[mannykannot]

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.

[VBprogrammer]

> 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.

[mannykannot]

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.

[VBprogrammer]

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.

[mannykannot]

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.

[cmurf]

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.

[VBprogrammer]

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.

[cmurf]

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.

[mpweiher]

> 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

[mannykannot]

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

[mpweiher]

> 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.

[mannykannot]

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.

[mpweiher]

Not the planes I've flown.

[mannykannot]

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