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Ethiopian airliner down in Africa

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Ethiopian airliner down in Africa

Old 30th Apr 2019, 23:48
  #4661 (permalink)  
 
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Originally Posted by gums View Post

My point is that they did not call out AoA vane for 3 or more minutes, even tho they had the ice light and so forth.

Gums sends....
I donít think anyone is disagreeing with your observation, and Murph and I attempted to offer an explanation as to why this may have happened. The crew was task saturated, so they didnít verbalize this annunciation earlier. It doesnít really seem to have anything to do with any action they did or did not take. Am I missing something here that you are trying to say?

Driver sends.....
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Old 1st May 2019, 00:19
  #4662 (permalink)  
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Avionista #4661
MCAS needs to be removed completely from the 737 Max as it is a far too powerful and dangerous solution to the issue of stick force gradient at high angles of attack.
We know why it's there, and more or less what it does, but I have to agree with this part of the above post. It's been nagging at me since November.

Why is such a powerful tool used to simply stop the PF heaving back too easily at a high AoA?
Boeing chose not to use conventional stall protection but instead devise a system that changes the aircraft's overall handling at a critical time. For a few moments, they are trying to make the MAX be something it's not. The necessary certifiable handling characteristics are synthesised by moving over 47 feet of flying surface - it's as though the design of the entire aircraft was being momentarily tweaked to cover one issue.

This vast surface is altered to give this synthetic nose down just at a time there's presumably a danger of the wings stalling. If it was capable of a lightning fast return to normal datums, it would just about be acceptable, but any return has to be done by the cranking of that jack. On that dark and stormy night, a last minute change of runway and a moment's inattention. Is that extra weight on the pole going to save the day? If it doesn't, even with a fully serviceable aircraft, it seems to me that the problem of an embarrassing stick shake could be turned into something orders of magnitude worse.
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Old 1st May 2019, 00:36
  #4663 (permalink)  
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Originally Posted by 737 Driver View Post


I donít think anyone is disagreeing with your observation, and Murph and I attempted to offer an explanation as to why this may have happened. The crew was task saturated, so they didnít verbalize this annunciation earlier. It doesnít really seem to have anything to do with any action they did or did not take. Am I missing something here that you are trying to say?

Driver sends.....
Agree with most of your comments relating to this post and others.It would be interesting to see if the crew of Lion Air and ET received any UPRT/UAT training in any previous recurrent training.
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Old 1st May 2019, 02:44
  #4664 (permalink)  
 
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Originally Posted by Loose rivets View Post
Avionista #4661


We know why it's there, and more or less what it does, but I have to agree with this part of the above post. It's been nagging at me since November.

Why is such a powerful tool used to simply stop the PF heaving back too easily at a high AoA?
Boeing chose not to use conventional stall protection but instead devise a system that changes the aircraft's overall handling at a critical time. For a few moments, they are trying to make the MAX be something it's not. The necessary certifiable handling characteristics are synthesised by moving over 47 feet of flying surface - it's as though the design of the entire aircraft was being momentarily tweaked to cover one issue.

This vast surface is altered to give this synthetic nose down just at a time there's presumably a danger of the wings stalling. If it was capable of a lightning fast return to normal datums, it would just about be acceptable, but any return has to be done by the cranking of that jack. On that dark and stormy night, a last minute change of runway and a moment's inattention. Is that extra weight on the pole going to save the day? If it doesn't, even with a fully serviceable aircraft, it seems to me that the problem of an embarrassing stick shake could be turned into something orders of magnitude worse.
It has been said time and time again: MCAS is not for stall protection.

MCAS is only there to provide the correct elevator feel at high angles of attack to meet the FAA certification requirements. You can still stall the aircraft, the only difference is how the elevator feels. With MCAS the elevator feels like every other aircraft you have flown. Without MCAS the elevator feels light at high angles of attack.

