The 737 Max is not an inherently unstable aircraft and to say so grossly misunderstands the term and what MCAS was doing.

MCAS existed to make an inherently stable aircraft handle in the same was as previous 737 generations. Design changes had forced the addition of this system in order to retain the same type rating because those design changes caused the aircraft to handle differently to previous generations.

MCAS was about keeping the same type rating and the same handling characteristics. It had absolutely nothing whatsoever to do with 'inherent instability' - the 737 Max is an inherently stable aircraft without MCAS.

Continuing to spread misinformation about what MCAS is and the 737 Max stability is detrimental to the discussion.

EDIT - There is clearly a lot of misunderstand about the terms 'stable' and 'unstable'. These has a defined meaning in aircraft handling. I encourage you to educate yourselves on the term, on what stability actually is in relation to this discussion.

https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...

The 737 Max is a stable aircraft. The 737 Max without MCAS activated is a stable aircraft. It is an inherently stable design.

The 737 Max can and was flown without MCAS activated (even by the same aircraft as crashed, the pilots of the previous flight pulled the stabiliser cut out switches and went to manual trim control).

It is a stable aircraft.

Boeing cut corners for costs and the FAA allowed them, in large part because they don't have the people power to do a lot of this testing. People died and faith has been lost in the US's oversight of airlines. This is a clear problem.

Most airliners now flying do have additional augmentation of their handling for a variety of reasons. Earlier versions of the 737 already had the elevator feel and centering unit and mach trim, but they are much less aggressive than MCAS, and, most importantly, their presence and full scope of effect was not obscured in order to avoid the cost of training pilots for their failure modes. Furthermore, in other cases where handling and stability augmentation is powerful, adequate redundancy is provided (even the 737 feel and centering unit has more redundancy than MCAS.) The presence of handling augmentation devices on other airliners does not justify the way Boeing introduced MCAS.

[1] https://leehamnews.com/2018/11/30/bjorns-corner-pitch-stabil...

Reminds me of the old futurama quote- technically correct is the best kind of correct.

That is, if the pilot is holding the airplane with a constant amount of force and the airplane is climbing at a certain angle, some turbulence or other disturbance could cause the nose to pitch up, and then continue pitching up?

It’s still aerodynamically stable (not neutral or divergent).

Meaning if the plane is under increased power and continues to pitch up to the point of stalling, is that an indicator of a negative stability regime?

The 737-Max does pitch up with increased thrust (as does any jet airplane with a low center of thrust), but it still reaches a stable pitch.

The problem in hand flight is that the controls on the Max don't get "progressively stiffer" in a linear fashion with increasing pitch. They still require positive force, but the force required to pull 2.0 G is less than double the force required to pull 1.5 G (as an example; I don't know the particulars of the Max stick force gradient curve).

I have not read anything to suggest that a normally trimmed, MCAS-disabled Max would pitch itself into a stall from thrust application. That would be bad, but is also almost surely far from the actual situation.

I can't vouch for this. It's possible I misunderstood it; it's possible the source was misinformed, though I seem to recall it was written by a pilot. But I have trouble imagining what lesser problem would have required such a heavy hand as MCAS to fix. If it were really just a matter of the controls going light, surely that could have been cured with motors that were not so powerful as to be almost impossible to overcome.

?? Most aircraft will do this. Even docile trainers.

https://www.youtube.com/watch?v=DZfOtvjJR-I [edit: watch this instead https://youtu.be/6RtVdmsx4qU?t=71 ]

There's been a couple 747 crashes due to this exact scenario. Or regarding the 737 in particular, to quote "Mike734" in 2007:

> In the B-737 too much nose up trim can make a go-around very exciting. The under wing engines create a very large pitch up when adding full power for a go-around. The pilot has to really push hard to stop the jet from pitching too far nose up.

and "Cac737":

> Now in the Boeings for example, B737 and bigger with underslung engines, this situation is aggravated even more because when you push the power levers forward for go-around thrust, due to where they are and their thrust lines, that act alone will cause the nose to rise very noticeably, and if you are light you can actually find yourself pushing forward on the control column on a go around. now trim "back" as you say and forgetting you did so, can find yourself in a very nosehigh attitude if not careful.

https://www.airlinepilotforums.com/hangar-talk/17446-trim-la...

That might be, but the video doesn't demonstrate it - the pilot flying pulls the elevator back quite a bit to induce the stall after he puts in full power.

Unfortunately there's two videos I found of the trim stall demonstration on youtube-- this one which shows most of it with a full cockpit view, and one which I think is a bit better technically but has the camera shaking everywhere.

See FAA AFM 4-12 for official training materials. https://www.faa.gov/regulations_policies/handbooks_manuals/a...

https://youtu.be/6RtVdmsx4qU?t=71

The plane goes way above the normal climb attitude. He doesn't take it to the full stall, but has to push heavily on the control column to arrest it-- he never pulls.

