Lion Air crash: Jakarta Boeing 737 'had prior instrument error'

[EEVblog]

The politics around this is nuts. Trade war fuelled, today: "Airbus SE secured a $35 billion jet deal from China"
Ouch.

Indonesian airline Garuda cancells 49 Boeing 737 MAX 8 $4.9 billion

Ouch

[EEVblog]

Wow, the same plane had the same incident the day before, and a pilot in the jump seat knew what to do and saved the plane:
https://www.smh.com.au/business/companies/pilot-who-hitched-a-ride-saved-boeing-737-max-a-day-before-it-crashed-20190320-p515sq.html

 

[Obi_Kwiet]

Stability/instability aren't really relevant there. In fact, this isn't even an issue of a strong non-linearity, it's just an issue of running into an actuation limit on the elevator control surface. The trim basically gives the elevators a boost.

Great theory, but here are the holes. If MCAS basically gives the elevators a necessary boost, you're suggesting full elevator down wouldn't even prevent the plane from runaway nose up stall when in a condition where the NG would be considered to still be in a normal part of its envelope. Where the NG would still be stable, and by your definition, stable means the NG would return towards straight even if you let go of the stick, let alone push the stick forward. So the MCAS has to kick in and help the pilot do something he wouldn't even do. If he thinks he can just let go of the stick, and the plane will be stable, why would he push full forward?

By your reasoning:
1. Ideally, the MCAS would not even be needed. They would make the elevator larger and more powerful. They could even limit the upwards range a bit, to ensure that upwards elevator is not increased. Only the downwards effect is increased. Of course this might not be possible without major alterations and cost and time and recertification, and perhaps due to these reasons it would be better to just build a new plane from scratch, even though it sounds simple.

2. If the MCAS is only supposed to boost or extend the pilot's response, then it would ideally only activate when the pilot is already at full elevator down. MCAS would not do anything if the pilot is not at full elevator down. In the case of these crashes, the pilots were obviously giving full elevator UP. But maybe trim control is too slow for this to work and/or maybe as I suggested earlier, an NG pilot wouldn't even know to push the stick that far forward until it was too late.

Any way you slice it, it would appear that MCAS is potentially a pathetic bandaid on a pretty serious wound.

So no. I think non-linearity and instability are potentially a big part of the problem. If Boeing were willing to eat it and redesign/recertify a new airframe, they would have made the plane with taller landing gear so the engines did not have to mount in a way that produced instability in a given AOA range where the plane still has laminar flow and lift. To me, "unstable" potentially means that even if the pilot had the range of control (with help of MCAS or otherwise) to reign the plane back from here, this area of AOA would be difficult to control and essentially unsafe/unusable without some active electronic aid that can respond relatively quickly and dynamically... not just to slam the nose back down, but to enable the pilot to utilize and fly in this range of AOA in a predictable and controllable manner if and when the need should arise. If and when then need should arise, slamming the nose down might be less than ideal.

You don't have a clear picture of what is going on. The elevators are flaps on the the tailplanes. MCAS adjust the trim, which rotates the entire tailplane.

I'm saying that the whole concept of stability/instability isn't even relevant here, and your definition of instability is vague and inaccurate from either a control systems or aerodynamics point of view. A power on stall isn't stable or unstable. It's just a flight characteristic, and the issue is that in some situations, the elevator alone might not have sufficient actuator force to reach a leveled out state if you keep the new more powerful engine as full thrust. If anything it's a reachability problem, except it isn't, because we have additional control surfaces that we can use to evolve the system to the desired state. People are saying that this is bad, but there aren't any reasons given for it, other than vague handwaveing. There's no reason that you couldn't implement this with a manual trim control, and if the aircraft had been designed fifty years ago, that's exactly what they'd have done. This doesn't require some kind of fast processing loop to stabilize a highly unstable dynamic system like the B2 or something. It's literally just an automatic switch that makes the tail plane more steep to help the elevators out. The only reason the pilots don't do it manually is to reduce workload on the pilot. That's the whole reason that automatic trim systems exist in the first place.

On top of that this was the crew's first flight on a MAX. All the controls are different. Video displays replacing gauges. I can spend an hour looking for something on my own bench that is in plain sight, as it is.

Yeah, but the control and gauge they needed to get at was the same at is has been for fifty years.

