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Comment by virgakwolfw | Hacker News Reader

Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated.

If plane at a low altitude at full power is stalling, just lower the nose. That is pilot training 101. Why need a system like MCAS to help. The pilot should be able to disengage the autopilot and take control of the airplane. This should be simple.

My suspicion is that there is an inherent, more fundamental problem with the MAX. Before Boeing rushes a software fix, idependendent entities “(remember we can no longer assume the FAA is independent based on what transpired over the last few days) need to investigate and makes sure the plane is safe.

I for one will not fly the MAX regardless of what the FAA and Boeing say. Their reputation for prudence and safety is gone.

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The short answer to your first sentence is that MCAS was needed to give acceptable handling characteristics at high angles of attack. the MAX was deemed similar enough to the NG, such that additional training was not thought necessary, only because of the difference MCAS made.

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

Stability is a more complicated issue than it might seem, for reasons such as aerodynamic stability being insufficiently damped, and the interaction between roll and yaw. An airplane that is statically stable could still be dynamically unstable, and go into a divergent series of oscillations if not corrected. It is very common for an airplane to be statically stable around all three axes, yet be prone to falling into a spiral dive if not corrected (this is probably what happened to JFK jr.) If you try to increase the stability to fix that, you get an airplane that is susceptible to an oscillation called dutch roll. Most swept-wing aircraft, including airliners, have gyroscopic yaw dampers that operate the rudder to counteract this.

Then there's helicopters... All I know about them is that its complicated.

> The short answer to your first sentence is that MCAS was needed in order to make the MAX so similar to the NG versions that it could be claimed that no additional training was needed.

I think that places the emphasis in the wrong place in a tabloid headline kind of way. The reason for MCAS is so that the 737 Max passes the certification requirement that the pitch controls cannot get lighter on the approach to a stall. It's correct in that clearly the previous 737 were certified as meeting this standard, but it's wrong in that even if the pervious 737 didn't exist this would still be a requirement of certification.

Yes, I had realized my mistake and corrected that, apparently while you were composing this reply. I think the current version covers your objection.

Bjorn's Corner in Leeham's has a lot of information on the topic.

It's probably splitting hairs at this point but I don't think that MCAS was nessasary to avoid having different training requirements between the NG and the Max.

The Max may not have been certified without MCAS.

The existence of MCAS certainly seems to have been brushed under the carpet. It's probably fair to say the reason was to avoid creating additional training requirements.

Perhaps the way to put it is that MCAS was necessary for certification, to correct a handling change caused by the installation of larger engines. Separately, the 737 MAX, in its as-produced form (which included MCAS), was considered similar enough to the NG that additional training was not thought necessary. Once Boeing had ruled out design choices such as a longer undercarriage with the engines moved back, or a larger stabilizer (if the latter would, in fact, have helped), the MAX without MCAS was not an option.

I follow all of this, but something doesn't pass the smell test. Disclaimer, I'm a pilot, former CFII, but I don't have much knowledge about aircraft certification requirements.

I cannot comprehend feature XYZ that helps achieve aircraft certification, that can also be disabled by the pilot. Either feature XYZ is mandatory for certification or it isn't.

I can imagine a feature that provides better handling behavior or safeguarding. But if it can go crazy in a way that it's routine to disable such a feature, it must be mandatory the pilot know about the feature's operation, and they must demonstrate competency at handling the aircraft when the feature is enabled and disabled.

And all of that tells me I don't know the full story yet.

I don't think disabling MCAS (or electric stab trim) should be routine. Given most airliners have given up putting trim wheels in the cockpit I'm sure the reliability of electric trim is very high.

Reading between the lines this system was added as a bit of an after thought. There are plenty of systems which have control of trim so I think they probably didn't give it the respect due to stabilizer trim.

There is always the possibility we haven't got the full story. I'm going to check the full preliminary report from Lion air but from what I've read there is some strange behaviours on the trim system that aren't fully explained yet, even by this half baked fix.

I don't see how MCAS obviates a positive static stability requirement, because it can be disabled. But I admit I'm not familiar with FAA requirements in this area.

If the airplane has substantially different pitch behavior, that usually means there'd be a type rating requirement on the pilot, not a lack of airworthiness certification for the airplane. So I'm not really clear on what behavioral requirement MCAS is mitigating. And further I'm not clear how something that can be disabled can help with either aircraft certification or a obviate a separate pilot type rating.

e.g. fly by wire airplanes have various layers of safeguards in place, and pilots type rated for a particular airplane (or models in that same type) are required to understand those safeguards and how the airplane behaves when they aren't in place.

