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Comment by PuffinBlue | Hacker News Reader

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

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.

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The 737 Max would not pass certification requirements without MCAS. Agreed?

Yes, I stated this already:

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

> the 737 Max flies just fine without MCAS

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.

Can you cite sources for the factual claims you're making here, which contradict what's claimed by Wikipedia and many media sources I've read?

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

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.

> Well, then we just disagree on the term.

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

The author in the NYT is a pilot.

"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. "

Mr. Wise is indeed a pilot, but not one for jetliners. Flying a high altitude jetliner is very different from flying a Cessna. Mr. Wise does not have a degree in aeronautical engineering, nor in engineering of any sort.

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

Note that he doesn't use the term "stable" incorrectly. He points out an issue (that would indeed preclude the 737 Max from being certified without MCAS, as I understand it).

> that would indeed preclude the 737 Max from being certified without MCAS

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

That's not my understanding. I understand (and PuffinBlue concurs, if I'm not mistaken) that the 737 Max would not be certifiable without MCAS.

> Because the 737 Max had been outfitted with larger new engines that could cause its nose to pitch dangerously skyward

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...

An accelerated stall is one of the most basic safety training exercises done. It's fundamentally familiarizing the pilot with the stall envelope of the aircraft.

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.

This doesn't have anything to do with inherent instability.

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.

>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.

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.

Read to the bottom of the answer on that page. Yes, the wind-up turn issue appears to have been the motivation for the initial introduction of MCAS, but (I have read elsewhere) the first version of MCAS did not yet have the motor torque required to override the pilot's force on the controls. But a second issue was found which occurred at low speeds — the article doesn't say what it was, but I'm fairly sure I've read somewhere that there was an actual instability — and in response to which Boeing expanded the allowable amount of stabilizer movement and removed the high-g sensor condition for MCAS to activate.

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.

I've been very specific in what I was saying - and I have been clear on the definition of stability.

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.

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