In my opinion, the original MCAS design was really not that big a deal. Boeing and FAA probably decided one Angle of Attack sensor was enough because it still flies just fine without MCAS. It just feels different. If the Angle of Attack sensor fails and MCAS operates incorrectly, you just trim it away with the thumb switches. (Lowering the flaps for landing also deactivates MCAS) If you get tired of playing with the trim switches to keep the nose up, then just turn off the Electric Stab switches and trim manually. No big deal right?

WRONG.... It seems that there are pilots who will have difficulty with this simple concept of FLY THE AIRCRAFT so now Boeing has refined MCAS to make it less likely to confuse these pilots. Now it takes 2 Angle of Attack sensors to agree before MCAS activates, AND it will not apply nose down trim repeatedly. That should make it safe for all pilots. YES?

But, for some reason, people are still not satisfied. It seems these people have a real hate-on for Boeing and the FAA. I think this is all an over reaction to a simple design underestimation that has an easy solution.
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Old 1st May 2019, 03:54
  #4665 (permalink)  
 
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Originally Posted by Lost in Saigon View Post
It has been said time and time again: MCAS is not for stall protection.

MCAS is only there to provide the correct elevator feel at high angles of attack to meet the FAA certification requirements. You can still stall the aircraft, the only difference is how the elevator feels. With MCAS the elevator feels like every other aircraft you have flown. Without MCAS the elevator feels light at high angles of attack.

In my opinion, the original MCAS design was really not that big a deal. Boeing and FAA probably decided one Angle of Attack sensor was enough because it still flies just fine without MCAS. It just feels different. If the Angle of Attack sensor fails and MCAS operates incorrectly, you just trim it away with the thumb switches. (Lowering the flaps for landing also deactivates MCAS) If you get tired of playing with the trim switches to keep the nose up, then just turn off the Electric Stab switches and trim manually. No big deal right?

WRONG.... It seems that there are pilots who will have difficulty with this simple concept of FLY THE AIRCRAFT so now Boeing has refined MCAS to make it less likely to confuse these pilots. Now it takes 2 Angle of Attack sensors to agree before MCAS activates, AND it will not apply nose down trim repeatedly. That should make it safe for all pilots. YES?

But, for some reason, people are still not satisfied. It seems these people have a real hate-on for Boeing and the FAA. I think this is all an over reaction to a simple design underestimation that has an easy solution.
I read several hundred posts ago MCAS uses a circuit that trims faster than the thumb switches, which is a key part of the problem. Just using the thumb switches is what the first victims did, and we have all been told that was obviously wrong and poor airmanship.

This was not a case of unaware pilots not noticing that the plane was trimming down, or not knowing the basics of how to fly an airplane. The most convincing arguments that I read here for pilot error is that they were too slow to recognize the problem and use the procedure that Boeing specifies (although even that gets fuzzy as Boeing used to specify a more detailed procedure that is now not taught because runaway trim was rare to non-existant before the MAX.)

The variety of "simple concepts" presented as solutions here that would have also crashed the MAX is concerning. My understanding of the current "party line (see Dominic Gates in the Seattle Times) is that the first pilots erred by not turning the electric trim off, and the second pilots erred by turning the electric trim off too soon. My unqualified understanding is that the ideal procedure is to use the electric trim to undo the MCAS input (while not letting go of the switch for a moment) and then (presumably without letting go of the trim button) turning off electric trim to disable MCAS. (I do not believe that is the standard response to a trim runaway which is an argument against the concept that the pilots erred by not applying the trim runaway procedure.)