In the 737 MAX case, it's very weird to me that MCAS is somehow a requirement on the one hand, but then it can be disabled without pilots understanding the alternate behavior on the other hand.

> prone to falling spiral dive if not corrected (this is probably what happened to JFK jr.)

Great post, however, my information was that JFK jr. most likely entered a Graveyard Spiral[1], which is a pilot issue, not a plane/aerodynamics issue.

In short, you think you are flying straight, but are in a turn (so banked). You notice you are losing altitude and gaining speed. In level flight, that means you are nose-down attitude, which you correct by pulling back on the yoke. This would fix both issues.

However, as you are in a bank, pulling back to yoke tightens the turn, meaning you lose altitude more quickly and gain more speed. Loop.

It's a situation that is now trained for in basic flight training.

[1] https://en.wikipedia.org/wiki/Graveyard_spiral

It is both a pilot and a plane/aerodynamics issue. Spatial disorientation and the tendency of the airplane to undergo spiral divergence combine to produce the graveyard spiral. If the airplane was unconditionally stable in roll and pitch, the actions you describe would not lead to the increasing bank and dropping nose of a spiral.

The point here is that spiral divergence is possible, without any contribution from the pilot (whether disoriented or not), even in airplanes having three-axis static stability.

https://www.history.nasa.gov/SP-367/chapt9.htm

> airplane to undergo spiral divergence

Hmm...I don't see the need for anything of the sort. The graveyard spiral can be achieved purely due to erroneous pilot inputs, the plane's behaviour is basic aerodynamics:

- you lose lift because the wings are at an angle. Nothing you can do about that relationship.

- the tightening of the spiral is also due to basic aerodynamics/geometry: once you are banked, the lift from the aerodynamic surfaces has a horizontal component in addition to a vertical component. You increase the lift from the surfaces by increasing the angle of attack, you get additional force in the horizontal component. Of course you also get vertical component, so in a normal turn this is fine.

Since there is continuous pilot input, even if the plane were stable in such a fashion as to automatically try to revert to straight and level (which most planes don't, you have to explicitly command exit from a turn), that wouldn't help you in a graveyard spiral.

Now the plane doing this by itself due to instability is an additional problem, sure, but it's not a necessary condition.

I don't think so. Firstly, in your scenario, and with an airplane that is unconditionally stable in roll, there is no tendency for the bank to increase. As the pilot pulls back, the airplane will slow down to the target speed, the pilot will adjust the elevator to maintain that speed, and the airplane will have entered a stable fixed-radius turn, albeit slowly descending because the power is set for straight-and-level flight at that speed. But there has been no aileron input, so the roll stability will bring the wings level. If the airplane was initially trimmed for straight and level flight, and the pilot gets it back to the target speed, it will resume straight and level flight, though not on its original heading.

It doesn't work out this way in practice precisely because the airplane is not unconditionally stable in roll, and exhibits spiral divergence.

Not the planes I've flown.

Well, were the planes you have flown immune to spiral divergence? - that's the point here.

No, that's not the point at all, because none of them were left alone long enough for that ever to matter.

Just as in the case of the Graveyard Spiral.

But that's not a point I seem to be able to get across, so we can just let it rest.

You have correctly described what happens to start a graveyard spiral, and when you say "the plane's behaviour is basic aerodynamics" you are correct - but it is the aerodynamics of an airplane undergoing the onset of spiral divergence, and it is that spiral divergence, together with the pilot's failure to notice what is happening and correct appropriately for it, that leads to the increasing bank and falling nose. The bank increases despite the fact that the airplane has some static roll stability and despite the fact that the pilot has taken no action to command it.

"Three types of airplane motion can result from the interaction of yaw and roll:

1. Spiral divergence results when the static directional stability is great in comparison to the static lateral stability (dihedral effect). If a wing is lowered, the directional stability is greater than the roll stability and the aircraft will not sideslip readily. Thus, the dihedral effect is weak and the wing will not rise to the level position. The airplane tends to enter an ever-tightening spiral dive commonly called a graveyard spiral.

Flight Theory and Aerodynamics: A Practical Guide for Operational Safety (Charles E. Dole & James E. Lewis), page 274.

https://books.google.com/books?id=8DkA9ZcrcisC&lpg=PA274&ots...

> Why is MCAS needed? If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS? The most relevant question is “is the MAX aerodynamically stable?” If it is, why do we need the MCAS? That needs to be investigated.

This isn't even a question. The 737 Max is completely aerodynamically stable. It does however exhibit control behaviours which are undesirable.