[MT]

Wow, the same plane had the same incident the day before, and a pilot in the jump seat knew what to do and saved the plane:
https://www.smh.com.au/business/companies/pilot-who-hitched-a-ride-saved-boeing-737-max-a-day-before-it-crashed-20190320-p515sq.html

You are slightly late to the show, but dont worry the fat lady haven't even been dressed up yet!   On page 20!
https://www.eevblog.com/forum/chat/lion-air-crash-jakarta-boeing-737-had-prior-instrument-error/msg2283120/#msg2283120

[KL27x]

Obi_Kwiet
Thanks for trying to enlighten me, but I disagree with most of what you wrote there. And I understand (at least since the last couple days) the difference between the elevators and the horizontal stabilizers. We do not agree what instability means in this application/context.

If you manually just adjust the trim down, this makes the plane stable at only one exact AOA. This is great if you want to leave the plane exactly there, and then continually baby sit it with manual inputs so it doesn't change. Operating in an unstable area means (to me) if you inadvertently let the plane's attitude drift slightly down, that amount of trim you dialed in is now too much, and the plane will nose down in increasing rate. And if the plane drifts up a little, it will not be enough, and the plane will nose up ever more increasing. Because the plane is not stable in this range of AOA. It's like you are correcting me with your abstract definitions of stable vs unstable. But when I say the same thing, but in the context of the plane, you disagree. In my understanding, you cannot correct for instability with a static change. Now if the horizontal stabilizer increased in effectiveness at a higher AOA at the same rate as the engine placement increased their aerodynamic lift (at all given speed, no less), than this could result in a stable configuration. Maybe that's the case... (huh, yeah, maybe).

You are viewing this as "just put the trim down a bit" then the plane is now stable. I see it as "put the trim down a bunch to get the nose back down, cuz the pilot is an idiot." If the full amount of stabilizer overpowers the full range of flaps, and MCAS does half that amount per firing, I see that as a huge amount of trim.

Also, if you suggest the full amount of the elevator can't recover (let alone maintain) the plane with the stabilizer in a "normal" position when in an AOA where the 737 NG is happy to be flown and controllable, then your idea of what Boeing has done is way worse than what I even imagine, lol. This is gone from greed to total insanity.

[Dundarave]

Wow, the same plane had the same incident the day before, and a pilot in the jump seat knew what to do and saved the plane:

Which means that only one pilot out of the seven who have actually experienced the flaw in real time (as opposed to the experts in this thread) knew what to do.  Two, plus the quick-witted jump-seat pilot in the first instance, two in the first crash, and two in the second crash, makes seven.

Just wanted to point that out with respect to the pilot-error argument.

[Obi_Kwiet]

Obi_Kwiet

Thanks for trying to enlighten me, but I disagree with most of what you wrote there. And I understand (at least since the last couple days) the difference between the elevators and the horizontal stabilizers. We do not agree what instability means in this context.

If you manually just adjust the trim down, this makes the plane stable at only one exact AOA. This is great if you want to leave the plane exactly there, and baby sit it so it doesn't change. Operating in an unstable area means (to me) if you inadvertently let the plane's attitude drift slightly down, that amount of trim you dialed in is now too much, and the plane will nose down in increasing rate. And if the plane drifts up a little, it will not be enough, and the plane will nose up ever more increasing. Because this it is unstable. It's like you are correcting me with your definitions of stable vs instable. But when I say the same thing, you disagree.

You are viewing this as "just put the trim down a bit" then the plane is now stable. I see it as "put the trim down a bunch to get the nose back down, cuz the pilot is an idiot." If the full amount of stabilizer overpowers the full range of flaps, and MCAS does half that amount per firing, I see that as a huge amount of trim.

Also, if you suggest the full amount of flaps can't recover the plane with the stabilizer in a "normal" position when in an AOA where the 737 is happy to be flown and controllable, then your idea of what Boeing has done is way worse than what I even imagine, lol. This is gone from greed to total insanity.

These aren't "my" definitions of stability, they are "the" definitions of stability. Every aircraft has unstable system states. It doesn't matter, and it's not relevant to what we are discussing. We don't care that the aircraft will always return to level flight or remain in its current attitude if given no control inputs, because it won't. We care that the aircraft can always be recovered to safe level flight within a reasonable flight envelope that is maintained by the judgement of the pilot. Secondarily, we'd like to be able to do that in a climb without cutting power the the engine, but that's really just a "nice to have", and it's what MCAS gives you.

The degree of deflection that the MCAS can command is unnecessary and a design flaw.