(Really, it's pretty astounding how hard you have to push, and how high the nose rises anyways-- even if you weren't fully trimmed for glide, and even in mild-handling airplanes... I tend to fully trim for whatever flight condition I'm in, --except glide-- and then am holding light backpressure just to control this).

https://www.law.cornell.edu/cfr/text/14/25.175

The requirements have the form of "In configuration <XYZ>, the stick force curve must have a stable slope at all speeds within a range which is the greater of <range definition> above and below the trim speed."

It is this linear stick force curve requirement that was failing that MCAS was implemented to address. The airplane is still aerodynamically stable.

Moving the engines backwards or forward doesn't affect thrust related pitch moment.

It didn't vary with engine thrust, but only with airspeed at a high attack angle.

The author isn’t a recreational pilot: https://en.m.wikipedia.org/wiki/Pilot_certification_in_the_U...

It wouldn’t matter if Chuck Yeager wrote the article; people have their minds made up. It’s either “Boeing should die because ‘big US corporation’ or “The crash pilots were a victim of deficient, assembly-line third world airline training.”

The answer is more nuanced and in the middle of those extremes.

https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...

Both things can be true: 1) Many pilots are trained in assembly-ling training 2) Boing did not do adequate testing.

Often discorse seems to follow the lines of bracket tournaments: only one thing can be true.

https://leehamnews.com/2019/02/15/bjorns-corner-pitch-stabil...

https://seekingalpha.com/article/4286602-boeing-737-max-misc...

> If test pilots from the OEM and subsequently from the FAA/EASA consider the feel in pitch for a part of the envelope above a certain rating, the aircraft will not pass Certification. It needs augmentation for this part of the flight envelope.

> The aircraft OEM is asked to propose fixes to the pitch characteristics of the aircraft so it can be judged acceptable for use by a minimum standard (Re. training and proficiency) pilot. This was the reason behind the MCAS pitch argumentation for the 737 MAX.

> The 737 can be flown just fine without MCAS turned on as it has absolutely nothing to do with making the aircraft stable.

which suggests that MCAS would not have been necessary for certification, if the MAX was being certified as a completely new aircraft type.

To be said to be stable, and certifiable as a Civil Transport Aircraft, an airframe must pass all relevant prescriptive tests as specified in the FAR's.

FAR 25.173(c) is not demonstrated without MCAS, and therefore in regulatory parlance, not necessarily aerodynamic parlance, the aircraft is unstable by virtue of failing to demonstrate longitudinal stability as specifically prescribed in the FARs.

The fact that MCAS was added to minimize the risk of losing grandfathered type cert, and the terrible engineering that went into it is just icing on the cake.

The specific behavioral failure occurs during a wind-up turn. If you look up the procedure for the test, one of the independent variables is trim setting. That you have to adjust the trim (an input variable) during the maneuver in order to pass the test (preservation of control stick force response curve characteristics) kind of invalidates the entire endeavor. Never mind that the mechanism you leveraged to pass that corner case can doom the plane in case of false positive activation.

So yes. It may be aerodynamically stable in 98% of the operating envelope, but in order to eke out that last 2 percent, caution was thrown to the winds, and implemented a kludge that was never safely integrated.

Note I'm not arguing against the point you're trying to make. Just pointing out where the confusion in terms is likely arising in translation from technical to layman.

The layman only cares that the regulatory definition is hit, as the regulation is the compromise point between aviation and everyone else's interest in not having craters.

You have correctly identified the true source of much 'professional' outrage at what Boeing has done here. The MCAS system was, at its core, about saving money.

Boeing thought it could retain the same type rating with an automated system that would make the aircraft fly the same.

It is an almost unforgivable and detestable course of action.

The way Boeing went about it is foolish, short-sighted, exceptionally dangerous, and very poor engineering.

Unfortunately, accounting methods and markets don't distinguish between these, especially where benefits (or profits) are immediately evident, but costs are occult and obfuscated. This is a situation which appears frequently, repeatedly, and notoriously in technological contexts. I'm seeing it increasingly as a fundamental conflict and paradox of market economics.

The issue can work in reverse as well: measures (practices, products, technologies, services) with immediately evident costs, but distant, obscure, or covert benefits, can be a very hard sell. Maintenance. Employee training. CO2 mitigation.

SpaceX's (almost) entire existence is centered on a cost cutting measure: reusing rockets. But they didn't approach it in a way that killed a bunch of people.

If you look at the history of flight, the biggest periods of progress killed people left and right. I'm not sure space can't go forward without doing the same.

1. Yes, the 737 Max is stable in the conventional aerodynamic sense.

2. No, MCAS was not introduced only to make it handle like the previous 737 generation. Without MCAS, the 737 Max would not fulfil certification criteria related to stick forces at high angle of attack.

(I've never figured out whether the 737 Max stick force is sub linear, but monotonically increasing at high AoA, or decreasing.)

If I take a Cessna Turbo 206 in the landing configuration and apply full power with no other input, you can stall or even spin the airplane. Critical factors include the position of the center of gravity.

The point is that you might be misunderstanding one of the critical rules of aviation — a stall is independent of speed or power, but entirely dependent on the exceeding the critical angle of attack. A stall can occur at any speed and any power setting.

