My father-in-law is the former chief test pilot for the RAF. And he was Head of Flight Test and Evaluation for large parts of the Eurofighter programme (now retired). He has decades of pilot, engineering and aircraft design experience.

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.