You are trying to claim that because a full power, flaps up climb may not be able to be leveled out with the elevators alone, that the plane is somehow unsafe and irresponsible. Why? The pilot and flight computer have access to multiple system variables that will level the aircraft. Why must it be fully adjustable with only the elevator? This is just an arbitrary assertion with no real justification behind it.

[KL27x]

These aren't "my" definitions of stability, they are "the" definitions of stability.

And I fully accept these definitions. And furthermore, I am applying these definitions to the real world, i.e. the plane. This is what engineers do. You want to state the definition of instability. Then say, "ok, forget about it. Now, let's talk about planes!"

You are trying to claim that because a full power, flaps up climb may not be able to be leveled out with the elevators alone, that the plane is somehow unsafe and irresponsible. Why? The pilot and flight computer have access to multiple system variables that will level the aircraft. Why must it be fully adjustable with only the elevator? This is just an arbitrary assertion with no real justification behind it.

Because... in order to get into a high AOA, I would assume you need to put the elevator UP? And going by your definition of stability you just wrote a few hours ago, in the original plane you would have to maintain some of this elevator up (or use trim) to even keep it there, else the AOA would decrease. But you say in the MAX, once you get the AOA up (to this same AOA), you would have to completely reverse the elevator to full down, but even this would not be enough by itself to prevent the plane from runaway flipping up and stalling...

So yeah, this sounds much worse than anything I ever imagined.

We don't care that the aircraft will always return to level flight or remain in its current attitude if given no control inputs, because it won't.

Yeah, it would be nice if our cars drove themselves. We have to steer them. Without cruise control, we have to press the gas pedal. But if we had to ride a knife edge where any deviation got increasingly worse, it would be impractical to drive a long distance. It might be nice to be able to say "Yeah, officer. I was doing 120, but my gas pedal is unstable! One second I'm doing 60, then 65, and then the next thing I know I'm doing 120!" Theoretically, it would be possible to ride a motorcycle or a bicycle with no rake on the front wheel. There are people with fast enough reflexes and innate balance they can ride a unicycle. But we put rake on the front steering fork so that the vehicle is self-stabilizing. It would be extremely taxing to have to constantly balance the vehicle.

You are trying to claim that because a full power, flaps up climb may not be able to be leveled out with the elevators alone, that the plane is somehow unsafe and irresponsible. Why? The pilot and flight computer have access to multiple system variables that will level the aircraft. Why must it be fully adjustable with only the elevator? This is just an arbitrary assertion with no real justification behind it.

Because the plane is supposed to be flyable without this stuff. What other "system variables" do you have access to by pressing the yoke? Now you want to press on the yoke while manually cranking the trim wheel for 20-30 seconds and also simultaneously pressing other buttons to modify "other system variables," and reducing thrust, all while monitoring air speed, pitch, and elevation, all because you wanted to use a part of the AOA range where with the NG, you just "pull up on the yoke a little?" And can you do this dance to keep the plane operating in this unstable region without accidentally tipping over into a stall, or alternately starting to nose down rapidly into the kind of flight you are not trying to do at this point in time? Please tell me when I get on a plane, my pilot doesn't need to be able to juggle while riding a unicycle. For 5 hours.

Please, if you can, tell me where I'm wrong, not that I'm wrong. We all know I'm wrong, already. Go ahead and tear this post apart.

[Towger]

I don't think it has beed posted yet, but Mentour Pilot demonstrates a MCAS runaway in a simulator.  Note how the co-pilot does not have the strength to wind it back with his left hand.  In the section before the clip he explains the dangers of trying to stop the trim wheels turning by hand.

https://youtu.be/xixM_cwSLcQ

[iMo]

When the WWII submarines wanted to dive quickly the captain called the crew to immediately run into the front section of the sub, and vice versa.
Maybe B will finally implement that procedure in their airplanes too. I bet the passengers will be pretty willing to follow such a command when the plane in a critical situation..

[Gyro]

When the WWII submarines wanted to dive quickly the captain called the crew to immediately run into the front section of the sub, and vice versa.
Maybe B will finally implement that procedure in their airplanes too. I bet the passengers will be pretty willing to follow such a command when the plane in a critical situation..

Not at a >30' downward angle they wouldn't.

[KL27x]

Mentour Pilot in the previously linked vid would not have tried that. He'd be calmly calling out to his copilot: "Memory item for crashing into the ocean." While copilot, who is more askeered of breaking a nail whilst trying (and failing) to crank the trim wheel than crashing, would realize he doesn't know that one, and glance at the manual wondering if that is listed under C for crashing.