Air France Flight 447 is a perfect example of this. They pulled back on the stick so much the AoA reached 40 degrees. The wings lost lift and the aircraft fell like a rock tail-first despite the engines running at full throttle. "But I've been at maximum nose-up for a while!"

https://en.wikipedia.org/wiki/Air_France_Flight_447

This really illustrated how poor the articles about this have been. the 737 MAX was not violating the "most basic rules of aviation". We keep hearing the design is unstable. But no one can tell us where in the flight envelope it is unstable.

It's not "just to retain the same type rating" that the MCAS worked as it worked.

The MCAS, as it was implemented in the planes which crashed had to be implemented for the plane to pass the certification requirements which exist because of the safety.

The plane is with its geometry inherently unsafe without some clever devices correcting the "inherent" behavior.

Anybody who claims it's not "unstable" does misdirection of clinging to the technicalities of wording of the specific claim (as the plane can be technically-speaking "stable", if nothing unexpected happens, while still being the death trap under non-average conditions, and that's what happened here), as there are more elephants in the room remaining, to be specific:

- The regulation exists to provide safety.

- Boeing 737 MAX is unsafe to fly without some additional hardware and software which does the compensation. Nobody can sell or certify the Boeing 737 MAX as it is designed without adding some hardware and software that would do something similar to MCAS. "Just turning off MCAS" permanently or "just removing it" makes the plane non certifiable safety-wise, not only losing its "type rating."

- MCAS design (its power) wasn't decided only once and implemented. The set of events was that only during the test flights the additional problems with the safety of the plane's response to the commands were detected, and the MCAS revisions added more power to MCAS (and less to the pilots) through the time. The result was that the MCAS which exists on all the planes sold (and the planes crashed) is not the MCAS from the initial specifications or claimed by Boeing after the crashes, as they still quoted the original characteristics.

- The goal of Boeing of "keeping the same type rating" is the motivation why they decided to lie about what they implemented and why they actively reduced the safety of the plane to the point of plane actively being the death trap.

- Specifically, Boeing misused the trust it had: through the years Boeing was allowed to do more and more of the certification work for their own planes.

- Not to mention disastrous public claims there's nothing wrong with their planes even after the two crashes, a lot of information already public and most of the world already having grounded the planes.

- Not only Boeing lied about what they did to the FAA and the rest of the world, even after the first crash happened, but while making the plane the engineering decisions have been made more than once just with the goal of preserving the false promises made by the Boeing managers as they have sold the planes: that's why there was the change in the behavior of the switches where there were separate switches before but they changed two switches to behave identically, only to actively hide what the new MCAS was doing. That's why the system was made glaringly non-redundant: adding redundancy would make it obvious that the system was (and it actually was) critical.

In short, it's not misinformation that 737 MAX was a death trap made as such under the guidance of Boeing's managers.

There are many levels of evilness Boeing managed to achieve in designing the plane and responding to the existing crashes. Some managers should be prosecuted for what they did. If that doesn't happen, the laws should be changed to allow that kind of responsibility the next time, or there will be even more victims.

No it isn't. It is a stable design. To claim otherwise misunderstands the term.

The 737 can be flown just fine without MCAS turned on as it has absolutely nothing to do with making the aircraft stable.

Many people seem to want to shit on Boeing for making an 'unstable' aircraft. They simply did not make an unstable aircraft. They made an inherently stable aircraft that handled a little bit differently and then to save money (in order to prevent the need for a new type rating and pilot retraining) they used MCAS to make it handle like previous generations.

To repeat, the 737 Max flies just fine without MCAS (indeed previous flights of one of the downed aircraft pulled the stabiliser cut out switches and flew just fine) and MCAS was about handling, keeping the same type rating and saving money.

https://news.ycombinator.com/item?id=21249017

No. Without the MCAS (but implemented correctly with the redundance etc) it behaves dangerously wrong in some ranges of its flight envelope: it effectively amplifies the small changes in the pilots control to the catastrophic results.

Also see that the final MCAS was delivered non-redundant and also modified to turn itself on under more than one condition and also to exert much more power in overriding pilot than in the initial design.

What you're claiming is a retelling of an initial belief about the MCAS as it was introduced, not about the MCAS as it was actually sold and as it was present in all the planes.

The claim wasn't true then and isn't true now.

Edit: What I'm talking about is documented here:

https://www.seattletimes.com/seattle-news/times-watchdog/the...

"This original version of MCAS, according to two people familiar with the details, was activated only if two distinct sensors indicated such an extreme maneuver: a high angle of attack and a high G-force."

"About a third of the way through flight testing in 2016, as first reported by The Seattle Times in March, Boeing made substantial changes to MCAS.

The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.

Because at low speed a control surface must be deflected more to have the same effect, engineers increased the power of the system at low speed from 0.6 degrees of stabilizer nose-down deflection to 2.5 degrees each time it was activated.

