How Safe is the Boeing 737 Max's MCAS System?

[cyboman]

https://www.seattletimes.com/business/boeing-aerospace/failed-certification-faa-missed-safety-issues-in-the-737-max-system-implicated-in-the-lion-air-crash/

And this opening paragraph:

As Boeing hustled in 2015 to catch up to Airbus and certify its new 737 MAX, Federal Aviation Administration (FAA) managers pushed the agency’s safety engineers to delegate safety assessments to Boeing itself, and to speedily approve the resulting analysis.

I mean, seriously? That's the whole point of the independence of the FAA. That's like asking VW to do their own emissions certification...wait...

Adding that it is “unable to comment … because of the ongoing investigation” into the crashes, Boeing did not respond directly to the detailed description of the flaws in MCAS certification, beyond saying that “there are some significant mischaracterizations.”

Euphemism kung fu there.

“There was constant pressure to re-evaluate our initial decisions,” the former engineer said. “And even after we had reassessed it … there was continued discussion by management about delegating even more items down to the Boeing Company.”

Boeing was obviously exerting the pressure. If they need to expedite certification for competitive reasons, why not give the FAA extra budget dollars for that project so they can hire or redirect more safety engineers, accelerating the process as much as possible without compromising safety?

The original Boeing document provided to the FAA included a description specifying a limit to how much the system could move the horizontal tail — a limit of 0.6 degrees, out of a physical maximum of just less than 5 degrees of nose-down movement.

So the actual stab limit of 2.5 degrees would give a physical maximum of 20.8 degrees of nose down movement (assuming they are directly proportional)? That seems like a pretty insane amount of potential pitch attitude change for a system that's supposed to be so invisible it's not worth mentioning. And, it can keep resetting and essentially max out the nose down trim of the stab. Is that origin of the name Max? j/j

I wonder what the maximum nose down trim angle of the stab is. I looked but couldn't find it. Thought it would be here: http://www.b737.org.uk/techspecsdetailed.htm but couldn't find it. One interesting thing you can see is that the stab in the Max is unchanged from the previous NG model. Maybe a redesign of the stab would of been worth the cost, as opposed to the MCAS software which mysteriously changes the control surface behind the scenes.

Like all 737s, the MAX actually has two of the sensors, one on each side of the fuselage near the cockpit. But the MCAS was designed to take a reading from only one of them.

Sheesh, they had the sensor there and didn't even use it? I say use 6, three on each side like on the big boys. Seems like a pretty important sensor.

 

[cyboman]

BTW, I'm currently learning to fly in a plane without a functioning stall horn. This discussion has me questioning whether that's wise...

I vote not-wise. Can't be too careful.

 

[cyboman]

Pilot / science writer they had on PBS had a pretty good summation of the Max situation:

 

[russ_watters]

I've been asserting basically what you've said here from the beginning. I feel like there is a bit more agreement now however. The crux of our disagreement as I see it was me seeing MCAS as effecting attitude and not just effecting yoke force. So when you say "yoke force is the effect", I also see a change in pitch force from the change in trim an effect, that has an effect on pitch, that's where I think we differ in our viewpoint of the system. I was arguing against the idea that it only applies force feedback and it's only the pilot which effects pitch attitude. I've been saying MCAS effects attitude by changing the aerodynamics a lot, that's sort of central. There were times when I was refuted, by saying "thinking of it changing aerodynamics is not the right way".

I admit my wording, "MCAS adds negative pitch attitude" is confusing and imperfect - that's how I was visualizing it. I don't mean it always has the effect of causing a negative pitch attitude, as in below the horizon (I think I stated that somewhere), but instead that it adds negative pitch (it does that by creating a pitch down moment (thx @PeterDonis)). So the moments created by yoke input, the moment of the stab (commanded by MCAS), and all the other moments in the equation in sum create the result force that determines pitch attitude of the plane.

I don't think we're quite there yet. You're still focused on the position and not the force. The pilot doesn't know what position the yoke is in, only what force he's applying, and when a change in trim happens, the yoke moves involuntarily in response because physiologically humans can't hold a position very well, only a force.

It is probably worth considering that for the plane I'm flying and any plane with trim tabs, if moving the tab were directly affecting the plane's attitude without a change in elevator position, the trim tab would move the nose in the wrong direction. The trim tab is there to apply aerodynamic force to the elevator. When the tab tilts up, it pushes the elevator down. If the elevator didn't move down as soon as the tab tilted up, the nose of the plane would rise instead of dropping.