I wonder if that was what it was like on that plane. Maybe on the first several problems. Maybe there were calmly leafing through the manual whilst the plane gradually slipped into the deep dive, unnoticed. I'm sure the last 30 seconds were not like that, though. On second thought, maybe it was... calm all they way to the end. Like the captain of the Titanic in the movies. On a big plane/ship, maybe everything happens in slow motion, and what is set in motion become inevitable/unstoppable way too far in advance to the point where the last 30 seconds is just resignation.

[Obi_Kwiet]

These aren't "my" definitions of stability, they are "the" definitions of stability.

And I fully accept these definitions. And furthermore, I am applying these definitions to the real world, i.e. the plane. This is what engineers do. You want to state the definition of instability. Then say, "ok, forget about it. Now, let's talk about planes!"

You are trying to claim that because a full power, flaps up climb may not be able to be leveled out with the elevators alone, that the plane is somehow unsafe and irresponsible. Why? The pilot and flight computer have access to multiple system variables that will level the aircraft. Why must it be fully adjustable with only the elevator? This is just an arbitrary assertion with no real justification behind it.

Because... in order to get into a high AOA, I would assume you need to put the elevator UP? And going by your definition of stability you just wrote a few hours ago, in the original plane you would have to maintain some of this elevator up (or use trim) to even keep it there, else the AOA would decrease. But you say in the MAX, once you get the AOA up (to this same AOA), you would have to completely reverse the elevator to full down, but even this would not be enough by itself to prevent the plane from runaway flipping up and stalling...

So yeah, this sounds much worse than anything I ever imagined.

Why? It's a flight characteristic. All planes with low slung engines will have more pitch up force on full throttle. The MAX just has a bit more. It's not like this is some sudden onset operating region where the dynamics change hugely. What happens is that you pull on back on the stick, and at full throttle, you'll notice stick pressure needed to maintain that AoA starts to lessen and invert. If you want to keep that AoA and full throttle, you need to push down, sometimes really hard. That's quite intuitive. The auto trim exists to help the aircraft maintain control input without physically exhausting the pilot. If you don't have trim, then you have to sit there and haul back super hard on the stick whenever you want to climb or descend. MCAS is just a special case of that for a specific situation.

Yeah, it would be nice if our cars drove themselves. We have to steer them. Without cruise control, we have to press the gas pedal. But if we had to ride a knife edge where any deviation got increasingly worse, it would be impractical to drive a long distance. It might be nice to be able to say "Yeah, officer. I was doing 120, but my gas pedal is unstable! One second I'm doing 60, then 65, and then the next thing I know I'm doing 120!" Theoretically, it would be possible to ride a motorcycle or a bicycle with no rake on the front wheel. There are people with fast enough reflexes and innate balance they can ride a unicycle. But we put rake on the front steering fork so that the vehicle is self-stabilizing. It would be extremely taxing to have to constantly balance the vehicle.

That's not what is happening with MCAS. It's not sudden and impossible react to. It's not an issue of reflexes. It's an issue of the fact that the plane is huge and hard to move. You need trim input because otherwise the control surfaces are physically too hard to move. It doesn't need to be automatic, but it is automatic to reduce pilot workload. All airliners already automatically adjust the trim for these exact reasons. MCAS is just one very specific case of automatic trim adjustment.

Because the plane is supposed to be flyable without this stuff. What other "system variables" do you have access to by pressing the yoke? Now you want to press on the yoke while manually cranking the trim wheel for 20-30 seconds and also simultaneously pressing other buttons to modify "other system variables," and reducing thrust, all while monitoring air speed, pitch, and elevation, all because you wanted to use a part of the AOA range where with the NG, you just "pull up on the yoke a little?" And can you do this dance to keep the plane operating in this unstable region without accidentally tipping over into a stall, or alternately starting to nose down rapidly into the kind of flight you are not trying to do at this point in time? Please tell me when I get on a plane, my pilot doesn't need to be able to juggle while riding a unicycle. For 5 hours.

Like I said before, flying with just the yoke would be exhausting in any airliner. So would manually cranking the trim wheel to reduce stick pressure. That's why they all have auto trim. It's not like the aircraft is super twitchy, it's more like driving a big antique buss with no power steering. It's slow and predictable, but physically hard to turn the wheel. MCAS is really only relevant during a very small portion of the flight during climb-out, because that's the only part of the flight where this would be an issue. Most of the flight is hands off with an auto-pilot keeping the course. Honestly, the "yoke jerk" to turn off auto trim seems like a bad idea to me. The yoke controls the ailerons and the elevators. It's a bit subjective, but I think it's better to keep distinct control surfaces attached to their separate interfaces.