On the stabilizer, maximum nose down is about 4.7 degrees away from level flight. So with the new increased authority to move the stabilizer, just a couple of iterations of the system could push it to that maximum.

Because there are no excessive G-forces at low speed, the engineers removed the G-force factor as a trigger. But that meant MCAS was now activated by a single angle-of-attack sensor."

"While the changes were dramatic, Boeing did not submit documentation of the revised system safety assessment to the FAA."

The "low-speed flight conditions" is effectively what was discovered only in 2016, what was changed in the implementation of the MCAS only then and what was not documented and what eventually caused the crash of both planes.

And they had to add it for safety, not for "type rating." And it seems PuffinBlue is either not aware of that part of the story or intentionally ignores these findings in the comments, sticking to the the first part of the story, the MCAS as it was initially planned, not as it was actually implemented.

The "type rating" motivation was why they lied about what they did. But the intention of the MCAS changes were to make it safer to fly, not to fit the type rating.

Well, then we just disagree on the term.

"Because the 737 Max had been outfitted with larger new engines that could cause its nose to pitch dangerously skyward" (Source: https://www.nytimes.com/2019/03/14/opinion/business-economic... )

I don't call this a stable design. You can crash every airplane (e.g. "stall") but the plane should not enter an unsafe flying mode during normal flying behavior. No good engineer would have chosen that design.

An airplane that can only be kept in the air with a computer must not be necessary unsafe. As far as I know the stealth bombers can only fly with massive computer help. But this must be a rational, conscious decision and the pilot must be aware of this. Not a quick fix to stick huge engines on a plane that was designed in the 60ies.

I can put a Ferrari engine in a Mercedes from the 20ies. And I am sure it will drive. But the car, the brakes, everything is not made for this combination.

When you're talking aeronautics, you should use the definition used by aeronautical engineers.

"Unfortunately, in the case of the 737 Max, it seems that Boeing has built a plane with a fundamental aeronautical issue that it thought would be resolved by adding a new automated system. "

http://jeffwise.net/about-the-author/

It would not prevent certification. It would require pilots to be separately rated for the MAX.

I'm stretching to the limit of my understanding of the matter here but I'll try outline why that statement is misleading.

To put it in basic terms, you want to avoid a situation where an aircraft can enter an 'accelerated stall'. I believe this to be the correct term.

In the original documents to the FAA I think the exact quote of the wording was:

> MCAS “was added to address potential nose-up pitching moment at high angles of attack at high airspeeds outside the normal flight envelope.”

Specifically this was in a banked downward spiral, known as a 'wind-up turn' where instead of maintaining constant resistance on the control column, at a certain point the resistance to pulling back would lessen and this could allow entering an accelerated stall situation.

NOTE - from there I think MCAS ballooned into other areas of control, but I believe that the original intent of MCAS.

The reason this possibility arose was because the engines were moved forward, altering their thrush relationship to the center of gravity of the aircraft and, as I understand it, by the design of the engine pods themselves (flat bottomed and can add lift).

Under normal flight conditions an airliner would never enter a wind up turn, but it is a test that must be passed as part of the certification process, as far as I understand it.

So yes, there are circumstances where the characteristics of the 737 Max can make it easier to enter a dangerous nose up pitch. Note only 'easier' i.e. less resistance on the control column so pulling back with the same force in the same circumstance through the duration of a wind-up turn could allow it to be easier to enter a stall state.

But in normal flight and under conditions an airliner would fly, the nose up pitching moment caused by the engines would be entirely 'normal' and in line with what many airliners do (i.e. control column resistance would remain linear) but would have caused the handling to be different is very specific circumstances and so would have needed a new type rating and pilot training to address.

MCAS sought to avoid the need for that type rating by automatically applying trim to prevent any situation arising, even outside the normal flight parameters, that would cause different handling characteristics.

Again, none of this relates to stability/instability as was my original point.

Sources:

I read about this level of detail here quite a long time ago:

https://www.seattletimes.com/seattle-news/times-watchdog/the...

In looking up this article I found this aviation.stackexchange.com answer that also comments on this:

https://aviation.stackexchange.com/questions/66799/what-is-m...

While I don't claim to know the truth of the matter, I think it's premature to assert forcefully and unconditionally, as you have been doing, that there was no actual instability anywhere in the flight envelope. What I recall having read is that during climb, if the pilots took the AoA to a certain point somewhat higher than the normal range, pitch-up forces would increase to an unacceptable degree. Now, I may have misunderstood, or the source (sorry, can't find it) may have been misinformed. But unless you can find me an authoritative, independent source specifically contradicting this possibility, I am not willing to dismiss it, and I don't think you should be waving it away either.

That said I accept your comment and would happily be corrected, but everything I have seen has stated that there is no inherent instability in the design of the 737 Max.

What I have seen (and is widely reported) is that low speed high AoA may have increased the possibility of stall compared to how previous gen 737's behaved, but we're again not talking about stability as I've been clear to define it. That increase in the possibility of stall appears to be the reason why MCAS became more powerful and operated at these critical phases of flight.