That said, I'm not sure if the action of the trim in the 737 is direct or reversed, but either way it doesn't matter because it instantly changes the force and the elevator and yoke moves in response.

This point was really the necleus of our disagreement the way I see it. It could be seen as unimportant I suppose. I guess the importance of the point is it stems from seeing the system as not ultimately having a direct effect on attitude. It seems to suggest MCAS doesn't have any pitch authority. Actually it has more than the pilot using only the yoke since it's moving the entire horizontal stab. That's likely why in the failures the pilots could not overcome the nose down pitch force from the stab.

Well, here you're mixing the normal and the failure scenarios, but also you are making some assumptions about the mechanics of the motions. The stabilizer and elevator can move together, separately, by the same amounts, by different amounts, however the designer chooses to link them. In a Cessna, when you move the elevator the trim tab moves with it, but not as far. But when you move the trim tab, the elevator doesn't move (mechanically speaking). I don't know how they are linked in the 737, but either way, in normal operation the MCAS won't overpower the pilot because that isn't its job.

Put that way, I think they are almost the same thing. Your wording, or the latter, again seems to suggest MCAS has no pitch authority.

You really think "harder" and "physically impossible" are the same thing? The difference is that with "harder" the plane falls out of the sky and with "physically impossible" it doesn't. That's a big difference!

In any case, could you please explain why you think any of this is important.

 

[cyboman]

In any case, could you please explain why you think any of this is important.

The post you just responded to of mine I think pretty clearly explains that. To summarize it quickly: Your perspective implies MCAS has no pitch authority, which I don't agree with.

 

[cyboman]

You really think "harder" and "physically impossible" are the same thing? The difference is that with "harder" the plane falls out of the sky and with "physically impossible" it doesn't. That's a big difference!

I'm referring to this statement you made: "So again, the difference between "trying to get the nose down" and "helping you avoid letting the nose rise too high" is not very critical."

 

[russ_watters]

The post you just responded to of mine I think pretty clearly explains that. To summarize it quickly: Your perspective implies MCAS has no pitch authority, which I don't agree with.

I get that that's your point, but that doesn't explain why you think it's important. What useful thing does it tell us about the crashes?

Looking back at some of the posts I missed over the weekend, evidently @PeterDonis had a virtually identical discussion with you. At this point, since it seems like just semantics with no value - nothing to tell us about the crashes, I don't see a need to continue it.

 

[cyboman]

I get that that's your point, but that doesn't explain why you think it's important. What useful thing does it tell us about the crashes?

Looking back at some of the posts I missed over the weekend, evidently @PeterDonis had a virtually identical discussion with you. At this point, since it seems like just semantics with no value - nothing to tell us about the crashes, I don't see a need to continue it.

Like I said, "It could be seen as unimportant I suppose. I guess the importance of the point is it stems from seeing the system as not ultimately having a direct effect on attitude. It seems to suggest MCAS doesn't have any pitch authority."
If you're asking why I think it's important to see MCAS that way, I guess the answer would be, because to see it in a different way wouldn't be accurate and would serve to inaccurately represent how MCAS operates.

I agree there's no point discussing it further, I'm comfortable with the points I made before this, I was just originally replying to your post, it wasn't my intent to revive the disagreement. It was already established you and Peter saw it the same way. I just thought it respectful to respond to your post since you took the time to write it.

I have to add when Boeing states: "...a limit of 0.6 degrees, out of a physical maximum of just less than 5 degrees of nose-down movement." It seems odd to still see the MCAS as not moving the nose down or effecting pitch.

 

[PeterDonis]

I'm not sure if the action of the trim in the 737 is direct or reversed

It seems pretty clear to me from what I've seen that the trim function on the 737 changes the angle of the entire horizontal stabilizer, which changes its default angle of attack ("default" meaning "if the elevators are in the neutral position") and therefore the default amount of lift force it exerts at the tail end of the plane. So it would be a different method of working from trim tabs.

 

[cyboman]

It seems pretty clear to me from what I've seen that the trim function on the 737 changes the angle of the entire horizontal stabilizer, which changes its default angle of attack ("default" meaning "if the elevators are in the neutral position") and therefore the default amount of lift force it exerts at the tail end of the plane. So it would be a different method of working from trim tabs.