We cannot forget that two planes from two different respectable airlines with legally qualified pilots crashed in short order, which is not something that happens very often. There is so far no reason to believe that those pilots were any more or less skilled than the thousands of their brethren so if this was just the sad fact that pilots don't know how to fly anymore, we should expect to see major airplane crashes every six months or so. We do not, which is why the plane was grounded despite Boeing's and the FAA's objections.
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Old 1st May 2019, 06:01
  #4666 (permalink)  
 
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Originally Posted by Water pilot View Post
I read several hundred posts ago MCAS uses a circuit that trims faster than the thumb switches, which is a key part of the problem. Just using the thumb switches is what the first victims did, and we have all been told that was obviously wrong and poor airmanship.
My understanding is that ANY use of the thumb switches disables MCAS. Any type of nose up trim stops the MCAS dead in it's tracks. If you release the thumb switch, a faulty "original" MCAS will start trimming again but only after 5 seconds. It should have been easy enough to keep stabbing the the nose up trim as much as you need to overcome the MCAS down trimming. At that point you would have to realize that the trim is not behaving like you want it to, and you should hit the Stab Trim Cut Out switches and just use the manual trim wheel.

Any "poor airmanship" criticism is just because they did not keep on using the thumb switches to keep nose up trim. There may be some underlying issues we don't know about that prevented them from operating the nose up trim. Until we get the full accident report, we don't really know if that is true.

Last edited by Lost in Saigon; 1st May 2019 at 06:25.
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Old 1st May 2019, 06:11
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Originally Posted by Organfreak View Post
Speaking as an informed passenger, try to see my POV: If there is something wrong with the airplane you're riding, wouldn't you want the very best piloting skills possible sitting up there in the pointy end??? I sure would! I am shocked that there's so much sentiment against "Just fly the damned plane!"
I know what you really mean but, as another informed passenger, if there is actually something wrong with the airplane, the pilots should NOT fly the damned plane. The plane should not leave the ground with passengers on board if the pilots are not confident that the aircraft is safe to fly.

In fact that's what I heard commercial pilots say when asked about the things they do to keep us, the passengers, safe: If they feel that there is anything wrong with the aircraft that could affect the safety of the flight, they will not take off until the issue is taken care of. Period. No ifs and buts.

But in this case, after the Lion Air accident, the pilots trusted Boeing, the FAA and the airlines when they claimed that the MAX is not unsafe, and just continued flying the damned plane. In my opinion that trust has been misplaced.

Anticipating accusations of being hysterical, I feel I need to recap some of the unbelievable things I have read in the last months about how Boeing failed at making safety their primary concern, while the FAA was a watchdog that didn't have teeth sharp enough to prevent it, and also some of the stuff the airlines did that contributed:

- the latest blunder about the AOA disagree warning being disabled "by accident" on the MAX
- recent whistleblower reports about damage to the wiring of the AOA sensors
- the redesign of the cutout switches that prevents disabling automatic trim independently from the manual electric trim
- relying on a single AOA sensor for the MCAS function
- increasing the amount of trim MCAS can apply in one run from 0.6 to 2.5 without re-evaluating its safety
- hiding the existence of the MCAS function and its behavior from the pilots
- a pilot that demanded more training for the MAX being ignored and then punished by his airline when he insisted: https://qz.com/1584233/boeing-737-ma...more-training/
- a pilot that said he didn't feel prepared and had significant issues on his first flight on the MAX: https://www.theatlantic.com/notes/20...37-max/584791/
- making the manual trim wheels smaller to fit the larger displays starting with the NG, and also making them harder to use
- Boeing accepting sub-standard hardware components from its suppliers, then literally hammering them in place or drilling additional holes in the components when they didn't fit properly, see "Problems with Boeing 737 next generation with structural dangers reported SBS​ dateline Australia":
- foreign objects found on aircraft delivered by Boeing
- airlines refusing to accept aircraft assembled in one of Boeing's factories due to quality issues
- Boeing ignoring employees that reported quality issues in its factories or the factories of their suppliers
- FAA mostly ignoring whistleblowers that did the same.
- Boeing firing the whistleblowers and the people reporting quality issues.
- The battery issues on the Dreamliner (the Al Jazeera investigation):

But with all that going on somehow some people are still extremely surprised about the criticism towards Boeing and the FAA.