It would obviously make no sense for Boeing to design an aircraft that was unbalanced. That would increase drag and reduce fuel efficiency. It's also understandable that the large and more powerful engines, plus their slightly further forward positioning, would create a slightly higher nose up pitch when compared to the previous 737's under certain phases of flight/flight configurations. That's not to say it made the aircraft unstable or unflyable, but it would suggest a reason why MCAS might be shoved into action there, in another attempt to recreate the same handling characteristics as the previous generations.

That seems far more likely to be where the greater authority for MCAS came from (it would need greater authority under that flight configuration/phase of flight).

It's also far more likely that there was a simple screw-up and MCAS was given too much authority and too little breadth of input.

Every bit of evidence so far suggests a fight for compromise over retaining type certification and use of MCAS. Nothing suggests inherent instability.

The pilot should have to increasingly fight the stick to approach the stall. If the operating characteristics of the plane allow it to approach an accelerated stall with only an increase in thrust, then that is inherently not a stable behavior.

There's not enough information around that I can find to give a concrete answer as to what was found under high speed, high bank angle and high AoA as would be tested in a Wind-Up Turn. Some things I've seem mention it's a simple 'stick-force-per-g' test. Other things I've read suggest there was maybe flow separation under these conditions, which would point to accelerated stall.

What we can say is accelerated stalls are stalls that occur at higher airspeeds than the aircraft would otherwise normally stall due to increased g load.

In the specific example I mentioned above, I believe (I can find scant info on this) that additional handling augmentation was required because of the non-linear force on the control column when conducting the wind-up turn. To be certified I believe the 737 Max needed to return a linear force input requirement throughout the entire wind-up turn.

Another comment has pointed out that it is a basic requirement for all aircraft to have this linear feedback curve, so this wasn't just specifically to make the 737 Max handle like other 737 under this particular scenario (though apparently it did evolve to that under other conditions but I'd have to look into that further to be sure).

Again, this is not about inherent instability.

There are some comments out there on the web I've seen that point to there being some issues at slow speeds and high AoA. I'd love to get more info on both things if anyone has it.

But really, we have to use the correct language for the field we are talking about and that means understanding stability/instability are specifically defined and relate to a very specific thing.

1)This is precisely how you induce the accelerated stall I mentioned. Slow down, nose up till you get to the bottom of the energy curve, then accelerate while continuing to nose up. Hold this straight back, or bank to one side, and you stall. It's one of the most basic training procedures there is, although maybe not for 737s.

2) If accelerating causes nose up on it's own (without pulling on the stick) while near the bottom of the energy curve, you could have a run away condition.

So are you saying a run-away condition can still be "stable" in aeronautical engineering terms?

Because I'm pretty sure that's wrong. You might be studying the wrong books re: the technical language.

In the context of aviation "stable" and "unstable" almost always refer to longitudinal static stability. Sure, it's a technicality but this is a term of art even beginner pilots should be quite familiar with.

>while still being the death trap under non-average conditions

Which has precisely nothing to do with stability, but with the plane deciding to fly into the ground.

Citation needed. No indication of that at https://en.wikipedia.org/wiki/Maneuvering_Characteristics_Au... and a quick Google turns up no suggestion that it's true.

Note the section "In flight test, MCAS changes"

I've posted the link in my other comment too.

The initial introduction of MCAS was motivated by "type rating" issues. The later changes were the real safety issues. The later were activated during the crashes.

But the certification rules for type rating are also there to provide safety, and not some crazy bureaucracy decision. It was designed to prevent the pilots flying the planes for which they aren't trained. Boeing just lied about what MAX is as it was selling it and as it preformed the certification.

So the narration that implies the plane was "inherently" safe is just wrong. It could have been in completely useless purely technical terms "stable" (as ryanlol in the other comment writes that that apparently technically means only "longitudinal static stability") but that is just a red herring in the whole context.

Not for nothing, how is this dismissal of terms helping forward understanding? This is in part a technical discussion with intelligent individuals. If avionics takes ‘stable’ to have a special meaning, then denying the term’s special meaning undermines credibility of those who are working so hard to hold Boeing to account.

I understand this is the attempt, but my understanding is that this is fundamentally impossible and the aircrafts fundamentally handle differently. This is the basis of the allegations of fraud.

Ok, but 737 MAX wihout MCAS is not the aircraft in question. 737 MAX is the aricraft and it comes with MCAS. And that is unstable, right?

In no way is the aircraft unstable with or without MCAS activated.

Delete 'unstable' from the discussion. It's is a completely incorrect term to be using.

MCAS - Movement Characteristics Augmentation System is a system to give the 737 Max the same handling characteristics as previous generation 737.

The use of larger engines on the 737 Max and the short undercarriage meant that the engines had to sit further forward on the wing. This caused a vary very small difference in handling characteristics but was enough to cause the aircraft to need a new type rating as it didn't handle in the exact same way as previous generations.

MCAS was born to make adjustments to the horizontal stabiliser in order to make the aircraft feel the same as before so pilots wouldn't need retraining and the aircraft could keep the same type rating.