A fair analogy for this is probably a car which oversteers, in general a normal family car is designed to understeer, because for your average driver that is safer. In the 737 Max the controls get lighter close to the stall because of lift generated by the engine nacelle. The certification requirements require that the controls don't get lighter. Boeing applied what now appears to be a poorly thought out fix.

What might surprise you is that there are plenty of certified aircraft which are actually aerodynamically unstable, at least along the longitudinal axis. For example the 757 has dual yaw dampers and at least one of them needs to be serviceable before flight. The consensus is that at cruise altitude it would depart controlled flight without one of them working.

I think a fairer car comparison would be this: a new power train is fitted to the car, but it sometimes causes torque steer to the right. To overcome that, the steering wheel gets trimmed to the left when torque steer is detected until torque steer is neutralized.

The system detecting the torque steer sometimes has false positive.

> The certification requirements require

Eh, let's not spin it as "the requirements made them do it", they chose to make the new model stick with the existing 737 certification because building a substantially different new plane would require pilots to be retrained, and they didn't want that as Airbus was ahead of them in the development of the A320neo and they needed that commercial advantage to remain competitive.

I'm not sure where you draw the line though? None of this is new, making modifications to existing designs is the aviation equivalent of developing a new feature. Hanging new engines from an existing fuselage, replacing the avionics and lengthening a fuselage have all been done before with reasonable results. Conversely, clean sheet designs have had terrible safety records initially.

Asking a manufacturer to make a clean sheet design every time they make a change is probably going to result in more accidents than it fixes (see the bathtub curve).

> The certification requirements

The 737 is self certified by Boeing, which is really a joke. Basically a Boeing engineer can sign off and say LGTM with no oversight or independent audit as I understand.

> Their reputation for prudence and safety is gone.

A lot of people felt this went a while ago, see the rudder issues that plagued them during the 1990's and the way they subsequently tried to deny responsibility: https://news.ycombinator.com/item?id=19389983

> If the MAX is so similar to old 737’s and no additional training is needed, then why the MCAS?

That's the problem. The MAX isn't similar enough to older 737s, so MCAS is necessary to change the handling characteristics to be more like older 737s. It's a fix to cover up a lie.

> If plane at a low altitude at full power is stalling, just lower the nose. That is pilot training 101. Why need a system like MCAS to help.

They know this, but different planes still do that in a different way. MCAS was supposed to cover up the fact that the MAX did this differently.

> The pilot should be able to disengage the autopilot and take control of the airplane.

But MCAS is not the autopilot, it's something that tried to make the plane behave like a regular 737 during manual control. So turning off the autopilot does nothing; it's already off.

The problem is clearly that Boeing wanted to pretend the MAX flies just like an older 737. It doesn't. If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary.

> If they'd just admitted that and given pilots extra training, all this mess wouldn't be necessary.

But if they'd done that they probably wouldn't have been able to sell very many of them. Airlines don't want to have to retrain pilots.

And that greed is ultimately what caused this situation. Had they been more honest, these crashes wouldn't have happened, but they would also have sold less planes.

The Max has more powerful engines, mounted further forward on the wings (out of geometric necessity), and as a result has an aerodynamic center of lift further forward.

From the addition of MCAS, I gather (but don’t know with certainty) that they couldn’t make some certification requirement without MCAS.

"If plane at a low altitude at full power is stalling, just lower the nose."

Except as can be seen time and again basic instinct can kick in "damn, plane is falling, I need to be higher" and pilots have been known to pull back on the stick to get height.

A pilot that deals with a stall by pulling up has failed their training.

As I recall, we practiced low speed flight and recovering from a low speed stall either the 2nd or 3rd time I ever went up in a Cessna. Power on stalls were a few days later, they are quite different.

I never did get my license, mainly because I experienced moderate nausea / motion sickness which I thought would abate after a dozen flights or so, but never really got over it.

The problem isn’t that pilots can’t or shouldn’t be relied upon to detect and recover from stalls or near-stalls by increasing throttle and decreasing pitch.

The problem appears to be that a new system, added for the purpose of making a new plane with different handling / characteristics behave the same as an older one for training purposes, is malfunctioning.

The plane could be perfectly safe without MCAS but pilots would have had to be recertified.

> The plane could be perfectly safe without MCAS

It depends on what you mean by "perfectly safe". Many people believe that having the yoke effort decrease at higher angles of attack instead of increase is not very safe. That's why the FAA certification requirements force manufacturers to do whatever is necessary to make sure the yoke force increases with increasing angle of attack, so the pilot has to exert more effort to pull up at higher angles of attack. Without MCAS, the 737 MAX as designed would not meet this requirement.

I agree! After Lion Air I would have been quite hesitant to fly on a Max. Now with all I'm reading - no way would I.

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