The problem goes deep, and just fixing MCAS is not enough. The factors that allowed the issues with MCAS to slip through the cracks, assuming they were not hidden intentionally, also need to be fixed. And based on what I learned so far I think those factors are the under-funding and maybe even corruption within the FAA, and having some unethical people in key positions at Boeing and some of the airlines, that prioritized short term profit over safety.
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Old 1st May 2019, 06:34
  #4668 (permalink)  
 
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Originally Posted by Water pilot View Post
I read several hundred posts ago MCAS uses a circuit that trims faster than the thumb switches, which is a key part of the problem. Just using the thumb switches is what the first victims did, and we have all been told that was obviously wrong and poor airmanship.
If you look at the FDR traces, there's not a huge difference between the rate at which the stab moved ANU in response to crew trim commands, and the rate that MCAS drove it AND. Time to knock this one on the head.



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Old 1st May 2019, 06:38
  #4669 (permalink)  
 
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Originally Posted by Organfreak View Post

Speaking as an informed passenger, try to see my POV: If there is something wrong with the airplane you're riding, wouldn't you want the very best piloting skills possible sitting up there in the pointy end??? I sure would! I am shocked that there's so much sentiment against "Just fly the damned plane!"
However, if the aircraft’s response to a single failure reduces the ability of any piloting skills to recover the aircraft, then discussing the quality of flying become irrelevant.

The assumed use of electric trim depends on understanding the nature of the failure - no direct warning alert. MCAS trim surpasses elect trim - 10 sec to 5 sec, then there is a point where elect trim becomes ineffective, it should be inhibited before then, there is a (coincident) point where manual wheel trim is ineffective (physically impossible to move), so that the residual control force and / or manoeuvre cannot be flown.

If speed of awareness and trim action are important, then the aircraft systems must support these; you cannot expect to cure a poor design by ‘improving’ every pilot, and expecting that enhanced performance every day in every situation.

Water pilot,
We cannot forget that two planes from two different respectable airlines with legally qualified pilots crashed in short order, which is not something that happens very often. There is so far no reason to believe that those pilots were any more or less skilled than the thousands of their brethren so if this was just the sad fact that pilots don't know how to fly anymore, we should expect to see major airplane crashes every six months or so. We do not, which is why the plane was grounded despite Boeing's and the FAA's objections.


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Old 1st May 2019, 07:35
  #4670 (permalink)  

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Even Chuck Yeager had to ride the rails a few times....

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Old 1st May 2019, 08:01
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Much of this discussion continues to obfuscate some fundamental and indisputable points.

In 2019, the loss of two aircraft to the same cause within less than two years of service entry is anomalous.

Whatever one may think about automation and piloting skills, developments in aircraft technology have paralleled not only a continuous improvement in safety, but an improvement that has been worldwide and near-universal. So far, we have not seen any evidence of an actual decline in piloting standards.

This specific failure - which can barely be called a chain - has a kill probability of 0.667. A surface-to-air missile development team would dream of anything like that.

If there were more incidents of AoA-vane failure triggering MCAS, but with the aircraft being recovered, there would be strong grounds for the FTFA-fundies' argument. As it is, their case is purely speculative.
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Old 1st May 2019, 08:53
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Originally Posted by LowObservable View Post
Much of this discussion continues to obfuscate some fundamental and indisputable points.

In 2019, the loss of two aircraft to the same cause within less than two years of service entry is anomalous.
No one is disputing that the current implementation of MCAS was dangerous and led to the 2 accidents.
If those 2 planes could have been saved by better pilot training is very much an orthogonal discussion.