Absolutely no part of the 737 Max is unstable and no part of MCAS has anything to do with stability.

This information is widely verifiable for anyone taking the time to look.

If you say "This information is widely verifiable" then post links, please.

We have had numerous discussions on this topic and any pilot with an understanding of the definition of the term will tell you that the 737 Max is inherently stable. My comments parrot his words.

Many here have to understand that learning about these things is not compatible with a 5 minute HackerNews comment. There is a good deal to educate yourselves about. One excellent series to do so comes from Juan Browne - a current 777 pilot:

https://www.youtube.com/watch?v=s3LrsvaCUoo&list=PL6SYmp3qb3...

Yes, that's a playlist, yes watching all of them is necessary to understanding the true nature of the problem. If understanding this was a simple 5 minute thing then you can expect pilots with years of direct training to be able to deal with it. The fact something has gone this catastrophically wrong should give sufficient indication that this is a complex and difficult subject.

There is also clearly a massive misunderstanding about what stability is. This site has a good overview:

https://www.boldmethod.com/learn-to-fly/aerodynamics/3-types...

To misunderstand this means one will misunderstand what went wrong with MCAS. MCAS is about trimming the aircraft (moving the horizontal stabniliser) in order to recreate the exact same handling characteristics as the previous generation 737's.

On the 737 Max (and most aircraft actually) there is a pitching moment that results in the engine nacelle producing lift. Most aircraft will do this to some extent - more power = increase in altitude for many aircraft. The 737 Max is not unusual or unstable in this regard. The important thing to know is that the pitching moment on the 737 Max was different enough due to design and engine changes that the aircraft handled slightly differently in certain phases of flight and in certain flight configurations - namely high angle of attack.

Because the aircraft handled differently to previous generation 737 it weould have needed a new type rating and that would have likely cost billions of dollars in certification and pilot retraining. Boeing saught to avoid this through the use of MCAS.

The important things to know are:

1) If an aircraft model retains the same type rating as previous generations then pilots don't need retraining, this saves money.

2) The 737 Max engines and mounting design produced a slightly higher pitching moment in certain phases of flight and under certain flight configurations than previous 737 designs. This was enough to make the handling different - not unstable, but different. In general terms, this would require slightly different trim level to 'balance' the aircraft under certain flight configurations (notably high angle of attack). Balance does not relate to stability/instability, it relates to balanced aerodynamic forces on flight control surfaces and ensures pilots don't have to continually input control stick movement to counteract an unbalanced setup. Trimming an aircraft is piloting 101 and noting to do with inherent stability.

3) To keep the same type rating, Boeing decided to create an automated system that adjusted the trim on the 737 Max automatically in order to cause the aircraft to handle in the exact same fashion as previous generations 737 under all phases of flight. This automated system allowed the 737 Max to retain the same type rating as pilots flew an aircraft with the exact same characteristic thanks to the automation provided by MCAS.

What went wrong:

MCAS had too much authority and took input of flight data from too few sources. This meant that when those inputs were erroneous, MCAS was able to apply greater nose down trim than could be overcome with other flight control surfaces. In order to recover from this pilots would have to pull the stabiliser trim cutout switches and manually trim the aircraft. Again, this is nothing to do with stability/instability and is piloting 101 stuff.

The problem was compounded further because the failure mode of this runaway trim condition was not clear to the pilots. It presented as a series air speed indication and other warnings with a the highly disconcerting addition of automatic nose down trim. This multi-failure presentation can quickly overwhelm a pilot and make identifying the correct course of action difficult.

Further, stabiliser trim cutout is a memory item, it's not contained in a checklist. So human factors, overwhelming error indications and the fact MCAS was not even explained fully in early manuals meant that pilots didn't immediately know what the issue was. When there is just a few seconds to identify the problem before crashing, these factors combine into a fatal outcome.

Again, this is not aimed at you but the general posting without underpinnings that goes on far too much.

My problem is that when this plane is discussed there are some strange posts that I don't trust (they try to spread blame to the pilots, to question other countries' flight safety, smear things out). Also there are claims from people who may have just done some reading, and kudos to them but they aren't experts. Without more info a post like yours may fall into either of these camps, or you may be an actual expert, or quoting one. Clearly the latter here, so I can trust your info now.

> about these things is not compatible with a 5 minute HackerNews comment

Actually they are. If a verified expert they can present a lifetime of experience in a short sentence, then do. With any luck these threads will become a fraction the length and much more informative.

> stabilizer trim cutout is a memory item, it's not contained in a checklist

All memory items are contained (in written form) above the dashed line on the checklists. Memory item means crews are expected, trained, and tested to execute those actions prior to referencing the written checklists. It does not mean they aren’t on the checklist.

Screenshot: https://i.stack.imgur.com/cRG4J.jpg

It also makes the airplane safer if pilots can transfer their experience and training from one aircraft to another, as it reduces the likelihood they'll do something right for one airplane but wrong for another (there have been multiple crashes due to this sort of mistake, even when they were correctly trained for each type).