Would it be erroneous to think of trim tabs as basically small rudders on the elevators? So in the Cessna example, the Cessna elevators would be more akin to the stab on the 737, and the Cessna trim tabs more akin to the 737's elevators?

EDIT: I guess that's imperfect since looking at the Cessna it still has a horizontal stabilizer too, which doesn't rotate.

 

[russ_watters]

It seems pretty clear to me from what I've seen that the trim function on the 737 changes the angle of the entire horizontal stabilizer, which changes its default angle of attack ("default" meaning "if the elevators are in the neutral position") and therefore the default amount of lift force it exerts at the tail end of the plane. So it would be a different method of working from trim tabs.

I do think it's direct acting, but I can envision a way it might not be: if adjusting the trim control moved both the stabilizer and the elevator, in opposite directions. Or if the trim adjustment just moves the stabilizer and the pilot responds by moving the elevator.

The advantage of moving both would be that the control column wouldn't need to move in response to a trim change. The pilot would still move it unconsciously, and then move it back, but back to the same position. Otherwise, every time you changed the trim, the neutral position of the yoke would be in a different place. Maybe that doesn't matter much because the neutral position isn't marked anywhere, but I don't know.

It's also worth noting that trim and feel for the pilots can be whatever the designers want; the control surfaces are actually moved hydraulically and the feel for the pilots doesn't need to match what the hydraulic cylinders are applying.

 

[russ_watters]

Would it be erroneous to think of trim tabs as basically small rudders on the elevators? So in the Cessna example, the Cessna elevators would be more akin to the stab on the 737, and the Cessna trim tabs more akin to the 737's elevators?

EDIT: I guess that's imperfect since looking at the Cessna it still has a horizontal stabilizer too, which doesn't rotate.

It's a similar idea. What creates the force against the control column is the elevator not being aligned with the stabilizer - it's the wind pushing it back down. The all-moving trim aligns the stabilizer and elevator by changing the stabilizer angle whereas the trim tab changes the force directly, by giving the elevator its own lift.

 

[PeterDonis]

It's also worth noting that trim and feel for the pilots can be whatever the designers want; the control surfaces are actually moved hydraulically and the feel for the pilots doesn't need to match what the hydraulic cylinders are applying.

AFAIK the 737 already does considerable artificial feel adjustment.

 

[russ_watters]

I have to add when Boeing states: "...a limit of 0.6 degrees, out of a physical maximum of just less than 5 degrees of nose-down movement." It seems odd to still see the MCAS as not moving the nose down or effecting pitch.

Erm, those are so the pilot doesn't have to think about the aerodynamics and mechanics of the stabilizer when he's manipulating the trim. Obviously, it doesn't lower then nose only by 5 degrees either. Similarly, in my Cessna, it doesn't say down trim to raise the nose - that would be confusing!

 

[russ_watters]

AFAIK the 737 already does considerable artificial feel adjustment.

Yes. From a link I posted earlier:

Incidentally, it looks like we're describing "up" and "down" incorrectly for the stabilizer? It says "Stab LE Down = airplane nose up" Not sure what "LE" stands for...

This is one of several such graphs. One was a simple speed vs angle. Honestly, these are pretty complicated and I haven't put forth the effort to think them through.

Either way, not that important: trim adjustment is made clear to the pilot in terms of what you want the nose to do, not the direction the control surface moves (per @cyboman's last post).

[edit] I believe the above is just telling us where the neutral position is, not what the automated trim adjustment is. Here's an example of automated trim adjustment:

 

[PeterDonis]

Not sure what "LE" stands for...

I would assume "leading edge".

 

[cyboman]

Incidentally, it looks like we're describing "up" and "down" incorrectly for the stabilizer? It says "Stab LE Down = airplane nose up" Not sure what "LE" stands for...

I think what it might be is when it says "Stab LE Down = airplane nose up". It's saying when the stab is rotated downward or forward, which is "down" from the horizontal, it results in the nose up moment and vise versa. We've been referring to the stab adding nose down trim. I don't think we were saying the stabilizer moves down for nose down trim.

EDIT: *..which is "down" from the horizontal looking at the LE / leading edge (thx @PeterDonis)

Like it says here: "MCAS moves the horizontal stab trim upward..."