And those fundamental and indisputable points have been discussed over and over, so why do you have a problem with people discussing pilot training?
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Old 1st May 2019, 09:28
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Originally Posted by Water pilot View Post
We cannot forget that two planes from two different respectable airlines with legally qualified pilots crashed in short order, which is not something that happens very often. There is so far no reason to believe that those pilots were any more or less skilled than the thousands of their brethren so if this was just the sad fact that pilots don't know how to fly anymore,
I nearly spat my porridge when I read the first line of that.
Water pilot, do yourself (do us all) a favour and look LionAir up on wiki - and tell me if it's accident and incident record - along litany of written off airframes and fatalities - plus bribery at governmental level and flying routes wholesale without licences constitutes a "respectable" airline in your book. Look at the crew + engineering actions and procedures on the days preceeding their accident too...
Also check Indonesia's national historical accident record, which explains why for so long all its airlines were banned from EU airspace, and how recently they were given a reprieve. Doubtless you'll assert that big cats can change their spots. I don't doubt it. What I do doubt is whether they have done so sufficiently.
Of course their latest accident just might be an extremely unfortunate anomaly that a newly reformed company didn't deserve, but with a statistical background like theirs and the evidence from the preceeding flights not many would be taking bets on that I fear.

I can't comment much on Ethiopian, they do seem to have a good record although that statement is often heard qualified, but the non-stop floods of reports over many years of grossly exceeding pilots' flying hours as a matter of routine, failures to honour contracts or provide pay on occasions plus the latest suggestions about failure to publish/incorporate Boeings safety bulletins and lack of systems awareness even after the Lion Air accident make it plain that parts of their operation at least are not up to the sort of standards we expect in N Europe and the US, but I don't think we can judge their overall training quality and standards from the actions of just two pilots though it must raise doubts.
Do not mistake this for racism as doubtless some with auto-offend enabled will do. It most certainly isn't. It's called being honest and realistic. The only bit that isn't proven to be factual yet is the Ethiopian amendment and awareness states, the rest is all hard fact, and even those doubts seem to be pretty much accepted if unproven as yet.

Both nations have historically beeen known for an almost total lack of democratic process, a history of repressive military rule (aka Dictatorship), a highly developed hierarchical society and steep if not near vertical authority gradients in the cockpit. None of these are condusive to the sort of open reporting culture of operations that so many of us here take for granted and it behoves us to take these cultural differences into consideration when we consider what's happened out there.
I've worked in Africa, including Ethiopia and things are simply not done the way we N Europeans expect out there. It shocks one at first but that's just the way it is, just as the way different nations have different driving habits and styles so they do too in Aviation. They also have different ideas on legal matters, so 'legally qualified' may mean one thng to you and something entirely different in a gynae clinic in a fovella in Rio...

This is just another aspect of what's turning into an extremely complex matter which doubtless will become even more so before any resolution is found.

Last edited by meleagertoo; 1st May 2019 at 14:02.
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Old 1st May 2019, 10:16
  #4674 (permalink)  
 
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@meleagertoo

Originally Posted by wiedehopf View Post
No one is disputing that the current implementation of MCAS was dangerous and led to the 2 accidents.
If those 2 planes could have been saved by better pilot training is very much an orthogonal discussion.

And those fundamental and indisputable points have been discussed over and over, so why do you have a problem with people discussing pilot training?
To take a contrarian viewpoint: If an aircraft is not safe to be operated by airlines in countries whose pilots are not top-notch, then it should not be sold to those airlines/countries. Or fix the aircraft, so that it is safe to be operated by less-than-perfect pilots. End of (my version of) the story...

The implication is that training outside the US and other "first world" countries is not up to standard. That may or may not be true, but nobody on this forum has control over all of those countries, and that discussion belongs elsewhere on the forum.
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Old 1st May 2019, 10:36
  #4675 (permalink)  
 