[KL27x]

Why? It's a flight characteristic. All planes with low slung engines will have more pitch up force on full throttle. The MAX just has a bit more. It's not like this is some sudden onset operating region where the dynamics change hugely. What happens is that you pull on back on the stick, and at full throttle, you'll notice stick pressure needed to maintain that AoA starts to lessen and invert. If you want to keep that AoA and full throttle, you need to push down, sometimes really hard. That's quite intuitive. The auto trim exists to help the aircraft maintain control input without physically exhausting the pilot. If you don't have trim, then you have to sit there and haul back super hard on the stick whenever you want to climb or descend. MCAS is just a special case of that for a specific situation.

Obi, you did not understand what I was talking about.

1. I am aware that the MAX induces more nose up under throttle. And they way I have read it is not "a bit more," it's actually "quite a bit" more. And yet, this is not what I'm referring to in my last couple of posts between us.

2. There is an additional aerodynamic problem that is caused by the larger engines. The plane was designed to be aerodynamically efficient and stable with the original engines. As AOA increases, the belly of the plane starts to increasingly get exposed to the oncoming air. And these larger engines, which are placed farther forward, are now getting hit at an angle. They are no longer just thrusting the plane. They cause lift and drag simply by their aerodynamic shape. But due to their farther forward position, the lift is not acting at the center of mass of the plane, anymore. It is closer to the nose and is creating a twisting force, causing the nose to pull up. Due to the higher engine placement, the drag is no longer acting below the wing as much, which should be counteracting this force, more, in the original placement.

3. In fact, the aerodynamic lift from the engines is acting on the plane through the wings. So this is increasing the torque on the wings (in addition to the normal loading and effect of engine thrust) and might induce a small fraction of an inch more than the normal twist to the wings, increasing the wings' AOA. This is certainly true (as an average, and to an engineer) on some level. Everything has flex. Whether it is significant or not, who knows.

4. Unlike the nose up effect that is caused by throttle, this nose up effect induced by the aerodynamics is not significant at lower AOA. But it increases ever more at higher AOA. This is what makes the high AOA reagion unstable on the MAX. The engine effect would be more static and controllable, as you say, by the pilot, simply by adding trim based on thrust. Unstable doesn't mean unflyable. If it is minor, it would just mean more manual labor/tweaking/monitoring to keep the plane in this region. Since it is probably relatively close to a stall, this might make it unsafe.

4b. Whenever you have some effect (aerodynamic lift of the engine) which causes something else (nose up of the plane) that increases its own effect (the aerodynamic lift of the engines is now greater), this is what might be called a positive feedback loop. Unchecked, it causes some degree of instability. You can't completely negate/counter this dynamic variable with a static change (like setting trim to X). 

5. MCAS does not adjust the trim when you change throttle. It reacts only to AOA. It therefore is not strictly there to counterbalance the effect of the throttle.

6. I might be wrong, but because the pilot makes trim adjustments for other reasons and for intentions unknown to the MCAS system, it would be impossible for MCAS to be able to dynamically and actively negate the aerodynamic effect of the engines without making some degree of unpredictability in handling to the pilot. This is why I believe the MCAS is more like a "Saw Stop." I would guess it acts more like a last ditch anti-stall, despite how Boeing wants to classify it. Also, to actively/dynamically negate the aerodynamic effect of the engines, MCAS would have to be keeping track of the changes it makes; which this either isn't the case (since it is known to be able to repeatedly fire, indefinitely), or there was an actual bug (whether coding or design oversight) in the original implementation. 

6b. For active electronic dynamic stability to work best, with the fewest compromises, IMO, requires fly-by-wire. Airbus. F16. W/e. The computer has to be able to take the user input and decide how to implement that in the moment (mixing this in with the juggling), so that the pilot gets a consistent response. There is a huge amount of work and risk and expense to get the response tuned/refined to be efficient to human pilots... because someone has to test it.

I state all these things like facts, but it is just what has been spread in the internet news. And I may be inferring some of it incorrectly. As for the physics/aerodynamics, full disclosure; I'm not an engineer in the state of Oregon, but I like to pretend I am, on the internet.

I'm sorry for the confusion you have had over this, thinking I'm talking about the engine thrust effect. And I thank you for adding your insights and for engaging my questions.