This is one of several such graphs. One was a simple speed vs angle. Honestly, these are pretty complicated and I haven't put forth the effort to think them through.

I was going to ask you to explain the graph actually, because I find it hard to understand.

 

[cyboman]

OK, this might be a little silly and over simplifying but I'm going to throw it out there.

If they just increased size of the stab and thereby it's trim changes would be magnified, could they perhaps just of changed the existing automated trim systems like auto speed trim and other flight software appropriately, they could of then avoided the MCAS "patch"? And eliminate the complication of over-engineering by adding a system like MCAS which introduces a host of new variables and failure modes?

EDIT: I guess it's not that silly because I've read elsewhere which I posted, that people have suggested they should of just redesigned the stab. I suppose I think I'm oversimplifying by "making it bigger" but maybe that's generally speaking, accurate?

 

[cyboman]

Me Tarzangineer, me think bigger engine, and bigger up pushing, me think need bigger back wing.

 

[cyboman]

Actually on this line of thought, even if they didn't change the stab. Why couldn't they just adjust the existing auto trim and force feedback systems to accomplish basically what MCAS does? Why is an additional system or "subroutine" or whatever needed? Sometimes it's qualified as an additional stall prevention system to deal with the engines. But why not just reconfigure the existing stall prevention systems?

 

[cyboman]

I think this largely, as is suggested in the articles, boils down to a cost assessment. Down to money. So redesigning the stab is expensive and then the clients need to worry about new parts for that and lengthy re-certification for changes to the airframe. Redesigning the existing flight systems like autotrim and elevator feel control, also requires re-certification and is also expensive. Adding a new control or augmentation system maybe falls into a different path of certification and has the lowest cost. BUT, is the FAA not considering how new systems like MCAS effects other systems and therefore their previous certifications for them is invalid? It almost seems like they may be viewing all these systems as disparate, but they actually all interact with each other, and changes to one effects all other systems and their expected performance.

 

[.Scott]

I'm not sure what Boeing's equivalent of Direct Law ...

The Airbus has several "Laws" that can be selected for control of the plane. Most fundamentally are Direct Law and Mechanical Law. They are very specific to Airbus - which normally takes pilot control input as an indication of the pilot's intention - and then moves the control surfaces with those intentions in "mind".
For other aircraft, "Direct Law" is approximately equivalent to normal manual flying.

 

[.Scott]

Not pitching down just means the airplane crashes tail first.

No. If you want to crash tail first, you need to put the center of gravity (CoG) well aft of the center of lift or execute some really wild maneuvers.
All normal aircraft are designed so that when the wings stall, the nose drops. Holding the yoke back will allow a stall to persist.
The aerobatic maneuver for inducing the plane to fly backwards is called a tail slide. I have piloted two different aerobatic aircraft (Citabria and Decathalon), but neither of them was certified for tail slides.

 

[.Scott]

I don't understand is how stall detection can be based only on angle of attack. A low speed and low angle of attack can still result in a stall. A high speed steep climb can be perfectly ok.

AM

There is only one cause for a stall - high AoA. Each aircraft has a "maneuvering speed", which is the fastest that the plane can go without being subject to damage by aggressive pilot inputs (primarily pitch up/down). Manuevering speed is not "slow" - though it is less than a normal operating speed. If you pull the nose of the plane up in straight and level flight at maneuvering speed, you will enter a stall. If you hold that stall for a moment and then slam on rudder (left or right), you will fully stall one wing (the one turned back) while generating maximum lift from the other. That's called a snap roll.

At very low speed (below the minimum controllable airspeed) with low angle of attack means that you are not generating enough lift to offset your weight - so you will be accelerating downwards. Perhaps you are following a parabolic trajectory.

 

[.Scott]

It is probably worth considering that for the plane I'm flying and any plane with trim tabs, if moving the tab were directly affecting the plane's attitude without a change in elevator position, the trim tab would move the nose in the wrong direction. The trim tab is there to apply aerodynamic force to the elevator. When the tab tilts up, it pushes the elevator down. If the elevator didn't move down as soon as the tab tilted up, the nose of the plane would rise instead of dropping.

That said, I'm not sure if the action of the trim in the 737 is direct or reversed, but either way it doesn't matter because it instantly changes the force and the elevator and yoke moves in response.

The action of the 737 trim is dramatically direct.