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Originally Posted by meleagertoo View Post
I can't comment much on Ethiopian, they do seem to have a good record although that statement is often heard qualified, but the non-stop floods of reports over many years of grossly exceeding pilots' flying hours as a matter of routine, failures to honour contracts or provide pay on occasions plus the latest suggestions about failure to publish/incorporate Boeings safety bulletins and lack of systems awareness even after the Lion Air accident.....
You can add to that list the findings on the crash of ET409, a totally serviceable 737, into the Med in 2010....
"the probable causes of the accident were the flight crew's mismanagement of the aircraft's speed, altitude, headings and attitude through inconsistent flight control inputs resulting in a loss of control and their failure to abide by CRM principles of mutual support and calling deviations."
More significant even than the probable cause is that both Ethiopian Airlines AND their CAA rejected the findings out of hand, Therefore it is reasonable to doubt they have acted to address the signficant shortcomings identified, and all the more reasonable that the performance of the crew of ET302 is discussed.
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Old 1st May 2019, 11:09
  #4676 (permalink)  
 
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Aerodynamic or Mechanical Fix

From Gums

The problem, Bill, et al, is that the plane required an aerodynamic fix for pitch moments approaching the stall AoA.

Hi Gums,
That's not what I have understood the certification problem to be - which was that stick back force did not increase enough at higher AoA to satisfy the requirements. That is not quite the same as a pitch up problem, although it could lead to one, depending on pilot reaction.
If what I understood back there is true - and it came from a very reliable source quite early on on this thread, there can be a few ways to increase stick back force, without going the aerodynamic route.
To my mind, this is preferable than putting large inputs into the stab and has the added advantage that if a fail input into a direct feel-controlling software or linkeage occurs, it is much easier to manage.
Of course if all works normally, the feeling in the design case (high AoA) for the pilot would be a similar one. Of course Boeing wouldn't deliberately put a time bomb in their system . but as we see, one was waiting there.
Greetings, B

PS The quote you used wasn't from me - although I subscribe to a lot of the sentiment in it.

Last edited by bill fly; 1st May 2019 at 11:27.
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Old 1st May 2019, 11:45
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Originally Posted by bill fly View Post
From Gums

The problem, Bill, et al, is that the plane required an aerodynamic fix for pitch moments approaching the stall AoA.

Hi Gums,
That's not what I have understood the certification problem to be - which was that stick back force did not increase enough at higher AoA to satisfy the requirements. That is not quite the same as a pitch up problem, although it could lead to one, depending on pilot reaction.
If what I understood back there is true - and it came from a very reliable source quite early on on this thread, there can be a few ways to increase stick back force, without going the aerodynamic route.
To my mind, this is preferable than putting large inputs into the stab and has the added advantage that if a fail input into a direct feel-controlling software or linkeage occurs, it is much easier to manage.
Of course if all works normally, the feeling in the design case (high AoA) for the pilot would be a similar one. Of course Boeing wouldn't deliberately put a time bomb in their system . but as we see, one was waiting there.
Greetings, B

PS The quote you used wasn't from me - although I subscribe to a lot of the sentiment in it.
It may (or may not) be helpful to remember that the autopilot does not require MCAS help.
This reinforces the fact that this is fundamentally a stick feel issue rather than a critical instability that could kick in under extreme but still in certified envelope conditions.
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Old 1st May 2019, 12:27
  #4678 (permalink)  
 
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[QUOTE=GordonR_Cape;10460385]@meleagertoo



To take a contrarian viewpoint: If an aircraft is not safe to be operated by airlines in countries whose pilots are not top-notch, then it should not be sold to those airlines/countries. Or fix the aircraft, so that it is safe to be operated by less-than-perfect pilots.