[langwadt]

Why? It's a flight characteristic. All planes with low slung engines will have more pitch up force on full throttle. The MAX just has a bit more. It's not like this is some sudden onset operating region where the dynamics change hugely. What happens is that you pull on back on the stick, and at full throttle, you'll notice stick pressure needed to maintain that AoA starts to lessen and invert. If you want to keep that AoA and full throttle, you need to push down, sometimes really hard. That's quite intuitive. The auto trim exists to help the aircraft maintain control input without physically exhausting the pilot. If you don't have trim, then you have to sit there and haul back super hard on the stick whenever you want to climb or descend. MCAS is just a special case of that for a specific situation.

Obi, you did not understand what I was talking about.

1. I am aware that the MAX induces more nose up under throttle. And they way I have read it is not "a bit more," it's actually "quite a bit" more. And yet, this is not what I'm referring to in my last couple of posts between us.

2. There is an additional aerodynamic problem that is caused by the larger engines. The plane was designed to be aerodynamically efficient and stable with the original engines. As AOA increases, the belly of the plane starts to increasingly get exposed to the oncoming air. And these larger engines, which are placed farther forward, are now getting hit at an angle. They are no longer just thrusting the plane. They cause lift and drag simply by their aerodynamic shape. But due to their farther forward position, they are not lifting at the Center of Mass of the plane. They provide a twisting force, causing the nose to pull up.

3. In fact, the aerodynamic lift from the engines is acting on the plane through the wings. So they are increasing the torque on the wings and might induce a small fraction of an inch more than the normal twist to the wings, increasing the wings' AOA. This is certainly true (as an average, and to an engineer) on some level. Everything has flex. Whether it is significant or not, who knows.

4. Unlike the nose up effect that is caused by throttle, this nose up effect induced by the aerodynamics is not significant at lower AOA. But it increases ever more at higher AOA. This is what makes high AOA unstable on the MAX. The engine effect would be more static and controllable, as you say, by the pilot, simply by adding trim based on thrust.

5. MCAS does not adjust the trim when you change throttle. It reacts only to AOA. It therefore is not strictly there to counterbalance the effect of the throttle.

I state all these things like facts, but it is just what has been spread in the internet news. And I may be inferring some of it incorrectly.

I'm sorry for the confusion you have had over this, thinking I'm talking about the engine thrust effect.

a "positive feedback" from the bigger engine nacelles at high AOA is also my understanding of the issue

so the MCAS is there to prevent something simliar this:

https://youtu.be/g8XxQkXCmsU?t=11s

https://youtu.be/e21ZjwZGjiQ?t=25s

[KL27x]

First link, i give a 9. But, wow, that second link is a perfect 10. Height, distance, and oodles of style.

[dcac]

LIVE: Senate airline safety hearing

[PartialDischarge]

LIVE: Senate airline safety hearing

The amount of lies in those hearings is off the charts even when measured in dB

[floobydust]

When you have a bad sensor, both the flight control software and the pilot are being lied to and make mistakes. Never mind Boeing's shit MCAS disaster.

I would add AI, something that models the aircraft using all available sensor data BUT only makes a recommendation to the pilot. All the machine intelligence in the world, but no overriding the pilot's ultimate authority. That just leaves crashes attributable to human error, but reduced due to the AI offering a suggestion.

This is highly relevant to each of us, soon there will be self-driving vehicles for all.

As an example with a self-driving vehicle, do you risk roll-over turning sharp OR hit the tree? Traction is unknown, ice, rain etc.
"Envelope protection" would not let the steering wheel get cranked to avoid the tree, because the prediction is vehicle roll-over. So you hit the tree.

I don't see either philosophy being so great.

[Obi_Kwiet]

Why? It's a flight characteristic. All planes with low slung engines will have more pitch up force on full throttle. The MAX just has a bit more. It's not like this is some sudden onset operating region where the dynamics change hugely. What happens is that you pull on back on the stick, and at full throttle, you'll notice stick pressure needed to maintain that AoA starts to lessen and invert. If you want to keep that AoA and full throttle, you need to push down, sometimes really hard. That's quite intuitive. The auto trim exists to help the aircraft maintain control input without physically exhausting the pilot. If you don't have trim, then you have to sit there and haul back super hard on the stick whenever you want to climb or descend. MCAS is just a special case of that for a specific situation.

Obi, you did not understand what I was talking about.

1. I am aware that the MAX induces more nose up under throttle. And they way I have read it is not "a bit more," it's actually "quite a bit" more. And yet, this is not what I'm referring to in my last couple of posts between us.