There may be some confusion here because the "trim" on a jumbo jet operates differently than the trim on most small planes. In the case of most small planes, the pitch force adjustment is not controlled by a surface as large as the horizontal stabilizer. In fact, for most sail planes - trim is controlled by the positioning a tie point of a long spring.

 

[.Scott]

This says "Using electric pitch trim will only pause MCAS, to deactivate it you need to switch off the STAB TRIM SUTOUT switches." This seems to indicate that the cutout switch the pilot comment we were discussing referred to does disable MCAS.

Right.
This is part of the problem. If you use the electric pitch trim - the MCAS (which also uses the electric pitch trim) will back off - but only for a short time (less than half a minute). Then it will turn the trim back to what it thinks is correct.
So you need to use the electric trim to put it where you want - then quickly kill it with cutout switch.

If you use the cutout, you're stuck with the manual wheel - which may operate too slowly to save the aircraft. Basically, this is a device in the cockpit that directly turns the jack screw that positions the horizontal stabilizer. So it take many turns to get just a little stabilizer motion.

The horizontal stabilizer are those small "wings" in the back. The elevators are along the trailing edge of the stabilizers. When the stabilizers are out of position, much more elevators deflection is required - and this mean more pitch up or down force by the pilot.

 

[.Scott]

Here is a pprune post from "Aloha_KSA" that goes a long way to understanding the problem presented to the pilots:
https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-102.html#post10423122

As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.

But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)

My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.

Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"

(1) Just after takeoff there is a lot going on with trim, power, configuration changes, and as noted above, the darn speed trim is always moving that trim wheel in seemingly random directions to the point that experienced NG pilots would treat its movement as background noise and normal ops. Movement of the trim wheel in awkward amounts and directions would not immediately trigger a memory item response of disconnecting the servos. No way.

(2) The pilots could very reasonably not have noticed the stab trim movement. Movement of the stab trim on the 737 is indicated by very loud clacking as the wheel rotates. On the -200 it was almost shockingly loud. On the NG, much less so. HOWEVER, the 737 cockpit is NOISY. It's one reason I am happy to not be flying it any more. The ergonomics are ridiculous. Especially at high speeds at low altitudes. With the wind noise, they may not have heard the trim wheel moving. The only other way to know it was moving would be yoke feel and to actually look at the trim setting on the center pedestal, which requires looking down and away from the windows and the instruments in a 'leans'-inducing head move. On the 717, for example, Ms. Douglas chimes in with an audible "Stablizer Motion" warning. There is no such indication on the 737.

(3) The fact that they were at high power and high speed tells me the stick shaker was activated. With that massive vibrator between your legs, alternating blue sky and brown out the window, your eye balls bouncing up and down in their sockets as the plane lurches up and down in positive and negative G's, it would have been a miracle if one of the pilots calmly reached down, flicked off the stab servo cutout switches, folded out the trim handle, and started grinding the wheel in the direction of normalcy. These pilots said over the radio that they had "unreliable airspeed". So they did not even know which instruments to rest their eyes on for reliable info. Their eyes were all over the cockpit looking for reliable info, the plane is all over the place like a wild boar in a blanket not behaving in any rational way. And the flying pilot may have been using the tiny standby IFDS for airspeed and attitude. Ouch.

Finally, runaway stab trim is a very, very rare occurence up until now. We trained it about once every other year in the sim because it is so rare. And when we did it was obvious. The nose was getting steadily heavier or steadily lighter with continuous movement of the trim wheel. That is a VERY different scenario than what these pilots faced.

We also trained for jammed stabilizer, the remedy for which is overcoming it with force. The information they were faced with could very reasonably have been interpreted that way, too.

An URGENT AD from the FAA/ Boeing after Lion Air would have helped get it back to the front of the pilot's minds for sure. Extra training by the airline or an urgent pilot memo would have helped. Maybe one was issued, we don't know yet.

A better question might be: given this nose down attitude, high speed, and fully nose down or almost fully nose down stab, how much altitude would they have had to have to be able to recover. I'm thinking at least 10000 feet to recognize the problem, disconnect the switches, fold out the handle and start frantically winding the stab back to normalcy while the flying pilot tries to gain control via the elevator. It's entirely possible that this scenario, if not recognized early on, is unrecoverable at any altitude.

 

[.Scott]

Regarding the attitude indicator:

Yes, I imagine that is a very mission critical instrument to a pilot. Not many failure modes for that sucker I imagine.