Minor side note: More power is supposed to increase altitude. (An aircraft is trimmed for a given airspeed; adding power injects more energy into the system. Then, it doesn't go faster, it goes up. To go faster, lower the nose. To go faster without losing altitude, you lower the pitch and increase power.)

For aircraft with engines below the center of mass, thrust tries to rotate the aircraft upwards around the center of mass. In un-accelerated flight condition, this is counteracted by other forces. But adding thrust increases the force and causes the aircraft to pitch up to a new angle based on the new thrust. This is aside from the more power = more altitude effect.

A second-order effect is that the engine nacelles act aerodynamically. (They're wings, albeit poor ones.) When pitched up, they generate more lift. Because the 737 MAX's nacelles are in front of the wing, their increased lift causes another pitching up force.

None of these forces affect stability; the aircraft will arrive in another stable state shortly after you change the power. However, they can lead to other effects like the control force changes you clearly describe in other comments.

Edit: Source: I slept in a Hilton last night.

MAX has more powerful engines with center of thrust lower than in NG, and they are both physically bigger and mounted such as to exaggerate an already increased aerodynamic pitching force.

The distinction is frequently neglected in discussion here. Both effects contribute to MAX's different handling characteristics, but differently in different flight regimes.

The result is that it is tricky to make a MAX fly like an NG, particularly while trying to conceal the system used to do it.

But even during the development and the test of the plane it turned out that it wasn't enough. MCAS was redesigned to correct for more than that initial flaw -- check the widely researched and published articles about that. There are different moments at which MCAS was modified to turn itself on, and the more power over the pilot was added, to the point of crashing two planes while the pilots fought against the wrongly implemented device (which was made non redundant "to preserve the type rating" -- i.e. to "not affect the sales").

737 MAX under any type rating can't be certified to fly without something doing something similar corrections to MCAS.

There are articles documenting that.

For example.

Vs. how Boeing describes it:

> The Maneuvering Characteristics Augmentation System (MCAS) flight control law was designed and certified for the 737 MAX to enhance the pitch stability of the airplane – so that it feels and flies like other 737s.

Also this: "The MCAS deflects the horizontal stabilizer four times farther than was stated in the initial safety analysis document..[19] Due to the amount of trim the system applies to the horizontal stabilizer, aerodynamic forces resist pilot control effort to raise the nose." Wikipedia

In that case we just disagree on what unstable means. Where I come from it means something that suddenly stops working and that thing happens too often. The plane is grounded because it suddenly stopped working few times. I call that unstable. Like Windows Vista with old GPU drivers was unstable.

(No retraining required. These pilots never crashed a non MCAS plane, and MCAS was actively moving the plane towards the ground). <--- How do you conclude this plane is stable?

That malfunction manifested as the plane pitching down consistently. No matter what the input of the stick, it'd point to the floor.

Now, if a big flashy light+warning buzzer had been sounding (like with a stall warning) then this would have never happened. But it didn't, and because the pilots were not trained for this specific feature (cause it had the same rating) They didn't know what to do.

Its nothing to do with stable. the MCAS silently pointed the plane at the floor regardless of user input.

http://bit-player.org/2019/737-the-max-mess

Lion Air crew did it the day before to save the aircraft. The Ethiopian Air crew almost saved their aircraft by executing that checklist. They then turned the trim back on and for whatever reason only initially commanded nose up electric trim and then stopped, leaving the system powered.

Boeing has significant fault here, but I don’t agree that pilots were not trained to deal with a stab trim runaway (obviously). The crux is whether existing stab trim runaway training was sufficient for the MCAS equipped Max. Evidence suggests it was not.

So, you agree with me? I'm confused.

They weren't trained specifically in what to do in the event of an MCAS failure, because the thinking at Boeing was that the presentation was one of stab trim runaway, something that is already trained for. I have a lot of sympathy for this point of view when looking forward from several years' ago. It turns out to have been proven wrong in the field.

Problem with MCAS, as implemented on MAX, is that it has too much authority and can prevent pilots from maintaining controlled flight under certain conditions such as if feed wrong input from angle of attack sensor.

There are aircraft that are aerodynamically unstable and require flight control systems to maintain controlled flight but they are not used in civil air transport.

"keeping the same type rating" lead to creating an "inherently unstable aircraft". As shown by the fact that over 100,000 hours of accumulated training didn't help.

Saying the "737 Max is an inherently stable aircraft without MCAS" is like saying its stable if it never leaves the ground. It has (well had) MCAS by default.

Minimising the size of this mistake is what is detrimental to the discussion. Boeing should go out of business. They are responsible for hundreds of deaths. If this were a person, this wouldn't even be a discussion. But corporations some how get a free pass on man-slaughter.

It's like if I started to describe your app's database as "unstable" because your website's down due to cloudflare configuration.

(With that said it's all just petty semantics; we all understand what the post is trying to convey)

Ordinarily I would agree with you, but this issue of stability is clouding the truly detestable course of action Boeing took.