2. There is an additional aerodynamic problem that is caused by the larger engines. The plane was designed to be aerodynamically efficient and stable with the original engines. As AOA increases, the belly of the plane starts to increasingly get exposed to the oncoming air. And these larger engines, which are placed farther forward, are now getting hit at an angle. They are no longer just thrusting the plane. They cause lift and drag simply by their aerodynamic shape. But due to their farther forward position, they are not lifting at the Center of Mass of the plane. They provide a twisting force, causing the nose to pull up.

3. In fact, the aerodynamic lift from the engines is acting on the plane through the wings. So they are increasing the torque on the wings and might induce a small fraction of an inch more than the normal twist to the wings, increasing the wings' AOA. This is certainly true (as an average, and to an engineer) on some level. Everything has flex. Whether it is significant or not, who knows.

4. Unlike the nose up effect that is caused by throttle, this nose up effect induced by the aerodynamics is not significant at lower AOA. But it increases ever more at higher AOA. This is what makes high AOA unstable on the MAX. The engine effect would be more static and controllable, as you say, by the pilot, simply by adding trim based on thrust.

4b. Whenever you have some effect (aerodynamic lift and drag of the engine) which causes something else (nose up of the plane) that increases its own effect (the aerodynamic lift and drag of the engines is now greater), this is what might be called a positive feedback loop. Unchecked, it causes some degree of instability. You can't completely negate/counter this dynamic variable with a static change (like setting trim to X). 

5. MCAS does not adjust the trim when you change throttle. It reacts only to AOA. It therefore is not strictly there to counterbalance the effect of the throttle.

I state all these things like facts, but it is just what has been spread in the internet news. And I may be inferring some of it incorrectly.

I'm sorry for the confusion you have had over this, thinking I'm talking about the engine thrust effect. And I thank you for adding your insights and engaging my questions.

 
2. From a stability perspective, the NG and the Max are apparently no different when it comes to getting into a stall. You have to be very careful on the throttle and push the nose back down on both models. The MCAS is there in case you make a mistake and get into a stall, you need some help from the trim to push the nose down and smooth out the stall. (If you keep the nose up, the stall will eventually correct itself, but it will be more violent. I doubt the engines are powerful enough to keep the nose up forever.) See this:  https://youtu.be/TlinocVHpzk?t=969

3. The Max has new wings that are designed to withstand a flight envelope that is far greater than is safe for the passengers. Structural failure shouldn't be a concern here.

4. These are all part the same effect and they are present on all models. It's just stronger after the stall has taken place on the Max due to engine thrust and location. You still have to be very careful on the throttle on the NG to avoid the stall for all of these reasons. In the NG, I think the effect was controllable using the elevators alone. The MAX needs a bit of trim in addition, and if the pilot is busy trying to fix a stall, it's better if he/she doesn't have to manually fuss with the trim at the same time. That's what the auto trim system is for. The first part of that video I linked explains it all.

4b. You say that you can't negate a positive feedback loop with a static change, but this isn't correct. If the position of the engines is what causes the problem in the first place, that's a static change to the aerodynamics of the aircraft. If a static change can cause a feedback loop, it can stop it. But we aren't trying to make the aircraft stable, we just want to make it possible for the pilot to level the aircraft out with the control yoke.

5. MCAS only activates what certain AoA, throttle, flaps, and I think airspeed conditions are met.

So, yes, there is a small operating region where the angle of attack will increase until the aircraft stalls. This is present on both model of the 737. My point isn't that this isn't true, but that it's not important. As I said before, all aircraft have unstable regions of of operation. Instability is only an issue for two reasons. One, the instability may such rapid control responses that the pilot cannot keep up with them. An extremely unstable aircraft might exceed the ability of the control surfaces to respond quickly enough to stabilize the aircraft, making it impossible for even a computer to fly it. Two, even if a person can keep up with the instability, it may be tedious to do for a long period of time. The fact that the 737 Max has an unstable operating region simply doesn't matter, and it's not the issue we are discussing.

a "positive feedback" from the bigger engine nacelles at high AOA is also my understanding of the issue

so the MCAS is there to prevent something simliar this:

https://youtu.be/g8XxQkXCmsU?t=11s

https://youtu.be/e21ZjwZGjiQ?t=25s

No, I don't think so. The engines would have to be absurdly powerful to do that. Once the AoA gets too high, the airspeed will drop, and the airflow won't be enough to sustain the AoA. You'll just end up with a much worse stall. 