The "artificial horizon", that black and blue ball in the middle of the control panel is as important as its real estate implies. It can fail when the power system (electric or vacuum) that its depends on fails - which is why there are often one of each in the cockpit.

https://www.boldmethod.com/learn-to...your-only-attitude-indicator-in-imc-now-what/

Other instruments can be used to compensate when there is no functional attitude indicator - and these procedures are practiced as part of instrument pilot training.

 

[pyroartist]

Does this plane have only one AOA sensor? If so, that is a problem right there. Everything that is flight critical should have redundancy.
(I am a pilot.)

 

[.Scott]

Does this plane have only one AOA sensor? If so, that is a problem right there. Everything that is flight critical should have redundancy.
(I am a pilot.)

According to a Boeing site, it has access to both AoA sensors, but only uses one at a time. Alternating from one to the other from flight to flight.
http://www.b737.org.uk/mcas.htm

 

[cyboman]

Does this plane have only one AOA sensor? If so, that is a problem right there. Everything that is flight critical should have redundancy.
(I am a pilot.)

It has two, but MCAS only reads one. I've read most airliners use 3 and the really big ones use 6. I think it's fairly well established that there is some big problems with the MCAS, not just the sensor. In a nutshell being, how much the system can adjust the stab (see the latest Seattle times article linked in the thread), how often it can do that, which appears to be infinitely as it resets and can max out the nose down trim of the stab. Add poor HCI with annunciations and instrumentation to communicate what's going on. It get's worse and worse. Much is covered in this thread.

 

[cyboman]

Here is a pprune post from "Aloha_KSA" that goes a long way to understanding the problem presented to the pilots:
https://www.pprune.org/rumours-news/619272-ethiopian-airliner-down-africa-102.html#post10423122

This was a great read and funny at times. Thanks!

Neat how he was able to fly with only trim on the cargo plane. I linked earlier to the wiki for trim and how in wartime with damaged primary controls, trim was often used to control the craft.

 

[cyboman]

Regarding the attitude indicator:
The "artificial horizon", that black and blue ball in the middle of the control panel is as important as its real estate implies. It can fail when the power system (electric or vacuum) that its depends on fails - which is why there are often one of each in the cockpit.

https://www.boldmethod.com/learn-to...your-only-attitude-indicator-in-imc-now-what/

Other instruments can be used to compensate when there is no functional attitude indicator - and these procedures are practiced as part of instrument pilot training.

Interesting, I sort of want one now. I thought they were like a compass and relatively simple mechanically. That's why I thought they wouldn't have many failure modes. Then I wiki'd them and looked what they cost on ebay. Not soo much...

I looks like it failed in this case: https://en.wikipedia.org/wiki/Aeroflot_Flight_2415

 

[cyboman]

There is only one cause for a stall - high AoA. Each aircraft has a "maneuvering speed", which is the fastest that the plane can go without being subject to damage by aggressive pilot inputs (primarily pitch up/down). Manuevering speed is not "slow" - though it is less than a normal operating speed. If you pull the nose of the plane up in straight and level flight at maneuvering speed, you will enter a stall. If you hold that stall for a moment and then slam on rudder (left or right), you will fully stall one wing (the one turned back) while generating maximum lift from the other. That's called a snap roll.

At very low speed (below the minimum controllable airspeed) with low angle of attack means that you are not generating enough lift to offset your weight - so you will be accelerating downwards. Perhaps you are following a parabolic trajectory.

I had a question before which is sort of related. I still don't understand it completely: If MCAS is meant to be active in scenarios of high AoA and pitch up conditions. Would it ever be active when the plane has a negative pitch attitude. It was suggested this could occur during a dive when the pilot is pitching up to pull out. But it doesn't seem likely adding nose down trim in that scenario would be beneficial so wouldn't it be improbable MCAS would be active then?

I guess it seems counter intuitive imagining MCAS operating in negative pitch attitude scenarios since it's often described as a stall prevention system.

 

[PeterDonis]

If MCAS is meant to be active in scenarios of high AoA and pitch up conditions.

High AoA, yes. Pitch up, not necessarily. As @russ_watters reminded me in post #238, AoA is not the same as pitch attitude. AoA is the angle between the wing and the relative wind; if the plane is descending fast enough, AoA can be high even if the nose is pitched down.