The implication is that training outside the US and other "first world" countries is not up to standard. That may or may not be true, but nobody on this forum has control over all of those countries, and that discussion belongs elsewhere on the forum.
That contrarian viewpoint, if I may say so, is naiive in the extreme. One can no more withold aircraft sales on those grounds than one could of cars or power tools - They claim to be operating to international standards, if they choose to backslide on that why/how is it Boeing's or Washington's job to judge - ? Wouldn't that be a particularly unpleasant form of self-righteous paternalism? (a phrase I never thought I'd actually use myself!) WE allow them into or airports - we can't then refuse to sell them aircraft, or de we prefer our skies filled with their legacy antique vodka burners dthat haven't seen a spanner in a year and with 28 bald tyres out of 28 instead?
No one but you suggeted that 'training outside the US is not up to standard", (do you include Europe, Canada, Japan, Ozealand in that too?) and discussion of national standards is, I predict, going to become a pivotal matter in this whole affair so I suggest this is exactly the right place to discuss it.
I gather there is a freakish viewpoint out there that Boeing is responsible to fix the aeroplane so even imcompetent pilots can't crash it but that's so grotesquely unrealistic it's simply laughable. How on earth such weird cotton-wool woo-woo ideas ever got into an aviation forum beats me! This is real life for God's sake, not the bloody Guardian's social pages!

Oggers, I refrained from including ET409 - although it may well prove related I don't think it is wise to include what could be a one-off, that's a poor basis for a general conclusion wheras LionAirs's history of disasters is so long the next one might almost be predictable on a time-passed basis. If Ethiopian were to have another similar I might change my tune.

re your valid concerns about the Ethiopian CAA's state of denial let's not judge them until we see the final report. However given that Ethiopia is a single-party socialist state and the government controls every aspect of life including the CAA and by extension the national airline too I'm not expecting to learn much very from it beyond being long on rhetoric and short on critical facts but we'll just have to wait and see. I sincerely hope I'm proved wrong.

This makes it all the more essential that Boeing appear as open and honest as possible.

Last edited by meleagertoo; 1st May 2019 at 14:05.
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Old 1st May 2019, 12:33
  #4679 (permalink)  
 
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Originally Posted by MurphyWasRight View Post
It may (or may not) be helpful to remember that the autopilot does not require MCAS help.
This reinforces the fact that this is fundamentally a stick feel issue rather than a critical instability that could kick in under extreme but still in certified envelope conditions.
An interesting point! I can only assume that this has something to do with the autopilot not using AOA as an input like MCAS does, but rather gyro pitch (and other parameters).

I previously asked the question: How could the autopilot ever get into a high AOA situation? One answer was if the autothrottle is disabled. The implication being that the autopilot could keep increasing the nose up pitch until the stall warning activates, and the crew intervenes. I hope that scenario has been carefully tested?

This also touches on the question of whether the MAX autopilot was specifically programmed for the region of high AOA characteristics covered by MCAS, and whether it was tested under actual flight conditions?
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Old 1st May 2019, 13:04
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Originally Posted by GordonR_Cape View Post
An interesting point! I can only assume that this has something to do with the autopilot not using AOA as an input like MCAS does, but rather gyro pitch (and other parameters).

I previously asked the question: How could the autopilot ever get into a high AOA situation? One answer was if the autothrottle is disabled. The implication being that the autopilot could keep increasing the nose up pitch until the stall warning activates, and the crew intervenes. I hope that scenario has been carefully tested?

This also touches on the question of whether the MAX autopilot was specifically programmed for the region of high AOA characteristics covered by MCAS, and whether it was tested under actual flight conditions?
It may be simpler than that, of course I could be wrong on autopilot details.

The autopilot 'knows' the desired column (and other controls) positions and puts them there, it does not rely on column feel for feedback.

It is manipulating the controls to achieve the desired aircraft state, when pitch is low set column position back until pitch OK at a defined 'gain', how much (not 'how hard) to pull is based on divergence.
This a classic feedback loop, the stick force is not part of the loop.

An imperfect analogy is cruise control in a car, it maintains desired speed directly whereas the driver uses more or less force on the accelerator to maintain speed.

Given the above I would assume (a tricky word in many fields) that no significant changes to autopilot for MAX were required, at most some tweaking of gains.

When I said 'critical' instability i referred to something like reversal of effect past a certain point or similar which would be catastrophic in a feedback loop.
MurphyWasRight is offline  

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