[tooki]

If I got this right, after al, this boils down to a philosophical question:
Should the final decision be left to a human, or to a machine?

Yep, that's a burning question, like the stupid trolley question in the philosophy 101 classes.  I know the industry already commited to 'give the final decision to the machine', but if you ask me, this is a wrong approach no matter the statistics.

But the aircraft industry (as a whole, at least) didn’t do that. In the 737 MAX, like in all 737s, there is a switch right on the center console to disable the motor for stabilizer trim, the motor used by the thumb controls on the yoke, the autopilot, and in the MAX, MCAS. Flip the switch to “disable” and the pilots are in full manual (fully mechanical, by cable) control of stabilizer trim, by turning the stabilizer trim wheels with their pop-out cranks. (It’s puzzling that the pilots in these two flights didn’t do this, since it’s standard procedure when the stabilizer trim is acting up, even if you don’t know why it’s acting up.)

Generally speaking, Boeing leans more towards “give humans final authority” than Airbus does, but the differences between the two camps are fairly small, all things considered. (The vast majority of automated systems on an Airbus can be overridden, and AFAIK every automated system on a Boeing can.)

[tooki]

I would add AI, something that models the aircraft using all available sensor data BUT only makes a recommendation to the pilot. All the machine intelligence in the world, but no overriding the pilot's ultimate authority. That just leaves crashes attributable to human error, but reduced due to the AI offering a suggestion.

That’s pretty much what all the safety systems already do. There’s always (or, in some Airbuses, almost always) a way to override things.

People don’t seem to understand that cockpit automation isn’t all-or-nothing, and that pilots don’t just spend their days twiddling their thumbs. They routinely fly manually, and the aircraft absolutely allow full manual control. It’s just that with so many things to keep an eye on, it makes sense to delegate as needed, and that’s what the various cockpit automation systems do. Want it to maintain level? Select that. Or autothrottle? Choose that. Navigation? Ok. But the pilot can take back authority over any or all of those things if and when desired. (There have been a few crashes caused by a pilot inadvertently putting some system on the wrong setting, such that they thought it was doing one thing, but was actually doing another...)

[KL27x]

2.

The MCAS is there in case you make a mistake and get into a stall, you need some help from the trim to push the nose down and smooth out the stall.

Interesting. This is what I guessed it probably did. Despite Boeing just now in a senate hearing denying it. Senator Sinema (lol) asked about the MCAS anti-stall, and was corrected by the Boeing execute that it is NOT an anti-stall. Whether that's the case in practice or just legal CY6, who knows.

But your answer is completely ignoring the aerodynamic lift/drag which has been discussed in the news.

3. This was just me being detailed. As I say, flex is present in all things. The idea was not that it was dangerous to the wings, I'm just highlighting the direction of torque would tend towards more AOA of the wings in this secondary (and probably trivial and stupid to mention) way.

4. I don't like you you said that. But I see what you mean. If when you set the trim down far enough, it also increases in drag/effect at increasing AOA roughly same amount as the engine increases in lift, then it could make this region stable. Good point.

No, I don't think so. The engines would have to be absurdly powerful to do that. Once the AoA gets too high, the airspeed will drop, and the airflow won't be enough to sustain the AoA. You'll just end up with a much worse stall. 

I think you are missing the point of this. Langwidt is highlighting the aerodynamic effect of the engines. When level, no effect. As the front of the car lifts, the lift increases. And like the engines on the MAX, this lift isn't centered under the COG, so the car noses up. Indy car needs to get together with Boeing to add MCAS* to the car, so instead of flipping it can do a Mario jump. 

*Oh irony. Boeing initially snuck MCAS onto the plane without even disclosing it to its customers. And now "MCAS" is a permanent part of my vocabulary.

[MT]

Lots of detailed info from Juan.

[langwadt]

I think you are missing the point of this. Langwidt is highlighting the aerodynamic effect of the engines. When level, no effect. As the front of the car lifts, the lift increases. And like the engines on the MAX, this lift isn't centered under the COG, so the car noses up. Indy car needs to get together with Boeing to add MCAS* to the car, so instead of flipping it can do a Mario jump. 

those were Le Man cars, they now have big holes in the fenders bleeding the air under that car so bottom doesn't create lift

NASCARS has a system of flaps on the roof of car that pops up if the car goes sideways or backwards spoiling the lift that previous tended to flip the cars, mechanical MCAS if you will