Boeing was trying to save money. They wanted to keep the same type rating, so they added MCAS to an inherently stable and flyable aircraft for purposes of retaining the same type rating.

The system they added included optional safety measures that airlines had to pay more to install [0]. Boeing new that MCAS would be safer with these features and charged more for them!

The true outrage here should be directed at this underlying cause and the erroneous discussion over stability should be put by the wayside.

[0] https://www.theguardian.com/business/2019/mar/21/doomed-boei...

Semantics is what words mean.

All mcas does is adjust the offset on the elevator.

it is a failure of UX and oversight. Pilots weren't trained, Alarms weren't installed. Its as simple as that.

This had nothing to do with the aircraft being unstable, but that MCAS had introduced a huge downwards trim as consequence of the sensor malfunction. Had MCAS not been active, none of the accidents had happened.

Any aircraft will make a hole in the ground if its nose is pushed down, which is what mcas was doing.

But the airframe is not mcas. The pitch up movement happens in the NG as well, to a lesser degree.

    > ...  it's an example of what happens when MBA execs, convinced that they can run "any business" take over from a rigourous engineering culture. The Harvard (and other) Business Schools have a lot to be blamed for.

Don't hold your breath for anyone to held accountable, don't expect that business will learn a lesson from this.

Is this documented? Did MBAs from HBS flood the FAA? And how would that lead to a plane falling out of the sky?

I'm with you on the general gist of your comment with regards to HBS - but I'd be careful about starting a witch hunt.

Yes, MCAS is a pernicious and deadly design.

Equally, Boeing's corporate governance and FAA's regulation and oversight are also "pernicious and deadly".

All must be fixed before this aircraft flies again.

If fixing culture is a prerequisite the MAX will never fly again.

From what I can tell flying is far too safe for its own good. Seriously:

Flying is regulated to ridiculous level of safety. At current levels, each marginal dollar spent on flight safety would do much better being spent on driving safety or more radically, preventing deaths from malaria.

If we apply unequal dollar amounts to how much we are willing to pay for each life saved in different areas of life, we are saving fewer lives than we could.

I tried to find some numbers about how much the marginal life saved in aviation costs vs other areas. https://en.m.wikipedia.org/wiki/Value_of_life is a good start, but doesn't have the numbers I was looking for.

The argument about paying two different prices for the same good buying you less than settling on a uniform price is basic economics. But it seems like something the LessWrong people or SlateStarCodex might have a write-up for.

The topic is difficult to discuss because huam life is a 'sacred good' and money is a 'profane good', and it is not socially acceptable to admit that we are making finite trade-offs between the two.

And I do have some sympathy for safety regulations of the aircraft themselves, as they are mostly based on actual engineering, as opposed to the safety theater when you board the plane.

But honestly, people aren't just trading off lives for money, they are trading off lives for minor inconveniences: you could make airplanes safer at no extra cost by mounting the seats backwards and removing the windows.

Windows are the essentially holes, they weakest points on the airframe. Removing them from the design would even save money.

Of course, you'd have a hard time finding enough passengers to pay to fly in you windowless plan.

(The military doesn't have a problem with that. Their passengers don't have much of a choice.)

I've been around computers since well before it was called UX, and since UX? trust me, I never blame users, I definitely blame UX.

we need to bring back UI.

https://en.wikipedia.org/wiki/Yaw_damper

Less stable is not the same as unstable.

https://en.wikipedia.org/wiki/Dutch_roll#Accidents

Based on my reading of this thread, in aviation, "stable" means that, given certain assumptions (fuel supply, etc), the plane will tend to return to equilibrium after pretensions along one particular axis, if you give it no inputs. Pilots in the thread, please correct me if I'm wrong.

You may think to yourself "that's stupid, stable means it doesn't crash!" But if you want to communicate with people in the field, you have to learn their language. If an astrophysicist said "what do you mean that program is stable?? It doesn't have enough hydrogen to roast a marshmallow, much less initiate and sustain fusion!" you would roll your eyes. Sure it's all the same English word, and there is even a common semantic root, but they all mean very different things in their contexts.

>a rudder power control unit malfunction led to a Dutch roll oscillatory instability

>a Dutch roll incident following structural failure of the rudder

>a trainee pilot's actions violently exacerbated the Dutch roll motion and caused three of the four engines to be torn from the wings

My understanding of dutch roll is it takes only a small perturbation to push it off of equilibrium and into oscillation, the oscillation is sickening to people, and it doesn't return to equilibrium without active correction (by the pilot or an active system). A small perturbation means it happens more or less continuously. Correcting the oscillation rather than making it catastrophically worse is something only a well trained pilot can do successfully.

The stable point is not straight and level, it's lurching around sickeningly. I mean that literally, at Boeing I was told about a 707 crossing the Atlantic when the yaw damper failed. The pilot didn't crash it, but the airplane was full of vomit by the time it arrived.

To be technical, it is not stable flying straight and level. The stability point is dutch roll, like a carbureted car does when the engine rpm "hunts" at idle.