in abnormal conditions - extreme tail angles, nor that trimming with an alleviating tail load (nose down) involves different forces against the load in failed conditions (nose up).

Conversely to your previous assertion; there is nothing in the entire history of the 737 trim system since the introduction of the NG (larger tail) that indicates that the Manual Wheel or Electric Trim would be able to recover from high aerodynamic load; excepting what has been deduced from two accidents.

Whereas you fail to qualify your assertion, the opposing argument is that where an aircraft is deliberately mis trimmed and the resultant force held on the control column, it is probable that a limiting force or ability to apply more trim is reached before the physical stops (flight test techniques AC25-7, within the aircraft flight envelope not necessarily at the extremes of physical tail angle).
But if the aircraft is already in a extreme condition, greater that previously identified during flight test to due to a malfunction, then there is nothing to say either way that the aircraft can be recovered - except accidents.
And this is where I need to ask if you understand that there is only one trim motor involved? That motor receives a signal for direction and speed either from an automated source (Speed Trim, Mach Trim, A/P, MCAS) or the pilot yoke switch. In the accident scenario, when MCAS was commanding the stab nose down the forces were increasing on the jackscrew mechanism. When the pilots were commanding nose up, the forces were decreasing. If the stab motor was going to stall under an aerodynamic load, it would have happened in the direction of increasing (MCAS), and not decreasing force (pilot trimming). On the contrary, MCAS had absolutely no problem using that one trim motor to drive against an ever increasing force until it hit the physical stops. There is no evidence that the motor was ever overtasked.

Add to this the fact that all the FDR traces for both accident aircraft which occur within the certified flight envelope show a direct match between each pilot trim input (yoke switch) and an actual stab movement. We also have the experience of the penultimate Lion Air flight in which both pilots successfully used the Main Electric Trim to counter MCAS. During the Lion Air accident flight, the Main Electric Trim was successfully used to counter MCAS up to the point that the Captain handed over control to the First Officer. You can't just ignore this evidence because it doesn't fit your narrative.

Finally, let's put ourselves in the place of the Ethiopian and Indonesian investigating authorities. Given the history and cultural inclinations of these two nations, they have great motivation to unearth any exculpatory evidence that would remove pilot actions as a cause of concern. They are in possession of the full set of CVR and FDR data and any relevant physical evidence. They have access to all the subject matter experts. If there was any legitimate evidence that there was a failure in the Main Electric Trim system or that it was unable to stop and counter the MCAS inputs, they would be shouting it from the rooftops. They are not, nor is anyone else.

It is time to move on from this line of argument. It is a dry hole.
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Old 30th Jun 2019, 19:33
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Hey Yoko...ask your boss if the Eaton stab trim motor is a brushless servo motor, and if so is its driver logic self-contained. Then ask him if its required step and direction signals come from a digital source, like an FCC.

Then get him to confirm whether the same system architecture with its apparently undersized computer exists on the NG.
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Old 30th Jun 2019, 20:30
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particularly when only two are required in order to identify that one is sending bad data and trigger the "AoA Disagree" warning that the Max already has provision for.
I don't follow that. Sure, if one sensor goes back, it's detected, and MCAS is disabled. But that's my point. If MCAS is needed for airworthiness, how big a deal is it when it goes inop in flight? If I was an xAA, it's a question I'd be asking.

Some quick mental arithmetic says the MAX fleet had accumulated ~1 million flight hours when grounded (within a factor of 10). So AoA sensor failures are happening every 500K flight hours (actually more since sometimes it's the inactive one and nobody notices). When there are 5000 aircraft out there, that's a failure every 20 days or so. If MCAS is necessary... what happens when an aircraft has a failure and then, say as a result of an RA, enters the condition MCAS is supposed to handle? And it's one of these low-time minimally-trained crews that are the reality of airlines today - maybe the resurrection of the PF on AF447?

As has been said many times, there are going to be a lot of red faces if there is another hull loss because of MCAS, or the underlying aerodynamic problems which prompted it. I would sure want to be careful not to be one of them.
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