First, my understanding of the issue -- gleaned both from public reports as well as private correspondence is this:
1. The mass of the engines (the LEAP engines are about 1500 pounds heavier, per engine, than the CFM56s they replaced) coupled with their location forward of the longitudinal center of gravity means that they impart a large inertial moment. Meaning once the airplane starts to rotate in pitch, the mass of the engines resists a change in rotation. Once it starts to rotate, it wants to keep rotating. To me, and I am just a software engineer, that sounds dynamic.
2. The engine nacelles (cowlings) generate lift. That lift is a function of the angle of attack of the cowling. The more the angle of attack, the more lift. The more lift, the more aerodynamic moment. Meaning, once the nacelles start to generate lift, they tend to rotate the aircraft to a higher angle of attack, which causes them to generate more lift. Again, to me that sounds dynamic, not static.
3. Thrust changes cause also a pitch up. I cannot go to why they cause a pitch up other than to note that pitch up with increasing thrust is common (my Cessna does it). This is perhaps, to me, the only non-dynamic part of things because the last thing I would want as a pilot would be an aircraft that pitches up in response to a thrust decrease (i.e. pitches down in response to a thrust increase) as that situation, again to me, would be dynamic.
By dynamic here I mostly mean to imply "makes the problem get worse."
I believe that the 737 MAX is statically stable. By that, I mean that if flying in trim and a pilot removes her hands from the controls, the airplane will continue to fly stably. It will seek its trim speed.
My best answer here is something I wrote in response to an email query. Here is the thread:
Hello, Mr. Travis. Some friends of mine and I have been having an argument over a couple of sentences in your article, namely this:
"Boeing then tried to mask the 737’s dynamic instability with a software system. Big strike No. 2."
Some of us maintain by the use of the word "mask" here, you mean that Boeing is "compensating" for the new instability, and they believe that there is nothing wrong with using software in this manner. Others of us believe you are using the word as in to hide or disguise and it is the action of hiding that is the problem, not necessarily the use of software per se. Can you settle this for us? What was your intent here?
To which I replied:
Much more the latter — it is the action of hiding MCAS that is the root of the ethical problem. As your friends note, compensating for an aerodynamic issue with some kind of machinery is fairly common practice. The use of yaw dampers on swept wing jets being the best example I can think of
Yaw dampers are a necessary evil (but evil still as they add complexity and thus fAilure modes). But no one ever deliberately tried to pretend they didn’t exist
Rhetorically I would argue that there are two destroyed aircraft and over 300 destroyed lives that would, if they could, take exception to the notion that AOA sensor failures are rare. Less emotionally, I would argue that irrespective of the failure rate, it is beyond foolish to design a system that a) relied on only a single sensor output to b) make configuration changes to the aircraft that render the aircraft uncontrollable.
I am glad you mentioned the static port. A static port is nothing more than a hole. My lowly Cessna has a static port. It also has an alternate (backup) static port, which I can switch to with a control in the cockpit. Yes, my Cessna has a backup hole in case the primary hole fails. That should give some idea of how reliable something needs to be before it doesn't need a backup, in aviation. It's something like 100% reliable, maybe a little more.
My understanding, based on my own experience plus that of talking with commercial pilots, is that AoA is pretty much a cosmetic indicator in commercial aircraft. Nobody flying a commercial airliner gives a **** about AoA. They, like me, fly by attitude (pitch) and airspeed. AoA is important in military fighting, but unless you want to take that 737 into a dogfight where you need to maneuver
as close as you can to the stall, it doesn't matter.
Regarding pilots seeing an uncommanded trim activation -- speculation along this line (as well as the speculation regarding pilot error in the article you cite at the top) really gets to me. There is the supposition that a) The pilots are at fault with b) Because they are not skilled enough (Boeing's marketing department does not want people telling airlines that only their most skilled (expensive) pilots should fly the 737 and c) They are not skilled enough because they are brown. In other words, there is a NASTY bit of implicit racism going on here that is entirely unfounded.
In high stress, lift-threatening situations, decision making ability and rational thought go RIGHT OUT OF THE WINDOW. How do I know? I've been hijacked, at gunpoint, while flying a plane (I'd put a link to the story on Medium here, but PilotsOfAmerica says I don't have enough hours to include links in posts, yet ) and it made my brain turn to mush. I don't think that any pilot, of any training, could have known that hitting the stabilizer cutout switches in the event of an MCAS malfunction was a thing to do. Why? First, because an MCAS malfunction does not present as a classic trim runaway. Second, because the pilots were not even told that MCAS existed, or under what circumstances it might malfunction and what that malfunction would look like.
I gotta say it: This guy looks like a PowerPoint engineer to me. A near-complete compendium of buzz words, lots of activities listed but few if any accomplishments, ... Technical Education: 30 YO undergraduate History degree. Patents: none Technical Publications: none.
